British Journal of Ophthalmology 1995; 79: 892-899

Fundus imaging in patients with cataract: role fora variable wavelength scanning laserophthalmoscope

J N P Kirkpatrick, A Manivannan, A K Gupta, J Hipwell, J V Forrester, P F Sharp

AbstractAims-An investigation was carried out tocompare the image quality of the ocularfundus obtained clinically, photographi-cally, and with the scanning laser ophthal-moscope (SLO) at visible and infraredwavelengths in patients with significantcataract.Methods-Nineteen patients admittedfor routine cataract extraction wereexamined clinically by two independentobservers to ascertain cataract type andclarity of fundus view with an indirectophthalmoscope. Fundus photographyand both confocal and direct (non-con-focal) SLO imaging at 590 nm, 670 nm,and 830 nm were carried out after pupil-lary dilatation. Images obtained weregraded independently using a recognisedgrading system.Results-Quality ofSLO images appearedto be superior to indirect ophthalmos-copy (p<0*01) and fundus photography(p<0O001) when graded subjectively.Quantitative analysis ofcontrast ofretinalvessels demonstrated significantly highercontrast for the SLO compared withdigitised fundus photographs at all wave-lengths tested (p<0.001), with highestcontrast at 590 nm. Use of a confocalaperture significantly improved vesselcontrast but may reduce overall imageintensity.Conclusions-Scanning laser ophthal-moscopy may offer a method to observeand record fine fundus detail in patientswho have marked cataract.(BrJr Ophthalmol 1995; 79: 892-899)

University ofAberdeen, MedicalSchool, Foresterhill,Aberdeen AB9 2ZD

Department ofOphthalmologyJ N P KirkpatrickA K GuptaJ V Forrester

Department ofBiomedical PhysicsA ManivannanJ HipwellP F Sharp

Correspondence to:Mr J N P Kirkpatrick,Department ofOphthalmology, Royal PerthHospital, GPO Box X2213,Perth 6001, WesternAustralia.

Accepted for publication18 July 1995

Various methods are available to the ophthal-mologist for examination of the fundus of thecataractous eye. Clinical methods such as in-direct ophthalmoscopy are rapid and easy touse giving an acceptable image quality inpatients with moderate cataract. A permanentrecord of the fundus may be achieved usingfundus photography but results may be disap-pointing. The scanning laser ophthalmoscope(SLO) may offer an alternative to fundusphotography in providing a high resolutiondigital image at a variety of incident wave-lengths. It is an investigational tool which hasallowed acquisition of real time fundus angio-graphy,1-3 infrared ophthalmoscopy,4 and,more recently, retinal and optic disc tomo-graphy.5 6 Initial observations using the SLOhave suggested that this instrument may offer

improved detail in patients with significantcataract.4 The use of a monochromatic sourceto reduce chromatic aberration together withthe reduced optical density of aging lens nucleito longer wavelengths7 8 may result inimproved image quality. In addition a fine,highly collimated laser beam might penetrateareas of the cataractous lens which are lessdense and thereby reduce light scatter.

This paper investigates the ability of a multi-wavelength confocal SLO to provide usefulfundus images in patients with cataract. Itcompares the subjective assessment of imagequality with images from clinical indirectophthalmoscopy and fundus photography. Inaddition, quantitative analysis of the contrastof retinal vessels is compared between SLOimages at different wavelengths and digitisedfundus photographs.

Patients and methods

PATIENT RECRUITMENTApproval was obtained from the joint ethicscommittee of the University of Aberdeen andAberdeen Royal Hospitals NHS Trust for thisstudy. From patients admitted for routinecataract surgery at Aberdeen Royal Infirmary,19 eyes of 19 patients were selected for in-clusion. Patients were excluded if there wasmature cataract, significant media opacityother than cataract, or if the pupil did notdilate to greater than 4 mm. All patients gavewritten informed consent.

CLINICAL EXAMINATIONAfter pupillary dilatation with tropicamide 1%and phenylephrine 2-5% patients wereexamined by slit-lamp biomicroscopy and in-direct ophthalmoscopy by two independentexperienced ophthalmologists (JNPK andAKG).The presence of cortical, nuclear, and

posterior subcapsular opacities was classifiedaccording to the LOCS II system by compari-son with a set of standard photographs.9 In thistechnique cortical cataract is graded on a scalefrom 1 to 4, nuclear colour on a scale of 1 to 3,nuclear opacity on a scale of 1 to 3, and pos-terior subcapsular cataract on a scale of 1 to 3.For the purposes of the present study thenuclear cataract grading was estimated bydegree of opacity alone and nuclear colour wasnot used for subsequent analysis.The clarity of detail seen at the posterior

pole with indirect ophthalmoscopy was

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Fundus imaging in patients with cataract: role for a variable wavelength scanning laser ophthalmoscope

Table 1 Grading system for optic disc image clarity(adaptedfrom Nussenblatt et a110)

Gradingscore Optic disc features

0 Fundus detail clear1 Slight blurring of optic disc margin and fine vessels2 Fine vessels on optic disc visible but blurred3 Only large disc vessels discernible. Disc margin

blurred4 Optic disc margins just discernible. Major vessels

not seen5 Red reflux present but optic disc not seen

estimated using a modification of a scoringsystem developed for grading of severity ofvitreous opacities in posterior uveitis.10 Inparticular the observers were concerned withthe appearance and-clarity of the optic disc andadjacent vessels (Table 1). Observers' resultsof clinical examination, including grading ofoptic disc clarity, were noted independentlyand no discussion between observers regardingtheir clinical assessments took place.

IMAGING PHOTOGRAPHYFundus photography and scanning laserophthalmoscopy were carried out at the same

sitting within a few minutes of one another.From each patient a minimum of four fundusphotographs was taken by one operator(JNPK) using a Nikon Retinapan-45 funduscamera using Fujichrome 100 film with theflash setting at maximum intensity and 1/60second exposure. A 35 degree field centred on

the optic disc was used. Colour transparencieswere graded independently by both observersby viewing on a light box with a X 10 loupe.The same grading system was used as forindirect ophthalmoscopy (Table 1).

For quantitative analysis the transparencywhich was adjudged to be ofbest image qualityfor each patient was digitised by projectionthrough a 550 nm interference filter using a

highly collimated light source and a 25 degreefield of the frame centred on the optic discwas focused onto an optically aligned, mono-

chrome Kodak MegaPlus charge coupleddevice (CCD) camera (1320X 1024 elements).Digital output from the camera was fed to theframe grabber of a Series 151 image processingsystem (Imaging Technology Inc, USA) per-mitting capture of each frame at a resolution of1024X 1024 pixels. The grey level for eachgrabbed transparency was adjusted so that themaximum intensity within the image was set ata level of 255. The system was controlled by a

SUN IPX workstation (Sun Microsystems,USA) through the VISILOG image processingsoftware (Noesis, France).

IMAGING - SCANNING LASEROPHTHALMOSCOPYScanning laser ophthalmoscopy was carriedout using the custom built Aberdeen scanninglaser ophthalmoscope."1 12 This instrument issimilar to commercially available instrumentsbut permits a greater degree of flexibility withrespect to confocal aperture size and wave-

length source. All images were of a 25 degree

field centred on the optic disc and consistedof 740X511 pixels. Patients were imaged at830 nm, 670 nm, and 590 nm using diodelaser sources for 830 nm and 670 nm while atunable rhodamine-6-G dye laser (SpectraPhysics-375, USA), pumped by a 1 W argonlaser (Spectra Physics Stabilitic-2017, USA)was used for the 590 nm image. The entranceand exit pupil sizes of this SLO were 1 mm and6 mm in diameter respectively and the incidentpower at the cornea for all laser sources wasset at 200 puW/cm2. Patients were imagedboth with and without a confocal aperture of400 ,um diameter and the gain control for thedetector was adjusted to provide an imagecentred on the optic disc with the optimumcontrast. A minimum of two images for eachaperture size and wavelength setting weregrabbed and transferred to the SUN IPXworkstation for subsequent grading (Table 1)and image analysis.

IMAGE PROCESSING AND QUANTITATIVEANALYSISFor convenience of image handling andstorage digitised fundus photographic imageswere reduced to 512X512 pixels. SLO imageswere converted to a similar format by cutfingunwanted pixels from the sides of the imagesand adding a row of pixels of zero value to thetop of the image. Despite attempts to registerthe images to allow automated processing theimage quality in many of the images was poorand both automated and manual registrationtechniques were considered impractical.A quantitative comparison of digitised

fundus photographs and SLO images wascarried out as follows. From the availableimages of each patient the best quality imagesfrom fundus photography, and SLO imaging at830 nm, 670 nm, and 590 nm, with and with-out the confocal aperture, were selected forprocessing. Using binary image drawing toolswithin the software program it was possible tocreate a series of binary 'mask' images whichcould select out a small portion of the originalimage for analysis. For each image a first orderbranch retinal arteriole and venule with animmediately adjacent area of backgroundfundus close to the optic disc was selected.Three separate masks of approximately 1000pixels in area were created to overlap arteriole,venule, and background respectively. For eachpatient studied the same portion of arteriole,venule, and background fundus was selectedmanually from all of the images, both photo-graphic and SLO images, and subjected toquantitative analysis. A typical SLO imagewith overlying image masks is shown inFigure 1. The statistical data concerning thegrey levels of the pixels in the original imagecovered by each mask could then be calcu-lated and transferred to a spreadsheet. Thedata recorded were the number of pixels,minimum and maximum grey levels, meangrey level, and standard deviation of the greylevels. Using these data a simple measure ofimage quality, object contrast, could bedefined as follows:

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Kirkpatrick, Manivannan, Gupta, Hipwell, Forrester, Sharp

Figure 1 Four scanning laser ophthalmoscope images ofoptic disc imaged at 590 nm withconfocal aperture. Overlying areas marked in green are the binary masks used to comparecontrast ofbackgroundfundus reflectance (top left) with adjacent arteriole (top right) andvenule (bottom left). Note image noise accentuated by photographic reproduction.

Mean grey level below 'background mask'Arteriole contrast=

Mean grey level below 'arteriole mask'

andMean grey level below 'background mask'

Venule contrast=

Mean grey level below 'venule mask'

where a value of 1 implies no contrast - that is,no discrimination between object and back-ground.

In addition, the ability to discriminatebetween two structures depends not only onthe ratio of mean grey level but also on thenoise or random variation in the two structuresbeing compared. Using data concerningvariance of pixel distributions, 95% confidencelimits for each mask area were calculated todetermine whether the two adjacent regionswere significantly different in grey level charac-teristics.

STATISTICAL ANALYSISInterobserver variability for grading of cataracttype was analysed by use of a weighted kappa(K) score. Interobserver variation for thegrading of image quality of indirect ophthal-moscopy, fundus photography, and SLOimaging was also calculated. Intraobservervariation for grading of image quality wascarried out on all of the SLO images arrangedin random order 4 weeks after the initialgrading process.

For subsequent comparison of subjectivegrading between image groups the mean

observer score from the two observers was usedand groups were compared using a non-para-metric Wilcoxon matched pairs analysis.

Correlation between cataract type gradingand subjective image quality scores wasanalysed using Kendall's rank correlation.

Comparison between arteriole and back-ground 'masks' and between venule and back-ground 'masks' for individual images wascarried out using 5% and 95% confidencelimits as these image regions were assumed tohave a normal distribution. For comparison ofcontrast ratios between digitised fundusphotographs and SLO images the Wilcoxonmatched pairs test was applied.

For all statistical tests significance wasassumed if the value of p was less than 0 05.

Results

QUALITATIVE DATAThe mean age of the patients included in thestudy was 76x3 (SD 9 1) years and there werefour males and 15 females. Details of thecataract type grading according to the LOCS IIsystem are shown in Table 2. Weighted kappascores for interobserver variability ofgrading ofcortical cataract, nuclear opacity, and posteriorsubcapsular cataract were 084, 0-76, and 0-83respectively, showing good agreement betweenobservers.From the 19 patients included in the study

at the time of clinical examination, 17 under-went satisfactory fundus photography with acamera failure leading to loss of fundus photo-graphs in two patients (patient numbers 17and 19). All patients underwent SLO imagingbut attempts were not successful in all patientsat all wavelengths. In one patient (patientnumber 8) non-confocal images at all threewavelengths were not recorded. This patient'sdata were excluded from any comparison ofconfocal and non-confocal imaging. For SLOimaging in general, it was more difficult toachieve a recordable image with confocal imag-ing compared with non-confocal techniquesand, furthermore, imaging at 590 nm and670 nm was found to be more difficult than at830 nm. For this reason the SLO image seriesis not complete for patient numbers, 4, 7, 12,13, 15, 16, and 17 (see Table 3).

Table 2 Grading of cataract type for 19 patientsaccording to LOCS II system9

Image grading score

Patient Cortical Nudear Posterior subcapsularNo (grade 0-4) (grade 0-3) (grade 0-3)

1 3 1 22 3 3 33 1-5 1 14 2-5 1 25 2-5 3 0-56 2-5 1 37 4 3 08 3 2 2-59 1 2-5 310 1 3 211 1 2 312 2 3 313 1 3 314 1 2-5 0-515 3 3 016 1 25 317 4 2 318 3 3 319 1 1-5 05

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Fundus imcaging in patients with cataract: role for a variable wavelength scanning laser ophthalmoscope

Table 3 Mean image grading score for 19 patients from clinical examination, fundusphotography, and scanning laser ophthalmoscope imaging at 590, 670, and 830 nm withand without confocal aperture

Scanning laser ophthalmoscopy

590 nm 670 nm 830 nmPatientNo Clinical Photo Confocal Direct Confocal Direct Confocal Direct

1 15 1 1 2 2 2 2 22 2 3 2 2 2 2 2 2-53 2 25 2 2 2 3 2 24 3 3 NR 1-5 NR 3 NR 35 1 3 3 1-5 2 2 1 5 26 3 3-5 3 2 2-5 2 2 27 4 5 NR NR NR NR NR 58 1 1-5 2 * 2 * 2 *9 1-5 2 2 1-5 2 2 2 210 2 3 2 2 2 3 2 311 1-5 2 1 1 1-5 1-5 2 212 4 4 NR 3 NR 4 NR 2-513 4 4 NR 2 NR 3 2 314 1 2-5 1 1 1-5 1-5 1 1-515 4 4 NR NR NR 3 NR 316 3 3 NR 2 2-5 2 2 217 3 * NR 5 2-5 2-5 1-5 218 4 3 2 1 2 2 1-5 219 1 * 3-5 1 2 1 1-5 2

NR=image not recordable; *=image recordable but not stored.

Mean values for clarity of optic disc detailgraded by the two observers for indirect oph-thalmoscopy, fundus photography, and SLOimaging with and without confocal aperture at590 nm, 670 nm, and 830 nm are shown inTable 3. For fundus photography the gradingscore is applied to the best of four images whilefor SLO images the grading score is that for thebetter of two images at each wavelength, withand without confocal aperture. Interobservervariation for indirect ophthalmoscopy, fundusphotograph, and SLO image grading was cal-culated to be 0-82, 0 77, and 0 55 respectivelywhich shows an acceptable level of agreement.Intraobserver variation for observer one forSLO images was found to be 0-62 which alsoshows a good degree of reproducibility.Taking the SLO images for each patient as a

group the lowest grading score (best quality)for any image is shown in Table 4 and com-pared with the scores for indirect ophthal-moscopy and fundus photography taken fromTable 3. In general the quality of fundus

Table 4 Summary of image grading score for 19 patientswith optimum scanning laser ophthalmoscopy (SLO)imaging wavelength recorded

Image quality grading score

Patient Optimum Wavelength details ofNo Clinical Photo SLO optimum SLO image

1 1-5 1 1 590c2 2 3 2 590c+d, 670c+d, 830c3 2 2-5 2 590c+d, 670c, 830c+d4 3 3 1-5 590d5 1 3 1-5 590d,830c6 3 3-5 2 590d, 670d, 830c+d7 4 5 5 N/A8 1 1-5 1-5 830d9 1-5 2 1-5 590d10 2 3 2 590c+d. 670c, 830c11 1-5 2 1 590c+d12 4 4 2-5 830d13 4 4 2 590d,830c14 1 2-5 1 590c+d, 830c15 4 4 3 670d,830d16 3 3 2 590d, 670d, 830c+d17 3 N/A 1-5 830c18 4 3 1 590d19 1 N/A 1 590d,670d

'c'=confocal; 'd'=non-confocal.

photographic detail is less than that seen witheither the indirect ophthalmoscope or SLO. Atypical example of a fundus photograph andSLO image (590 nm confocal) is shown inFigure 2. Using the Wilcoxon matched pairstest to compare groups, with the exclusion ofdata from patients 17 and 19 for photography,it is seen that SLO imaging shows a significantimprovement over indirect ophthalmoscopy(p<0 01) and both SLO imaging and indirectophthalmoscopy are significantly superior tofundus photography (p<0 001 and p<0 02respectively). Scatter diagrams represent thesefindings in Figure 3.

Table 4 also shows which SLO imagingtechnique resulted in the highest quality imagefor each patient. A number of patients showedsimilar optimum image quality at more thanone wavelength (patient numbers 2, 3, 5, 6, 10,13, 14, 15, 16, and 19). For those 17 patientsin whom it was possible to achieve images at590 nm it was seen that this wavelength gavethe highest optic disc score in 14. In the

V

Figure 2 Comparison of colourfundus photograph of optic disc (left) and scanning laser ophthalmoscope (SLO) image at;590 nm (right) with confocal aperture. Both images have been reduced to approximately 10 degree field. Contrast of vesselsand optic cup is greaterfor SLO image.

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ophthalmoscopyFigure 3 Scatter diagram showing qualitative gradingscores for indirect ophthalmoscopy, fundus photography,and scanning laser ophthalmoscopy (SLO). Best imagequality is given lowest grading score.

remaining five patients it was seen that imagingat 830 nm yielded best results in four patientswhile one patient had no useful SLO imagedata (patient 7). In no patient was 670 nmexclusively the optimum wavelength for fun-dus imaging.

Correlation of image quality with cataracttype has been performed using Kendall's rankcorrelation coefficient, T. By adding the scoresfor cortical, nuclear, and posterior subcapsularcataract an overall cataract score with apossible maximum of 10 can be assigned to agiven patient. Correlation of this score withimage quality is significant for indirectophthalmoscopy only (p=0-025). If the scorefor posterior subcapsular cataract is notincluded in this total then both indirect oph-thalmoscopy and fundus photography showsignificant correlation with the cataract gradingscore (p=003 for both) (Fig 4). Quality of theoptimum SLO image (Table 4) appears toshow some correlation with nuclear cataractdensity (p=004) but not with a combinedcataract density score.By looking at image grading scores for indi-

vidual SLO wavelengths (Table 2), there is acorrelation between the image quality and totalcataract score for both 590 nm (p=0-04) and670 nm (p=0-03) which is enhanced by remov-ing the posterior subcapsular cataract scores(p=0 01 for 590 nm and p=0 005 for 670 nm).This correlation is not apparent at 830 nm.

QUANTITATIVE DATAContrast between first order retinal arteriolesas they emerge from the disc and adjacentareas of background retina was calculated andexpressed as a ratio of the mean grey levelswithin the two areas. Similar calculations werecarried out for the accompanying retinalvenule nearby. For all images from a singlepatient the same portion of vessel and back-ground was used to allow comparison.Contrast values for retinal arterioles are shownin Table 5 and for retinal venules in Table 6.Given that these areas were of approximately1000 pixels in size, calculation of 95%

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Figure 4 Scatter diagram of cataract type versus imagesgrading score for indirect ophthalmoscopy (A), fundusphotography (B), and scanning laser ophthalmoscopy (C).Cataract scores are the sums of the cortical and nuclearcataract grades for each patient.

confidence limits for the grey levels shows asignificant difference between all vessels andthe adjacent background with the exception ofone value (patient 17, 590 nm confocal image,Table 5). Contrast values are plotted on scatterdiagrams for retinal arterioles (Fig 5) andretinal venules (Fig 6).

In general, the contrast achieved for a givenvessel is significantly greater for any SLOimaging wavelength, with or without confocal

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Fundus imaging in patients with cataract: role for a variable wavelength scanning laser ophthalmoscope

Table S Contrast measurements ofgrey levels ofpenpapillaryvessels compared with those ofadjacent backgroundfundusfordigitisedfundus photographs and scanning laser ophthalmoscopyimages - arterole to background contrast

Scanning laser ophthalmoscopy

590 nm 670 nm 830 nmPatientNo Photo Confocal Direct Confocal Direct Confocal Direct

1 1-037 1-36 1-64 1-70 1-73 1-88 1-522 1-025 4-19 2-08 2-18 1-75 1-47 1-203 0 994 2-61 1-39 1-34 1-18 1-21 1-154 1-004 N/A 1-62 N/A 1-47 N/A 1-515 0-965 3-08 3-85 1-50 1-47 1 90 1-486 1-054 1-82 1-59 1-45 1-20 1-56 1-467 N/A N/A N/A N/A N/A N/A N/A8 1-185 1-77 N/A 1-77 N/A 1-42 N/A9 1-208 13-79 3-36 1-29 0 95 1-79 1-4110 1-047 3-56 1 10 1-12 1-17 1-34 1 1011 1-152 1-53 1-81 1-27 1 19 1-59 1-3412 0-956 N/A 4-36 N/A 1-89 N/A 1-1813 1-029 N/A 1-63 N/A 1.19 1.19 1 1014 1-140 2-49 1-72 1-49 1-06 1-34 1-2515 0-931 N/A N/A N/A 1 43 N/A 1-2616 1-108 N/A 4-87 2-61 1-44 1-74 1-4917 N/A N/A 1 90 2-30 1-62 1-84 1-8518 1-159 3-65 2-56 1-74 1-47 2-27 1-5619 N/A 1-89 2-22 0 50 1-68 1-41 1-29

All contrast values p<005.

Table 6 Contrast measurements ofgrey levels ofpenpapiUaryvessels compared with those ofadjacent backgroundfundusfordigitisedfundus photographs and scanning laserophdzalmoscopyimages - venule to background contrast

Scanning laser ophthalmoscopy

590 nm 670 nm 830 nmPatientNo Photo Confocal Direct Confocal Direct Confocal Direct

1 1-256 3-20 2-26 1-38 1-49 1-58 1-352 1-043 6-93 4-34 5-15 1-82 1-58 1-333 1-204 10-84 2-87 3 00 1-20 1-35 1-154 0-855 N/A 7-32 N/A 1-87 N/A 2-045 1-200 9-81 4-67 2-29 2-60 2-75 1-946 1-124 9-97 4-67 2-06 1-48 2-30 1-457 N/A N/A N/A N/A N/A N/A N/A8 1-264 7-26 N/A 2-29 N/A 2-18 N/A9 1-201 17-22 3 59 1-51 1-35 1-70 1-6410 1 091 6-29 2-14 1-48 1-28 1-44 1-2311 1-320 7-69 8-45 1 90 1-49 1-88 1-6312 1-032 N/A 3-33 N/A 1-65 N/A 1-1213 1-035 N/A 3-15 N/A 1-50 1-64 1-2514 1-255 17-30 3-32 1-73 1-75 1-59 1-5015 1-116 N/A N/A N/A 1-33 N/A 1-4516 0-965 N/A 4-62 3-52 1-73 2-28 1-6517 N/A N/A 1-73* 2-11 1-64 1-72 1-3118 1-272 10-58 7-77 2-23 1-85 2-98 1-7619 N/A 8-60 4-23 5-22 1-67 1-66 1-32

All contrast values except those marked (*) p<0 05.

aperture, when compared with digitised fundusphotographs as determined by the Wilcoxonmatched pairs analysis. A typical digitised fun-dus image and accompanying SLO images are

shown in Figure 7. Greater vessel contrast isapparent when a confocal aperture is used com-pared with no aperture (p<0 001 for arteriolesand venules). Furthermore, vessel contrast issignificantly greater when SLO images are

obtained at 590 nm compared with 670 nm or830 nm whether they be confocal or non-con-focal images (p<0 005 for all data subsets).There may be a trend towards slightly improvedvessel contrast at 670 nm compared with 830nm but this only achieves significance for retinalvenules imaged without a confocal aperture(p=0-02).

DiscussionThis study has made use of subjective clinicalgrading systems for the assessment of cataract9and the degree of blurring of the optic disc and

retinal vasculature.10 The latter system wasdeemed appropriate as it is readily applied toany condition which obscures the fundus view,although it is commonly used for patients withposterior uveitis. Clinical grading systems areinherently subjective and thus subject to inter-preter variation which may render them insen-sitive to real changes in the subjects studied. Inthe present study observer variation was con-sidered and measured. Interobserver variationfor the grading of cataract type was good, aswas that measured for the interpretation offundus images by all three methods.Intraobserver variation for observer one wasmeasured by repeat grading of SLO imagesand was also found to be acceptable.To allow for variations in image quality

caused by minor patient movement or opticalmisalignment it was felt that the best of fourfundus photographs and the best of two SLOimages at each setting should be used for sub-sequent grading and image analysis.

Qualitative data from the results of gradingof optic disc clarity suggest a number of points.Firstly, the image quality achieved with theSLO is evidently superior to that achieved withthe 35 degree fundus camera used in thisstudy. Secondly, compared with the imagequality of indirect ophthalmoscopy, the SLOappears to show more fine fundus detail. Thismay be attributable to the disparity in field ofview of these methods since the SLO has anarrower field and might therefore have animproved resolution.Each of the imaging methods has its draw-

backs. Indirect ophthalmoscopy provides awide field of view but with lesser magnificationthan the other methods. Furthermore theresultant image is unrecordable. Fundus pho-tography is the mainstay of permanent oph-thalmic fundus recording but may not result inacceptable image quality, presumably as aresult of increased light scatter in cataractpatients. The SLO uses a smaller field of viewand provides a monochromatic image whichrequires some degree of familiarity for accurateinterpretation, particularly with infrared wave-lengths. With the SLO the optic disc has anunusually bright optic cup and features of thefundus such as chorioretinal pigment abnor-malities may appear pronounced.4 The SLO isrelatively non-portable and is also more expen-sive.The question of optimum SLO wavelength

for imaging is not entirely clear. For the major-ity of patients in the study, all of whom hadsufficient cataract to warrant surgery, images at590 nm yielded the best grading scores. This isto be expected since the resolution of the SLOdecreases for an increase in wavelengthl2 and,moreover, at 590 nm the absorbance ofhaemoglobin and hence the contrast of retinalvessels is many times greater compared with670 nm or 830 nm.13 However, there was agroup of patients whose images at 590 nmwere unrecordable as no reflected signal couldbe detected. In these patients 830 nmappeared to offer a satisfactory view of the fun-dus in many cases. It was also found that thegain settings for the SLO were invariably

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confocal optics offers an additional way toavoid light scattering by the lens and resultantdegradation of the received image.16 With thistechnique the SLO has the ability to create anoptical sectioning or tomographic effect and sofilter out light scattered from different layers,such as the lens, which would act to reduceimage contrast. The 400 ,um confocal apertureused in this study creates an optical sectionthickness of 2600 pum for 830 nm light'2 and

* this is not expected to vary significantly for the* other wavelengths employed. Results compar-

ing the confocal and direct (non-confocal)A images show that while vessel contrast is* ° increased significantly the clinical grading

I + scores show no significant improvement. This* ..i.." -.4....f. j is not remarkable since there were six patients*A ~ in whom the use of the confocal aperture

caused increased difficulty in achieving arecordable image particularly at 590 nm, thus

I reducing the statistical power. In general, there-590 D 670 C 670 D 830 C 830 D fore, a confocal aperture may allow improved/background contrast ratios for indirect image quality in some patients but at theand scanning laser ophthalmoscopy (SLO). Mediane dotted lines. Note no data for indirect expense of the itensity of the received image.ibers on the x axis refer to SLO imaging wavelength Correlation of image quality grading withdirect (non-confocal) respectively -for example, cataract type in this study should be inter-g 590 nm laser and confocal aperture. preted cautiously owing to the small number of

patients and the preponderance of mixed~r for 590 nm than 830 nm, usually with cataract classifications graded by the LOCS IInm in between. Low signal returns at system.9 Rank correlation with a summatednm may be due to the increased optical score of cataract degree showed that indirectity of the lens at lower wavelengths, a ophthalmoscopy images were related some-re of the normal lens which is exaggerated what to the score awarded to each cataract byging7 14 15 and this may be more pro- the observers. A significant correlation withIced in patients with cataract. In addition, cataract density could be achieved for indirectLetector used in this version of the SLO has ophthalmoscopy and fundus photography ifLk sensitivity in the infrared range. only scores for cortical and nuclear cataractgeneral terms while one might expect the were used. It may be that while posterior sub-to improve fundus image quality by capsular cataract is responsible for significant

iinating the fundus through a small, clear functional visual loss, it contributes less to theon of the crystalline lens, the principle of image degradation seen when imaging in

patients with a dilated pupil. The SLO showedno correlation with cataract grading scores andit is worthwhile pointing out that some patientsadjudged to have high cataract density scoresstill had good fundus image quality scores(Fig 2). The suggestion that the SLO is bestsuited to a particular cataract type should bethe subject of further investigation on a largergroup of patients.

Quantitative data from digitised imagesserve to back up the qualitative data alreadydiscussed. The use of a ratio of vessel to back-

. ground fundus as a measure of contrast is* simple and applicable to these data since the

baseline for both fundus photographs and SLOimages was zero and no offset existed. The

A method of image capture of the fundus photo-| graphs using red free light to allow digital pro-

cessing was similar to that used in other*I--- * studiesI7 18 and leads to minimal loss of con-"A* , 8 trast of retinal features. For those patients in

* ...,... o whom images are obtainable, significant...... ,. .. 4@ jimprovement in vessel contrast is achieved by

using the SLO at any of the wavelengths testedcompared with digitised fundus photographs.

590 D 670 C 670 D 830 C 830 D It appears that a striking contrast is achieved at

background contrast ratios for indirect 590 nm for both arterioles and venules whichand scanning laser ophthalmoscopy. Median values may be explained by selective use of a wave-Notes on axis labelling are similar to Figure 5. length at which haemoglobin is optically dense.

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Fundus imaging in patients with cataract: role for a variable wavelength scanning laser ophthalmoscope

Figure 7 Comparison of digitised colourffundus photograph (top left) with scanning laser

ophthalmoscopy images: 590 nm confocal image (top right), 670 nm confocal image(bottom left) and 830 nm confocal image (bottom right). Vessel contrast is greatest at

590 nm. Note colour fundus photograph is rendered monochrome during digitisationprocess.

Thus the reflected signal from vessel areas is

extremely low compared with backgroundfundus. Vessel contrast is also significantlyimproved with the use of a confocal aperture, a

finding which was not significant when

analysing the qualitative data.In conclusion, the SLO appears to offer an

alternative to conventional imaging in exami-

nation of the fundi of cataract patients. For

many patients the contrast of retinal features is

subjectively and objectively improved com-

pared with fundus photography. The optimumwavelength for SLO imaging from those tested

appears to be 590 nm but because of low

reflectance in some patients at this wavelength,useful data can be achieved at higher, particu-larly infrared, wavelengths. A confocal aper-ture will improve contrast but this may be at

the expense of overall image brightness and itsusefulness may vary from patient to patient.The type of cataract is which SLO imagingoffers most advantage is somewhat unclear andrequires further study.The authors are grateful to their sponsors for assistance withthis project. Mr Kirkpatrick was supported by an ACTR fellow-ship from the Scottish Home and Health Department. DrManivannan and the image processing work station are fundedby Scotia Pharmaceuticals. The scanning laser ophthalmoscopewas built from funds provided by the Wellcome Trust.The authors are grateful to Mr R Hutcheon for photographic

assistance and to the technical staff of the Department ofBiomedical Physics.

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