Finally, the current resolution and the absence of staining limit IVCM to morphological analyses. Further developments in microscope objectives and image processing will certainly increase the resolution for a more accurate morphological analysis. IVCM of the future should also include the use of staining procedures administered by instillation to facilitate image interpretation and the diagnostic ability of IVCM.

Anterior Segment Ocular Coherence Tomography (OCT)

Analysis of the anterior segment structures is an integral part of ophthalmologic investigations and many clinical situations require a precise assessment not only of the spatial relationships but also of the dimensions of the structures making up this segment of the eye. In everyday clinical practice, corneal imaging is usually carried out with the slit lamp. However, this technique does not provide the means for objective quantitative assessment. New imaging techniques such as anterior segment OCT have thus been developed to overcome these problems.

Principles of Anterior Segment Optical Coherence Tomography

(AS-OCT)

Optical coherence tomography is a noninvasive technique which uses a low coherence interferometry principle cross sections of tissue structures in vivo. The technique measures the delay and intensity of the light reßected from the structure being analyzed and compares it with the light reßected by a reference mirror, and the combination of these two signals results in a phenomenon termed interference. The signal intensity depends on the optical properties of the tissues, and the device uses this record to construct a sagittal cross section of the structure being examined. In fact, the operating principle behind OCT is similar to that of the ultrasound scan except that a light is used instead of ultrasound.

Since the speed of light is a million times faster than that of ultrasound, this type of imaging allows longitudinal resolution levels of the order of a few microns to be achieved. Marketed for the Þrst time in 1995 by Carl Zeiss Meditec (Dublin, USA), OCT has until now mostly been used in ophthalmology to produce images of the posterior segment of the eye. The Þrst optical coherence images of the anterior segment were made in 1994 using an OCT with a wavelength of 820 nm designed for examination of the posterior segment. This new imaging technique was then used to assess anatomical changes during refractive surgery and, adapted for use with the slit lamp, for routine clinical examination of the anterior segment. However, the anterior segment images obtained with this system were not always very good in quality. This is because the time taken for image acquisition was between 1 and 5 s, meaning that distortion due to micro-movements of the eye could not be avoided. In addition, the image calculation algorithm designed for the (concave) retina could produce distortions in the images of the (convex) cornea and anterior segment. Finally, the wavelength of 820 nm gave only limited penetration of the signal through the sclera, limbus or iris, making it impossible to view more posterior structures.