Detector sensitivity influences blood flow sampling in scanning laser Doppler flowmetry

Purpose: To establish the effect of photodiode sensitivity on the DC (brigh tness) value and the resultant blood flow measurements of retina and rim ti ssue using a scanning laser Doppler flowmeter (SLDF). Methods: The sample c onsisted of one eye of each of 15 healthy subjects (mean age 27.8 +/- 6.1 y ears). Using the Heidelberg Retina Flowmeter (HRF), three 10-deg images of the superior temporal retina and three further images of the superior tempo ral rim were acquired for each of five DC bands: band 1: 30-70; band 2: 70- 110; band 3: 110-150; band 4 150-190; band 5: 190-230. Retinal blood volume , flow and velocity were determined for each image using a 10 x 10 pixel sq uare grid located at a predetermined location on the retina and rim for eac h subject. Following image acquisition, the DC values corresponding to each pre-assigned retinal or rim location were determined. The mean and standar d deviation were determined for the blood flow parameters within each DC ba nd for each subject in both locations. Analysis of variance was used to ide ntify significant change in the data as a function of the DC value (P<0.05) . Results: Analysis of variance revealed that retinal blood flow measures a cquired within DC band 5 resulted in significantly lower measures of blood flow and velocity (P=0.035 and P=0.049 respectively) than at lower DC value s. Band 5 values of flow, volume and velocity in the neuroretinal rim were also significantly low (P=0.016, P= 0.003 and P=0.026 respectively). Peak n euroretinal rim blood flow was recorded when the DC value was between 70 an d 110. For blood flow measurement at the retina and neuroretinal rim the DC value should not exceed 190. Conclusion: Photodiode sensitivity as indicat ed by the DC value affects measurements of ocular blood flow using the HRF.