Ординатура / Офтальмология / Английские материалы / Corneal Endothelial Transplant (DSAEK, DMEK & DLEK)_John_2010

Introduction

Direct visualization of a patient’s anatomy for diagnosis and surgery is an important component of medicine. A vast array of optical instrumentation that provide a high quality three-dimensional (or binocular) view are employed in medical subspecialties such as ophthalmology, ear, nose and throat (ENT) and neurosurgery, where anatomical microstructures are examined and operated upon using surgical microscopes that provide a highly magnified view of the tissues of interest. The current technology employed in devices such as the surgical microscopes, slit-lamps and endoscopes require binocular (two sets of) optics for obtaining three-dimensional views. These instruments provide magnification, illumination and mechanical manipulation so that a surgeon has an excellent visual access to the specific anatomy of the patient. Surgical microscopes have been in use for at least 80 years. Their use in ENT surgery and ophthalmic surgery enabled several innovative, less invasive surgical procedures. These instruments have seen several improvements since the early 1960’s such as a variable focus, ceiling mounted microscope systems, X-Y mechanism and fiber-optic illumination. Today, worldwide, it is estimated that there are about 50,000 surgical microscopes with different levels of functionality.

Carl Friedrich Zeiss (1816-1888), an inventive optical designer from Jena in Germany, is credited with some of the early microscope designs. His company designed the first commercial compound microscope that incorporated the objective and eye piece into a single integrated instrument. Later Ernst Abbe (1840 -1905), Schott (18511935) and Horatio Greenough made significant contributions to advance the optical microscope technology. As early as 1887, the first stereo microscope was introduced. Stereo surgical microscopes have been in use for more than eighty years in microsurgery.

Despite their wide acceptance, current optical microscope technologies have several limitations of functionality in terms of ergonomics, flexibility, image quality and archival storage. Most current instrument concepts do not derive any benefit from the recent advances in digital imaging technology, image processing software and related hardware. These newer digital technologies provide a significant potential for improvement in real time surgical imaging through image enhancements, interactive intelligence, storage and retrieval. Several recent advances in imaging technology such as high resolution sensors, high performance processors and image processing software allow acquisition of high resolution color images at frame rates that are very rapid. The digital display technologies can now display high resolution images in

brilliant colors at high frame rates. Three-dimensional images can now be displayed on two-dimensional display systems with advanced stereoscopic display systems. Several holographic three-dimensional display technologies are emerging for medical applications.

Modern microsurgery places intense physical and mental demands on a surgeon and the surgical team. Besides the expected hand-eye coordination and manual dexterity, a surgeon is required to process vast amounts of visual and tactile information from the surgical field and environment, analyze this data and make timely decisions on the course of action. Ophthalmic and other microsurgeries require excellent coordination between all members of a surgical team. Conventional surgical microscope technology provides the surgeon with a good magnified view of the surgical field with limited options, namely, zoom and focus. Only the surgeon and assistant(s) can view through binocular eyepieces, placing the rest of the team at a disadvantage. These microscopes are “passive,” meaning, they can only present the image without any ability to provide further information or enhancements that could aid the surgeon. This is a serious limitation of the current state of the art equipment. There have been no significant technological innovations in the field of surgical microscopy for the past fifty years.

Digital 3D Microscope System

Digital Microsurgical Workstation

The authors are developing a digital microsurgical workstation (Figures 9-1A and B) based on various advances in digital imaging technology. Our concept may be parallel to the current revolution evident in the digital photography. Digital photography has garnered a large fraction of the photography market through its versatility, flexibility and superior image quality. Unlike the film where the acquired images can only be enhanced through chemical process in a laboratory, digital photography allows user to manipulate images through inexpensive software that is widely available. Post-processing allows users to obtain images with the desired attributes. This key concept can be carried over to the operating room where a surgeon can obtain a desired view with post-processing. Combination of real time view and processed images provides additional information to the surgeon, and such additional information could be critical for achieving better surgical outcomes.

The major components of the system are illustrated in Figure 9-1B. The advent of digital (silicon-based) photography, novel display technologies and image processing software presents an excellent opportunity for

the development of a new type of surgical imaging system that not only provides a high quality image but also much needed supporting information that aids the surgical process. The key elements of such technology are:

1.A three-dimensional image capture system provided through a stereo camera system. The camera system will have motor driven zoom and focus features. Magnification and stereo separation will be optimized to produce a high resolution three-dimensional image in true color.

2.A three-dimensional display system that is flexible and allows multiple users the same view as that of the surgeon. Such a system should also allow for different forms of display such as head mounted goggles, video monitors and large format screen for larger audiences.

3.An optional image processing system that incorporates hardware and software for image enhancement and other tasks provide additional information which may assist the surgeon. Such information could be presented as overlays over the real time image, measurements or

navigation through a complex procedure. One could call such systems “smart” and applications as “intelligent”. Such “application” software could be procedure specific in that it provides guidance, metrology and overlay information as and when it is needed during the surgery.

4.Physical configuration that provides best ergonomic design features and flexibility.

Image Acquisition

Stereoscopic image acquisition is accomplished through a dual camera system (Figures 9-2 and 9-3) with desired stereo separation for optimal depth perception. The sensor is typically a CMOS or CCD chip. The system has a single objective lens with a magnification of 8-12x for the chosen working distance and an optical zoom system that may provide 4x to 6x zoom. Any resolution over 1024 × 1024 is desirable in ophthalmology where the surgical field tends to be smaller than 60 mm. A frame rate of 30 or more frames per second per sensor is desirable for obtaining a flickerless image on the display device. The display frame rate is twice that of sensors as the dual frames are fused on to a single display screen.

Figure 9-2: Stereoscopic image acquisition through a dual camera system.

Display System

A common display system for stereoscopic display is a conventional CRT monitor (Figure 9-3) with a polarizer screen (commonly known as a Z-Screen). The dual images

Figure 9-3: Dual camera system, “active” polarizing CRT monitor attached to a movable, on-wheels, floor-design stand.

from the stereo camera are displayed in alternate frames (a frame sequential mode) and the polarization of the Z-screen is altered (in orthogonal axes) in synchronization with the frame data. Such screens are known as “active” polarizing screens. The surgeon (observer) wears a spectacle with lenses polarized in orthogonal axes (same as the Z-screen) thus each eye is presented with the corresponding camera sensor view. As an alternative system may utilize a passive polarizing screen (over the monitor) with an active spectacle lenses (worn by the observer). High resolution and high brightness screen are required for stereo viewing, since polarizer screen can block almost 50% of the screen illumination.

In other stereoscopic displays, a projection system (with dual projectors converging on to a single screen) is employed. LCD or DLP projectors are ideally suited for this. With conventional commercial projectors the resolution may be limited to 1200 ×1200 pixel range. Such systems require rigorous alignment of two projectors to achieve excellent pixel registration.

Stereoscopic head mounted displays are yet another alternative display system where a 3D imaging mode is achieved without the need for convergence of dual images. The stereo image pair is projected or displayed on two small screens built into the head mounted displays. Some of the lower resolution displays can be fit into an oversized goggle configuration. The major disadvantage of these devices is that they are single observer displays and technologies for lower cost and higher resolution displays are still in the developmental stage.

Displays with lenticular screens are yet another alternative for stereoscopic displays. These displays tend to be low resolution as lenticular films attached to the screens have the native resolution of the display. They also tend to be highly directional with a narrow angle of view. These displays require image information to be “interlaced” to conform to the screen configuration. This imposed some additional processing burden on the system. Newer display technologies may allow for flatter displays at higher resolutions and brightness in the future. Organic-LED, dual-LCD and compact high resolution DLP technologies hold great promise for future high performance 3D display systems. This will allow for configuration flexibility in the future 3D display systems.

Benefits of 3D Surgical Microscope

There are several major benefits of such a digital microsurgical workstation, as stated below:

1.Ergonomics

2.Low light levels

3.Depth of focus

4.Form factor

5.Working distance

6.Intelligent microscopy

a.Image enhancement tools

b.Archival system

c.Image framing

d.Metrology

e.Registration

f.Diagnostics

g.Computer aided surgery.

Ergonomics

In an optical microscope the physical (or ergonomic) relationship between surgeon, patient and the system are fixed during the surgery. There is very little flexibility in positioning any one of these components thus requiring the surgeon to stay at a fixed position for optimum viewing. This may impose an inordinate amount of stress on the musculoskeletal system. The injury rate among surgeons who spend long hours in surgery is significantly high. Some surgeons have called this a “silent epidemic.” Several surgeons have curtailed their surgical time in operating room to minimize personal injury. A few surgeons have opted out of surgery. The injury rates reported by OSHA (that apply to all microscope users) support the fatigue factor of surgical microscope use (Table 9-1).

With a digital microsurgical workstation a surgeon views the image on a monitor or a flat screen display monitor at a

TABLE 9-1: The injury rates among all microscope users, as reported by OSHA

desired position and distance thus providing relief from a fixed position. This reduces pressure on the back and the neck, thus preventing injury. There is no need for adaptation between the view offered through the microscope and normal “room view”. This reduces potential errors. A surgeon and his team will have the same view. The digital workstation has twice the working distance as compared to an average optical microscope. This feature allows the surgeon to directly view the patient if necessary. This is often desirable for keeping track of the patient’s status.

Low Light Levels

Conventional microscope systems require powerful light sources. Beam-splitters employed for assistant scopes and video camera feeds “steal” the light output, hence these microscopes require increased light to be delivered to the surgical field. In a typical system only 12 -25% of the light from the lamp actually is needed to view the field. Potential retinal light toxicity is a major concern with the conventional operating microscopes. A low-light budget reduces the total heat and noise (from fans). In a digital system assistant microscopes are not needed as the entire surgical team will have a full 3D view of the surgical filed on the screen or monitor. The light output on to the surgical field could be as low as about 30% of that needed for a typical optical microscope (Editorial Note: This is a major step forward in reducing potential light-induced retinal toxicity during ophthalmic surgery).

Depth of Focus

Lower light levels needed in a digital system allow for smaller apertures for the delivery and collection of light delivered (and reflected from) to the surgical field. A smaller aperture results in higher depth of field. This allows surgeons to view most of the surgical field (up to 3 cm for eye surgery) without moving the microscope focusing optics.

Form Factor

In a digital workstation heavy optics are replaced by electronics thus reducing the size, weight and volume by a significant factor. This has a beneficial effect of reducing handling costs and installation costs.

Working Distance

In a conventional optical microscope, the ergonomics, namely, the relative position of eyepieces to the working surface imposes restrictions on the working distance. The normal working distance tends to be in the range of 1520 mm. In a digital system where the surgeon is “freed” from the eyepieces the distance is more determined by the available light budget and the sensitivity of the sensor element chosen. With CMOS or CCD sensors one can easily achieve working distances in the range of 300 to 400 mm.

Intelligent Microscopy

The basic operative concept is that by introduction of an “image processor” between the image capture unit (camera) and the display system, the image is processed, analyzed, interpreted and then the enhanced image is displayed on the display system. Such image processing is computer bandwidth intensive as all the data contains highresolution, real time images at fairly high repetitive rates. We may have to incorporate unique pipeline processors that relieve the computer of certain routine processing chores. We call such image processing capability “image intelligence”.

We plan to introduce various levels of functionality in gradual steps of sophistication and utility. We call this a “layering” methodology. It would be important for the surgeons to determine what kinds of information is essential and helps in the process and what would be either interfering or overwhelming in its content and presentation. This human (operator) interface will be decided in consultation with prominent surgeons. Some of the elements of such a system will incorporate training features to aid the learning process.

The following is a partial list of functionality that can be incorporated as various layers of intelligence.

Image Enhancement Tools

This set of tools and processes will improve the image as seen by surgeons. Such controls can be activated by voice commands. Improvements in contrast, edge sharpness and color filtering can be provided. The display selection commands such as zoom and pan will also be made available.

Archival System

Surgical procedures or components of a procedure can be stored in a central storage for later retrieval and review. Since the images are stored in 3D, a realistic view of the surgery is provided. Digitized images or procedures also provide a capability for random or partial retrieval unlike the analog video systems. The system provides a true documentation system for training and insurance purposes. Ability to recall prior surgical images during a surgery may aid in surgical decisions.

Image Framing

With features such as “picture-in-picture” and “splitscreen” we will have the ability to present on the screen, duringthesurgery,thesameprocedureperformedpreviously bythesamesurgeon,oranotherleadingsurgeonspecializing insuchprocedures,inconjunctionwiththecurrentsurgery. Thisabilitytoviewa“reference”sourcewillbeofsignificant benefit to the young surgeons. As newer procedures are developed the technique can be learned more easily if the comparative images are available during the surgery. We think this is an important training tool and will be adapted as standard tool in teaching programs. It will be possible to provide access to an entire library of surgical techniques and procedures.

Metrology

With proper calibration of the microscope optics it would be practical to make actual measurement of tissue structures. This will save surgeons a significant amount of time during the surgery. It also minimizes the error rate. The software can calculate areas and volumes as necessary and display such information in real time. The range of measurements can extend to measurement of topography, refractive power and linear area of measurements. Some of these functions may require an additional accessory to be attached to the system. For example, the measurement of corneal topography under surgery may require an accessory that will project a simple Placido image on the cornea while the dual camera system will acquire the frame image for processing.

Registration

This feature will allow a surgeon to place two (current real time view and a stored image) “images” of an anatomy, with one of the layers being a transparent layer, over the other with perfect registration. As the surgery progresses at different depths of tissue, the surgeon will have the ability to view on the screens, these sections, as layers. This

positioning feature will enable surgeons to correctly locate appropriate sites of treatment. Any variations in anatomy specific to a patient can be viewed more vividly. As an example, such technology will immensely improve the accuracy of laser treatment for diabetic retinopathy.

In a successful experiment that could help benefit the way surgery is performed, surgeons have begun viewing and manipulating 3D medical images of the very patients on whom they are operating while the operation is in progress. Early results indicate the potential for improving success rates on such procedures as the removal of cancerous tumors. In the first procedure of its kind, doctors at Britain’s Manchester Royal Infirmary used a standard laptop computer in the operating room to project and manipulate, in real time, a complex, three-dimensional image of a patient’s organ.

Diagnostics

More sophisticated image processing techniques such as pattern recognition can be employed for image interpretation applications. In this methodology the surgical field image is interpreted for certain preset adverse states. Such alerts could relieve the surgeon from processing a multitude of images during surgery, and thus he can continue to stay focused on the surgical procedure. The processing algorithm can “spot” changes such as undesirable colors and dimensional changes and report them to the operating surgeon.

Computer-aided Surgery

The advancements in computerized image processing enable surgeons to see the anatomy in 3D at full resolution. Data derived from recent medical imaging systems can be utilized for planning, simulating, and validating surgical procedures. The modern frameless stereotactic techniques enable surgeons to visualize the surgical site by providing interactive and intuitive access to surgical/anatomical images during the course of the surgical procedure. In this method the spatial relationship of moving surgical devices to the target lesion is viewed through the system. This technique allows surgeons to navigate through and around the site of surgery to determine the best surgical approach.

The first step in computer-aided surgery is the acquisition of the medical images by conventional imaging technologies outside the operating room. Data is then downloaded to an appropriate computer allowing surgeons to view the images taken before the procedure. They can easily manipulate the volume and position views interactively and plan the surgical procedure by modeling multiple cuts of their choice. Software designed for computer-aided surgery offers surgeons interactive

perspective volume rendering for clarity of view, full tissue texture and detail, volume sculpting for simulating incisions, and electronic contrasting by enhancing the volume display among regions of equivalence.

The next step is to set up the system for the intervention. Then the intervention can begin; the monitoring system shows the surgical instruments in real time on an image chosen by the operator and utilizing alternative incisions. Surgical navigation, monitoring and training through virtual surgery are some of the applications that can be developed from the above technology.

Surgical Applications

Removal of Unwanted Corneal Reflections and Artifacts

There are times when a corneal reflection is useful clinically. Although the size of the reflection, in most clinical settings is small, it can be magnified and the meticulously analyzed. For example the whole field of corneal topography and keratometry depend on such quantitative analysis.

However, the reflection can also be an obstacle to the surgeon. For example, the size of the corneal reflection, as produced by the operating microscope light measures 1 to 2 mm. If the reflection is situated over a critical structure during the surgery, the reflection will obscure that structure. Thus, removal of the reflection is important in the operating room.

Artifact Removal

Removal of the corneal reflection is not a new problem for ophthalmologists. For example, many ophthalmoscopes use polarizing filters to remove the reflected image, while some fundus cameras overlay a black dot on the corneal reflection. However, this latter approach also obscures the structure under the black dot.

The 3D Vision System takes advantage of the advances in computer processing of digital images to remove the reflection in a new way. The present digital system described in this chapter can replace the reflection with an image of that area taken earlier, when the light source was moved, thus moving the reflection to another location. This is all possible because the surgeon does not view an optical image of the surgical field but a digital image of the field on a screen. A program can be built into the system to simply cancel any image that has the characteristics of the unwanted corneal reflection and substitute an earlier image of the area in question. Figure 9-4 shows an illustration of the way the surgical field appears when the 3D Digital Image System ‘erases’ the unwanted corneal reflection.

Figure 9-4: Display of the 3D Vision System that has “erased” the unwanted corneal light reflection from the operating microscope light thus facilitating the surgical procedure.

Variable Magnification within the Surgical Field

In general, increasing the magnification of the surgical field reduces the field of view. A related analogy would be watching a football game with binoculars. If you focus on the quarterback with the magnification of most binoculars, this will result in loosing sight of the other important events taking place on the field. In other words, the observer is looking at reduced field of view. It would certainly be helpful to have a binocular that places a magnified image of the quarterback in the center of the field, and have this image surrounded by another image that shows the rest of the playing field at a lower magnification.

The 3D system achieves this effect by limiting the magnification to only the center of the surgical field. This effect can only be achieved if the surgeon looks at a virtual digital image of the operative field on the screen instead of the real time optical image. Thus, the computer program that processes the image can be made to only zoom in on the central portion of the field and leave the rest of the field at a lower level of magnification. In this form of digital magnification (versus optical magnification) an image point that normally would cover one pixel is stretched out to cover many pixel areas (this accounts for a variable digital zoom effect). Figure 9-5 shows an operative field in which the center is at a higher magnification and the less important periphery is at a lower magnification.

Using a Template for Capsulorhexis

An anterior capsulorhexis is the mechanical removal of the central portion of the capsule of the crystalline lens. The opening produced, allows the surgeon to enter the

Figure 9-5: 3D Vision System showing variable magnification of the operative field. The area of surgical interest namely, central magnified image is surrounded by a second image at a lower magnification of the same operative field.

interior of the lens in order to remove the cataractous nucleus and cortex by phacoemulsification followed by irrigation and aspiration. One of the unwanted result of the procedure is to produce a capsulorhexis of the wrong size. If the capsulorhexis is too small, then the implantation of the IOL becomes more difficult. If the capsulorhexis is too large, there is a greater chance of the IOL dislocation as well as a greater chance of a radial tear of the capsule with possible zonular disinsertion. To avoid these potential complications, a superimposed circular template of the correct capsulorhexis size would be a helpful guide to the surgeon.

Using the 3D digital microscope, a capsulorhexis template (yellow circle) of the desired size can be superimposed on the digital image of the anterior lens capsule during the surgery (Figure 9-6), thus guiding the surgeon

Figure 9-6: Shows the capulorhexis template (yellow dotted circle), whose size can be controlled by the surgeon, is superimposed over the center of the anterior lens capsule of the cataractous lens. Also seen is the initial step of the surgical capsulorhexis path (blue dots).

to perform the capsulorhexis of the desired opening. This ‘template’ for the correct size and shape of the capsulorhexis will always move so as to remain in the same location on the lens if the lens moves during the surgical procedure.

Metrology

Presently, ocular entities such as pupil size, incision length, and lesion dimensions are measured by placing a caliper in the surgical field over the object to be measured. A less invasive, but also less accurate method, would involve superimposing a measuring reticule in the eyepiece of the microscope over the object of interest.

The 3D system, by using a digital image of the surgical field, can employ a program which measures the dimensions of the object of interest by counting the pixels underlying the object of regard. Thus the diameter of the pupil is measured, in the computer program, by counting the pixels between the edges of the pupil and converting them into millimeters (Figure 9-7). The metrology of the 3D digital microscope system function can also be extended to more complex measurements which may involve range finding, triangulation, and topography. Intraoperative measurements of keratometry and refractive power are also possible with the digital technology.

Figure 9-7: Showing the pixel ruler of the 3D digital microscope system that is used to measure the pupillary diameter.

A Mentor System of Dealing with Complications

Wouldn’t it be useful to have an experienced surgeon at your side when you are presented with a complication? The 3D system has developed a unique way to take advantage of the digital image presentation to simulate the help of an experienced surgeon (the mentor). It has in its memory, 3D videos of the mentors performing various

Figure 9-8: The mentor system is demonstrated. The surgeon performs a phacoemulsification procedure, while a video of the same operation performed by the mentor is displayed in a split-screen mode, and it appears in another convenient part of the screen.

surgical procedures. These videos can be played in a splitscreen fashion (Figure 9-8). Thus, the young surgeon can view the real time surgical field on the central part of the screen while a 3D video of the mentor surgeon performing the same operation is seen on another part of the screen. This video of the experienced surgeon can be stopped or made to go fast or slow, forward or backward, to help review the steps taken to avoid or correct complications.

The mentor system has broad implications in surgical training and education. It is possible to develop a training methodology that includes rating (or grading), interactive learning with either preceptor or computer aided interpretation. The system can be integrated with any surgical simulation systems.

Real Time Corneal Topography

The design of an easy to use operating keratometer has been a legitimate goal for many years. The current methods of measuring the intraoperative corneal curvature includes placing a lighted circular target over the cornea and attempt to measure the dimensions of the corneal reflection to determine the real time corneal curvature. More recently, a portable corneal topography unit has been introduced, that is held over the cornea and a printed map created with the quantitative information. All of these systems require the surgical action to come to a halt while the circular lighted target is placed in position and a measurement of the reflection is taken.

The 3D system takes advantage of the 3D image being continually created by the 2 digital cameras and uses a technique known as stereogrammetry to quantitate the corneal contour (Figure 9-9). Such a two camera

Figure 9-9: Shows real-time intraoperative, corneal topography map that is superimposed on the cornea using the 3D digital microscope system.

photographic system has been used for many years to quantitate the contour of the 3D images of mountain ranges. Such a system can be useful in creating the proper tension in tying sutures or making relaxing corneal incisions of the proper length and depth in correcting corneal astigmatism.

Current Status and Future Directions

The prototype systems have been used in actual surgery on humans (Figures 9-10A and B). The surgeon adaptation time was found to be negligible. Surgeons who used the systems in general had favorable impression of the system.

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Figures 9-10A and B: (A) Actual use of the prototype systems of 3D digital microscope, and microsurgical workstation on humans;

(B) Projection screen displaying the surgery in real-time.

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