Exp. Eye Res. (1981) 33, 597-602

An Objective Staging for Cortical Cataract in vivo Aided by Pattern-analysing Computer

HECTOR MACLEAN* AND CHRIS J. TAYLOR t

* University of Melbourne, Department of Ophthalmology, 32 Gisborne Street, East Melbourne, Australia, 3002 and "~ University of Manchester, Department of Medical

Biophysics, Stopford Building, Oxford Road, Manchester MI3 9PT, England

(Received 7 February and accepted 15 June 1981, New York)

A subjective staging system for cortical cataracts, based on retro-illumination through a dilated pupil, has been adapted for objective assessment on a pattern analysing computer (Magiscan), using transparencies taken with a modified Zeiss slit lamp camera. The area of the pupil occupied by densities transmitting 90 % or less of the background illumination is presented as a print-out of the percentage of the pupil area occupied by cataract, together with a map of the involved area. The mean annual increase of cortical cataract in a series of patients followed for up to four years was 1"6 %.

Key words: cortical cataract ; computer aided staging; retro-illumination cataract photography; progression analysis.

1. Introduct ion

Clinical assessment of changes with t ime in cortical ca tarac t is difficult. Photographic recording can provide a baseline from which change ma y be observed. Different methods of photography may show apparen t ly different degrees of cataractous change, so t ha t technical s tandard iza t ion is essential. Silhouettes against the red reflex are simpler to record, and more reproducible, t han mult iple slit images. Stereoscopic photographs have an advan tage over monoscopic images by allowing a check on the depth of specific features wi th in the lens. Changes in the areas of in teres t mus t be plot ted accurately. Convent ional ly , this would be done by project ing the photographic image on to squared paper and count ing the area of involvement . The development of pa t t e rn analys ing computers has enabled such tasks to be au tomated , and combines t ime-saving with increased accuracy. This paper describes a method for tak ing sui table photographs and explains how these are analysed.

2. M e t h o d s

Photographic method Pupils are dilated with l0 % phenylephrine and 0"5 % tropicamide. A modified Zeiss photo

slit lamp set for a magnification of l0 x and a constant flash output is used to make consecutive stereo pairs. The slit beam is pre-set to 1"5 x 5 mm. The slit lamp microscope is focussed on the central anterior lens capsule. The first stereo pair is taken with the beam axially displaced to the left side of the pupil with its outer corners just touching the pupil edge. A second pair is made similarly but with the beam towards the right side of the pupil. By using the optimally illuminated transparencies from each pair, a consecutive stereo pair is created with each photograph showing the information hidden in the flash reflex of the other. The procedure is repeated on the fellow eye. Kodak Ektachrome Professional 200 ASA film is used with standard processing by a laboratory with consistent quality control. A step wedge

* Reprint requests to Dr Hector Maclean, 32 Gisborne Street, East Melbourne, Victoria 3002, Australia.

0014-4835/81/120597 + 06 $01.00/0 �9 1981 Academic Press Inc. (London) Limited

597

598 H. MACLEAN AND C. TAYLOR

was photographed on each film to aid confirmation that density gradients remained constant from film to film, and batch to batch. A two-year supply of film of the same batch number was bought and stored under refrigeration.

Slit lamp modifications A Zeiss photo slit lamp was modified to increase its power output to 820 Wsec with an

electronic interlock to prevent the flash being fired until the storage capacitors acquired a charge to within 2 % of this level. A metering circuit was added to monitor the actual flash output as a precaution against loss of light from ageing of the flash tube. No fill-in flash was used.

The mountings of the head rest of the slit lamp are adjusted to give greater rigidity with an extra bearing against the edges of the instrument table. The standard fixation device is replaced by a modified Haag--Streit target click-stopped for reproducible positioning. Optimum slit beam size proved to be from using the Waterhouse stops at the 5 mm diameter setting with the slit width control at the same position, giving a slit image 5 x 1"5 ram. A Zeiss 1:7 beam splitter (30.15.13) is fitted and coupled through two Zeiss photographic tubes and 2 x adaptors to a pair of Minolta motor drive SRM bodies. This allows the image from the majority of pupils to fill the frame vertically using the l0 x setting of the slit lamp objectives. The f/14 aperture setting of the photo tubes is used. The footswitch is replaced by a microswitch fitted into the joystick.

With the patient's left eye gazing at the fixation device, the slit lamp is set with the aid of fiducial marks made on the instrument table 32'5 mm to the left side of the centre of its range of movement and at 90 ~ to the table edge. The joystick is then moved so that the first Purkinje image of the projector mirror in the slit lamp housing is centred on a cross graticule in the right hand eyepiece. Using the beam decentring control, the slit beam is moved towards the right edge of the pupil for the first photograph and then to the left edge for the other. This process is then repeated moving the slit lamp 65 mm to its right for photography of the left eye (and moving the fixation device to the right eye).

Computer methods The computer used was a Magiscan Image Analysis System (Joyce-Loebl Limited,

Gateshead, England). For analysis, a transparency is set on a light box and imaged by a television camera

connected to the computer. Suitable extension rings or tubes are used with the camera lens, to obtain a full frame picture. No filtering is necessary, even with tungsten filament illumination. The television camera scan is controlled by the computer which is aware of the coordinates of the spot at any instant, as it is also of the electron beam current in the tube. This current corresponds to the brightness of the image at a particular point. Brightness is quantitized to 64 levels on an array of 512 x 512 points. A monitor screen displays the picture from the camera and allows interaction by the operator. The system is software controlled and a program has been written to analyse optical density on a 128 • 128 matrix of points, recording those with a transmission of 90 % or less with respect to the image of the underlying red reflex.

Initial operator interaction uses a light pen to mark three points on the circumference of the pupil (Fig. 1). From simple Euclidean principles, this allows the circumference of a circle and its centre (and thus its radius) to be calculated. Next, a minimum of six and a maximum of 256 points free from cataract change are indicated with the light pen to enable the complex background illumination curve to be automatically calculated and stored (Fig. 2). Then points within the circle where the transmission is less than 90 % of this background are shown on the screen (Fig. 3). Highlights, e.g. the flash reflex, are ignored. Blink artefacts or non-circular areas of the pupil may be difficult to separate from areas of cataract near the lens equator and if recorded as cataract can be manually erased with the aid of the light pen or, if the operator prefers, the process can be repeated from the beginning, choosing three different pupil points and new background points. Alternatively, the whole circumference may be uniformly shrunk by reducing the radius in steps corresponding to two matrix points. When operator interaction is complete the image is stored in the computer's memory. The chosen transparency of the second pair on the same eye is similarly analysed and its image stored.

C O M P U T E R C A T A R A C T S T A G I N G 599

FIG. 1. Using a light pen to indicate circumference of chosen circle (edge of dilated pupil).

FIo. 2. Indicating areas of background free from cataractous change.

FIG. 3. Analysis from one transparency.

Fro. 4. The same patient's fellow eye. Electronically merged images. The top line of coding indicates the patient's name, date of visit and whether right or left eye. The next two lines relate to program functioning and the final line is the percentage of pupil area occupied by cataract.

The t~vo images are then electronically shuffled with respect to the centres of the circles by the equiva len t of four television lines vert ical ly and horizontally. With the best match obtained, the two images are electronically merged ( ' logical or ' ) and presented as a map on the screen. A code number is added to indicate the pat ient ' s name, date of visi t and eye position (Fig. 4).

Photographs from subsequent visits are similarly analysed and stored. When a pr in t -out is called for, the smallest radius in the series for tha t eye is taken and made common for the series. Calculation then gives, at each visit, the amount of ca taract in a circle of tha t radius and is presented on the screen as the percentage of the area corresponding to this radius (and centre) occupied by cataract , together with a map of tha t cataract .

600 H. MACLEAN AND C. TAYLOR

3. Results

This technique was devised for an objective analysis of possible change in cortical cataract of patients undertaking a long-term trial with the anti-cataract drug 'Catal in ' and as a refinement of the photographic methods previously described (Maclean, 1977, 1978). Analyses were completed on 38 patients over a follow-up period of 1-4 years. A small difference favourable to Catalin but not statistically significant, occurred between treatment and placebo groups. Overall, a gradual increase in cataract occurred. The annual increase as measured by the method described was 1"60__ 3"3 %, calculated from 38 test eyes analysed on a mean follow-up period of 2"0 years.

4. D i scuss ion

Increasing interest in the possibilities for medical therapy of cataract requires the development of means to objectively assess cataract. Increasing colour of the nucleus as an index of the worsening cataract was proposed by Pirie (1968). This has been refined subsequently, (Truscott and Augusteyn, 1977 ; Mareantonio, Duncan, Davies and Bushell, 1980), and an a t tempt made to extend it to in vivo readings by reflection densitometry (Sigelman, Trokel and Spector, 1974). The requirement for a staging system of cortical cataracts in vivo has been met by a retrorillumination method (Maclean, 1977, 1978) This provides for four basic stages according to the number of quarters of the pupil area occupied by cataract, and each stage may be subdivided. A standard series is readily formed by a set of slides. Gross comparisons may be made with such a series (to __ 5 %) and blink comparisons allow smaller degrees of change to be noted, but such techniques are tedious and lack the objectivity of the computer method.

The method of photography described was evolved by trial and error. I t bears resembance to one described earlier by Hayashi (1970), in the Japanese literature, using only a single image and single camera. I t lacks the depth of field of Brown's camera (Brown, 1972), or the elegant multi-slit derivation from that by Hoekwin's team (Dragorimescu, Hockwin, Koch and Sasaki, 1978), but it is much easier to apply for both patient and researcher, and records the whole lens in two shots. I f the limited depth of field restricts recording of posterior cortical opacities, a separate pair of photographs may be taken focussed on the posterior lens capsule and electronically merged with those already taken. This limited depth of field effectively prevents results being influenced by posterior subcapsular cataract which, being so out of focus, merely causes a reduction in overall background illumination on the transparency. Even Stage 1 and Stage 2 nuclear cataract do not influence the results, although they do make setting the background much more difficult. I f it is essential to record both anterior and posterior cortex in detail on one pair of photographs, a greater depth of field may be obtained by omitting the 2 • televertors and using a lower flash output. I t is advisable to retain the fast film, however, because of its ability to handle an extended contrast range.

Use of sequential stereo does result in loss of some photographs because of patient eye movement. A single photograph, however, means loss of any detail covered by the flash reflex, as in Hayashi 's method (Hayashi, 1970). Use of a retinal camera {Hockwin, Weigelin, Hendrikson and Koch, 1975), gives a smaller flash image, but

COMPUTER CATARACTSTAGING 601

has the disadvantage of a curved field and a power supply much more difficult to regulate.

Software for the Magiscan system can be developed in several ways. For simple applications, SPEL, an interactive menu selection program may be used. A wider range of facilities and a more organizational flexibility are features of MAGIC, a development of the widely understood BASIC language. For applications which require access to the full flexibility of the instrument, programming in FORTRAN is used with all s tandard functions supported by an extensive subroutine library. In this particular case, the problem of correcting for the gross non-uniformity of illumination was sufficiently complex that programs were written in FORTRAN. Full details of the methods used will be published elsewhere.

The choice of a matr ix of 128 x 128 points offers a sensible compromise between recording of very small localized opacities and the time taken for analysis. Use of a 64 • 64 matr ix gave a much faster but less accurate map of cataract and a 256 • 256 matrix, while very accurate, was too time consuming. The degree of ' shuffling' was also a compromise. Occasional transparency pairs showed eye movement or rotation (cyclotorsion) differences. To increase the amount of shift possible between the images when the computer was seeking the best match, caused unacceptable increases in the running time of the program, and excessive demands on the memory store. Similarly, no a t tempt was made to deal with rotated images in this particular program and no use was made of the stereological abilities of the instrument.

The choice of 90 % of background transmission as the cut-off point for cataract change was, like the choice of number and size of matrix points, arrived at through experiment. The earliest development of the program recorded actual densities. Plotting areas of change was difficult with such a scheme. Using a relative transmission of 95 %, or even of 98 ~o, is feasible but requires the operator to spend much more time marking the spots for calculation of the background illumination. Interference from nuclear change is much more likely with these levels, and this risk increases during a long-term project such as the one for which the method was developed.

The method has provided reproducible results, whether using the same transparencies for analysis on different occasions, or from transparencies of the same eye taken at two differing but closely related visits, e.g. two months apart . I t allows comparison with, and indeed may be extended to deal with, post-operative gradings as described by Chylack (1978). An ult imate refinement may be immediate in vivo readings from a television camera at tached to a beam splitter in the optical path of a slit lamp microscope.

R E F E R E N C E S

Brown, N. (1972). Quantitative slit image photography of the lens. Trans. Ophthalmol. Soc. U.K. 92, 303-7.

Chylack, L. T. (1978). Classification of human cataracts. Arch. Ophthalmol. 95, 888-92. Dragorimescu, V., Hockwin, 0., Koch, H.-R. and Sasaki, K. (1978). Development of a new

equipment for rotating slit image photography according to Scheimpfiug's principle. Interdiscip. Top. Gerontol. 13, 1-13.

Hayashi, H. (1970). Transillumination photography of the lens--utilisation of slit lamp microscopic photography. Opthalmology (Japan) 12, 788-92.

Hockwin, 0., Weigelin, E., Hendrickson, P. and Koch, H.-R. (1975). Photographische Method zur Kontrolle des Trubungsverlaufs bei der Cataracta Senilis (in Japanese with German summary). Folia Ophthalmol. Jap. 25, 88-93.

602 H. MACLEAN AND C. TAYLOR

Maclean, H. (1977). The Melbourne Catalin trial. Current protocol and some early results. Austr. J. Ophthalmol. 5, 183-7.

Maclean, H. (1978). A controlled trial of Catalin in senile cortical cataract. Acta X X I I I Int. Congr. Ophthalmol. Pp. 1357-61. Excerpta Medica, Amsterdam.

Marcantonio, J. M., Duncan, G., Davies, P. D. and Bushell, A. R. (1980). Classification of human senile cataracts by nuclear colour and sodium content. Exp. Eye Res. 31,227-37.

Pirie, A. (1968). Colour and solubility of the protein of human cataracts. Invest. Ophthalmol. 7, 634-42.

Sigelman, J., Trokel, S. L. and Spector, A. (1974). Quantitative biomicroscopy of lens light back scatter. Arch. Ophthalmol. 92, 437-42.

Truscott, R. S. W. and Augusteyn, R.C. (1977). Changes in human lens proteins during nuclear cataract formation. Exp. Eye Res. 24, 159-70.