Optic nerve imaging

New imaging technologies and understanding their limitations

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Ultra-high resolution optical coherence tomography (OCT) is a nearly ideal technology for imaging the optic nerve in terms of reliability, broad applicability and ease of use, but it retains some limitations in terms of image contrast and its ability to distinguish between certain types of tissue, according to Leopold Schmetterer PhD, of the Medical University of Vienna, Austria. “OCT’s sensitivity and specificity in detecting early glaucoma is better than with older technology, but is as yet not good enough for screening purposes. We get fairly objective readings that we could not obtain by other means, it can be applied to many different diseases, has a very short measurement time and is easy to operate,” he told the 12th European Glaucoma Society Congress in Prague, Czech Republic. OCT is now used in ophthalmology as a type of in vivo histology that can identify the layers of tissue similarly to conventional histology, he said. However, what the OCT images actually represent is the difference between the refractive index of the tissue in each adjoining layer. Therefore, the sharpness of the imaging of the layers depends on the difference between the refractive index of the tissues that compose them. That, in turn, means that when the difference in refractive index is slight, the image contrast will be poor, making segmentation of the layers more difficult. HIGHER SENSITIVITY He noted that using Bruch’s membrane opening to identify minimum rim width provides a high contrast image of retinal thickness because of the difference in refractive index between the layers. Published research indicates that Bruch’s membrane opening-minimum rim width measurements may provide higher sensitivity and specificity than retinal nerve fibre layer (RNFL) thickness measurements in the diagnosis of glaucoma. Different ultra-high resolution OCT instruments yield different absolute RNFL measurements, although they tend to have similar sensitivity and specificity, around 80 per cent, he noted. Most of the difference in measurements may be due to the instruments’ software rather than hardware. Research has demonstrated that independently developed software brings the measurements of the Spectralis and Cirrus OCT devices into much closer concordance. An important limitation with OCT is the floor effect which arises from the inability of the devices to distinguish between neural and non-neural tissue in the RNFL. As a result, the RNFL as measured by OCT will not be reduced to zero even in the absence of neural tissues. OCT angiography may help provide a clearer picture, although its sensitivity and specificity in glaucoma diagnosis has not been determined. Other optic nerve imaging approaches under investigation include the combination of adaptive optics with OCT to view the lamina cribrosa, Dr Schmetterer said. Leopold Schmetterer: leopold.schmetterer@meduniwien.ac.at
Tags: optic nerve, optical coherence tomography, tissue types