A method for corneal nerves automatic segmentation and morphometric analysis
Highlights
► We developed a method for automatic segmentation of corneal nerves. ► The method uses phase-symmetry for nerve identification. ► The method sensitivity is near 90%. ► The average percentage of false recognitions is 5.3%. ► Morphology of corneal nerves can be used to diagnosis diabetic neuropathy.
Introduction
There is a growing interest on the segmentation and analysis of corneal nerves images. These images, obtained in vivo, in conscious patients, through corneal confocal microscopy (CCM), can document corneal nerve changes. There is increasing evidence that it is possible to diagnosis and assess peripheral diabetic neuropathy through the morphometric analysis of the sub-basal nerve plexus of the cornea.
The development of a simple, non-invasive method for diagnosis and follow-up of peripheral diabetic neuropathy will have great social impact. Peripheral diabetic neuropathy, is a major cause of morbidity in diabetic patients [1], [2] affecting 50% of diabetic patients 25 years after diagnosis of diabetes [3], and being implicated in 50–75% of non-traumatic amputations [4], [5]. Early diagnosis and assessment of disease progression are important to accurately define at risk patients, evaluate therapies and decrease patient morbidity [6]. Currently, this diagnosis often fails or occurs only when patients became symptomatic [7]. The follow-up, through the documentation of small-fiber damage, is only possible with invasive biopsy [8], [9], [10].
Being one of the most innervated tissues in humans and directly assessable to inspection using light, the cornea became a natural target for the diagnosis of peripheral diabetic neuropathy. This is particularly true since the development of CCM allowed non-invasive in vivo imaging of corneal sub-basal nerve plexus. CCM has been used to quantify the corneal nerve morphologic features [11], nerve density in normal and diabetic patients [12], and the extent of degeneration and regeneration of corneal nerve fibers in diabetic patients with increasing neuropathic severity [13], [14]. It was also demonstrated that the number of fibers in the cornea sub-basal nerve plexus of diabetic patients was significantly lower than in healthy humans, even for short diabetes duration [15], [16]. So, there is evidence to suggest that the quantification of sub-basal plexus morphometric parameters may constitute a basis for diabetic neuropathy diagnosis [17], [18].
At present, the analysis of sub-basal corneal nerve parameters is usually based on a manual laborious task [11], [12], [13], [19] that is subjective and prone to errors. Fully automatic methods, requiring no specific expertise from the user, were recently proposed. Holmes et al. [20] developed a method to automatically trace the nerve fibers in corneal micrographs and used it to show a relationship between the presence of peripheral neuropathy and the density and shape of corneal nerve fibers.
As far as we know, Scarpa et al. [21] were the first to propose automatic methods for the recognition and tracing of the corneal nerve structures, in CCM images [22].
In this work, we propose and evaluate the performance of a segmentation method for automatic extraction of sub-basal corneal nerves morphometric parameters. Our method starts by normalizing the image contrast and reducing the noise, using contrast equalization, a phase symmetry-based method and histogram procedure. Then a fast search is done to locate candidate regions that are expanded to identify each corneal nerve. Discarding false positives and merging all remaining nerves results in the identification of complete nerve structure [23], [24].
Section snippets
Corneal nerves segmentation algorithm
The segmentation algorithm method is schematically described by the functional block diagram of Fig. 1. The algorithm comprises several steps:
- 1)
Local equalization, for enhancing image details.
- 2)
Phase symmetry, using wavelet transform filter, to identify the nerves structures.
- 3)
Nerve reconstruction using morphologic operations.
These steps are detailed in the next sections.
Materials
The method described herein was implemented in Matlab and was applied on 47 images (36 healthy controls and 11 diabetic patients) from a database available online [22], containing 90 images. Two images with stromal nerves were discarded. Images whose en face section traverses obliquely several corneal layers were not considered. In these images, the sub-basal nerves appear only on a small region, with the remaining field of view containing other corneal features, such as stromal keratocytes or
Results
Corneal nerves segmentation results for the developed algorithm are illustrated in Fig. 3, where the first and second rows show the best results, while the third row shows the worst result, in terms of sensitivity.
Table 1 shows the values of automatically and manually segmented nerve lengths and densities. Densities values are obtained dividing the nerve length by the image area. The results show that the automatic algorithm underestimates the nerve length. They also show that the nerve lengths
Discussion and conclusions
In this work we presented a fully automatic segmentation algorithm for the identification of corneal nerves in CCM images. We evaluated the algorithm performance in a set of 47 images of the sub-basal nerve plexus from non-diabetic and diabetic corneas. This set was extracted from a larger set of 90 images available online, by rejecting low quality images and images whose en face section traverses obliquely several corneal layers, in order to have a set representative of the images obtained by
Acknowledgements
This work was supported by the following Grants: Portuguese Foundation for the Science and Technology (FCT), PTDC/SAU-BEB/73425/2006 and PTDC/SAU-BEB/104183/2008.
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