Optical coherence tomography (OCT) is a noninvasive, noncontact, trans-pupillary imaging technology that can image retinal structures in vivo with a resolution superior to other in vivo imaging techniques such as scanning laser ophthalmoscopy, B-mode ultrasound and ultrasound biomicroscopy (Huang et al. 1991; Hee et al. 1995; Puliafito et al. 1996). This technique provides cross-sectional and tomographic images of the posterior segment of the eye produced using optical backscattering of light in a fashion analogous to B-scan ultrasonography. Operation of OCT is analogous to ultrasound, but OCT utilizes reflections of infrared light waves from different structures in the eye rather than acoustic waves. OCT is based on the principle of low-coherence interferometry, which measures the time of flight delay of light reflected from different structures of the eye. The probe beam is directed into the eye, and reflections from tissue interfaces provides information about the distances and thickness of the ocular structures. The longitudinal resolution (experimentally measured as 14 µ in air and 10 µ at the retina) of OCT is based on the coherence length of the source light. Two dimensional B-mode images are created by performing serial longitudinal scans in transverse direction, and each tomogram is composed of a sequence of 100 A-scans, acquired in 1 s. The final image of optical reflectivity is displayed in false colors. Although the main applications of OCT are represented by retinal diseases, and alterations of the vitreo–retinal interfaces, many reports have been published on glaucoma applications.
OCT has been previously demonstrated to detect changes in tissue thickness with micrometer-scale sensitivity (Izatt et al. 1994). The RNFL is highly backscattering and therefore is contrasted from the intermediate retinal layers because the nerve axons are oriented perpendicularly to the OCT probe beam. With a prototype instrument, OCT data were collected and reported to correlate with the topography of humans retinas (Hee et al. 1995). OCT has also demonstrated to be able in detecting and well evaluating the induced RNFL changes in monkey (Toth et al. 1997). Many Authors have reported on OCT reproducibility, and have demonstrated Standard Deviations (SD) of RNFL and retinal thickness measurements of approximately 10–20 µ (10–20%) in normal and in glaucomatous eyes (Schuman et al. 1995; Schuman et al. 1996; Baumann et al. 1998; Bowd et al. 2000). OCT measurements of nerve fiber layer thickness correlates with functional status of the optic nerve, as measured by visual field examination (Schuman et al. 1995), and it appears promising as a tool for early diagnosis of glaucoma (the soon incoming OCT3 instrument, with a resolution of 8 µ, will guarantee better information).
Many retinal diseases induce a change in retinal thickness that can be monitored by OCT. Based on the rationale that alterations in thickness reflect a change in retinal volume, the estimation of the macular volume may be useful for the follow-up of most retinal diseases (Puliafito 2000). The possibility to calculate the macular volume based on thickness and in an uncomplicated way may have clinical utility. The purpose of our study is to evaluate macular volume in normal and glaucomatous eyes using OCT, and to assess the correlation of macular volume with the glaucoma status (Schuman et al. 2001).
OCT images were obtained with a commercial version OCT scanner (OCT 2000 version). A cross sectional image was displayed in a false-colour scale, which indicated the different reflectivity of the retinal layers. A computer algorithm was used to profile the inner and the outer retinal boundaries, and the retinal thickness was computed automatically from these boundaries by assuming a constant refractive index.
The macular map consists of six radial scans intersecting at the fovea, with a scanning diameter of 6 mm, and on each scan the thickness has been measured at 100 points: the software analyses 600 total thickness measurements in order to obtain the macular volume.
The volumetric study is based on the rationale that 600 thickness measurements represented as average thickness in nine regions, and the weighted average thickness of the nine regions multiplied by the scanning area provide volume estimate (the weighted average thickness (Πr2) = volume)(11).