Microperimetry

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11 INTRODUCTION TO MICROPERIMETRY

Leonardo Mastropasqua MD

Mauro Campigotto MSEECS

Raff aella Bisson MSEE

A Brief History

History of Fundus Related Perimetry

In examination of the retina, it has always been important to estab-lish the relationship between measurements of vision and ability to perform everyday tasks. To assess visual abilities, the main function-al approach has historically been to measure the visual acuity, still considered the standard in clinical practice. In the past, quantifi able 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 defi ciencies on quality of life and on daily activities. In an attempt to

Handbook of Microperimetry In Visual Rehabilitation

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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 fi eld examination, is that the retinal defects are not mapped exactly on the correspond-ing retinal locations. This means that there is only a quantifi cation 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 stim-uli in not precisely known positions of the fundus. Thus, with unstable or extrafoveal fi xation, it becomes impossible to accurately assess the sensitivity map. In low-vision patients, the quantifi cation and contextual localization of retinal sensitivity defects and tracking of fi xation are fundamental in establishing not only the ability to make out letters with a certain size and contrast, but even the identifi cation of the size, shape and depth of scotomas. (2) Moreover, the eye-tracking system is funda-mental in providing the visually-impaired with a correct and custom-ized rehabilitation protocol. For these reasons VA and conventional visual fi eld examination are tThese considerations led engineers to fi nd an instrument that was able to overlap the functional map (reti-nal 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 fi rst fundus-related perimeter. (4)This instrument allowed a clear, real-time 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 in-dependent of fi xation and any other eye movement. The respective map contained the fi xation area, the fi xation target, and the threshold data. This SLO tool was called microperimetry, and the name is cur-

INTRODUCTION TO MICROPERIMETRY

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rently a synonym of fundus (related) perimetry, which probably bet-ter defi nes this diagnostic technique.(5)

With the introduction of this instrument, it fi nally became possible to quantify the location and the stability of fi xation, thanks to its fi rst-time system of compensation for eye movements. With this feature, SLO microperimetry overcame the limitation of unstable fi xation, 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 fi xation lo-cus and the opportunity to regulate the stimuli pattern at the desired threshold, quantifi able 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 auto-matic fundus-related perimetry with a high-frequency 25Hz eye-tracker, 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 fi rst examination, providing either static or dynamic microperimetry. MP-1 guarantees a high quality color retinography, over which the sensitivity map and the fi xation stability results can be displayed.New microperimeters have been developed since MP-1 was commer-cialized. The OTI is one of them. It is a SLO (Scanning Laser Oph-thalmoscope) 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 abil-ity 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, fi xating at times the illumination source instead of the correct stimuli. Further-more, the eye-tracking system, being 8Hz, is slower than that of the MP-1 Microperimeter, thus resulting in a loss of precision during eye-movement monitoring and in the registration phase. In addition, the fi eld of view is smaller than the MP-1, being 12°x12°, while the MP-1 has a 45° round fi eld 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 fi xation and the fact that, instead of a full-color retinography, it provides only a black-and-white fundus image. Fur-thermore, 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 fi xa-tion stability analysis in the case of Maia and OTI is limited to the classical one (quantifi cation of fi xation stability according to the percentage of fi xation 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 specifi c and sensitive technique than the traditional method. (8)

The MP-1 is the fi rst microperimeter to introduce structured reha-bilitation based on audible feedback, while OTI doesn’t have it at all, and Maia uses a common feedback exam. The new MP-1 Micrope-rimeter feedback examination allows the ophthalmologist to train the patient to fi xate the target with the new PRL, while stimulating it with a fl ickering checkerboard (structured stimulation). Patients are asked to move their gaze according to audio feedback, which tells them whether they are getting closer to a fi xation position chosen by the ophthalmologist. Cortical neurons located in the retinotopic posi-tion 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 fi ndings, 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 fi rst time in the MP-1S Scotopic Microperimeter. This ophthalmic instrument can be used for patient diagnosis and to assess treatment effi cacy 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 fi xation analysis. In 2011, Crossland et al (Department of Visual Neuroscience, UCL Institute of Ophthalmology) made some tests on a modifi ed HFA (Humphrey Field Analyzer) with perimetry adjusted for scotopic ex-amination (prototype set-up), because scotopic visual function has traditionally been measured using it. However, this system does not control for fi xation errors or poor fi xation stability. (10) At the same time, they modifi ed a version of MP-1 by adding some fi lters to ob-tain a scotopic stimulation of the retina. The MP-1 method proved to be quicker than the modifi ed HFA technique, because it corrects for poor fi xation and it shows the retinal position of the scotopic scotoma. (11) For this reason, MP-1 now has the scotopic microperimetry and fi xation 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 post-processing 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 in-cluding fundus auto-fl uorescence patterns.


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Integration of Fundus Camera

and Computerized Perimetry

Historically, technology has always been a strong support for medi-cal 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, fl uorescein angiography, confocal microscopy and polarimetry, etc. are the best applications for analyzing the morphol-ogy 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 cur-rent, example of functional analysis.In comparison with the progress made in the morphological fi eld, the functional one has taken a different path, the visual fi eld 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 reti-nal regions: the visual fi eld test, or perimetry. The purpose of such examination is to fi nd 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 fi eld can be performed manually (now in-creasingly 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 specifi c retinal zones, while kinetic perimetry is particularly useful for quickly studying the extension of pathological areas and in the identifi cation 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 fi eld examination, as a result the progress made in perimetry has always been weaker than the strides made in morphological stud-ies, 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 fi eld is performed using the so-called Standard Automated Perimetry but it remains, in substance, the same examination introduced decades ago. The conventional visual fi eld, 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 compro-mised foveal area that suffers from unstable fi xation and/or is located in an eccentric region. The impossibility of fi xation analysis regarding its location and stability, and the assumption that we can have accu-rate results only in the presence of stable fi xations, makes computer-ized perimetry unsuitable for accurate analysis of retinal sensitivity (see Sunness et al).

MICROPERIMETRY: THE TURNING POINT

As previously discussed, classical perimetry suffers from two cru-cial issues that place this functional test at a lower level of accuracy and repeatability in comparison to the morphological analysis break-through. The two drawbacks to overcome continue to be the possibil-ity of sending the light stimuli in precise and repeatable positions over the retina and the ability to analyze locus and stability of fi xation. It is therefore necessary to introduce two innovative concepts in func-tional analysis: the constant monitoring of retinal movements and the


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matching of the results on retinal fundus. The matching of the functional analysis on retinal fundus fi nds its fi rst application in the 1970’s through the introduction on the ophthalmol-ogy market of the Rodenstock Scanning Laser Ophthalmoscope.

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 sen-sitivity in specifi c 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 pe-rimeter was introduced on the market. It improved and made more precise, repeatable, and complete the analysis of retinal sensitivity and fi xation. 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 can-not give any indication about fi xation location and stability. In fact, conventional perimetry supposes that the fi xation is always foveal

Figure 1 - Scanning Laser Ophthalmoscope, Rodenstock


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and stable. This supposition is not able to take into consideration the error caused by unstable fi xations that inevitably brings to approxi-mations 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 fi xation 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 inac-curate 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 fi xation. These great advantages make micro-perimetry a highly accurate and repeatable functional test. Finally, the opportunity to correlate these results with the morphological ex-amination 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.

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

Italy)


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THE NIDEK MICROPERIMETER

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 func-tional and morphological data. Thanks to its versatility, the user-friendly MP-1 is the “gold standard” today in the microperim-etry fi eld. The core and strength of the MP-1 is its retinal Eye-Tracking system based on a real time analysis of retinal position per-formed 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 fi xation. Another point of strength is the light pro-jection system which utilizes a dedicated LCD screen that allows extreme variability in the projection of fi xation target and light stimulation thanks to a perfect correlation with the Eye-Tracking system. Finally, thanks to the possibility of acquir-ing a digital color image of the fundus cov-ering 45° of visual fi eld, the matching be-tween the functional data of fi xation 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

Retinography

Fixation Exam

Microperimetry Exam

Figure 3 – Exam types


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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 Eye-Tracking system in a different way, the MP-1 has introduced a true revolution in the low-vision fi eld introducing the new concept of ac-tive visual rehabilitation. The diagnostic application of the device is able to identify a region of the retina with improved characteristics compared to the cur-rent locus of fi xation, or P.R.L. (Preferred Retinal Locus). The re-habilitative 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 specifi c rehabilitation protocol, of it becom-ing the new P.R.L. of the patient. Thanks to the careful and active choice of the area to be rehabili-tated and the stimulation protocol, the fi xation behaviors as well as the residual vision of the low-vision patient are improved, as is his/her quality of life.

Figure 4 – Enh ance morphology mapping

MicroperimetryOver Fluorescein

Angiography

Microperimetry over Indocyanine

Angiography

Microperimetry over OCT map


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Given the extreme usefulness of the morpho-functional matching introduced with the MP-1 Microperimeter, this concept was expand-ed, 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 Microperi meter 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 approxi-mately 10 minutes before actually performing the fi rst microperime-try, 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 fi rmly on the relative rests. The patient’s position in front of the device can be set in 3 different ways: adjusting the elec-tric table top, moving the chin rest by its wheel, rotating the joystick for fi ne refi nement. In order to fi x the chin rest in the optimal posi-tion, 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 vice-versa for the right eye. The patient must look in the direction of the front lens as requested by the user and fi xate at the fi xation target all throughout the examination. The user needs to receive confi rmation 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 Al igning

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 se-lected, 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 Examina-

tion 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 pa-tient’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 in-cluding 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 in-frared light by means of the IR Power slider (see Figure 6, section “B

The following step is very important. Adjustment of the infrared pow-er through the IR Power slider is essential to improving the quality of the infrared image and to proceeding with an effi cient examination procedure.


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Then move the optical head back and forth until two light spots ap-pear (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 astig-matism, 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 fi xation target is in the center of the fi eld 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 fi xation target position hori-zontally in order to properly include the optic disk in the fi eld 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 fi xation target in order to better include the optic disk and the main vessels in the fi eld 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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Fixation Target Selection

The MP-1 can use different shapes as a fi xation 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 fi xation exam, the best starting fi xation 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 fi xation 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 fi xation target. Again, the user may change dimension and thickness in order to be sure that the fi xation target can be seen well by the patient.The best fi xation 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 ac-cordingly. The purpose of this target is to have at least one cross vis-ible to the patient. The user has to be warned that the points of fi xation at the end of the exam are placed at the center of the 4 crosses even if the patient looked only at a specifi c cross.

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

spots. (c) Correctly aligned and focused.

(a) (b) (c)


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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 be-tween fi xation target and stimuli. If it is necessary to project a stimu-lus in the center of the pattern, the cross is not the correct fi xation target because its center may be overlapped with the central stimu-lus. 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 fi xation target in order to avoid overlapping dur-ing the examination that can affect the reliability of the sensitivity values due to the interference between light intensity of the stimuli and the fi xation target.

A very useful shortcut is the use of the keyboard arrow keys that per-mit the user to change shape, dimension and thickness of the fi xation target directly on the exam panel.

In order to change the fi xation target characteristics, the user can ac-cess 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 simi-lar 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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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.

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 tra-

cking bar during exam with exam information for eye selection, remaining

stimuli and patient’s view. (D) Test selection and exam confi guration.

A

B

C

D


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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 identifi cation 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 or-der to compensate the direction of the light stimulation and analyze fi xation 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 re-gions, due to the presence of specifi c pathologies, can be good R.O.I. regions, thanks to their high refl ectivity to infrared light.


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Just before the selection of the R.O.I., remind the patient to look at the fi xation 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 fi xa-tion 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 pa-tient’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.

Figure 7 – R.O.I. “goodness” index


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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 fi nd 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 fi xa-tion information is acquired and no light stimuli are projected, until the Eye Tracking system is able to once again fi nd 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 dura-tion 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 fi xation exam in order to analyze the fi xation 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 fi xation 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 fi rst alignment is made following instructions in section 1.2.3, the user has to select the “Fixa-tion” 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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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 fi xation 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 fi xation points. This means that after 30 seconds the system has acquired 750 points, which become 1,500 after 1 minute: this is suffi cient for understand-ing where and how the patient is fi xating. The fi xation exam has to be physically stopped by the operator by pressing the “Stop” button.If the purpose of the examination is only fi xation analysis and no other examination (i.e. microperimetry) has to be done, the user may con-tinue on the examination page by taking a retinography exam and reg-istering the fi xation examination with it in order to have the fi xation data over the more detailed color photo of the fundus. Instead, if the

Figure 8

(A) Fixation exam on infrared retin-

ography.

(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 micro-

perimetry exam through the off-line

registration procedure.

A

B

C

D


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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 neces-sary before performing a microperimetry examination (usually about half an hour until the fl ashing 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 fi xation data through the off-line registration procedure.

MICROPERIMETRY EXAM

In order to do a microperimetry exam, the user must fi rst 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 confi guration, 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 “Cus-tomize” button in the Examination page, eliminating the need to ac-cess the Test editing page (see Figure 6, part “D”). Depending on Microperimetry type, some steps could be added be-fore 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 registra-tion 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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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., re-gions 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

Figure 9 - Registration page


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“OK” button again in order to fi nish 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 fi rst choice of reference areas fails, it is highly recommended to try the Semi-Automatic procedure several times before passing to the Manual one.

The current Registration page refers to the Manual registration pro-cedure.

Figure 10 - Manual Registration page

Microperimetry

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Transcript of Microperimetry