Is digital image compression acceptable within diabetic retinopathy screening?

Anterior ophthalmic imaging

Improvements in imaging chips and computer processing power have brought major advances in imaging of the anterior eye. Digitally captured images can be visualised immediately and can be stored and retrieved easily. Anterior ocular imaging techniques using slitlamp biomicroscopy, corneal topography, confocal microscopy, optical coherence tomography (OCT), ultrasonic biomicroscopy, computerised tomography (CT) and magnetic resonance imaging (MRI) are reviewed. Conventional photographic imaging can be used to quantify corneal topography, corneal thickness and transparency, anterior chamber depth and lateral angle and crystalline lens position, curvature, thickness and transparency. Additionally, the effects of tumours, foreign bodies and trauma can be localised, the corneal layers can be examined and the tear film thickness assessed.

On the quantitative effects of compression of retinal fundus images on morphometric vascular measurements in VAMPIRE

BACKGROUND AND OBJECTIVES

This paper reports a quantitative analysis of the effects of joint photographic experts group (JPEG) image compression of retinal fundus camera images on automatic vessel segmentation and on morphometric vascular measurements derived from it, including vessel width, tortuosity and fractal dimension.

METHODS

Measurements are computed with vascular assessment and measurement platform for images of the retina (VAMPIRE), a specialized software application adopted in many international studies on retinal biomarkers. For reproducibility, we use three public archives of fundus images (digital retinal images for vessel extraction (DRIVE), automated retinal image analyzer (ARIA), high-resolution fundus (HRF)). We generate compressed versions of original images in a range of representative levels.

RESULTS

We compare the resulting vessel segmentations with ground truth maps and morphological measurements of the vascular network with those obtained from the original (uncompressed) images. We assess the segmentation quality with sensitivity, specificity, accuracy, area under the curve and Dice coefficient. We assess the agreement between VAMPIRE measurements from compressed and uncompressed images with correlation, intra-class correlation and Bland-Altman analysis.

CONCLUSIONS

Results suggest that VAMPIRE width-related measurements (central retinal artery equivalent (CRAE), central retinal vein equivalent (CRVE), arteriolar-venular width ratio (AVR)), the fractal dimension (FD) and arteriolar tortuosity have excellent agreement with those from the original images, remaining substantially stable even for strong loss of quality (20% of the original), suggesting the suitability of VAMPIRE in association studies with compressed images.

Design, Development, and Evaluation of a Teleophthalmology System Using a Low-Cost Fundus Camera

Introduction:

The increasing prevalence of diabetic retinopathy in developing countries has become a worldwide concern. This problem is preventable by timely diagnosis and treatment; however, in the majority of cases, patients attend the eye clinics very late because of a lack of specialists and travel difficulties. Running a teleophthalmology system would significantly help to manage this disease.

Aim:

This study seeks to assess the accuracy of the teleophthalmology system and its effect on reducing unnecessary referrals in Iran.

Methods:

This study was conducted on 125 diabetic patients. First, the patients were examined by a retina specialist using a slit lamp and, then, single-field digital photos were captured by a portable, low-cost fundus camera. The images were uploaded onto a website and, after two months, were assessed by two retina specialists and two general practitioners (GPs). Finally, the diagnoses based on the digital photos were contrasted with the diagnoses established through face-to-face visits as a gold standard.

Results:

Out of 125 diabetic patients, eight (6.4%) were removed because of low-quality images and a total of 117 were evaluated. The sensitivity and specificity of each retina specialist presented with the photographs produced success rates of 90% and 97% respectively when judged against the gold standard of face-to-face visits. The rates of sensitivity for retinopathy referrals from the retina specialists were 92% and 85%. The sensitivity and specificity of their diagnoses of clinically significant macular edema (CSME) were calculated at 93% and 100%. The rates of sensitivity for each GP were 95% and 93% and the level of specificity was estimated to be approximately 98% for both GPs. The diagnosis rate for GPs when viewing the photographs as opposed to hosting face-to-face visits was more than 90%. Generally, with the implementation of this system, between 40% and 55% of referrals were calculated to have been avoidable.

Conclusion:

Our results from the first-ever research conducted on this topic in Iran showed that the teleophthalmology system is extremely accurate, that it can prevent unnecessary referrals and that it is useful for locating treatable patients. The results of this study could be of assistance in the running and expansion of such systems throughout Iran and Kerman Province to reduce eye damage arising from diabetes, decrease avoidable referrals to clinics, increase the availability of specialist visits for people in remote and rural areas and optimize the use of clinical infrastructures for patients in emergencies.

Influence of image compression on the interpretation of spectral-domain optical coherence tomography in exudative age-related macular degeneration

PURPOSE

To evaluate the effect of image compression of spectral-domain optical coherence tomography (OCT) images in the examination of eyes with exudative age-related macular degeneration (AMD).

METHODS

Thirty eyes from 30 patients who were diagnosed with exudative AMD were included in this retrospective observational case series. The horizontal OCT scans centered at the center of the fovea were conducted using spectral-domain OCT. The images were exported to Tag Image File Format (TIFF) and 100, 75, 50, 25 and 10% quality of Joint Photographic Experts Group (JPEG) format. OCT images were taken before and after intravitreal ranibizumab injections, and after relapse. The prevalence of subretinal and intraretinal fluids was determined. Differences in choroidal thickness between the TIFF and JPEG images were compared with the intra-observer variability.

RESULTS

The prevalence of subretinal and intraretinal fluids was comparable regardless of the degree of compression. However, the chorio-scleral interface was not clearly identified in many images with a high degree of compression. In images with 25 and 10% quality of JPEG, the difference in choroidal thickness between the TIFF images and the respective JPEG images was significantly greater than the intra-observer variability of the TIFF images (P=0.029 and P=0.024, respectively).

CONCLUSIONS

In OCT images of eyes with AMD, 50% of the quality of the JPEG format would be an optimal degree of compression for efficient data storage and transfer without sacrificing image quality.

Grading diabetic retinopathy severity from compressed digital retinal images compared with uncompressed images and film

PURPOSE

To compare research grading of diabetic retinopathy (DR) severity level from compressed digital images versus uncompressed images and film.

METHODS

Compressed (JPEG2000, 37:1) digital images (C) were compared with uncompressed digital (U) and film (F) stereoscopic photographs from a 152-eye cohort with full-spectrum Early Treatment Diabetic Retinopathy Study severity levels for agreement on severity level, DR presence with ascending severity threshold, presence of DR index lesions, and repeatability of grading.

RESULTS

Classification of Early Treatment Diabetic Retinopathy Study severity levels from C images agreed substantially with results from F images (κ = 0.60, κ(w) [linear weighted] = 0.86) and uncompressed digital images (κ = 0.76, κ(w) = 0.92). For agreement of uncompressed digital versus F images, κ = 0.62 and κ(w) = 0.86. Distribution of Early Treatment Diabetic Retinopathy Study levels was not significantly different between C and F images (P = 0.09, Bhapkar's test for marginal homogeneity). For presence/absence of DR at 8 ascending severity thresholds, agreement between C and F was "almost perfect" (κ ≥ 0.8). Agreement on severity level between readers with C images was at least as good as that with uncompressed digital image or F. Repeatability of severity threshold grading between readers was similar using C or F images. For identifying individual DR lesions, agreement between C and F ranged from "moderate" to "perfect." Agreement of grading venous beading from C was slightly lower than from F.

CONCLUSION

Full Early Treatment Diabetic Retinopathy Study scale DR severity level grading using C images is comparable to that using U images or film.

Agreement between clinical estimation and a new quantitative analysis by Photoshop software in fundus and angiographic image variables

PURPOSE

To evaluate the validity of a new method for the quantitative analysis of fundus or angiographic images using Photoshop 7.0 (Adobe, USA) software by comparing with clinical evaluation.

METHODS

Four hundred and eighteen fundus and angiographic images of diabetic patients were evaluated by three retina specialists and then by computing using Photoshop 7.0 software. Four variables were selected for comparison: amount of hard exudates (HE) on color pictures, amount of HE on red-free pictures, severity of leakage, and the size of the foveal avascular zone (FAZ).

RESULTS

The coefficient of agreement (Kappa) between the two methods in the amount of HE on color and red-free photographs were 85% (0.69) and 79% (0.59), respectively. The agreement for severity of leakage was 72% (0.46). In the two methods for the evaluation of the FAZ size using the magic and lasso software tools, the agreement was 54% (0.09) and 89% (0.77), respectively. Agreement in the estimation of the FAZ size by the lasso magnetic tool was excellent and was almost as good in the quantification of HE on color and on red-free images.

CONCLUSION

Considering the agreement of this new technique for the measurement of variables in fundus images using Photoshop software with the clinical evaluation, this method seems to have sufficient validity to be used for the quantitative analysis of HE, leakage, and FAZ size on the angiograms of diabetic patients.

The potential of digital monochrome images versus colour slides in telescreening for diabetic retinopathy

We explored the potential of digital monochrome images as an alternative to colour slides in screening for diabetic retinopathy. Twenty-eight patients with diabetes were recruited for the study and 20 actually participated. Using a fundus camera (Nikon 505AF) one set of three digital images and one set of three colour slides were taken per eye. Two independent ophthalmologists graded the colour slides and the digital images for diabetic retinopathy. The ophthalmologists spent about two minutes grading each set of images, suggesting that specialists could potentially screen a large number of patients. The agreement between the two screening methods was 0.95 and 0.89, with respect to disease or no disease. The agreement (kappa) between the two ophthalmologists for grade of retinopathy was 0.47 when colour slides were employed and 0.61 when digital monochrome images were employed. The results indicate that digital red-free monochrome images represent a superior screening tool for diabetic retinopathy. Tele-screening may be beneficial when patients have to travel substantial distances to visit an ophthalmologist.

OPHDIAT: a telemedical network screening system for diabetic retinopathy in the Ile-de-France

OBJECTIVE

International and national guidelines recommend an annual funduscopic examination for all diabetic patients, but such annual fundus examinations are not sufficiently performed in France. Non-mydriatic fundus photography is a valid method of evaluation for diabetic retinopathy (DR) and a viable alternative to ophthalmoscopy. After two pilot studies demonstrated the feasibility of telemedical screening for diabetic retinopathy in both hospital and primary-care settings, we developed a regional telemedical network, OPHDIAT, designed to facilitate access to regular annual evaluations of patients with diabetes while saving medical time.

MATERIALS AND METHODS

OPHDIAT comprises peripheral screening centres equipped with non-mydriatic cameras, where fundus photographs are taken by technicians linked by telemedicine to a reference centre, where ophthalmologists grade the images. Currently in the Ile-de-France region, 16 screening centres are linked through a central server to an ophthalmologic reading centre and includes 11 centres located in the diabetes departments of 11 hospitals, one diabetic retinopathy screening centre located in northern Paris, three in healthcare centres and one in a prison.

RESULTS

During the 28-month evaluation period, 15,307 DR screening examinations were performed. Retinal photographs of at least one eye could not be graded in 1332 patients (9.7%) and diabetic retinopathy was detected in 3350 patients (23.4%). After the screening examination, 3478 patients (25.2%) were referred to an ophthalmologist for either DR, cataract and/or non-gradable photographs.

CONCLUSION

Fundus photography combined with telemedicine has the potential to improve the regular annual evaluation for diabetic retinopathy. The organization of the network around a central reading centre serves to guarantee quality control.

Image quality characteristics of a novel colour scanning digital ophthalmoscope (SDO) compared with fundus photography

PURPOSE

To establish a set of quality parameters for grading image quality and apply those to evaluate the fundus image quality obtained by a new scanning digital ophthalmoscope (SDO) compared with standard slide photography.

METHODS

On visual analogue scales a total of eight image characteristics were defined: overall quality, contrast, colour brilliance, focus (sharpness), resolution and details, noise, artefacts and validity of clinical assessment. Grading was repeated after 4 months to assess repeatability. Fundus images of 23 patients imaged digitally by SDO and by Zeiss 450FF fundus camera using Kodak film were graded side-by-side by three graders. Lens opacity was quantified with the Interzeag Lens Opacity Meter 701.

RESULTS

For all of the eight scales of image quality, good repeatability within the graders (mean Kendall's W 0.69) was obtained after 4 months. Inter-grader agreement ranged between 0.31 and 0.66. Despite the SDO's limited nominal image resolution of 720 x 576 pixels, the Zeiss FF 450 camera performed better in only two of the subscales - noise (p = 0.001) and artefacts (p = 0.01). Lens opacities significantly influenced only the two subscales 'resolution' and 'details', which deteriorated with increasing media opacities for both imaging systems.

CONCLUSIONS

Distinct scales to grade image characteristics of different origin were developed and validated. Overall SDO digital imaging was found to provide fundus pictures of a similarly high level of quality as expert photography on slides.

Classification simplifiée de la rétinopathie diabétique adaptée au dépistage par photographies du fond d’œil

AIM

Fundus photographs using a nonmydriatic digital camera are the reference method for diabetic retinopathy screening today. The aim of this study was to validate a simplified diabetic retinopathy classification, adapted for diabetic retinopathy screening, including all diabetic retinopathy severity scales.

PATIENTS AND METHODS

One hundred and twenty-one diabetic patients had three digital color fundus photographs taken, which were graded by four independent ophthalmologists according to the proposed screening classification, and a reference examination (nine field fundus photographs and optical coherence tomography examination of the macula).

RESULTS

The proposed diabetic retinopathy screening classification was easy to use, as it was based on a visual comparison between the three digital color fundus photographs and standard retinal photographs. This classification provided the diagnosis of severe levels of diabetic retinopathy with high sensitivity and accurate specificity (100% and 50%-58%,respectively, for a moderate nonproliferative diabetic retinopathy level or higher on the screening examination) and the diagnosis of macular edema with a high sensitivity and specificity (96%-97% and 89%-91%, respectively). The results of the screening examination using this classification were highly reproducible (weighted k for interobserver agreement: 0.78-0.93).

CONCLUSION

These results suggest that this new simplified classification is accurate. The screening method should not be used above the threshold level of mild nonproliferative diabetic retinopathy. Thus, from the moderate nonproliferative level (inclusive), a complete fundus examination should be undertaken. Severe levels of diabetic retinopathy will be referred to an ophthalmologist without delay for laser treatment.

Evaluation of the effect of JPEG and JPEG2000 image compression on the detection of diabetic retinopathy

AIMS

To compare the effect of classic Joint Photographic Experts Group (JPEG) and JPEG2000 compression algorithms on detection of diabetic retinopathy (DR) lesions.

METHODS

In total, 45 colour fundus photographs obtained with a digital nonmydriatic fundus camera were saved in uncompressed Tagged Interchanged Files Format (TIFF) format (1.26 MB). They were graded jointly by two retinal specialists at a 1 month interval for soft exudates, hard exudates, macular oedema, newvessels, intraretinal microvascular abnormalities (IRMA), and retinal haemorrhages and/or microaneurysms. They were compressed to 118, 58, 41, and 27 KB by both algorithms and 24 KB by classic JPEG, placed in random order and graded again jointly by the two retina specialists. Subjective image quality was graded, and sensitivity, specificity, positive and negative predictive values, and kappa statistic were calculated for all lesions at all compression ratios.

RESULTS

Compression to 118 KB showed no effect on image quality and kappa values were high (0.94-1). Image degradation became important at 27 KB for both algorithms. At high compression levels, IRMA and HMA detection were most affected with JPEG2000 performing slightly better than classic JPEG.

CONCLUSION

Performance of classic JPEG and JPEG2000 algorithms is equivalent when compressing digital images of DR lesions from 1.26 MB to 118 KB and 58 KB. Higher compression ratios show slightly better results with JPEG2000 compression, but may be insufficient for screening purposes.

Retinopathy screening in individuals with type 2 diabetes: who, how, how often, and at what cost—an epidemiologic review

BACKGROUND

Despite treatment regimens, which can reduce blindness, diabetic retinopathy remains the leading cause of irreversible blindness for persons less than 75 years. Screening for diabetic retinopathy in type 1 diabetes mellitus has proven to be both efficacious and cost effective. Although treatment has been shown to be equally efficacious for type 2 diabetes, the effectiveness and cost benefit of screening are less clear.

METHODS

A systematic review of the recent literature on the epidemiology of retinopathy in type 2 diabetics was carried out. Specific issues related to the changing epidemiology of diabetes and diabetic retinopathy, as well as the effectiveness, and cost benefit of screening were evaluated.

CONCLUSIONS

Diabetic retinopathy is at the convergence of several epidemiologic trends. Although the prevalence of diabetic retinopathy within the population of diagnosed diabetics may be decreasing because of aggressive risk factor reduction, any gains achieved are likely to be negated by the large increase in the prevalence of type 2 diabetes in the population owing to obesity. The epidemiology findings suggest that certain persons with well-controlled type 2 diabetes may not warrant yearly examination for diabetic retinopathy; however, given the low percentage of diabetics who are currently screened for retinopathy, it would seem unwise to recommend changes in the timing of examination. Evidence appears to suggest that photoscreening is a viable alternative to a screening examination.

The effect of digital image resolution and compression on anterior eye imaging

AIM

To determine the theoretical and clinical minimum image pixel resolution and maximum compression appropriate for anterior eye image storage.

METHODS

Clinical images of the bulbar conjunctiva, palpebral conjunctiva, and corneal staining were taken at the maximum resolution of Nikon:CoolPix990 (2048x1360 pixels), DVC:1312C (1280x811), and JAI:CV-S3200 (767x569) single chip cameras and the JVC:KYF58 (767x569) three chip camera. The images were stored in TIFF format and further copies created with reduced resolution or compressed. The images were then ranked for clarity on a 15 inch monitor (resolution 1280 x 1024) by 20 optometrists and analysed by objective image analysis grading. Theoretical calculation of the resolution necessary to detect the smallest objects of clinical interest was also conducted.

RESULTS

Theoretical calculation suggested that the minimum resolution should be > or = 579 horizontal pixels at 25x magnification. Image quality was perceived subjectively as being reduced when the pixel resolution was lower than 767 x 569 (p<0.005) or the image was compressed as a BMP or <50% quality JPEG (p<0.005). Objective image analysis techniques were less susceptible to changes in image quality, particularly when using colour extraction techniques.

CONCLUSION

It is appropriate to store anterior eye images at between 1280 x 811 and 767 x 569 pixel resolution and at up to 1:70 JPEG compression.

Retinal image analysis: concepts, applications and potential

As digital imaging and computing power increasingly develop, so too does the potential to use these technologies in ophthalmology. Image processing, analysis and computer vision techniques are increasing in prominence in all fields of medical science, and are especially pertinent to modern ophthalmology, as it is heavily dependent on visually oriented signs. The retinal microvasculature is unique in that it is the only part of the human circulation that can be directly visualised non-invasively in vivo, readily photographed and subject to digital image analysis. Exciting developments in image processing relevant to ophthalmology over the past 15 years includes the progress being made towards developing automated diagnostic systems for conditions, such as diabetic retinopathy, age-related macular degeneration and retinopathy of prematurity. These diagnostic systems offer the potential to be used in large-scale screening programs, with the potential for significant resource savings, as well as being free from observer bias and fatigue. In addition, quantitative measurements of retinal vascular topography using digital image analysis from retinal photography have been used as research tools to better understand the relationship between the retinal microvasculature and cardiovascular disease. Furthermore, advances in electronic media transmission increase the relevance of using image processing in 'teleophthalmology' as an aid in clinical decision-making, with particular relevance to large rural-based communities. In this review, we outline the principles upon which retinal digital image analysis is based. We discuss current techniques used to automatically detect landmark features of the fundus, such as the optic disc, fovea and blood vessels. We review the use of image analysis in the automated diagnosis of pathology (with particular reference to diabetic retinopathy). We also review its role in defining and performing quantitative measurements of vascular topography, how these entities are based on 'optimisation' principles and how they have helped to describe the relationship between systemic cardiovascular disease and retinal vascular changes. We also review the potential future use of fundal image analysis in telemedicine.

Screening for diabetic retinopathy: the first telemedical approach in a primary care setting in France

OBJECTIVES

Diabetic retinopathy (DR) remains a major cause of visual impairment in France, due to insufficient regular annual screening. Fundus photography is a sensitive alternative to ophthalmoscopy for DR screening. The aim of our study was to report the first telemedical approach to this screening in a primary care setting in France.

METHODS

A DR screening centre equipped with a nonmydriatic camera was opened in the 18th district of northern Paris and placed at the disposal of general practitioners (GPs) of the Réseau de Santé Paris Nord (North Paris Health Network). These GPs were invited to send their diabetic patients who had no known DR and had had no fundus examination for more than one year to this screening center. Retinal photographs were taken by an orthoptist without pupillary dilation and sent for grading through the Internet to the Lariboisière Hopital Ophthalmology Department.

RESULTS

During an 18-month period, 912 DR screening examinations were performed in 868 diabetic patients referred to the DR screening center by 240 GPs. Patients' mean +/- SD age was 59.9 +/- 11.1 years. Of these 868 patients, 260 (30%) said they never have had an ophthalmological examination. Diabetic retinopathy was detected in 197 patients (22.7%). The proportion of patients for whom fundus photographs of one or both eyes could not be assessed was 10.1%. 159 patients (18.3%) required referral to an ophthalmologist.

CONCLUSION

Nonmydriatic photography, combined with teletransmission to a reading centre, proved to be a feasible valid method for the detection of DR. This screening method allowed the identification of patients requiring prompt referral to an ophthalmologist for further complete eye examination.