Macular retinal circulation in healthy eyes examined by optical coherence tomography angiography extended interscan time analysis

PURPOSE: To examine whether extended interscan time (IST) on optical coherence tomography angiography (OCTA) can detect slow retinal blood flow, which is undetectable on default IST, in the healthy macula. METHODS: OCTA (OCT-A1, Canon Inc.) scanning of a macular area measuring 4 × 4 mm(2) of 14 healthy eyes of 14 healthy volunteers with no history or evidence of systemic and macular diseases was performed. ISTs were set at 7.6 (IST(7.6), default setting), 12.0 (IST(12.0)), and 20.6 msec (IST(20.6)). Ten OCTA images were acquired at each IST, and an averaged image was created. For each averaged OCTA image obtained at IST(7.6), IST(12.0), and IST(20.6), we defined the area surrounded by the innermost capillary ring as the foveal avascular zone (FAZ). We qualitatively evaluated the delineation of the capillaries consisting of the FAZ and quantitatively measured the FAZ area at each IST. RESULTS: Extensions from IST(7.6) to IST(12.0) and IST(20.6) could newly delineated retinal capillaries that were undetectable at the default IST; new capillaries were detected in 10 (71%) eyes at IST(12.0) and 11 (78%) eyes at IST(20.0). The FAZ areas were 0.334 ± 0.137 mm(2), 0.320 ± 0.132 mm(2), and 0.319 ± 0.129 mm(2) for IST(7.6), IST(12.0), and IST(20.0), respectively; the FAZ areas at IST(12.0) and IST(20.0) were significantly decreased compared with that at IST(7.6) (p = 0.004 and 0.002, respectively). CONCLUSION: In OCTA for healthy participants, extensions of the ISTs newly detected retinal capillaries with slow blood flow around FAZ. The FAZ shapes varied with different ISTs. Thus, the blood flow dynamics are not physiologically uniform around FAZ. Compared with conventional OCTA, this protocol enables a more detailed evaluation of retinal circulation and provides a better understanding of the physiological circulatory status of the healthy retina, and may enable the assessment of circulation in the very early stages in diseased eyes.