Computer-aided Analysis of Fundus Photographs Retinal Lesion Changes Over Time
This article focuses on automated computer-aided analysis of fundus photographs to identify retinal changes occurring over time, especially microaneurysm turnover.
Image Co-registration
A mandatory step for automatically comparing images is their co-registration, one against the other. That is, one of the images needs to be projected to the image space of the other, which acts as a reference image. Following this procedure, both share a common reference making it possible to establish a direct pixel-to-pixel correspondence.
To achieve the required image co-registration, it is necessary to identify eye fundus natural landmarks, intrinsic fiducial marks and compute the transformation matrix that, applied to one image, will project it to the image space of the reference image.
Two major steps are involved in the above concept. One relates to the identification and classification of the fiducial markers, while the other relates to linking similar fiducial markers between any two images to co-register.
A natural source for fiducial markers is the retinal vascular network, an imprint for each human eye. Vessel characteristics, bifurcations and crossovers make it possible to establish links between any two images from the same eye. After finding the true links for several fiducial markers, the respective transformation matrix can be computed. These two steps are implemented using a proprietary technique while the vascular network is segmented into contourlets, following an initial approach of using differential geometry.1
Automated Monitoring of Diabetic Retinopathy Progression – Microaneurysm Turnover It is of fundamental importance to monitor disease progression in specific patients and identify if they shows signs of rapid progression and to which phenotype of progression they belong. Some patients need special attention and timely intervention to avoid development of DR complications, macular oedema or proliferative DR.
The main changes that occur in non-proliferative diabetic retinopathy (NPDR) and need to be monitored are microaneurysm dynamics (their formation and disappearance), vascular leakage with subsequent oedema and hard exudates formation and capillary closure.
Visual function loss occurs characteristically late in DR because the eye has a large functional reserve of vision and DR initially affects the inner layers of the retina away from the photoreceptors. Therefore, structural changes are detected in DR earlier than functional changes. Evidence of structural changes need to be the focus to follow progression in the earliest stages of DR.
One of the best candidates for non-invasive imaging of the eye fundus is fundus digital photography because retinal cameras are widely available and the data obtained may be supported and analysed by computer-assisted procedures.
To identify progression it is essential to collect sequential series of images and these images must be compared. The need for co-registration of these sequences of images is, therefore, of great relevance. By applying novel image co-registration procedures and
EUROPEAN OPHTHALMIC REVIEW
Each detected microaneurysm (MA) is colour coded as new, old or disappeared (based on the proprietary co-registration algorithm.
Figure 2: Retmarker Software Automatically Calculates Microaneurysm Formation and Disappearance Rates
Figure 1: Automatic Microaneurysm Tracking Over Time
This patient had a microaneurysm (MA) formation rate of 5 MA/year over a 24-month follow-up.
automated comparative analysis software it is now possible to perform reliable sequential comparisons of fundus digital photography images.
RetmarkerDR (Critical Health, Portugal) is software that is currently available and is able to automatically detect changes occurring in eye fundus digital images. This software compares successive visits to the reference image, in each eye, based on co-registration and co-localisation of the changes (Figures 1 and 2).
Using fundus photography, microaneurysms and small haemorrhages are the initial changes detected in the diabetic retina. Retinal microaneurysms may be counted and this technique has previously been suggested as an appropriate marker of retinopathy progression.6,7 Retinal microaneurysms are important lesions of diabetic retinopathy and even one or two microaneurysms in an eye should not be regarded as insignificant.8
Similar findings were presented by Klein et al., who looked at the relationship between retinal microaneurysms and the progression of diabetic retinopathy over a four-year period.9
Kingdom Proliferative Diabetes Study (UKPDS) cohort that had either no retinopathy or microaneurysms only at study entry, the number of microaneurysms were found to have a high predictive value for worsening retinopathy at three, six, nine and 12 years after entry into the study.8
In this
study the number of microaneurysms at the baseline examination was positively associated with significant progression of retinopathy. More recently, Sjølie et al. showed that microaneurysm counts were predictive of an increased risk of retinopathy progression.10
Our studies have shown that it is not the absolute total number of microaneurysms at a certain point in time that may provide the best indication of retinopathy progression, but the rate of microaneurysm
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When examining 1,809 patients in the United
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