Results: Surgical microscopy has been significantly advanced in the technical aspects of high-end optics, bright and shadow-free illumination, stable and flexible mechanical design, and versatile visualization. New imaging modalities, such as hyperspectral imaging, OCT, fluorescence imaging, photoacoustic microscopy, and laser speckle contrast imaging, are being integrated with surgical microscopes. Advanced visualization and AR are being added to surgical microscopes as new features that are changing clinical practices in the operating room.
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Approach: More than 500 references were collected and reviewed. A timeline of important milestones during the evolution of surgical microscope is provided in this study. An in-depth technical overview of the optical system, mechanical system, illumination, visualization, and integration with advanced imaging modalities is provided. Various medical applications of surgical microscopes in neurosurgery and spine surgery, ophthalmic surgery, ear-nose-throat (ENT) surgery, endodontics, and plastic and reconstructive surgery are described.
Aim: This comprehensive review is based on the literature of over 500 papers that cover the technology development and applications of surgical microscopy over the past century. The aim of this review is threefold: (i) providing a comprehensive technical overview of surgical microscopes, (ii) providing critical references for microscope selection and system development, and (iii) providing an overview of various medical applications.
We aim to introduce and explain the surgical microscope and to give an overview of the literature on surgical microscope history, technologies, and applications. (1) We start with the history of the surgical microscope and give a timeline of milestones during the development of the surgical microscope. (2) We introduce the surgical microscope from technical aspects, including its optical system, illumination system, mechanical system, visualization system, and combination with other technologies, such as AR, fluorescence imaging, and OCT. (3) The section on applications refers to the available literature on how surgical microscopes are utilized in surgery. These applications mainly cover neurosurgeries and spine surgeries, ENT surgeries, ophthalmic surgeries, dental operations, as well as plastic and reconstructive surgeries. (4) We conclude with a discussion about the limitations and future directions of surgical microscopes.
First, the integration of HSI, LSCI, PAM, and polarization imaging with surgical microscopes and the related imaging processing methods will become more mature and well-developed, to provide surgical guidance in addition to that of fluorescence imaging and OCT. HSI and LSCI are particularly promising since they are noncontact and label-free, not requiring any injection of any contrast or dye. They can be used anytime during the surgery without administration time and provide abundant quantitative diagnostic information in real-time. Meanwhile, both HSI and LSCI have a very simple system for adaptation and an easy interpretation of images. Therefore, it takes minimal effort for physicians to adopt these methods. The endoscopic tool in the newest robotic visualization system also opens opportunities for some imaging modalities that are not as easy to be adapted with conventional surgical microscopes, such as confocal microscopy and Raman spectroscopy. Second, the visualization of surgical microscopes will be expanded. It will not be limited to a clear view shared only by the team in the operating room. New ways of visualization will give surgeons the freedom to visualize the procedures anywhere through monitors, headsets, smartphones, and large screens in the conference room. With advanced communication technologies and well-developed AR-assisted platforms, a larger group will be able to participate in the procedures remotely. Finally, it is anticipated that surgical microscopes will be increasingly used in more applications such as orthopedic spine surgery and cataract surgery. In particular, surgical microscopes have brought a revolution in dentistry, but they were mainly used in endodontics. Therefore, it is promising for surgical microscopes to be more adopted in other dental applications such as periodontal surgery. In addition, the new endoscopic tool and the picture-in-picture visualization mode give users access to more deep structures, which greatly increases the competitiveness of surgical microscopes in various ENT surgeries.
Surgical microscopes have gone through a long evolution and development. Because of numerous attractive features and new imaging modalities, surgical microscopes will not stop here but will continue to thrive. The limitations of large volume, high cost, and potential tissue damage by high-power illumination will be further addressed with robotic positioning, increasing utilization, and better light management. Three main future directions of the surgical microscope include being integrated with more advanced technologies, launching new ways for visualization, and being increasingly used in more clinical applications.
With numerous advantages such as clear and bright visualization, easy documentation and adaptation, stability, maneuverability, and improved ergonomics, surgical microscopes have been applied in various types of surgeries, including neuro and spine surgery, 8 , 10 , 50 , 59 &#; 61 ENT surgery, 5 , 20 , 51 , 62 &#; 64 dentistry, 11 , 13 , 65 &#; 77 ophthalmology, 15 , 36 &#; 38 , 40 &#; 42 , 78 &#; 80 and plastic and reconstructive surgery. 14 , 81 &#; 86 For example, they have been used for brain tumor resection, 27 aneurysm surgery, 87 nasal surgery, 88 head and neck cancer resection, 89 corneal keratoplasty, 37 vitreoretinal macular hole repair, 90 root therapy, 91 root coverage procedure, 92 craniosynostosis surgery, 93 and hepatic artery reconstruction. 94 For different applications, microscopes are modified into slightly different optical configurations and equipped with specific imaging modalities. The end-users of surgical microscopes include hospitals, dental clinics, other outpatient settings, and some research institutes. 95
Surgical microscopes of the time have been refined to a precision instrument with several appealing features. 19 They have high-precision optics and high-power coaxial illumination, which provide surgeons with adjustable magnifications, proper working distance, and an unobstructed view of the entire operating field. 10 The well-designed mechanical system offers stability and maneuverability, while the heads-up display improves ergonomics. 19 , 21 &#; 25 Stereopsis provides the third dimension of the field of view (FOV) thereby increasing the safety for surgery. 26 Multiple optical ports are available on the microscope for assistant observers or adaptation of video cameras. Moreover, contemporary surgical microscopes are enriched with various intraoperative imaging modules such as fluorescence imaging 27 &#; 35 and optical coherence tomography (OCT), 36 &#; 42 and they are open for adaptation of other imaging modalities including hyperspectral imaging (HSI), photoacoustic microscopy (PAM), 43 &#; 48 and laser speckle contrast imaging (LSCI). Augmented reality (AR) 49 &#; 54 has been actively evaluated on the surgical microscope and has offered huge convenience for surgery as an intraoperative diagnostic tool. Furthermore, high-definition (HD) display, 22 , 55 , 56 image injection techniques, 50 , 57 , 58 and three-dimensional (3D) display facilitate better visualization of both the surgical field and the multimodality images.
A surgical microscope, also known as an operating microscope, is an optical microscope specifically designed to be used in a surgical setting, especially requisite for microsurgery. 17 Although the compound microscope had been invented in 18 , 19 and was used for examination of wounds and scars in the late 17th century, 16 it had several limitations including the heavy weight, large size, and low image quality due to chromatic and spherical aberrations. Therefore, despite the high magnification, it was not widely adopted in clinical applications until the solutions to the aberrations were found. In the late 19th century, Ernst Abbe proposed numerical aperture and greatly enhanced the resolution of microscopes. Later on, the monocular and binocular microscopes were merged with tripods and attached light sources and were used for various examinations. 15 However, it was not until that a monocular microscope truly entered the operating room for an aural surgery. One year later, this idea was modified using a binocular microscope. 20 Ever since, surgical microscopes have been evolving with a wider range of magnification, longer working distance, better illumination, and more stable supporting structures. The benefits were soon acknowledged by otolaryngology surgeons and gradually recognized by surgeons in other fields.
Before the advent of the surgical microscope, surgeons had been using various magnifying systems mounted on spectacles or headbands. These systems can be grouped into three categories, namely single-lens magnifiers, prismatic binocular magnifiers, and telescopic systems. 15 , 16 Single-lens loupes used convex lenses for magnifying with a fixed magnification and a very short working distance. With the desire to have more magnification at a longer working distance, telescopic systems came into use. One of the first closed Galilean telescope systems had a 3 × magnification and a working distance of 15 cm. The Keeler Galilean system introduced in had a 2 × magnification at 25 cm. In addition, a set of five different telescopes, which could be separately fixed on a spectacle frame via screws, offered a choice of magnification from 1.75 × to 9 × and a working distance from 34 to 16.5 cm. The binocular loupe, which uses prism oculars and lenses to achieve stereopsis, was first developed by Westien and modified by von Zehender for the examination of the eye. Later, the Carl Zeiss company presented a binocular loupe with a working distance of 25 cm, which opened the door to modern microsurgery. 16 However, a head-mounted magnifying system suffers from unstable focusing due to the absence of the supporting structure. In addition, increasing the magnification or adding a light source can also increase the size and weight of the system, making it less comfortable for surgeons to wear.
Various diseases, including cancer, require surgery as a prime treatment method. 1 &#; 7 One key factor for surgeons to operate accurately is a clear visualization of the anatomical structures. 8 However, this has never been easy. On the one hand, some anatomical structures are very small, varying from millimeters to microns, 9 and they might have close proximity to other organs or tissue. 10 A clear view of these structures requires a resolution well beyond that of human eyes. 11 On the other hand, the lack of illumination in narrow cavities and deep channels, which are very common in neurosurgery, ear-nose-throat (ENT) surgery, and endodontics, results in a dim visualization with shadows. 10 , 12 , 13 Poor visualization may lead to inappropriate operation on anatomical structures or a nearby organ, which will affect the surgical outcome, reduce organ preservation, or even cause life-threatening consequences. 14 Therefore, sufficient magnification and proper illumination are vital for the success of surgery.
Contemporary microscopes have wide magnification options, sufficient illumination, satisfying balance and stability, and multiple choices of documentation. They are integrated with high-precision automation, as well as sophisticated imaging capabilities, some of them as shows. All these developments led to a terminology evolution as a &#;robotic visualization platform,&#; which indicates the system with significantly more functionalities than a conventional surgical microscope. 60 The new system uses a camera to capture the whole surgical field and replaces the direct interrogation of the light path by a high-resolution, all-digital way of visualization. This gives the surgeon additional freedom of movement and enables the whole operating room team to appreciate the detailed structures. It is particularly beneficial in minimally invasive robotic surgery so the surgeon can be reassured to operate standing by the robot. In this system, endoscopic assistance with a micro-inspection tool is integrated, which helps the surgeon to observe the deep structures and resection cavities and identify blind spots. Moreover, the surgeon-controlled robots make it possible to &#;bookmark&#; a position of the surgical field as well as to visualize the same structure at different angles, providing advantages in time, functionality, and ergonomics. The advent of this new system not only enriches the concept of a surgical microscope with multiple cutting-edge technologies but also unlocks many other improvements and new potential technologies. For example, the simplification of the optical head increases the working distance, providing more space for the use of various microsurgical instruments and the adaptation of imaging modules. The absence of oculars reduces the amount of light required for the assistant observers, thus lowering the intensity of illumination, which in current surgical microscopes may cause damage to underlying tissue. The endoscopic micro-inspection tool offers possibilities for the adaptation and intraoperative use of more imaging modalities such as confocal microscopy and Raman spectroscopy. Moreover, robotic surgery is able to overcome the preexisting limitations of minimally invasive procedures and has led to the possibility of remote surgery. However, the cost of robotic systems greatly limits the popularization of remote surgery. An AR platform developed for virtual surgical collaboration has enabled the cost-effective AR-assisted remote surgery. 113 If integrated with such platforms, this digital visualization system may promote remote surgery in more clinical applications.
In the later generations of microscopes, a trend of integrating a navigation system and advanced imaging techniques became popular. In , a frameless navigation system based on AR was developed for the surgical microscope in neurosurgery. 106 In , a frameless navigation device, namely the multicoordinate manipulator, 107 came into use as an accessory of the OPMI ES neurosurgical microscope. In addition, another navigation system called Multivision was equipped in OPMI Neuro in . OCT as a noncontact optical imaging technique was evaluated with an ophthalmic surgical microscope in . 108 A surgical microscope was modified for fluorescence imaging in . 6 In the following 20 years, the AR-based navigation systems and various imaging techniques were gradually transferred into microscope-integrated modules, which have greatly facilitated image-guided surgery, such as fluorescence-guided brain tumor removal, 109 indocyanine green (ICG)-based intraoperative angiography, 32 and OCT-assisted keratoplasty. 42 Images from intraoperative imaging modules or preoperative magnetic resonance imaging (MRI) and computed tomography (CT) images can be displayed in oculars or on monitors to help surgeons to make fast and accurate decisions. 50 , 110 Meanwhile, more imaging modalities such as HSI 43 and photoacoustic imaging 44 have been evaluated with the surgical microscope.
The OPMI 4 featured a deeper field focusing and 16-mm motion picturing, while OPMI 5 was produced in to overcome the large size of the Zeiss Diploscope. 19 The OPMI 7P/H was capable of allowing three surgeons to work simultaneously with its stereoscopic co-observer accessory. 97 With the increase of observer amount, the light that goes to each observer decreases. Therefore, the OPMI 7P/H applied a high-intensity light source to prevent dimness of the images.
Zeiss OPMI 2, which featured motorized zoom and focus, was manufactured in . 19 The components of OPMI 1 and OPMI 2 were interchangeable. One year later, OPMI 3 was produced, 15 and then several accessories were attached to it, including a measurement scale in the slit, a rotating prism, suturing reticules in the eyepiece, the rotary Galilean device of the OPMI 1 for magnification switch, and a device to sterilize the microscope. 19 shows the OPMI 2 and OPMI 3 microscopes.
The OPMI 1 microscope had a detachable binocular tube that could be replaced by an angled binocular tube. For the stand, which contained a counterbalancing weight and rotating arm, Littman adopted Wullstein&#;s idea but achieved better stability and operability. Later, an electric motor was added to the stand to provide up-and-down motion with a foot pedal. The microscope was capable of being attached with cameras or a second eyepiece with a beam divider between the optical head and the binocular tube. In the same year, three ophthalmological surgeons used this model for surgery. 102 shows the photo and optical diagram of Zeiss OPMI 1.
The invention of Zeiss OPMI 1 in was a momentum in the development history of surgical microscope. 19 Its coaxial illumination had superior performance than other contemporaneous microscopes. 18 Afterward, lots of refinements have been made to improve the operation of the surgical microscope including stability, flexibility, documentation, and share of view.
A changeable magnification, which is a primary feature of modern surgical microscopes, was first achieved in by changing the eyepieces on a modified Bausch & Lomb slit-lamp microscope. 19 This microscope had a working distance of 127 mm and variable magnifications of 3 × , 5 × , 7 × , or 10.5 × . In , Hans Littman (&#;) invented the microscope that was capable of changing magnifications without changing the focal length. 19 , 20 The so-called &#;Zeiss-Opton&#; microscope ( ) with a working distance of 200 mm and magnification options of 4 × , 6 × , 10 × , 16 × , 25 × , 40 × , and 63 × was a start of a new era.
The working distance of a surgical microscope gives a surgeon space to handle surgical instruments. The working distance of the first monocular microscope was 60 mm, and the first binocular microscope had a working distance of 75 mm. They were acceptable but shorter than the ideal length of 200 mm proven by most otology operators at that time. 20 In , the available working distance was improved to 228 mm, and 250 mm in . Zeiss OPMI 1 came out in and had a working distance of 100 to 405 mm. 19 Since then, working distance has been improving to meet the need of different types of surgeries varying from 200 to 500 mm. 10 , 103 &#; 105
In addition to the heavy tripod, Tullio and Calicetti&#;s invention in also included mounting prisms between the oculars and objectives, so that the identical view of the surgical field was successfully shared to an assistant surgeon. 16 And in , Littman adapted a beam splitter to a Zeiss microscope to allow a second surgeon to assist and named the microscope &#;Diploscope.&#; 19
The first modern surgical microscope tripod was built in when P. Tullio and P. Calicetti at the University of Parma constructed a heavy tripod with counterweights for the optical unit. The tripod was able to hang the optical unit freely above the surgical table and stabilize the image, thus it offered comfortable distance and mobility during the procedures. 16 In , the V. Mueller & Co. started to market a microscope with a suspension system consisting of a weighted table stand, then a ceiling-mount microscope suspension system was designed by Joaquin Barraquer in . 19 In addition, Horst L. Wullstein from Gottingen, Germany, built a microscope mounted on a stand equipped with a rotating arm. This idea greatly improved the mechanical flexibility of the microscope and was employed in the &#;Zeiss OPMI 1&#; (Zeiss Operating Microscope 1), 19 , 102 which was a milestone in the history of the surgical microscope.
The advent of the surgical microscope and its early applications in aural surgeries offered better surgical outcomes, and the improved vision helped relieve other diseases in the temporal bone. 20 Nevertheless, surgical microscopes in the early times had several weaknesses. The low stability of the tripod decreased the quality of images at high magnification, 19 and the fixed magnification, small FOV, as well as insufficient illumination limited the effective vision of the surgical field. 20 Furthermore, the old microscopes only allowed one surgeon to view the surgical field, which was inconvenient for assistance. In the following years, refinements have been made for surgical microscopes and made it a real precision and fundamental instrument in the operating room.
In , Carl Olof Nylén (&#;), a young otolaryngology surgeon at the University of Stockholm, used a monocular Brinell&#;Leitz microscope during surgery on a patient with chronic otitis. 20 , 96 It was reported as the first surgical microscope in the operating room. One year later, Gunnar Holmgren modified this idea with a binocular microscope attached with a light source. 19 , 20 The binocular microscope provided depth perception that was absent in a monocular microscope, and the attached light source overcame the dimness of the image with increased magnification. 19 , 20 , 97 shows Nylén&#;s first surgical monocular microscope and the binocular microscope of Holmgren. After being used in ENT surgeries, the binocular surgical microscope was then introduced into ophthalmology by the ophthalmologic surgeon Richard A. Perritt in . 98 Although nowadays neurosurgeries form the leading market of surgical microscopes, this instrument was not introduced in the neurosurgical operating room until , when Theodor Kurze at the University of Southern California, Los Angeles, removed a neurilemoma from cranial nerve VII in a 5-year-old patient. 19 Kurze was inspired by House, who removed an acoustic neurinoma and published his experience in . 10 , 16 Two years later, Pool and Colton used the microscope for intracranial aneurysmal surgery. 10 In , Fritz Zöllner reported using a binocular surgical microscope in 120 attic-antrotomies with plastic operations. 99 In , Wolfgang Walz from Heidenheim (Brenz), Germany, reported his experience with microsurgical reconstructive surgery of an occluded fallopian tube. 16 , 17 The introduction of the surgical microscope in endodontics was the latest. In , the first dental surgical microscope (DSM) was produced by Apotheker and Jako, 100 and in the design was incorporated into the first commercially available surgical microscope for dentistry. 101
A surgical microscope can be roughly divided into a microscope body, a light source, and a supporting structure,72,73,114 and each of these is vital for the performance of the microscope. Besides these three conventional parts, modern microscopes have adopted advanced technologies to facilitate visualization and surgical navigation.
The microscope body has all the high-precision optics that provide a clear magnified image with the minimum distortion.115 The binoculars mounted on the microscope head offer stereopsis.116 Multiple optical ports are open for adaptation of imaging devices such as video cameras or for assistants to share the identical FOV.25,117
The light source is installed away from the microscope to avoid heating the microscope optics or the surgical site.8 Commonly used light sources for surgical microscope are xenon light bulbs, halogen light bulbs, or light-emitting diodes (LED).118&#;120 Illumination from the light source is transmitted to the microscope through a fiber guide, then passes through the objective lens and illuminates the surgical site at a distance that is subject to the focal length.115 A good illumination arrangement, such as coaxial illumination, overcomes the shadow and dimness of the FOV.74,114 Meanwhile, the advanced light management ensures the stability of illumination as well as the safety of tissue.
Based on the configuration, there are four types of surgical microscopes: (i) on casters, (ii) wall mounted, (iii) table top, and (iv) ceiling mounted. The on-caster stand is the most popular supporting structure due to its better mobility, but a ceiling mount or wall mount can help with space management.73,95 The supporting structure of a modern microscope has precision motorized mechanics so the microscope can be balanced easily and adjusted flexibly to the right position. It is also a fundamental task for supporting structure to keep the microscope stable. Various controlling methods have been developed for &#;hands-free&#; operation, and the improved ergonomics reduces surgeons&#; strain during long surgery hours. Furthermore, some new microscope stands have HD display and documentation devices that facilitate the sharing of the operation process.
The adoption and modularization of advanced technologies for image-guided surgery have been actively evaluated in recent decades. On the one hand, the intraoperative imaging modalities have been evaluated with surgical microscopes to provide real-time diagnostic information. The imaging modalities utilize certain properties of human tissue and reveal information that is beyond what human eyes can see, even the deeper structures beneath the tissue surface. To apply these imaging modalities, certain system adaptations have been done for the microscope. The goal of system adaptation is to enable and disable these imaging functions easily without interrupting the surgical workflow or decrease the performance of the microscope. On the other hand, AR has been playing an important role in new generation microscopes, especially with the development of minimally invasive surgery. It helps surgeons relate the preoperative two-dimensional (2D) images with the real 3D surgical site intraoperatively for navigation. AR can work with various image modalities, either preoperative or intraoperative, and overlay the images onto the surgical site so the surgeons do not need to switch their sight between the surgical site and images. In addition, the overlay of images reveals not only the 3D model but also the anatomical structures beneath the patient&#;s skin. With proper system adaptation, accurate calibration and registration, and convenient visualization methods, AR could greatly aid in the clinic for surgery.
In this section, we provide detailed technical descriptions of a surgical microscope, including its optical system, illumination, mechanical system, and visualization. Advanced technologies employed with surgical microscopes for image-guided surgery will be explained, namely the AR, intraoperative fluorescence imaging, microscope-mounted OCT, HSI, and photoacoustic imaging. The purpose of this section is to provide a comprehensive explanation of the principle of the surgical microscope and how advanced technologies are adopted. It provides references for microscope selection and system development.
The optical system of the microscope is the main determinant of the imaging quality that a system can achieve. It is basically a binocular (with eyepieces on top) with a close-up lens, namely the optical components including the objective lens and the magnification changer (or zoom changer).26,115 The focal length of the objective lens fully determines the value of working distance, which is the distance from the objective lens to the point of focus of the optical system. The zoom changer is either a series of lenses moving in and out of the viewing axis or a system that changes the relative positions of lens elements.115 The binocular is equivalent to two telescopes hinged together, wherein prisms are used for a compact size of the unit. Stereopsis, which introduces the depth information into the surgeon&#;s vision, is an important feature brought by binocular and will be discussed in the visualization section.
Clinicians from different fields have recognized the usefulness of magnification.10,25,62,72,82,103,114,121
The total magnification (Mtotal) of a surgical microscope is determined by all the four optical components in the microscope, namely the focal length of the objective lens (fOBJ), zoom value (MZOOM), the focal length of binocular (fTUBE), and the magnifying power of eyepieces (MEP),115 as Eq. (1)
Mtotal=fTUBEfOBJ×MEP×MZOOM.
(1)
Magnification of modern surgical microscopes varies from 4× to 40× 10,73,122 and is usually selected through a manual or motorized magnification changer. The zoom value is usually 6:1 but can be as high as 8:1.123 For some microscopes, an additional magnification multiplier is applicable, which provides 40% more magnification.124 Resolution measures the acuity improved by magnification. It is the ability of an optical system to distinguish two separate entities.74 Human eyes have an inherent resolution of 0.2 mm125 but with 20× magnification, it can be increased to 0.01 mm.126 This can add more confidence to surgeons, enhance the advanced surgical skills, and enable the use of many fine surgical instrumentations when they operate on fine anatomical structures.121
The design of optics is vital to the image quality of a surgical microscope. Aberration is an inherent property of optical systems, and it causes the blur or distortion of images, which is adverse to the desire for a clear view. Monochromatic aberrations such as spherical aberration, coma, and astigmatism can be corrected but usually only for one color.127 Chromatic aberration is a failure of a lens to focus all colors to the same point, because of which images show color fringes and lose sharpness. Chromatic aberration correction is necessary for optics in a surgical microscope not only because of the wide-band light source used but also due to the image enhancement in cameras, such as sharpening and edge enhancement, which enhances the image edge as well as the color fringes.128 Achromatic lens, which is a combination of converging and diverging lens elements, was employed in early surgical microscopes to correct the primary spectrum, leaving the secondary spectrum being the main factor limiting the image quality.16,19 The apochromatic lens is the answer to that problem. It not only corrects for two wavelengths (red and blue) to reduce spherical aberration but also utilizes the exceptional quality optical materials that have unusual and desirable characteristics to reduce chromatic aberration for three wavelengths (red, green, and blue).19,129
Focusing is essential for a clear view. Surgeons would want the surgical site to be in focus throughout the surgery. However, the shape of organs or the deep cavities makes it impossible for the whole surgical site to be perfectly on the focal plane. Depth of focus, in other words, depth of field (DOF), is a term that indicates the area in front of and behind the point of perfect focus where the sharp focus is maintained. It depends on many factors, including but not limited to the quality of optical design, the size of objective lens aperture relative to the focal length of the objective lens, and the magnification of the object, and it is reciprocal of the resolution.115 A good surgical microscope should have an adequate depth of focus without sacrificing too much resolution to keep the scene sharp. Another important term is parfocal, which means an optical system can stay in focus even with magnification changes.130 Due to the need of switching magnification during surgery, a surgical microscope being parfocal saves surgeons from repeated refocusing.
Microscopes need to be well focused before the operation, and when the position of the microscope is adjusted during surgery, refocus is needed. A fast focusing capability can save setup time for surgery. Various methods have been proposed for the automatic focusing of the surgical microscope. Nohda131 proposed an automatic focusing device for a stereoscopic microscope, which detects the position of the reflected image of infrared LED (IR-LED) using a focusing screen. The positions of the IR-LED and the focusing screen are conjugate with the in-focus position of the sample; hence, the reflected image of the infrared diode is at the center of the focusing screen when the sample is in focus. Jorgens and Faltermeier132 proposed using the interaction of an active light-projecting system and a passive video system to focus on both covered and uncovered objects illuminated by the transmitted and reflected lights. Vry et al.133 proposed a high-precision optical arrangement for stereomicroscope autofocusing, where a cylinder optic is employed to project a bar-shaped mark onto the object. Many current microscopes are equipped with fast autofocusing optics, which uses two laser beams acting as a focusing reference to find a focus point rapidly. The focus point works for not only the main viewing position but also the assistant position and camera. Furthermore, methods have been developed to maintain the surgical microscope in focus at different viewing points. For example, Heller134 proposed a mechanical control unit for a surgical microscope support stand, and the unit constrains the microscope to move along a spherical surface so the focusing status can be maintained.
Illumination is another key factor besides the optical system for the imaging quality of a microscope. Successful surgical illumination has four key factors, namely the luminance, shadow management, volume of light, and heat. A bright view of the whole surgical site throughout the surgery is always desired. The original illuminator in the earliest surgical microscopes was an independent bulb externally mounted on the side of the microscope. Light transmitted to the surgical site likely creates shadows, and thus illumination of deep cavities was hardly possible.20,115 Modern microscopes have adopted high-power light sources with stable light intensity and close-to-sunlight color temperature.73,122,135 With the built-in coaxial illuminator, light is rerouted to the viewing axis and projected down through the objective lens.115 It is beneficial to remove shadows in cavities and complex structures and especially cause a red glow of the retina that assists cataract surgery. Light management methods have also been developed to guarantee a stable and relatively safe illumination regardless of any change of magnification or working distance. In addition, some modern surgical microscopes offer an option to set up various lighting profiles for different tissues, so by controlling the light sensitivity of the integrated recording camera, a change of color temperature may appear as images of the surgical field are displayed on the monitor.
Despite numerous advantages of surgical microscope illumination, it is still worth noting that many up-to-date neuro and spine surgical microscopes use light sources of the highest intensity to provide the best brightness and clearness for human eyes regardless of magnification and working distance. However, the high power can damage the underlying tissue. Though manufacturers of surgical microscopes provide safety warnings of possible damage, specific settings of the illumination are not regulated.136 Nevertheless, the International Organization for Standardization (ISO) -2 standard and the American National Standards Institute (ANSI) Z80.38 standard have set requirements for the maximum retinal exposure limit and the stability of light intensity of ophthalmic surgical microscopes.137,138 Besides, the International Electrotechnical Commission (IEC) has set general requirements for the characteristics of surgical lighting, including a central illuminance of 40,000 to 160,000 lux, a color rendering index between 85 and 100 Ra, and a color temperature of to K. This standard does not apply to the lights for surgical microscopes since they are excluded as &#;special purpose lights with different conditions of use,&#; but these requirements may offer a general idea for the requirement of surgical illumination.
Except for the traditional incandescent bulbs used in old surgical microscopes, there are mainly three types of light sources, i.e., xenon lamp, halogen lamp, and the LED. LED can provide illumination in the visible wavelength range with good brightness, good stability, longer life, less power consumption, and extremely low heat; therefore, it is preferred in many ophthalmic and ENT microscopes.139 However, LED as a surgical light source also has disadvantages: the higher color temperature and narrower wavelength range make the light not as close to sunlight; its spectrum is insufficient for fluorescence-guided applications especially ICG imaging, where an excitation light in the NIR range is needed; moreover, it is not easy to replace.
Xenon lamp and halogen lamp are two options to address these needs. Xenon lamp emits light with a broad spectrum from ultraviolet (UV) (185 nm) to infrared ( nm). The spectrum is relatively smooth in the visible range, but it has some spikes in the near-infrared (NIR) range. Xenon light has a color temperature of to K, which is similar to sunlight. Therefore, the bright-white light is able to offer a naturally colored view of the anatomy. Halogen lamp also covers a wide and continuous spectrum including visible and NIR light, but it has a slightly lower color temperature ( to K), which means the light does not look as &#;white&#; as xenon light. Both xenon and halogen lamps can provide a stable illumination with DC regulation power employed. Nevertheless, the surgical microscopes do not use all the wavelength range of xenon lamp or halogen lamp. Actually, UV light and infrared light above &#;&#;&#;nm are filtered out for surgical microscopes to avoid various possible damages to the patient&#;s skin or eye caused by exposure in this wavelength range.140,141 Xenon and halogen light sources are commonly used in neurosurgical microscopes because of the need for intraoperative fluorescence imaging. They are also utilized in some ophthalmic and plastic microscopes. For example, some ophthalmic microscopes may employ a dual-illumination system combining LED and halogen for Red Reflex and normal illumination. Both halogen lamps and xenon lamps emit much heat. Therefore, in a surgical microscope, the light source is installed away from optics, and a fiber guide is used to transmit light from the light source to optics without carrying the heat.
The tissue surface being viewed under a surgical microscope during operation is usually wet and highly reflective. The light that comes from an angle can be easily reflected away and cause a dark view, as shows. Coaxial illumination is the solution to this situation. Different from lateral illumination where light comes from the side, coaxial illumination matches the optical axes of illumination and visualization (lens).142,143 Illumination from the light source that locates on the side is diverted and projected almost parallel to the axis of the lens, as shown in . Therefore, light vertically illuminates the tissue surface and is reflected directly to the lens, not having much loss. Coaxial illumination reduces the diameter of the illuminated area,144 moreover, it can be directed into narrow and deep cavities, which is helpful for neurosurgery, ENT surgery, and endodontics.72,144
Open in a separate windowThe light path for coaxial illumination in nonophthalmic surgical microscopes, such as neuro or ENT microscopes, usually forms a small angle with the observation axis in the range of 6°.144&#;146 In some contemporary surgical microscopes, it is called small angle illumination (SAI),124 which provides a concentrated and evenly distributed light beam, a bright view, and an improved depth perception, as shows. With SAI, the shadow that appears at the edge of the viewing field is significantly reduced. Illumination with an even smaller angle is important when it comes to certain ophthalmic interventions especially cataract operations, where the vertically impinging light gets diffusely reflected by the fundus and the pupil under operation shines reddish, which is called red reflex.144,145,147 The production of red reflex requires a small angle between the illumination beam path and observation beam path, in the range of 0 deg to 2 deg.144,146 Although the red reflex helps under certain circumstances, it does not help as much in revealing good plasticity without the shadows on the structures in the interior eye.145 Therefore, both types of illumination, namely the 6 deg and the 0 deg illumination, are usually equipped in ophthalmic surgical microscopes.
Open in a separate windowA desirable illumination for a surgical microscope should provide a stable brightness for the viewing area regardless of the change of working distance or magnification. In fact, irradiance (irradiation of a surface, W/m2) of a microscope light source increases with decreasing spot size and decreasing working distance.136 With an unchanged illumination setting, the increased working distance can cause insufficient irradiance, while the decreased working distance excessive irradiance. Insufficient irradiance makes the view unclear, while excessive irradiance may cause soft tissue burns.148 Similarly, decreased magnification, which enlarges the FOV, may lead to the dimness, while increased magnification may burn tissue outside of the FOV. To address this issue, many contemporary surgical microscopes are equipped with smart light management, which adjusts light intensity automatically with the change of working distance [ ] or magnification [ ].
Open in a separate windowThe structure of the whole surgical microscope system can be delicate and complicated. It assembles every part of the system and makes them work together harmoniously. A well-designed system can assist surgeons with good stability, sterility, easy operation, as well as comfort. Mechanical stability is the second most important criterion in selecting a surgical microscope.130 The drift or vibrating of a microscope after positioning distracts surgeons&#; focus on the surgical site. Therefore, superior suspension and balancing mechanisms are important. Microscope draping is a necessary requirement for sterilization in the OR. A good draping design saves the setup time for the microscope and avoids the effect of glare.149 Various controlling methods have been developed to enable hands-free operation for surgeons.134,150&#;152 Moreover, different parts of a surgical microscope have been designed to improve its ergonomics and maneuverability.60,153 This section will discuss some important features that affect the operation of surgical microscopes involving its mechanical design and electrical automation.
As is known, surgical microscopes should be quick and effortless to move and remain stationary once the position is established.130 Balancing of the forces and moments from all directions should be achieved, otherwise, brakes or bracing devices are needed to hold the microscope in its position.154 Many suspension structures and balancing apparatus have been developed for a fast and reliable balancing of microscope.154&#;160 Modern surgical microscopes have made it an easy and time-saving process to balance. All six axes can get fully balanced with two pushes of a button, and intraoperative rebalance can be quickly and accurately accomplished with a single push of button on handgrip.
In recent years, a robotic autopositioning feature has been added to state-of-art surgical microscopes.161&#;163 The robotic ability enables the microscope to orient the angle or change its focal length so surgeons can target a specific point of interest, which is probably identified in a preoperative imaging study. Oppenlander et al.162 developed the automatic positioning movement control with three options. The first option is &#;auto lock current point,&#; which makes surgeons lock onto a target by changing the angle and focal length of the microscope to keep it in focus at one point while being manually moved. The second option is &#;align parallel to plan,&#; which positions the microscope to a preset angle and focus without any need to adjust the microscope. The last one is &#;point to plan target,&#; which automatically adjusts the focus on a predefined target. In a newly developed robotic visualization system, two robotic positioning features, namely &#;point lock&#; and &#;position memory,&#; have brought many advantages in time, functionality, and ergonomics.60 With &#;point lock,&#; the microscope head stays in focus when being manually or automatically moved during surgery, so the surgeon can visualize different angles of the same structure. &#;Position memory&#; makes the system able to &#;bookmark&#; positions and transit quickly and smoothly back to these positions with no need to rediscover. In some circumstances where the scope needs to be moved around to observe different structures or be temporarily removed to get an x-ray, &#;position memory&#; can save plenty of time getting the scope back to the same position. Previously, it was reported that around 40% of surgical duration was spent on adjusting the microscope. But with all these robotic positioning features, the surgical duration can be greatly reduced.60
Microscope drape is a very thin, transparent, and heat-resistant plastic film that houses the whole surgical microscope, and it includes a transparent optical lens enclosing objective lens and ocular-housing extensions.164&#;166 The drape is seamed for sterile packaging to assure the microscope sterility during surgery.167 To save the setup time of a surgical microscope, instant readiness is required. Meanwhile, the drape must have adequate ocular pockets, not reduce the working distance, not interfere with surgeons&#; operation or obstruct the view. Bala168 invented a microscope drape assembly, which has four ocular pockets for different needs and does not affect the working distance or visualization by locating the objective lens window support within the objective lens barrel.
Glare is one problem that comes with draping and illumination. As light passes through the objective lens and illuminates the surgical field, some of the light would be reflected by the lens cover on the drape, which can cause chromatic and spherical aberrations.149 Removing the cover, however, can cause the contamination of surgical instruments. A dome-shaped objective lens cover169 can not only reduce the reflection but also compromises the magnifying performance. Surgical microscope manufacturer has brought up a solution by replacing the sterile lens cover with a slanted one, and another attempt solution is to include the slanted lens cover in the sterile microscope drape. Both methods can eliminate glare, with the price of increased costs or system complexity. Langley149 has invented a glare elimination device for surgical microscopes. The device includes three parts: the first part is to be attached to the surgical microscope, the second part is for the sterile drape, and the third part is a body to connect the other two and provide an angular offset. The device can be semipermanently attached to a surgical microscope with more convenience. Weaver et al.170 proposed an apparatus that provides a secondary holder for a cover to be applied to the objective lens barrel. The additional cover can be rocked and rotated easily to a position where the view is not affected by glare.
Surgical microscopes can be controlled in various ways to facilitate easy use of the microscope and to free surgeons&#; hands during surgery. Contemporary surgical microscopes are often equipped with footswitch devices171 for generating control commands, touch-screen58,172,173 for operation mode selection or switching images intraoperatively, or joystick control174 for highly precise micropositioning. Mouth switch175&#;177 is a commonly employed controlling method. Surgeons can use the mouth switch to change signals simply by holding the levers with a mouth, in which way operation errors can be reduced, even with a large number of functions to control.175 Eye controlling is another trend for surgical microscopes. Charlier et al.178 proposed an eye-controlled surgical microscope, which used an IR-LED to illuminate the surgeon&#;s eye and a charge-coupled device (CCD) sensor to detect the reflected infrared light from the surgeon&#;s eye for movement tracking. Similarly, Roduit et al.152 proposed an eye-guided controlling technology, which used a CCD camera mounted on the right ocular of a microscope and continuously monitored the surgeon&#;s eye. With the eye-guided control function, surgeons can use their eyes to perform multiple tasks including access to built-in data display, laser aiming, and control of autofocusing. Voice control is a sterile remote control that facilitates operator in either sterile or nonsterile region and does not require the operator to take action.151 Furthermore, Pitskhelauri et al.150 developed a device named Mari, which allows hands-free utilization of surgical microscopes. The device was attached to the eyepieces of a surgical microscope, and operators can use the joystick and electric switch to do multifunction control of the microscope.
Except premium optics, good illumination, and various image-guided surgical functions, one nonnegligible benefit of surgical microscope compared with traditional loupes is the ergonomics,25,73,130 which guarantees a comfortable and flexible working position for surgeons and reduces the risk of back and neck musculoskeletal injuries.25 Meanwhile, maneuverability is valued for the simplification of microscope operations.130 Therefore, the microscopes of the time are equipped with a full range of movement and tilt of the optics carrier, as well as a selection of binoculars with full 360-deg rotation for different heights and positioning needs. Some microscopes have large HD monitors so that surgeons can all work with an upright position. Eye-to-object distance115 indicates the distance from the observer&#;s eye to the focus point of the microscope. Surgeons are likely to be more comfortable with a longer eye-to-object distance. In addition, new designs of surgical microscopes are trying to provide longer working distances up to 600 mm179 to offer better ergonomics, easy maneuvering, and more working space that allows long instruments. For example, Horizontal Optics Technology, which is employed in the state-of-art surgical microscope, enables a compact optics carrier and further ergonomics.180 shows how a surgical microscope can improve ergonomics for an endodontic surgeon as an example.
Open in a separate windowClear and bright visualization of the surgical site is the ultimate goal of using a surgical microscope. Except the good image quality provided by high-precision optics and sufficient illumination, the stereoscopic view that offers depth information is another non-negligible benefit of the binocular surgical microscope. Despite that stereopsis is the result of optical design, it influences how surgeons obtain information from and feel about what they see.
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Users of surgical microscopes can observe the surgical site in various ways. A microscope head usually has one main observation port and one rear or lateral port for co-observers, who can be assistants, students, or trainees. Cameras182 or other imaging systems183 can also be adapted to these optical ports for video recording or photography of the ongoing surgery. All optical ports offer an identical FOV, which beats surgical loupes and enables &#;cosurgery.&#;25,184 With the image injection technique, not only the white-light image of patient tissue but also pre- and intraoperative images can be overlaid accurately with the white-light image for navigation.57,58 HD display105,185 and 3D display22 have been employed in the surgical microscope for sharing of the view with high resolution and enlarged stereoscopic images. Other visual methods, such as using smartphones for recording and virtual reality (VR) headsets for visualization, have also been developed.186 With the advanced technologies applied, surgical microscopes can help surgeons see much easier than ever before.
Stereopsis is a key feature of binocular surgical microscopes. While the monocular depth cues lie in perspective projection, occlusion, size, shading, and motion parallax, the stereoscopic depth is based on the slight disparities between two images presented to two eyes.187 Stereo microscopes use two afocal relay zoom lens systems for the two channels of a binocular tube, and their axes are parallel to and offset from the axis of the objective lens.188 The light coming out of the objective lens is divided into two parts and forms two slightly different images into two channels. In surgery, especially when working with magnification, perspective, and size cues may be lost; therefore, the stereopsis brought by binocular is essential to provide a 3D impression of the surgical field. The depth information can aid the detection of diagnostically relevant shapes, orientations, and positions of anatomical features, especially when monocular cues are absent or unreliable.187 For example, it is vital for dentists to construct 3D structures in patients&#; mouth74 and for neurosurgeons to understand complex volumetric relationships of neuroanatomical structures.60
An optical design that enhances stereo visualization for surgical microscopes is FushionOptics technology,189 which sets two separate beam paths in the optical head, providing the DOF and high resolution, respectively. The two paths are then merged in the observer&#;s brain into a single, optical spatial image. Because of this combination of depth and resolution, the interruptions for refocusing can be avoided.
There is usually more than one observer during surgery, which makes the &#;share of view&#; an important and necessary feature for surgical microscopes. In some procedures, meaningful assistance has to be given by a cosurgeon sharing the same view with surgical microscope.25,117,190 It aids not only assistance but also teaching and participation of trainees.25 The simplest way to share the identical surgical view is to use an optical splitter to split the light into two eyepieces.142 Nowadays, surgical microscopes can have multiple optical ports for the main observer, assistant observer, and external cameras. Some models have integrated HD cameras and monitors so the whole team can share the view on the screen.19,25,117,191
Many new surgical microscope models, especially neurosurgical microscopes and ophthalmic microscopes, are equipped with HD video cameras and large HD monitors, so subtle details can be viewed more clearly and shared by the whole team.56,58 In addition, 3D screens, which employ passive linear polarization technology, have been brought to the operating room to deliver depth perception.22,192&#;194 Observers need to wear goggles to have a real-time 3D view, which gives a realistic appraisal of certain features. It was reported that screens possibly offer better contrast of the visual field than eyepieces and image injection in some cases.58 Moreover, utilizing screens enables the heads-up display, which is beneficial for surgeons&#; spinal health during long procedures.
A screen can show not only the white-light image of the surgical site but also other images, such as intraoperative OCT images, for surgical guidance. The images can be shown separately, overlaid on the white-light image,58 or even in picture-in-picture endoscopic assistance view60 for endoscopic microinspection tools, as shown in .
Open in a separate windowOCT is a noncontact and minimally invasive imaging modality that measures the scattered light from tissue.275,276 It is able to provide submillimeter-scale spatial resolution as well as subsurface information,51,275 which is vital for structural evaluation and instrument positioning during surgical procedures. Because OCT is based on optical communication technology, it is also more cost-effective.275 Compared with other tomographic imaging modalities, such as MRI and CT, OCT has much better intraoperative performance, such as higher resolution, no danger of ionizing radiation, and more compatibility with surgical instruments.275,277
OCT was first demonstrated for cross-sectional retinal imaging by Fujimoto&#;s team at Massachusetts Institute of Technology.276 Since the first perioperative study of OCT on epiretinal membranes,278 OCT has been proved to be capable of structural assessment, not only for the pathological structures but also surgically induced alternations.279&#;285 Nowadays, OCT is commonly used in ocular surgeries, since it can provide high-resolution images of tissue layers in both anterior and posterior segment that are poorly visualized with other conventional imaging modalities.23 Specifically, OCT has become critical for the diagnosis and management of ocular pathology80 and have been used widely in ophthalmic surgeries for the visualization of keratoplasty,37,42,286&#;289 epiretinal membranes,90,290 macular holes,38,39,90,290,291 retinal detachments,38,80,292 and vitreomacular traction.38,39,291,293 Moreover, it can also be used for breast cancer imaging,277 brain tumor imaging,294 nerve and microvascular biopsy,279 and otolaryngology,51,295&#;298 especially during tumor resection when noneffective evaluation of subsurface structure would cause improper margin assessment.275 Over the years, this technique has advanced with the improvement of laser sources, beam delivery instrument, and detection schemes,299 and has gradually become mature for clinical use. The integration of intraoperative OCT and surgical microscope not only enables the visualization of delicate structures but also keep the surgical workflow uninterrupted.300
The principle of OCT is basically low-coherence interferometry. It typically uses NIR low-coherence (in other words, broad-band) light source, which can be superluminescent diodes, ultrashort pulsed lasers, or supercontinuum lasers.277,301 The light in the OCT system is split into two arms, namely the reference arm and the sample arm. After the light being reflected from tissue, the light beams from two arms form an interference pattern only if the difference of distances they have traveled is less than a coherence length. To take advantage of this interference, scanning is needed to obtain the location of the microstructure of the sample. According to the domain in which scanning is fulfilled, OCT is classified as time-domain OCT (TD-OCT), and frequency-domain OCT (FD-OCT) also known as spectral-domain OCT (SD-OCT). In TD-OCT, the path length of the reference arm changes with time, while in FD-OCT (SD-OCT) the broadband interference is acquired with a spectral variation using either a spectrally scanning light source or a dispersive detector. The reflective profile from a single axial depth scan is called A-scan. To achieve a 2D visualization of the FOV, a lateral combination of a series of A-scans is required for a cross-sectional tomography (B-scan).
Intraoperative OCT has three types of devices: handheld OCT (HHOCT), needle-based probes, and microscope-integrated OCT (MIOCT).275,286,302 When using HHOCT devices, surgeons have to remove the surgical microscope to place the HHOCT device over the patient&#;s eye. Needle-based probes overcome the above problem; however, it requires an assistant to hold.275 Microscope integration is a key advancement of OCT. Since this technology is now available integrated to ophthalmic surgical microscopes with an HD display to view intraoperative OCT images, it is possible for surgeons to give real-time feedback during the surgery. The integration achieves increased stability that handheld devices do not possess. Moreover, the X-Y-Z-foot-pedal control enabled by the microscope foot pedal facilitates rapid imaging with enhanced image reproducibility.303 Considering the fast scanning time of MIOCT, the minimal surgical pause is needed to switch between the surgical microscope and other OCT devices during the procedure, which greatly facilitates a smooth workflow. Furthermore, MIOCT would benefit procedures (1) with a reduced anterior chamber view, (2) where transparent or very thin structures are not readily visible with a surgical microscope, (3) that need to be monitored, and (4) where the intraoperative projection of preoperative imaging data is preferred.300
The early reported MIOCT systems have the limitation of an A-scan rate of <2&#;&#;kHz,296&#;298,304,305 then it was improved to about 20 kHz.306&#;308 Although they achieved axial resolution as good as 7&#;&#;μm297 and imaging depth of 2 mm,298 an acquisition time of 1 to 4 s was still required for a densely sampled volume. Moreover, they were incapable of real-time rendering of the volumetric data. Using a continuous high-resolution B-scan system, which consists of either two orthogonal B-scans or one single B-scan, live images could be generated of surgical procedures and maneuvers.289,298,306,308&#;311 With the outstanding computational speed of graphics processing units (GPU), live 3D OCT has been enabled for real-time volumetric data rendering.312&#;314 Four-dimensional MIOCT (4D MIOCT) system with dual-channel head-up display was reported since to be capable of live volumetric imaging through time at the speed of up to 10-μm-scale volumes per second,78,315,316 which is promising for ophthalmic surgery improvement.
The essentials of a microscope-integrated iOCT system are the microscope integration, a HUDs, and the scan control of OCT. The integration of the OCT module with a microscope is a challenging job, as it compromises the performance of either the OCT or the microscope to some extent.
In the first demonstrated MIOCT system, the optics of iOCT were separate from those of the microscope, and the MIOCT system was coupled to the surgical microscope with a dichroic mirror after the microscope objective lens, which resulted in a reduction of working distance.275,317 In the following designs, the MIOCT system (or part of the system) was coupled into the microscope using either a dichroic mirror or a beam splitter cube. Based on the position of the dichroic mirror (or beam splitter) in the microscope and the optics shared by MIOCT and microscope, the integration method of MIOCT can be divided into two types.
The first design couples the MIOCT system using a dichroic mirror located prior to the microscope zoom module.296,318,319 The optical beam of MIOCT is folded by the dichroic mirror into the path of one of the oculars, and then the merged beam goes through the zoom system and the main objective lens. Specifically, a real-time projection unit can be applied to project the generated OCT image to the other eyepiece.319 The MIOCT beam can be coupled from a camera port of the microscope. Therefore, the MIOCT system does not change the height and the working distance of the microscope. This design can achieve a lateral resolution of 23 to 47&#;&#;μm and an FOV (lateral scan size) of 4 to 28 mm, with the microscope working distance in the range of 232 to 290 mm. The resolution and FOV are mainly limited by the diffraction, the shared magnification, and the working distance of the microscope.296
In the second design, the MIOCT beam is coupled into the microscope using a dichroic mirror prior to the objective lens but after the zoom module, as shows.23,108,188,309,310,318,320&#;327 In this way, the MIOCT beam is decoupled from the microscope ocular path, and the objective lens is the only optic that is shared by the microscope and the MIOCT. In some designs, the OCT is integrated with the illumination module, so the OCT beam is coupled into the illuminating beam path.108,320&#;322 The other form of MIOCT is an independent unit that is assembled onto the microscope.188,323,324,327 Since the MIOCT does not utilize the microscope magnification anymore, independent control of beam waist diameter and beam waist position is usually added.188 Moreover, a telescope made of a reduction lens and a magnifier lens is applied after the microscope objective lens to magnify and provide wide-field image.23,188,309,310,325 Furthermore, using reflective elements and a tunable focus lens can help improve the transmission and the integration of MIOCT to the microscope.23,310 However, this approach increases the height of the microscope and the distance between the surgical field and microscope ocular, both of which could have an adverse effect on the microscope ergonomics.275
Open in a separate windowHSI, or multispectral imaging (MSI), is an emerging optical imaging technique, which is noncontact, noninvasive, nonionizing, and label-free.328 It acquires a 3D data cube, namely the hypercube, which contains a series of 2D grayscale images across a wide electromagnetic spectrum from UV to mid-infrared (MIR).329,330 The main difference between HSI and MSI is the number of spectral bands within the data cube. A data cube acquired by MSI usually contains 3 to 10 bands, while that acquired by HSI has tens or even hundreds of narrow spectral bands. HSI was originally developed for remote sensing, but it has been gaining attention in the medical imaging field. HSI utilizes the optical properties of human tissue such as reflectance and fluorescence, which are related to tissue microenvironment or biochemical constitutes.330 It can extend surgeons&#; vision beyond the visible region,331 deliver near real-time biomarker information, and provide tissue pathophysiology through spectral characteristics.330 Therefore, it is promising for disease diagnosis and surgical guidance.328,330,332
One important application of HSI in the medical field is to extract the spectral signature of the recorded fluorescence signal to improve the accuracy of disease detection or differentiation and intraoperative metastatic diagnosis.333 On the one hand, HSI can be used to differentiate multiple distinct yet spectrally overlaid emissions of different dyes via spectral decomposition. On the other hand, it can be used to compute the quantitative estimates of fluorophore concentration through the processing of the recorded signal, so it can increase the system sensitivity to the fluorophore, enhance the contrast of fluorescence image, and improve the accuracy of disease detection or tissue differentiation.333&#;335
Meanwhile, various studies have revealed the potential of HSI as a dye-free imaging method to provide surgical guidance.332,336&#;352 The heterogeneous composition of biological tissue results in the spatial variations of optical properties, yielding distinct spectral signatures of tissue reflectance. With appropriate data processing methods, HSI alone is able to serve as an intraoperative tool to help surgeons visualize the surgical bed under the blood, facilitate residual tumor detection, monitor the tissue oxygen saturation, and enable the visualization of the anatomy of vasculature and organs.330 Without the requirement of any injection of contrast or dye, it can be used on-demand as many times as needed during surgery.
HSI can be classified into four types according to its acquisition mode, namely point-scanning, spectral scanning (wavelength-scanning), line-scanning (push-broom), and snapshot.353,354 The point-scanning mode is to scan the object point-by-point, which is slow and not commonly used anymore. Line-scanning mode is for the camera to scan the object along one spatial axis, and each time it acquires a complete spectrum for each pixel in a row of pixels. Line-scanning hyperspectral cameras have a good spatial and spectral resolution; however, it requires relative motion between the camera and the object (patient). The spectral-scanning method steps through the wavelength range and captures one grayscale image of the whole FOV at each step. Snapshot hyperspectral camera has filters integrated on the imaging sensor, and it is able to capture the spatial and spectral information simultaneously. The resolution of hyperspectral images depends on the imaging system, but generally, the spectral-scanning method achieves better spatial and spectral resolution than snapshot, while the snapshot method has a fast, video-rate acquisition speed. Both methods are applicable for adaptation with surgical microscopes.
Snapshot hyperspectral camera is small in size and light-weighted, and it can be easily adapted to the surgical microscope through a video adapter. Pichette et al.45 mounted a 16-band snapshot camera on the side port of a surgical microscope for intraoperative hemodynamic response assessment. The camera covered a 481- to 632-nm range with an image size of 256×512, and it was operated at 20 frame per second (fps) during the intraoperative imaging.
Spectral scanning HSI systems are made of a monochromatic sensor and a spectral-scanning component, which can be either electrical tunable filter46,183,355,356 or filter wheel.43,48,357,358 When a tunable filter is used, the imaging system (tunable filter and monochromatic sensor) is mounted on one of the microscope optical ports via a video adapter. Van Brakel et al.183 fitted a tunable filter and a high-resolution camera onto a surgical microscope for the observation of oral mucosa. The system could capture hypercube within 30 s with an image size of × and a wavelength range of 440 to 720 nm. Martin et al.46 developed a similar system for in vivo larynx imaging, which acquired images with 30 bands within 390 to 680 nm. For a filter wheel, it can be placed after the illuminator or in front of the imaging sensor. Roblyer et al.358 developed a spectral-scanning multispectral fluorescence surgical microscope using two four-filter wheels for oral neoplasia detection. The first illumination filter wheel was placed after the external mercury arc lamp to provide narrow-band illumination, and the second emitting filter wheel was mounted on the microscope optical port in front of the CCD camera to select autofluorescence signal. Wisotzky et al.47 employed a digital surgical microscope and a filter wheel with 19 band-pass filters for HSI within 400 to 700 nm. The wheel was located after the light source. Specific wavelength bands were selected by the filters, then the filtered light was transmitted to the microscope optics through a fiber guide to illuminate the surgical site. For comparison, the tunable filter is faster and easier for adaptation than a filter wheel, and it is capable of scanning more spectral bands. However, a filter wheel can provide higher resolution in some cases.43 shows three surgical microscopes adapted with different kinds of HSI systems.
Open in a separate windowPAM, also named optoacoustic imaging, is a promising optical imaging method, which combines laser irradiation and acoustic wave to reveal subsurface structural, functional, and molecular information at microscale in real-time.44,359 The tissue being irradiated by nanosecond-range laser pulses absorbs the light and has a local temperature rise. The heat induces the thermal elastic expansion namely the photoacoustic effect and generates an acoustic wave, which is then received by an ultrasound detector.359,360 With some data processing methods, the distribution of light absorption can be obtained, which reflects the distribution of reflectors.361 During surgery, PAM can be helpful for the observation of fine structures under surface, such as microvascular mapping362 and ocular structure.363
Han and Lee44,364,365 combined a commercial surgical microscope with a laser scanning PAM system for surgical guidance in ex vivo and in vivo studies. The PAM system shared the same optical path with the microscope, thus both the photoacoustic image and the white-light microscopic image were obtained simultaneously. The PAM scanning system was mounted under the microscope objective lens, consisting of two galvo scanners, an objective lens, and a beam splitter. The photoacoustic signal was detected by a needle-type transducer, which facilitates the application in the real surgery environment. In the first version, a laser of 532 nm was applied, and an axial resolution of 131&#;&#;μm and a lateral resolution of 17&#;&#;μm were achieved. The maximum penetration depth tested in a living mouse thigh was &#;900&#;&#;μm. However, a portion of the green laser was projected to the oculars, which appears as a green spot in the microscopic images and disturbed the surgeon&#;s vision. Afterward, they adopted a -nm pulsed laser and achieved an axial resolution of 65&#;&#;μm and a lateral resolution of 41&#;&#;μm. The invisible laser resolved the sight interrupt problem and laser safety concern for surgeons. Moreover, the 2D cross-sectional B-scan PAM images were projected onto the microscope view plan for AR. One year later, Lee et al.366 improved the aforementioned system by combining OCT and PAM altogether with a conventional surgical microscope. The presented near-infrared virtual intraoperative photoacoustic optical coherence tomography (NIR-VISPAOCT) system is shown in . The NIR-VISPAOCT system utilized both light absorption and scattering, and it was able to provide real-time information about tumor margins and tissue structure, as well as a magnified view of the region of interest. In an in vivo mouse melanoma resection surgery, the highly scattering skin layers were clearly shown in the OCT images, while the light-absorbing melanoma was very obvious in the PAM images. The surgical instruments were clearly seen in both images. The PAM system had an axial and lateral resolution of 63 and 35&#;&#;μm, and those of the OCT system were 7.8 and 23&#;&#;μm. By synchronizing both imaging systems, a real-time display with 23 fps was achieved for the B-scan PAOCT images.
Open in a separate windowLSCI, also named laser speckle imaging, laser speckle perfusion imaging, or laser speckle contrast analysis,367&#;369 is a technique that can be used for real-time assessment of perfusion. It is based on a phenomenon, where the backscattered light from a scattering medium that is illuminated by coherent light forms a random interference pattern, namely the speckle pattern. The movement of the scattering particles inside the medium causes fluctuations in the speckle pattern, which may result in blurring of speckle images.370&#;372 When illuminating tissue with a coherent laser and acquiring images of the tissue with adequate exposure time, the movement of red blood cells can cause fluctuation in speckle patterns, thus the blurring of the images can be related to the blood flow.373 This blurring of the recorded pattern is used to calculate the speckle contrast, which is useful for the quantitative analysis of blood flow. The speckle contrast of each pixel can be calculated in the spatial domain or the temporal domain. The spatial contrast of a pixel location is calculated using the intensity of all pixels in a defined neighborhood in one frame, while the temporal contrast is calculated using the same pixel in multiple frames in a time window.374 LSCI is noninvasive, relatively simple, and cost-effective.370 Most importantly, compared with the single-point laser Doppler flowmetry and the laser Doppler imaging that is relatively slow, LSCI is fast and full-field, providing a 2D perfusion map without scanning.375 Therefore, it can be very useful for intraoperative and postoperative visualization of tissue perfusion and measurement of blood flow velocity for retina,376 brain,368,377 skin microvasculature,375 liver,378 large intestine,379,380 and oral cavity.381,382
Richards et al.383 combined LSCI with a surgical microscope by attaching a laser adapter to the bottom of the microscope head and connecting a camera to one of the side ports. The laser adapter contained a laser diode of 660-nm wavelength and a curved mirror to diffuse the laser light. The system could achieve an FOV of &#;2×1.5&#;&#;cm with the maximum zoom of the microscope and a frame rate of 100 fps with an exposure time of 5 ms. Moreover, the laser adapter and the camera, which were attached to the microscope preoperatively, did not interfere with sterile draping or normal use of the microscope. In this study, the system was used to image the cortical area of 10 patients. LSCI images were recorded for 10 to 15 min for each patient. The patient&#;s electrocardiogram (ECG) waveform, as well as camera exposure signal, were recorded simultaneously during the image acquisition. With a retrospective motion correction by ECG filtering and image registration, the system achieved satisfying accuracy and sensitivity and could facilitate clear visualization of small cerebral blood flow (CBF) changes.
Mangraviti et al.384 developed the SurgeON system, which could continuously monitor the CBF and display the high-resolution LSCI images directly onto the eyepiece of the surgical microscope, as shown in . The components that were attached to the surgical microscope include a laser adapter with a laser source of 830-nm wavelength, a custom NIR camera module, and an LSCI projection module. The system had a lateral resolution of 512×512 and a circular FOV with a diameter of 10 to 40 mm, which is determined by the microscope zoom. The LSCI images captured by the NIR camera were processed in a GPU-based computer in real-time to estimate the blood flow velocity index (BFVI) of each pixel, and then the generated BFVI images were projected onto the microscope eyepiece. The frame rate of the system was continuously >60&#;&#;Hz. The results of a series of preclinical studies show that the SurgeON system could enable the detection of acute perfusion changes as well as temporal response patterns and degrees of flow changes in various microvascular settings. Furthermore, by comparing LSCI with ICG videoangiography, this study demonstrated that the dye-free LSCI could not only provide more information such as the CBF variation but also guide the surgery in real-time.
Open in a separate windowErgoPractice News &#; May
There are two main benefits of using loupes. The first is to improve our working posture: to avoid work-related pain and injuries.1 The second is to improve our vision: to see the details of our work.2-3
The key ergonomic factor for loupes is the declination angle. We showed how to determine the optimum declination angle for your next pair of custom loupes in the April issue of ErgoPractice News. The key vision factor for loupes is the magnification power. In this May issue, we will help you decide the magnification level of your next pair of custom loupes by answering key questions about loupe magnification powers.
If line separation (LS) is a measure of the level of detail our eyes can perceive, LS can be determined by a formula as a function of working distance (WD) and magnification power (MP) of telescopic loupes:4
LS = 3.3 x WD/MP,
where LS is in µm and WD is in cm.
According to this formula, we can improve our vision in two ways. The first way is to reduce our working distance, which can be done simply by leaning in very close to an object (Figure 1b). This is more instinctual to us than using a tool such as loupes. The other way is to increase our magnification power (up from 1x with no loupes) with which we can maintain an ergonomic, upright position (Figure 1a). The two students in Figure 1 may be achieving similar visual acuity as the image size of an object formed on the retina by both ways is similar. The main difference is their working distances and working postures. If the student &#;a&#; maintains this ideal posture, she will have a long and healthy career. If student &#;b&#; continues to work like this, she may suffer neck and upper back pain and injuries in a very short time.
Resolution indicates the sharpness of images and magnification indicates the enhanced size of the perceived image of an object. Loupe selection guides by some loupe manufacturers indicate that resolution is more important than magnification. But actually, both resolution and magnification are equally important because our eyes have a limited resolving ability. Even if loupe optics form images with an infinite resolution, our eyes cannot see beyond a certain level of detail. The image needs to be &#;brought closer&#; to our eyes through magnification to take advantage of a good image resolution.
It should be noted that no matter the resolution of individual ocular lenses, the experienced resolution of a binocular optical system strongly depends on the alignment of the two oculars. Precision alignment depends on the sturdiness of frames for Through-The-Lens (TTL) loupes and the stability of ocular mounting rack/arm assembly for Front-Lens-Mounted (FLM) loupes. Most major loupe manufacturers are able to make individual oculars with a good resolution, but if the alignment of two oculars is not precise, the resulting image resolution is poor. As magnification power increases, frames and rack/arm assemblies need to be even sturdier and more stable. It is very important to always consider frame quality for TTL loupes and the stabilizing features of rack/arm assembly for FLM loupes.
As a loupe&#;s working distance changes, the magnification power of the loupe changes. Because of this, the Reference Magnification quoted by the manufacturer refers to a specific focal distance. At a closer or farther working distance than this reference point, the experienced magnification may be greater or less than the reference magnification.
For example, 2.5x loupes offered by various manufacturers may not have the same magnification power because reference focal distances used by manufacturers are not the same. Since some companies may even use unrealistic reference points to enhance their quoted magnification level. You should not rely entirely on the reference magnification powers quoted by manufacturers. One of the most effective magnification tests you can do is simply compare loupes using a test target. Doing this you can compare magnification power as well as resolution (image sharpness).
One of your objectives of using loupes is to improve vision. As pointed out earlier, we can improve our vision by getting closer to objects. The result is that the projected image of an object onto our retina will be enlarged by reducing our distance. The same principle can be applied to the use of loupes.
The Effective Magnification of your loupes compared to the Reference Magnification may be very different. This is because your working distance is most likely different from the reference distance at which the magnification power was computed. The image size of objects projected onto the retina will enlarge or shrink as the working distance changes.
Figure 2 shows the effective magnification of a Galilean loupe with a reference point at 40cm (near 16 inches) for 2.5x magnification. At 30cm (near 12 inches) the effective magnification of this loupe is 3.4x and at 50cm (near 20 inches) the effective magnification is 2.1x. Figure 3 shows the effective magnification of several working distances for two prism loupes with reference points at 40 cm (near 16 inches). You can see, as the working distance decreases the effective magnification power increases significantly.
The two-dimensional (2D) magnification is given by the square of one-dimensional (1D) magnification (Figure 4). The 2D image size obtained with a 2.5x loupe will be 6.25 times of the image obtained with unaided eyes and the 2D image obtained with an 8.0x loupe will be 64 times the image obtained with unaided eyes. The information obtained with the 8.0x loupe is more than 10 times of the information obtained with the 2.5x loupe. As you can see, the 1D measurement does not indicate the reality of the 2D increase in image detail. Although the 1D magnification has been used to rate loupes, 2D magnification may be the true presentation of the visual perception improved through the use of loupes.
Digital cameras use a measure called Megapixels which counts the total pixels, or points of detail, recorded. This is an example of a 2D measurement as it is obtained by multiplying details in the height and width.
As a result of better ergonomic education, more clinicians are sitting or standing with a much straighter back and neck positions. As a result, we have noticed that the average working distances of dental professionals has significantly increased over time. Our data indicates that the average working distance of dental professionals has increased from about 15 inches a decade ago to more than 18 inches today. Many dental students and hygiene students even decide to work at more than 20 inches. The increased working distance helps with ergonomics and may even reduce cross-contamination, but 20 inches simply requires a higher magnification power to see the same level of detail seen at 15 inches.
2.5x loupes have been widely recommended for dental professionals as a starting magnification. Can 2.5x loupes continue to be an industry standard with working distances increasing as they are? The answer is definitely No: we need more magnification. If the working distance increases 15 inches to 18 inches or 20 inches, the magnification power should increase from 2.5x to 3.0x or 3.4x+.
Several dental schools have started to recommend 3.0x or 3.5x loupes to their students as startup loupes, skipping 2.5x all together. Based on the working distance trends, their recommendation is well justified. And since the field of view of Galilean loupes with powers higher than 3.0x becomes prohibitively small, prism loupes are recommended for 3.5x or higher magnification.
You may wish to use different working distances or different magnifications to perform various procedures. So SurgiTel invented two concepts to create multiple working distances and the resulting magnification powers on one pair of loupes. The first are working distance adjustment caps which can change the working distance of Galilean loupes. The second are interchangeable working distance caps (about 10 inches to more than 25 inches) which work for Keplerian (prism) loupes.
The reason loupes are not infinitely adjustable by working distance change is that as working distance changes, inter-pupillary distance changes. So the range of working distance adjustment with TTL loupes is limited more than FLM loupes because the inter-pupillary distance cannot be adjusted with the TTL style. Reducing the working distance is less limited than increasing the working distance. If you intended to use your loupes with several different working distances, we recommend you order your custom loupe with the longest intended working distance.
Prism loupes are becoming more popular not only for precision procedures but also for beginner loupes. In the past, these options were limited because traditional prism designs have several drawbacks: long length of oculars and heavyweight. To overcome these problems, SurgiTel invented a new design concept (patented) which significantly reduced the length of oculars and created the compact line of prism loupes. Recently, SurgiTel introduced micro-line prism loupes (3.0x, 3.5x, 4.5x) which are even lighter than traditional Galilean loupes. In addition to these improvements, SurgiTel invented the interchangeable working distance caps which allow a loupe to have multiple working distances of about 10 inches to more than 25 inches.
Question #1: I am a dental student, who is 6 feet tall, and my working distance is about 20 inches. What magnification power do you recommend?
Answer: One can recognize a 50µm line separation at 15 inches using 2.5x power loupes. If you want to recognize the 50µm line separation at 20 inches, you will need more than 3.3x power loupes. So we recommend 3.5x or higher prism loupe. Recently several dental schools started to recommend 3.5x or higher prism loupes as standard for all students at all working distances.
Question #2: I am a veterinarian. My practice includes both surgical and dental procedures. My optimum working distance with dental procedures is several inches shorter than surgical procedures. Can I use the same loupe for both procedures?
Answer: The answer is yes. You can adjust the working distance of your loupe with working distance caps. You may order your next custom loupe for the surgical procedures and a pair of working distance caps for the dental procedures. Using the working distance caps you will not need to compromise your working posture during any of your procedures.
Question #3: I am a general dentist and perform various procedures, including endodontic procedures. I have used a 2.5x loupes for the last five years. Several friends told me that I should upgrade to a higher power loupe. What power loupe do you recommend?
Answer: Many of our current customers have been upgrading to a high-power prism loupes such as 3.5x or 4.5x and later some of them to up to 8.0x. I recommend 3.5x or 4.5x as your first upgrade. It should be noted that recently several dental schools started to recommend 3.5x prism loupes to their students. With your 2.5x loupes, you can achieve an improved working posture, but you do not experience the true benefits of magnification.
For more Micro Prisms for Medical Imaginginformation, please contact us. We will provide professional answers.