Abstract:
The HRT II (Heidelberg Retina Tomograph) is a non-invasive, confocal scanning laser imaging system that transforms a series of optical sections into a layered, three-dimensional image of the retina. The HRT II sequentially acquires 16 two dimensional (in x and y planes) confocal section images per millimetre along the optical axis (z-axis). Each HRT II scan consists of 3 sequential 15° x 15° tomographic images of the retina that are averaged to display a mean topography image. The mean topography image that is subsequently analyzed has a resolution of 384 x 384 pixels.
As the reflected laser light intensity is measured at each point on the retinal surface, a confocal intensity profile can be created. The analysis of this light intensity profile allows the computation of edema and reflectance maps that provide the diagnostic tools necessary to assess the presence of, and assist in, the clinical management of macular oedema and other retinal pathologies
The edema index analysis developed by Flanagan and Hudson (Kit Guan et al, 2004) has been incorporated within the HRT II as the Macular Edema Module (MEM). The average edema index is derived for a 1500µm radius circle centred on the fovea using the MEM (Macular oedema module) software. The MEM software allows the clinician to quantify the extent of edema in all areas of the retina and to follow any fluctuation in oedema over time. It has been previously shown that MEM demonstrates good correlation with contact lens fundus biomicroscopy in detecting diabetic macular oedema (Hudson C. et al, 2006) Suggestions have also been made that MEM could serve as a sensitive diagnostic tool in the detection of early and progressive diabetic macular oedema (Hudson C. et al 1998).
However, as far as we are aware no studies have shown the effect of age on the MEM; therefore, the aim of this study was to investigate the effects of age on the MEM. This is also important when considering the HRT II role in screening of patients with diabetes. An age-matched database of normal macular scans will allow direct comparisons to be made with macular scans of those diabetic patients with and without macular oedema.
Currently, the impact of cataract on the HRT II is poorly understood. Hence, this study also aimed to determine the impact of artificial light scatter and light reduction on HRT II estimates of macular topography.