04 Final Images in the Hrt.

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CHAPTER 4

FINAL IMAGES IN THE HRT.

ANALYSIS AND DESCRIPTION

4.1 Map of HRT software version 2.01

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4.2 Show result

Show result is the first option of the Processing menu that allows you to see the final

images of the HRT. Two images appear on screen when this option is chosen. The image

on the right is called extended focus image or intensity image. It is the result of the addi-

tion of the 32 planes reflectivity and it is the most similar image to that of the eye fundus

ophthalmologists are familiar with. If the examination is carried out correctly, a very

clear image appears. The rims should be very definite, not blurry. In the case of a mean

topography, the standard deviation, which as we have already mentioned should be less

than 30 m, will appear in the upper right margin. When this happens the image is usu-

ally very clear, in spite of the presence of irregularities on the rims, which are part of the

alignment process of the three original series in a single final image.

The image on the left is the topographical image. It shows the topography of the

studied sector. For this purpose, the anterior or more superficial parts (in dark colors) areshown in colors ranging from orange to black, and the posterior or deeper structures (in

light colors), in a white to yellow range. These colors belong to a false color scale which

is coded by a real one of 256 tones of gray.

Since the scale of topographic levels and of original levels of intensity is a gray

scale, it is very useful to choose in the options menu, the option gray scale, with which

greater definition in the contour of structures can sometimes be obtained. The neuroreti-

nal rim can be clearly seen with the option inverse color scale or inverse gray scale.

For instance, when looking for localized fiber defects (damaged bundles) in the im-

age on the right, these appear in a dark tone due to a reflectivity loss, whereas in the im-

age on the left they appear in a lighter tone, due to a decrease of the retina in that sector,

which causes its surface to shift to a more posterior plane.

If we observe a normal optic nerve in the topographical image, we will find a

dark color indicating retinal elevation in the superior and inferior parapapillary zone due

to the higher number of fibers coming out at that level. Also, the cup surface is orange,

not white, because it is not very deep and it does not have a definite limit with the inter-

nal margin of the neuroretinal rim. This is because its slope is curved and progressive,

with no steep falls (figure 4.1).

If, on the other hand, we observe a pathological optic nerve in an advanced glau-

coma stage in the topographical image, we will find that the parapapillary zone is more

homogenous due to fiber bundle disappearance at the poles, with a white cup indicating

its great depth and with a well outlined limit between the cup and the internal margin of

the neuroretinal rim. This is due to a steep fall, of almost 90 degrees, of the fibers in the

cup, which is related with the presence of bayonet-shaped vessels (figure 4.2).These two images are the ones used to draw the contour line. Based on what was

discussed earlier, it should be stressed that this must be done upon the observation of

both images, because the computer automatically and simultaneously draws the course of

the line on the contralateral image.


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4.3 Display Image Series

This option in the processing menu has a submenu with four options: series, movie,

single and 3D movie. Their functions are useful for imaging as well as for the study of

the optic nerve and its alterations.

The series option displays the original series on the screen, with its 32 imagesgrouped in 4 lines of 8 images each, from left to right and from top to bottom, numbered

from 1 to 32 (figure 4.3).

As shown by figures 2.5 and 2.6, the presence of great brightness in many planes in-

dicates that the study depth should be increased, while poor luminosity shows that the

depth should be decreased. The plane where brightness begins, can be changed with the

refraction control in full or quarter diopters.

The movie option is also involved in image acquisition, because once the series is

taken, this option helps the observer to know if the patient has lost fixation or not as it

Fig. 4.1

Fig. 4.2


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displays the images on the screen one after another like a movie. If the patient did move

his eye, an abrupt and ample movement of the studied sector will appear. When this hap-

pens the tomography should be performed again and the previous one should not be

stored.

On the other hand, when a fiber defect is found, which sometimes looks like a

shadow in a specific sector of the parapapillary retina, the movie option may be used to

verify if the defect appears in many planes or if the shadow was caused by a reflectivity

difference. The same technique can be used to distinguish these fiber defects from vitre-

ous opacities, which are characterized by moving in an isolated manner, from the retinal

image.

With the single option, the 32 planes appear on the screen in two columns. Each

plane can be seen close up one at a time using the mouse. This is very useful for fiber

defect studies. If the defect is in any of the first planes it means that it is only superficial,

but if it extends over various planes it means that the defect is deeper and that it involves

a greater number of fibers anteroposteriorly. If the defect does not appear in various

planes then the defect may be due to a reflectivity difference (figure 4.4). In this case a

papil-edema is displayed.

The 3D movie option is to be used only for result analysis and not for image acqui-

sition improvement, since this option becomes available only after the 32 images have

been processed. This option offers optic disc animation, shifting the studied sector fromright to left, so that it becomes almost lateral at the ends. It is very useful because the

internal optic disc edges can be observed in detail, as well as the slope, which forms the

neuroretinal rim with the cup. In the case of a pathological cup with bayonet-shaped ves-

sels, the way they bend behind the slope due to the absence of glia in the neuroretinal

rim, can be observed (figure 4.5).

In localized fiber defect studies, it is very useful to observe in the 20 degree tomog-

raphy, if the bundle continues to be canal-shaped after going through the cup slope. In

these situations, a channel caused by the lack of retinal fibers can be seen.

Fig. 4.3


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Fig. 4.4

Fig. 4.5

4.4 Analyze Topography

This is the most important and most widely used option for optic disc analysis with

the HRT software. It provides a submenu with four main options, each containing another

final submenu. The options are: InteractiveMeas (A), StereoMeas (B), Display (C) andTopogrMap (D). The stereomeas option, unlike the other three, has four final submenus.

The following is an analysis of each option with its submenu:

A) InteractiveMeas:

In this menu the display option has a final submenu with the following options: x

profile, y profile, z profile, distance, topography, intensity, z in mm, and z in dpt.

With x profile and y profile, a line can be moved horizontally and vertically, so that

a frontal section anteroposterior to the optic disc is always obtained. This section shows


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the optic disc profile as well as its slope, its depth and the thickness of the retinal fiber

layer. The scale on one side of the profile can be used for finding out the cup depth.

The distance option has almost the same functions as profiles x and y, but it can

display the section with magnification, and the section line can be drawn in any direction.

In the lower part of the profile there appear two values in mm: one refers to the

length of the line shown on the screen (distance between the two end points projected

into the xy plane), the other refers to the distance between the two points at each end of

the line following the three-dimensional shape of the sections surface. The distance fol-lowing the surface will thus always be greater than the lineal distance between the two

points. Figure 4.6 shows two sections belonging to a normal optic disc and a pathological

one.

The z profile option is a very important function because it measures depth in rela-

tion to a reflectivity scale. In this way, coordinates x and y are used to find a specific

point and the reflectivity value at that point and along the z axis is displayed. Inside the

cup, the highest reflectivity occurs at high z values, while on the retina the highest reflec-

tivity is at smaller z values. Note that the reflectivity of the retina decreases at localized

fiber defects. This parameter is just a demonstration of how important a three-

dimensional measurement of an optic nerve is. Figure 4.7 is a graph showing how a re-

flectivity curve is plotted, and this option is applied to a 20 degree plaque of an optic discwith localized fiber defects in the papillary and parapapillary regions (figure 4.8).

The topography and intensity options are used for selecting the topographical or

extended focus image (summation image) to be displayed on the left side of the screen.

The z in mm and z in dpt options are used for changing the scale of the profile im-

age (image on the right) from mm to diopters and vice versa.

B) StereoMeas

This is the most important and most used menu under the analyze topography sub-

menu. It contains five options with all the tools necessary for contour line handling, for

Fig. 4.6


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its different ways of presentation, the reference plane, and the attainment of two-

dimensional and three-dimensional measurements, whether related to the reference plane

or not. It also contains an analysis by sectors and a 360 degree color analysis of surfaces

and volumes.

The four submenus are: contour line, diagram, coordinates, reference and meas-ure.

The following options are under the final contour line submenu: draw (for contour

line drawing), circle (to draw the contour as a circle), erase (to erase a contour), accept

(to accept a contour line and use it to perform optic disc analysis), change size (to change

the size of contour line that was defined by using the circle option or the draw option),

export (to export the contour line of a processed simple tomography to a mean or to a

subsequent examination), import (to import the contour line to the present examination),

Fig. 4.7

Fig. 4.8


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image selection (to choose any of the 32 planes on the image on the right for completing

a correct line drawing).

In the final submenu diagram, there are two options: scaling (used for changing the

lateral scale of the contour line height variation diagram), and polar diagram (used for

displaying the contour line height variation diagram in polar coordinates, with the height

displayed on the radial axis and the angle along circumference of the disc as the angular

coordinate).

The final submenu coordinates allows to switch between absolute coordinates (z

coordinate will be displayed with the origin at the eyes focal plane) and relative coordi-

nates (z coordinate will be displayed with the origin at the mean height of the peripapil-

lary retinal surface). Also, it is possible to select the tilted coordinate system with the xy

plane parallel to the plane of the peripapillary retinal surface.

The final submenu reference, the option set std. reference applies the standard ref-erence height (50 m posteriorly to the mean contour height in the 4 to 10 degree seg-

ment), while the option set reference height enables to enter any other desired location

of the reference plane. In addition, the option define reference region allows to define

location of the reference plane at the average height of any selected region in the topog-

raphy. The option show reference ring indicates on the screen which part of the retina is

used to compute the height and tilt of the peripapillary retinal surface.

Finally, in the final submenu measure, there are two options: predefined segments

(which offers every parameter measured in quadrants and octants) (figure 4.9), and seg-

ment. The latter option displays all parameters measured in the 360 degrees of the optic

disc. There are three more screens: page 1/3 (basic parameters with clinical names), page

2/3 (basic and secondary parameters with technical names) and page 3/3 (the area of each

section performed from 0 microns in depth to micron 1550, in 50 m intervals). Figures4.10, 4.11 and 4.12 show these three data pages with the contour line diagram. They also

show the surface analyses by colors that will be studied in the next chapter together with

stereometric analysis (parameters).

Once the contour line is accepted with the option accept of the contour line sub-

menu and the option segment is selected in the measure submenu, the computer will ask

if the examiner wishes the stereometric analysis to be carried out in the 360 degrees. If

not, the sector to be studied should be indicated by means of its beginning and ending

angles. By instructing 0 to 90, for example, the sector between 0 and 90 degrees will be

studied. In this case, the stereometric analysis, the parameters and the surface analysis in

colors will be carried out only in that sector. An example of this option, where the optic

disc is studied from 0 to 180 degrees, is shown by figure 3.8.C) Display

The display option contains a final submenu with three options: pseudo 3D, r/g ste-

reo, and stereo pair.

The Pseudo 3D option diagrams a three-dimensional graph on the screen that shows

the studied region's topography. It is very useful for observing the cup and comparing

one examination with another in a longitudinal study. The graph can be turned from right

to left and from top to bottom with the keyboard cursor keys (figure 4.13).


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With the r/g stereo option, an image may be observed with red and green glasses for

a stereo effect (figure 4.14).

In the stereo pair option, a pair of stereo projections of the intensity image is com-

puted and displayed on the screen. The distance between the images can be varied using

the keyboard cursor keys (figure 4.15).

D) Topogrmap

This option contains a final submenu with two important options: map and map

std.dev. The map option shows a topographic map of the retina, divided into 256 sym-

metric squares each representing a depth value related with the reference plane or not,

depending on whether the relative or absolute coordinates menu is being used.

Fig. 4.9: Parameters in 360 degrees and Fig. 4.10: The parameters in 360in quadrants and octants are analyzed degrees are analyzed and given their

individually with the predefined segments clinical names in the page 1/3

option. option.

Fig. 4.11: Primary and secondary para- Fig. 4.12: The correlation between optic

meters are analyzed and given their disc depth and surface in sections

technical names in page 2/3. separated by intervals of 50m of depth,

can be observed in page 3/3.


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Each of these squares is, in turn, made up of other 256 smaller squares (the original

pixels) that can be seen when choosing one of the first squares with the mouse. In this

way, the topography of this square can be seen with high resolution. Its relative depth

value represents the average value of the depths of the 256 small squares that make up the

first square (figure 4.16).

The map std. dev. option can only be studied when a mean topography is analyzed

or when the determine topographic difference menu has been chosen. Its major use is

for a longitudinal study of two tomographies taken on different dates.

Fig. 4.13: Pseudo 3-D shows the topo- Fig. 4.14: R/G stereo must be observedgraphy of the optic disc and the peri- with red and green colored glasses to

papillary region. The more posterior the achieve the stereoscopic effect.

structure, the darker it is displayed.

Fig. 4.15: Stereo Pair shows two intensity Fig. 4.16: Topo Map shows in each

images that can be inverted. By moving square, the depth relative to the

closer or further away from the screen, reference plane and this is, in turn

they create a stereoscopic effect that the average of the relative depths of

simulates depth within the optic disc. the other 256 squares.


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In this case the computer will show the standard deviation of that region in each

square, which is a result of the height variation in the retinal surface between the first andsecond tomographies.

It is important to mention that these last ways to present results should be used only

as a complement to the results obtained by stereometric analysis and by the contour line.

It is impossible to evaluate the optic nerve head with images alone and without a quanti-

tative analysis. Likewise, it is dangerous to make a diagnosis by taking only the parame-

ter values into account - the qualitative aspects must be taken into account as well.

4.5 The Topographic Examination : Initial Report

This print out is given by the computer by accessing to the data base menu, print

report or batch print report submenu. There are two features that the observer has to

control:a) Two or three examinations must be performed to obtain a mean image.

b) The standard deviation value for that mean must be lower than 30 m.

This report is very useful, but it should be followed with a written report by the ex-

aminer and sometimes with a video printer color copy showing the most characteristic

images of the patient. The explanation of this report is shown in figure 4.17.

Fig. 4.17


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Bibliography

Zinser G: Heidelberg Retina Tomograph Operation Manual, Software Version 2.01.

Heidelberg Engineering GmbH, Heidelberg, Germany, January 1997.


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04 Final Images in the Hrt.

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