Microperimetry

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INTRODUCTION TO MICROPERIMETRY Leonardo Mastropasqua MD Mauro Campigotto MSEECS Raffaella Bisson MSEE

A Brief History History of Fundus Related Perimetry In examination of the retina, it has always been important to establish the relationship between measurements of vision and ability to perform everyday tasks. To assess visual abilities, the main functional approach has historically been to measure the visual acuity, still considered the standard in clinical practice. In the past, quantifiable measurements of distance and near visual acuity were assumed to be equated to one’s ability to function in the seeing world. (1) However, these variables do not provide a true representation of visual ability, because they do not allow for detection of the real impact of retinal deficiencies on quality of life and on daily activities. In an attempt to

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Handbook of Microperimetry In Visual Rehabilitation

quantify the local defect map of the retina, the retinal thresholds to light stimuli, static and kinetic perimetry were later added to the gold standard of VA. The main limit of perimetry, or conventional visual field examination, is that the retinal defects are not mapped exactly on the corresponding retinal locations. This means that there is only a quantification of the retina sensitivity, but not an accurate functional evaluation of the foveal functions, for example. Furthermore, lacking an eye-tracking system, the perimeters have the disadvantage of projecting the stimuli in not precisely known positions of the fundus. Thus, with unstable or extrafoveal fixation, it becomes impossible to accurately assess the sensitivity map.

In low-vision patients, the quantification and contextual localization of retinal sensitivity defects and tracking of fixation are fundamental in establishing not only the ability to make out letters with a certain size and contrast, but even the identification of the size, shape and depth of scotomas. (2) Moreover, the eye-tracking system is fundamental in providing the visually-impaired with a correct and customized rehabilitation protocol. For these reasons VA and conventional visual field examination are tThese considerations led engineers to find an instrument that was able to overlap the functional map (retinal light thresholds) with the fundus photography, using systems to compensate for the eye movements: the so-called Fundus-Related Perimeter or Microperimeter.

Many fundus perimeters have been developed and used in the past 30 years. (3) The scanning laser ophthalmoscope (SLO) was the first fundus-related perimeter. (4)This instrument allowed a clear, realtime examination of the retina utilizing an infrared (IR) device and the manual projection of visual stimuli of different shapes, sizes, and intensities over selected retinal areas. The sensitivity map, obtained according to the stimulation pattern (in dB or pseudocolors), was available at the end of the examination, because light projection was related only to previously-selected anatomical landmarks, and was independent of fixation and any other eye movement. The respective map contained the fixation area, the fixation target, and the threshold data. This SLO tool was called microperimetry, and the name is cur-

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rently a synonym of fundus (related) perimetry, which probably better defines this diagnostic technique.(5) With the introduction of this instrument, it finally became possible to quantify the location and the stability of fixation, thanks to its firsttime system of compensation for eye movements. With this feature, SLO microperimetry overcame the limitation of unstable fixation, thus providing a fundus-related sensitivity map for patients with any kind of VA. The use of the Infrared laser light to illuminate the fundus, in combination with the visible helium neon (HeNe) red laser used for the light stimuli, allowed the exact determination of the fixation locus and the opportunity to regulate the stimuli pattern at the desired threshold, quantifiable by the SLO.(6) The main limit of this technology was that only manual examination could be performed. In other words, there was no opportunity for follow-up, or repetition of the same exam in the same retinal loci.

Furthermore, the fundus image was in black-and-white, given the monochromatic illumination source, thus limiting the observation of the fundus in its true colors.

The MP-1 Microperimeter solves this problem, guaranteeing automatic fundus-related perimetry with a high-frequency 25Hz eyetracker, thus eliminating the need for any manual procedures either to compensate for the eye movements or to deliver the stimuli in a proper location. The track is created thanks to pre-registered retinal landmarks assigned in an initial frame (registration procedure). This feature allows an automatic follow-up examination exactly on the same loci of the first examination, providing either static or dynamic microperimetry. MP-1 guarantees a high quality color retinography, over which the sensitivity map and the fixation stability results can be displayed.

New microperimeters have been developed since MP-1 was commercialized. The OTI is one of them. It is a SLO (Scanning Laser Ophthalmoscope) joining the OCT (Optical Coherence Tomography) with an optional microperimetry module. This Spectral OCT/SLO with the add-on Microperimetry module provides the clinician with the ability to display a combined image of the 3-D OCT Topographic Map and the Retinal Function Map. In any case, this microperimeter has

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some limits due to the use of a laser to scan the fundus and to provide the stimuli. The patient is disturbed by the scanning laser, fixating at times the illumination source instead of the correct stimuli. Furthermore, the eye-tracking system, being 8Hz, is slower than that of the MP-1 Microperimeter, thus resulting in a loss of precision during eyemovement monitoring and in the registration phase. In addition, the field of view is smaller than the MP-1, being 12°x12°, while the MP-1 has a 45° round field of view.

A few years ago another microperimeter was introduced: Maia, a Scanning Ophthalmoscope using a Super Laser Diode (SLD) as its illumination source. The main disadvantage is, as for OTI, the use of a SLD that affects fixation and the fact that, instead of a full-color retinography, it provides only a black-and-white fundus image. Furthermore, the number of testing patterns is limited and not open as in MP-1, where everything is customizable by the operator, even if standard patterns are suggested for beginners. In addition, the fixation stability analysis in the case of Maia and OTI is limited to the classical one (quantification of fixation stability according to the percentage of fixation points present in the 2° and 4° central areas, classification of fixation stability based on the system described by Fujii et al. (7) ). Instead, the MP-1 provides even the Bivariate Contour Ellipse Analysis (BCEA), a more specific and sensitive technique than the traditional method. (8) The MP-1 is the first microperimeter to introduce structured rehabilitation based on audible feedback, while OTI doesn’t have it at all, and Maia uses a common feedback exam. The new MP-1 Microperimeter feedback examination allows the ophthalmologist to train the patient to fixate the target with the new PRL, while stimulating it with a flickering checkerboard (structured stimulation). Patients are asked to move their gaze according to audio feedback, which tells them whether they are getting closer to a fixation position chosen by the ophthalmologist. Cortical neurons located in the retinotopic position corresponding to the scotoma receive some degree of activity from the unimpaired neurons in the area surrounding the lesion. (9) Recent clinical studies have shown that rod photoreceptor sensitivity declines more rapidly than cone sensitivity in some retinal neurode-

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generative diseases including Age-related Macular Degeneration and at least one form of Retinitis Pigmentosa. Based on these findings, there is an unmet medical need to measure rod thresholds under dark-adapted test conditions, in addition to cone thresholds under light-adapted test conditions. Rod threshold measurements require new hardware and software capabilities that are now introduced for the first time in the MP-1S Scotopic Microperimeter. This ophthalmic instrument can be used for patient diagnosis and to assess treatment efficacy in clinical studies of new therapeutics for retinal diseases that cause rod function impairments.

So MP-1S has been enriched with the scotopic analysis of the eye, which is the ability to stimulate mainly the rods’ sensitivity and to map their local sensitivity, along with the scotopic fixation analysis.

In 2011, Crossland et al (Department of Visual Neuroscience, UCL Institute of Ophthalmology) made some tests on a modified HFA (Humphrey Field Analyzer) with perimetry adjusted for scotopic examination (prototype set-up), because scotopic visual function has traditionally been measured using it. However, this system does not control for fixation errors or poor fixation stability. (10) At the same time, they modified a version of MP-1 by adding some filters to obtain a scotopic stimulation of the retina. The MP-1 method proved to be quicker than the modified HFA technique, because it corrects for poor fixation and it shows the retinal position of the scotopic scotoma.

(11) For this reason, MP-1 now has the scotopic microperimetry and fixation analysis fully integrated as part of its features.

The MP-1S also provides a new co-registration software application that spatially aligns MP-1S visual stimulus coordinates with postprocessing image data from cSLO retinal imaging systems. This will help clinical investigators study the correlations between localized rod and cone sensitivity and local imaging features in the macula including fundus auto-fluorescence patterns.

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Handbook of Microperimetry In Visual Rehabilitation

Integration of Fundus Camera and Computerized Perimetry Historically, technology has always been a strong support for medical science. This support was, and is, all the more important when it comes to eye care, a branch of medicine that more than any other has gained from technology in the form of diagnostic and surgical tools that currently help all ophthalmologists in their work.

Focusing on the study of the posterior pole, many innovations have been introduced to analyze the morphological state of the retina.

Tests such as optical coherence tomography (OCT), retinography, ophthalmoscopy, fluorescein angiography, confocal microscopy and polarimetry, etc. are the best applications for analyzing the morphology of the posterior segment making it possible to carry out even an in vivo histology of retinal structures.

Besides morphological aspects, the analysis of the posterior segment must also include the study of its functional state. This type of study is commonly done during any vision test, with examination of visual acuity through the reading of an optotype: the simplest, and still current, example of functional analysis.

In comparison with the progress made in the morphological field, the functional one has taken a different path, the visual field remaining the most in-depth examination on retinal function.

VISUAL FIELD TEST Historically the visual acuity test is aided by another, more detailed analysis to better understand the functional status of the various retinal regions: the visual field test, or perimetry. The purpose of such examination is to find the threshold of differential sensitivity to light, or to analyze which contrast is necessary to perceive a stimulus of a certain light intensity, form and position in reference to a background of known brightness. The measure of such threshold is associated with retinal sensitivity and is expressed numerically.

Examination of the visual field can be performed manually (now increasingly rare in clinical practice) or automatically. The projection

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of light stimuli can be done statically or kinetically. Static perimetry is more common and its purpose is to map the sensitivity of specific retinal zones, while kinetic perimetry is particularly useful for quickly studying the extension of pathological areas and in the identification of scotomatous areas.

This kind of exam has historically been of great value in the diagnosis and follow-up of ocular pathologies offering as a result a map that can describe the distribution and extension of functional dysfunctions.

Until a few years ago, functional analysis stopped its evolution at the visual field examination, as a result the progress made in perimetry has always been weaker than the strides made in morphological studies, as mentioned before.

Also perimetry over time has evolved thanks to the introduction of computer-based devices, which have further standardized the use of this examination. Currently, the examination of the visual field is performed using the so-called Standard Automated Perimetry but it remains, in substance, the same examination introduced decades ago.

The conventional visual field, even if carried out through automated perimeters, tends to be an examination with weak accuracy for the analysis of macular diseases especially in patients having a compromised foveal area that suffers from unstable fixation and/or is located in an eccentric region. The impossibility of fixation analysis regarding its location and stability, and the assumption that we can have accurate results only in the presence of stable fixations, makes computerized perimetry unsuitable for accurate analysis of retinal sensitivity (see Sunness et al).

MICROPERIMETRY: THE TURNING POINT As previously discussed, classical perimetry suffers from two crucial issues that place this functional test at a lower level of accuracy and repeatability in comparison to the morphological analysis breakthrough. The two drawbacks to overcome continue to be the possibility of sending the light stimuli in precise and repeatable positions over the retina and the ability to analyze locus and stability of fixation. It is therefore necessary to introduce two innovative concepts in functional analysis: the constant monitoring of retinal movements and the

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Handbook of Microperimetry In Visual Rehabilitation

matching of the results on retinal fundus.

The matching of the functional analysis on retinal fundus finds its first application in the 1970’s through the introduction on the ophthalmology market of the Rodenstock Scanning Laser Ophthalmoscope.

Figure 1 - Scanning Laser Ophthalmoscope, Rodenstock

With this device it was possible to carry out a real time viewing of the retinal fundus through a red infrared-laser source to which light stimuli could be manually sent in order to appreciate the retinal sensitivity in specific locations.

By that time, the market started to talk about perimetry correlated to the fundus, or fundus-related perimetry.

Many years later, precisely in 2004, a new type of fundus-related perimeter was introduced on the market. It improved and made more precise, repeatable, and complete the analysis of retinal sensitivity and fixation. It was called MP-1 Microperimeter and was developed and manufactured by NIDEK Technologies Srl, the Italian branch of the Japanese NIDEK Co., Ltd. It is still present on the market today.

As previously reported (Sunness at al.), conventional perimetry is not able to monitor retinal movements during the examination, so it cannot give any indication about fixation location and stability. In fact, conventional perimetry supposes that the fixation is always foveal

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Figure 2 - NIDEK MP-1 Microperimeter (NIDEK Technologies Srl, Padova, Italy)

and stable. This supposition is not able to take into consideration the error caused by unstable fixations that inevitably brings to approximations on the retinal sensibility measures, while it supposes that the inaccurate sensitivity map is always centered on the fovea due to the impossibility to verify its eccentricity.

If we consider that usually an unstable fixation is often located in an eccentric region, especially in maculopathy as in the majority of macular diseases (AMD, geographic atrophies, macular holes, etc.), the intrinsic limit of conventional perimetry is clear, in that an inaccurate sensitivity map cannot be topographically mapped over the retinal fundus.

Thanks to the real-time analysis of the retinal fundus, microperimetry is able to perform a live correction of the light stimuli direction based on the actual position of the Retina and is likewise able to analyze the characteristics of fixation. These great advantages make microperimetry a highly accurate and repeatable functional test. Finally, the opportunity to correlate these results with the morphological examination provided by the image of the retinal fundus (infrared on Rodenstock SLO, high-resolution color on the NIDEK MP-1), makes the quality of this exam higher than any other functional analysis in the assessment of retinal pathologies and their follow-up.

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THE NIDEK MICROPERIMETER

Retinography

Fixation Exam

Microperimetry Exam

Figure 3 – Exam types

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The NIDEK MP-1 Microperimeter takes the concept of automatic computerized perimetry correlated to the fundus of the Retina to the utmost heights, creating a perfect fusion between accurate functional and morphological data. Thanks to its versatility, the user-friendly MP-1 is the “gold standard” today in the microperimetry field.

The core and strength of the MP-1 is its retinal Eye-Tracking system based on a real time analysis of retinal position performed using an infrared halogen light source. Thanks to the 25- frame/second tracking speed, the MP-1 Eye-Tracking system results compatible with the real speed of eye movements in the presence of particularly unstable fixation.

Another point of strength is the light projection system which utilizes a dedicated LCD screen that allows extreme variability in the projection of fixation target and light stimulation thanks to a perfect correlation with the Eye-Tracking system.

Finally, thanks to the possibility of acquiring a digital color image of the fundus covering 45° of visual field, the matching between the functional data of fixation and sensibility results particularly enhanced by the high quality of the morphological image of the fundus.

The possibility of carrying out both static and kinetic microperimetry combined with the high repeatability of the automatic

INTRODUCTION TO MICROPERIMETRY

follow-up completes the functional analysis from both clinical and surgical points of view.

Thanks to the biofeedback test, the MP-1 is now established as the main rehabilitation device for low-vision care. Using the retinal EyeTracking system in a different way, the MP-1 has introduced a true revolution in the low-vision field introducing the new concept of active visual rehabilitation.

The diagnostic application of the device is able to identify a region of the retina with improved characteristics compared to the current locus of fixation, or P.R.L. (Preferred Retinal Locus). The rehabilitative feature is performed using the biofeedback examination through the use of properly guided auditory stimuli and structured visual stimulation in order to stimulate the selected retinal area with the purpose, through a specific rehabilitation protocol, of it becoming the new P.R.L. of the patient.

Thanks to the careful and active choice of the area to be rehabilitated and the stimulation protocol, the fixation behaviors as well as the residual vision of the low-vision patient are improved, as is his/ her quality of life.

Microperimetry Over Fluorescein Angiography

Microperimetry over Indocyanine Angiography

Microperimetry over OCT map

Figure 4 – Enhance morphology mapping

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Given the extreme usefulness of the morpho-functional matching introduced with the MP-1 Microperimeter, this concept was expanded, thanks to the possibility of importing images of retinal fundus coming from different technologies and related devices such as OCT, Fluorescein Angiographer, Indocyanine images, laser scanning, etc. In this way, it is possible to join the accuracy of the functional analysis with the best morphological sources in order to enhance the quality of clinical data in favor of a more knowledgeable response from the medical staff.

How a Microperimeter works Introduction The purpose of this section is to summarize the basic procedures on how to set, perform and check the modus operandi about the use of a microperimeter. The following procedures are based on the MP-1 Microperimeter (NIDEK Technologies Srl, Padova, Italy).

Preparing the Patient After switching on the instrument, it is advisable to wait approximately 10 minutes before actually performing the first microperimetry, as background and stimuli luminance stabilize after such time has elapsed. The device can be used in a dimly lit room, it is not strictly mandatory that the room be dark.

During the examination the patient must be relaxed, with forehead and chin resting firmly on the relative rests. The patient’s position in front of the device can be set in 3 different ways: adjusting the electric table top, moving the chin rest by its wheel, rotating the joystick for fine refinement. In order to fix the chin rest in the optimal position, move it up or down until the patient’s eyes are at the same level as the line printed on the housing.

The patient has to follow the same preparation as for perimetry. In particular, the untested eye needs to be patched. A different chin rest

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is provided for each eye to be tested. If the left eye is to be examined, the patient must place his/her chin on the rest to the right, and viceversa for the right eye. The patient must look in the direction of the front lens as requested by the user and fixate at the fixation target all throughout the examination. The user needs to receive confirmation from the patient that the target is visible and well-focused.

In case of microperimetry exam, ensure that the patient is holding the trigger command.

Starting and Aligning When the Personal Computer and the Microperimeter have warmed up, the NAVIS access page should appear. Once the operator has logged in, the application can be started. If no patient has been selected, NAVIS will open the list of patients to allow the user to select a patient already included in the database or to create a new one. Once the patient has been selected, direct access is given to the Examination page (see Figure 6).

In order to align to the patient’s retina, move the MP-1 head to the backmost position with the use of the joystick, then center the patient’s eye on the screen (vertical up/down movement is possible by rotating the joystick).

Once the patient’s pupil has been framed, move the joystick forward, gradually adjusting up/down and right/left alignment until the focal distance (50 mm) is reached and a portion of the retina, usually including part of the optic disc, has been framed. Should the image be too dark or saturated, it is possible to adjust the intensity of the infrared light by means of the IR Power slider (see Figure 6, section “B

The following step is very important. Adjustment of the infrared power through the IR Power slider is essential to improving the quality of the infrared image and to proceeding with an efficient examination procedure.

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Then move the optical head back and forth until two light spots appear (Figure 5a), move the optical head slightly back and forth until the two spots are focused (Figure 5b), move the optical head slightly left/right and up/down until the two spots are aligned vertically with the two blue lines and horizontally centered (Figure 5c).

Any refractive defect found can be corrected with the use of the Spherical error command (see Figure 6, section “B”). In case of astigmatism, the spherical equivalent should be added. It is also possible to type in the spherical defect, instead of using the arrows.

Unlike the correction made with a perimeter that takes into account the near distance, with the MP-1 the correction for far distance has to be used.

At the beginning of the alignment, the fixation target is in the center of the field of view, usually this position places the optic disk and the main vessel in the periphery of the live image. The optic disk and the main vessels are usually very useful regions where it is possible to choose the reference area for the Eye Tracking.

It is recommended to slightly move the fixation target position horizontally in order to properly include the optic disk in the field of view.

This step will help the selection of the tracking reference area.

Figure 5 even shows the slight shift (on the left in case of right eye and vice-versa) of the fixation target in order to better include the optic disk and the main vessels in the field of view.

The system is now ready to start any of the available examinations.

For any further information, please refer to section 5.5 of the MP-1 Microperimeter Operator’s Manual.

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(a)

(b)

(c)

Figure 5 - (a) Shifted and unfocused alignment spots. (b) Shifted and focused alignment spots. (c) Correctly aligned and focused.

Fixation Target Selection The MP-1 can use different shapes as a fixation target: a cross, a circle, 4 crosses and custom images. Besides the latter option, that can be used for visual rehabilitation or pediatric purposes, the main shapes that are used are the cross and the circle. Especially for a fixation exam, the best starting fixation target is the cross because the center of the target is well recognizable. Once the cross target is selected, the user may change dimensions and thickness in order to be sure that the fixation target can be seen well by the patient. If the center of the cross is not recognizable by the patient due to the pathological condition of the retina, especially in the case of central scotoma, it is advisable to switch to a circle fixation target. Again, the user may change dimension and thickness in order to be sure that the fixation target can be seen well by the patient.

The best fixation target dimension and thickness are the minimum ones able to be well recognized by the patient.

If neither the circle target is recognizable, the last option is the 4 crosses. Again, the user may change dimensions and thickness accordingly. The purpose of this target is to have at least one cross visible to the patient.

The user has to be warned that the points of fixation at the end of the exam are placed at the center of the 4 crosses even if the patient looked only at a specific cross.

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In the case of the microperimetry exam, the decision process could be quite different because the choice of the target has to take into account the position of the stimuli in order to avoid overlapping between fixation target and stimuli. If it is necessary to project a stimulus in the center of the pattern, the cross is not the correct fixation target because its center may be overlapped with the central stimulus. As a general rule, in the case of microperimetry, the user must check the position of the stimuli compared to the shape, dimension and thickness of the fixation target in order to avoid overlapping during the examination that can affect the reliability of the sensitivity values due to the interference between light intensity of the stimuli and the fixation target.

A very useful shortcut is the use of the keyboard arrow keys that permit the user to change shape, dimension and thickness of the fixation target directly on the exam panel.

In order to change the fixation target characteristics, the user can access the test editor page by clicking on the “Customize” button.

For a detailed explanation, please refer to section 5.3 of the MP-1 Operator’s Manual.

Eye Tracking setup Once aligned on the patient’s retina as described above, the user can select which exam to perform.

Apart from some differences, the procedure to start an exam is similar for any kind of examination, excluding solely the Retinography.

In order to perform an exam, the user has to select the appropriate button (“Fixation” or “Microperimetry” or “Feedback”) on the exam toolbar, after which the alignment spots disappear.

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A C

B

D

Figure 6 - Examination page. (A) Main window and Attenuation bar in dB. (B) Command toolbar for exam selection and control for Infrared power, Flash power and Spherical error correction. (C) Tracking window and tracking bar during exam with exam information for eye selection, remaining stimuli and patient’s view. (D) Test selection and exam configuration.

STILL FRAME By clicking with the mouse in any point of the main window, the live image will be “frozen” and a still frame is acquired.

The choice of the still frame is fundamental because it is the basic image on which the Eye Tracking will base its work. To the greatest possible extent, a high quality still frame choice is mandatory.

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If the still frame includes dark parts, saturated ones or the overall quality is not good enough, it is highly recommended to acquire it again. To do so, you can simply click the “Stop” button and repeat the procedure described above.

REFERENCE AREA FOR EYE TRACKING Due to the presence of the Eye Tracking system, once the still frame is grabbed, the next step is the identification of the reference area, also called R.O.I. (Region of Interest), which is the area that the Eye Tracking system is looking for during the examination. By identifying this area in each of the 25 images/second grabbed during the exam, the Eye Tracking system is able to monitor the eye movements in order to compensate the direction of the light stimulation and analyze fixation position and stability.

The selection of this region is made by moving the mouse over the still frame. When doing so, a white square around the mouse pointer becomes visible and a number appears outside the top left corner of the box. This is the goodness index, representing the quality of the still frame portion inside the square. This numerical index may vary from 0 to 10, if the value is below “2.0”, the squared region becomes red, meaning that the region is not good enough for the Eye Tracking system and it cannot be selected. If the index value is over “2.0”, the region becomes green and can be selected as R.O.I. The green region can be selected by simply clicking on it with the left mouse button.

Theoretically, the goodness index should be maximized in order to look for the highest value, basically the aim is to move the square over a region rich in details that is usually referred to as the optic disk or main vessel arcades outside the optic disk. Sometimes atrophic regions, due to the presence of specific pathologies, can be good R.O.I. regions, thanks to their high reflectivity to infrared light.

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Figure 7 – R.O.I. “goodness” index

Just before the selection of the R.O.I., remind the patient to look at the fixation target.

Depending on the kind of examination, usually after the selection of the R.O.I. the exam will start.

The live image goes to the top right corner window and the still frame remains on the main window where the light stimuli and/or the fixation points (light blue dots) start to appear (see Figure 6, part “A”).

The window on the top right corner, called Tracking window, shows the live image of the exam (see Figure 6, part “C”).

The user has to pay attention to the Tracking window in order to keep the live image as similar as possible to the still frame, in order tokeep the tracking bar green. If the tracking bar turns red due to the patient’s eye movements, the user may help the Eye Tracking system by slightly moving the optical head using the joystick so as to bring the Tracking bar back to green.

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Below the Tracking window there is the so-called Tracking bar that shows one of two colors. If the Tracking bar is green, it means that the Eye Tracking system is able to track the eye movements. If it is red, it means that the Eye Tracking system is not able to find the R.O.I. on the live image and it is not able to locate the position of the retina. In this case, the exam goes automatically into the pause phase, no fixation information is acquired and no light stimuli are projected, until the Eye Tracking system is able to once again find the R.O.I. on the live image. For this reason the Tracking bar shows 2 different times: the left one shows the elapsed time and represents the overall duration of the exam, the right one shows the tracked time and represents the sum of the time with eye tracking on.

Performing examination Excluding the need to take a retinography alone, and although the interest may only regard microperimetry, the approach should be started with the patient performing a simple fixation exam in order to analyze the fixation region, called P.R.L. (Preferred Retinal Locus) over the black-and-white infrared image. After that, a microperimetry exam could be performed as usual on the infrared fundus image, and lastly, a color retinography can be taken. The registration procedure can be performed between microperimetry and color retinography in order to create a second microperimetry exam over the color photo.

To complete the analysis, through the off-line registration procedure explained below, it is possible to use the color photo taken at the end of the microperimetry exam to overlap the result of the initial fixation exam in order to have both exams overlapped over the color photo of the fundus (Figure 8).

FIXATION EXAM In order to perform a Fixation exam, once the first alignment is made following instructions in section 1.2.3, the user has to select the “Fixation” button (see Figure 6, part “B”) and follow the steps described in section 1.2.4. At this point, the exam starts and the user has to pay at-

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Figure 8 (A) Fixation exam on infrared retinography.

(B) Microperimetry exam on infrared retinography.

(C) Microperimetry exam registered over color retinography taken at the end of the microperimetry exam.

(D) Fixation exam registered over the color retinography of the microperimetry exam through the off-line registration procedure.

A

B

C

D

tention to the Tracking window and related bottom bar (see Figure 6, part “C”) in order to support the Eye Tracking system, if needed.

The typical duration of a fixation exam is from 30 sec to 1 minute of real tracked time. Due to the Eye Tracking system speed, for every second of tracked time the system acquires 25 fixation points. This means that after 30 seconds the system has acquired 750 points, which become 1,500 after 1 minute: this is sufficient for understanding where and how the patient is fixating. The fixation exam has to be physically stopped by the operator by pressing the “Stop” button.

If the purpose of the examination is only fixation analysis and no other examination (i.e. microperimetry) has to be done, the user may continue on the examination page by taking a retinography exam and registering the fixation examination with it in order to have the fixation data over the more detailed color photo of the fundus. Instead, if the

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purpose is to proceed with another examination like microperimetry, it is inappropriate to take a retinography because a wait time is necessary before performing a microperimetry examination (usually about half an hour until the flashing effect disappears). As explained above, in this case the user can proceed with the microperimetry exam, take the color photo at the end, and use the same color photo to overlap the fixation data through the off-line registration procedure.

MICROPERIMETRY EXAM In order to do a microperimetry exam, the user must first decide which kind of microperimetry to perform. To do so, the user may access the Test editing page by clicking on the “Customize” button located in the bottom right corner. Through the test editor, every parameter can be set and eventually saved into a particular configuration, called “test”, which can be accessed by simply selecting the corresponding test in the left column list.

If the user already knows the meaning of the tests in the left column list, the same test names appear in a drop-down box near the “Customize” button in the Examination page, eliminating the need to access the Test editing page (see Figure 6, part “D”).

Depending on Microperimetry type, some steps could be added before the start of the exam.

REGISTRATION OVER A COLOR RETINOGRAPHY Immediately after an exam, typically a microperimetry, a color photo can be taken. To do so, the user has to once again align the MP-1 with the fundus (see section 1.2.3) and click on the “Retinography” button or simply press the joystick button. After that, a dialog box appears to allow for the registration of the last exam and the color photo. If the user agrees, the Registration page appears.

The current Registration page refers to the Semi-Automatic registration procedure.

Thanks to the hint panel at the bottom, the user is guided to choose two reference areas over the infrared image through a process similar

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INTRODUCTION TO MICROPERIMETRY

Figure 9 - Registration page

to the selection of the tracking R.O.I. A reference area can be chosen if it is green, which means when the goodness index is, in this case, higher than “5.0”. Similarly to the selection of the tracking R.O.I., regions near the optic disk or containing the main vessels are usually good choices. The two regions can be only partially overlapped, but it is a good rule to choose them far from each other.

Even if the border of the infrared image may be highlighted as a good region to be selected, the reference areas must be chosen only inside the visible part of the image.

Once the two areas are selected, by pressing the “OK” button the software looks for that region over the color photo. If the selected areas are correctly recognized over the color photo, the registration procedure has been successful and the user has to simply press the

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Handbook of Microperimetry In Visual Rehabilitation

“OK” button again in order to finish the registration procedure. If one or both reference areas are not correctly recognized, the user has to select different areas over the infrared image by repeating the steps described above.

If this procedure is unsuccessful, it means that there is no way to recognize the two reference areas over the color photo. A Manual procedure can be performed by clicking the “Manual” button.

If the first choice of reference areas fails, it is highly recommended to try the Semi-Automatic procedure several times before passing to the Manual one.

Figure 10 - Manual Registration page

The current Registration page refers to the Manual registration procedure.

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