Abstract
Abstract: :
Purpose: To compare the macular edema modules of the Heidelberg Retinal Tomograph (HRT), the Heidelberg Retinal Tomograph II (HRT II), and the Optical Coherence Tomograph 3 (OCT3) in their abilities to assess and map retinal thickness and macular volume in normal subjects. Methods: Twenty eyes of 20 normal, healthy volunteers were included in the study sample in this cross–sectional, comparative instrument validation study (mean age=34, SD=10.0, range=19–57). All subjects were without retinopathy or prior history of intraocular surgery or laser. Several fovea–centered scans were obtained with each machine and the best quality image was selected for analysis. On the HRT, volume was calculated by defining a reference plane at the presumed posterior retinal location and determining the volume above this reference plane. On the HRT II, the Macular Edema Module (version 1.2.0.0) was used to determine the edema index. On the OCT3, the built–in software was used to calculate retinal volume. The following were then compared: a) OCT3 vs. HRT macular volume calculations, b) Macular Map (MM) vs. Fast Macular Map (FMM) protocols of the OCT3, and c) retinal thickness and Edema Index in the fovea using the Macular Edema Module of HRT II vs. the MM protocol of the OCT3. Results: a) The macular volume measured by OCT (mean=2.476, SD=.130) was significantly greater (p<.01) than that measured by HRT (mean=1.558, SD=.304); minimal correlation (r=.14) was found between the two samples. We also found the intrinsic variation of the HRT measurements to be significantly greater than that of the OCT measurements (coeff. of variation=0.19 vs. 0.05, p=0.006). b) There was no significant difference in macular volume as computed by the MM and FMM protocols of the OCT (p=.90); values showed high correlation (r=.96). c) Foveal thickness, as calculated by OCT3 (mean=192.9, SD=19.2) and HRT II (mean=203.2, SD=30.5), was not significantly different (p=.22); the HRT II Edema Index was poorly correlated with foveal volume by OCT 3 (r=.08). Conclusions: The Fast Macular Map (FMM) and the Macular Map (MM) protocols of the OCT offer similar volume measurements. The FMM provides the option of faster image acquisition time for patients unable to maintain fixation. We observed poor correlation between OCT volume measurements and confocal scanning laser tomography measurements in normal volunteers. Since the exact measure of retinal volume is unknown, we are unable to determine which instrument is more accurate. The difference might be explained by differences in lateral sampling points and axial resolution.
Keywords: imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • macula/fovea • retina