Resolution of retinal digital colour images

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.

A comparison of colour micrographs obtained with a charged couple device (CCD) camera and a 35-mm camera

PURPOSE

The purpose of this study was to determine if micrographs produced with a colour charged couple device (CCD) camera were comparable in quality to micrographs obtained with a 35-mm colour film camera. The Axiocam (Zeiss, Jena, Germany) CCD camera produced images of 3900 x 3090 pixels.

METHODS

Each picture in a series of 50 pairs of film and digital micrographs was rated independently by each of three observers on the categories of colour, detail and overall impression. A total of 28 of the 50 pairs of micrographs were discarded due to colour differences.

RESULTS

Digital micrographs were rated lower on colour and higher on detail compared to film micrographs. The film and digital micrographs were rated equally on overall impression. The difference in colour between film and digital micrographs can be compensated for using image processing software on the digital image. However, such manipulation was not carried out as part of the present study. The study originally included 50 digital micrographs and 50 film micrographs. The micrographs were paired and 22 of the 50 pairs of micrographs were assessed. Thus 28 pairs were not evaluated due to colour differences in the non-manipulated pictures. The interobserver difference on colour in the remaining 22 pairs given as mean square +/- standard error of the mean was - 0.27 +/- 0.10, 0.62 +/- 0.10 and 0.90 +/- 0.11. The interobserver difference on detail was - 0.48 +/- 0.10, 0.95 +/- 0.10 and 1.43 +/- 0.10. The interobserver difference on overall impression was - 0.70 +/- 0.10, 0.57 +/- 0.10 and 1.27 +/- 0.10. All the results differed significantly (p < 0.001).

CONCLUSION

Digital micrographs are sufficient in detail, colour and overall impression to replace the traditional 35-mm film recordings for use in histopathology.

Diameter variations of retinal blood vessels during and after treatment with hyperbaric oxygen

AIMS

To quantify retinal vascular change during and after hyperbaric oxygenation (HO) for 6x5 weekly 90 minute treatments.

METHODS

Fundus photographs were taken before, during, and after HO at 2.5 atmospheres absolute pressure (ATA) on days 1, 2, 3, 10, 20, 29, and 30 of treatment on three patients using a specially developed hand held ophthalmoscope with a digital colour camera. Blood vessel diameter was estimated on red free retinal images. The mean of three measurements of arterioles and venoles close to the optic disc was calculated. Consistency and repeatability of the method was verified by estimating the diameter of the vessels by three measurements in each of seven images taken within 70 seconds on the same person. Analysis of variance with Bonferroni correction for multiple comparisons was conducted to ascertain whether significant intergroup differences existed.

RESULTS

Breathing 100% oxygen at 2.5 ATA constricts retinal arterioles by 9.6% (standard deviation 0.3%) and venoles by 20.6% (SD 0.3%) of their size in air at ambient pressure. Constriction escalates during treatment. Ten minutes after the HO, arterioles dilate to 94.5% (SD 0.3%) and venoles to 89.0% (SD 0.3%) of their primary size. This pattern is the same for each day of measurement. Heart frequency falls continually during HO. Systolic, diastolic, and mean arterial pressures stay constant.

CONCLUSION

Exposure to hyperbaric oxygen causes constriction of the retinal vessels. It is found that this constriction is constant through the series of treatments. This suggests that oxygen or products thereof are responsible for the vascular changes during and after hyperbaric oxygenation probably through autoregulation of the retinal vessels.

Effect of video display on the grading of diabetic retinopathy

PURPOSE

Digital images are increasingly being used in ophthalmology. These may be viewed either on thin-film transistor (TFT) or on cathode-ray tube (CRT) displays. However, there is little data showing which is superior. In this study, we compared the performance of CRT and TFT displays for grading of both compressed and uncompressed images of diabetic retinopathy.

METHODS

A total of 49 35 mm transparencies of diabetic retinopathy were scanned and compressed. The images comprised 17 with no retinopathy, eight with background, five with preproliferative, and 19 with proliferative retinopathy. Four levels of compression were used: 0, 70, 80, and 90%. A total of 196 randomised images were presented to two masked graders using both TFT and CRT displays under uniform lighting conditions, 2 months apart. The grade of retinopathy was assessed. Statistical analysis of grading accuracy was performed using receiver operator characteristic curves of sensitivity and specificity and the Stuart-Maxwell test for paired, nonparametric data.

RESULTS

Both displays showed high sensitivity and specificity for the detection of any retinopathy. For the specific grade of retinopathy, the CRT performed slightly better with a sensitivity of 0.80 for uncompressed images, compared with 0.75 using the TFT. Compression reduced these sensitivities to 0.73 on the CRT and 0.63 on the TFT. Grading of uncompressed images magnified to four times their original size was more accurate on the TFT.

CONCLUSIONS

Grading on both displays met sensitivity and specificity criteria proposed by Diabetes UK (formerly British Diabetic Association) for screening of diabetic retinopathy. The CRT generally performed slightly better than the TFT in relation to the detection of the specific grade of retinopathy.

Comparing light sensitivity, linearity and step response of electronic cameras for ophthalmology

PURPOSE

To develop and test a procedure to measure and compare light sensitivity, linearity and step response of electronic cameras.

METHODS

The pixel value (PV) of digitized images as a function of light intensity (I) was measured. The sensitivity was calculated from the slope of the P(I) function, the linearity was estimated from the correlation coefficient of this function. To measure the step response, a short sequence of images was acquired. During acquisition, a light source was switched on and off using a fast shutter. The resulting PV was calculated for each video field of the sequence.

RESULTS

A CCD camera optimized for the near-infrared (IR) spectrum showed the highest sensitivity for both, visible and IR light. There are little differences in linearity. The step response depends on the procedure of integration and read out.

Two-field photography can identify patients with vision-threatening diabetic retinopathy: a screening approach in the primary care setting

OBJECTIVE

To compare the effectiveness of two 45 degrees photographic fields per eye in the screening for diabetic retinopathy with the routine ophthalmologist's examination and to study the effectiveness of visual acuity measurement in the detection of diabetic macular edema.

RESEARCH DESIGN AND METHODS

Type 1 and 2 diabetic patients without a known history of more than minimal retinopathy (n = 469) had a routine examination by an ophthalmologist, including visual acuity measurement, indirect retinoscopy, and biomicroscopy. At the same time, two-field nonstereoscopic retinal photographs were made of both eyes and assessed in a masked fashion by a retinal specialist. The results were compared.

RESULTS

The prevalence was 4.3% for vision-threatening retinopathy and 24% for any retinopathy. The sensitivity of two-field photography in identifying diabetic patients with sight-threatening retinopathy was 95% (specificity 99%) and sensitivity for detecting any retinopathy was 83% (specificity 88%). The percentage of referrals to an ophthalmologist was 6.2%. All patients with macular edema detected by biomicroscopy were classified as having vision-threatening retinopathy on the photographs.

CONCLUSIONS

Two-field retinal photography is a promising alternative to the routine ophthalmologist's examination in the screening for diabetic retinopathy. Visual acuity measurement is not a sensitive tool for the detection of macular edema. Screening for diabetic retinopathy using two-field retinal photography is feasible in a primary care setting and can substantially lower the number of ophthalmic referrals.

Effect of digital image compression on screening for diabetic retinopathy

BACKGROUND/AIMS

Digital imaging is widely used for diabetic retinopathy screening. The storage and transmission of digital images can be facilitated by image compression. The authors aimed to assess the effect of image compression on the accuracy of grading diabetic retinopathy.

METHODS

Forty nine 35 mm transparencies (17 with no retinopathy, eight with background, five with preproliferative, and 19 with proliferative retinopathy) were digitised and subjected to JPEG compression by 90%, 80%, 70%, and 0%. The 196 images were randomised and graded on a portable computer. Two masked graders assessed the images for grade of retinopathy and image quality (0-10). The sensitivity and specificity of retinopathy grading were calculated with a weighted kappa for grading agreement between levels of compression.

RESULTS

The sensitivity of retinopathy grading was reduced by JPEG compression. At 90%, 80%, 70%, and 0% compression the sensitivities were 0.38, 0.50, 0.65, and 0.72, respectively; the specificity results were 1.00, 1.00, 0.83, and 0.84, respectively; and the weighted kappa scores were 0.60, 0.75, 0.77, and 0.84, respectively. The quality scores for 90%, 80%, 70%, 0% compression were 2.9 (SD 1.1, 95% CI; 2.7-3.2), 4.6 (SD 1.1, 95% CI; 3.0-5.6), 5.8 (SD1.5, 95% CI 5.0-6.6), 6.3 (SD1.4, 95% CI; 5.4-7.2) (p<0.01 for each intergroup comparison).

CONCLUSION

The results demonstrate significant loss of sensitivity to the features of diabetic retinopathy with JPEG compression; this was compounded by the thin film transistor (TFT) screen. The authors found the quality of uncompressed images on TFT screens too poor to give grading sensitivities which reach current guidelines for diabetic retinopathy screening.