Brightness alters Heidelberg retinal flowmeter measurements in an in vitromodel

PURPOSE. The Heidelberg Retinal Flowmeter (HRF), a laser Doppler flowmetry device, has captured interest as a research and clinical tool for measureme nt of ocular blood flow. Concerns remain about the range and accuracy of th e values that it reports. METHODS. An in vitro blood-flow model was constructed to provide well-contr olled laminar flow through a glass capillary for assessment by HRF. A chang e in material behind the glass capillary was used to simulate changing brig htness conditions between eyes. RESULTS. Velocities reported by the HRF correlated linearly to true velocit ies below 8.8 mm/sec. Beyond 8.8 mm/sec, HRF readings fluctuated randomly. True velocity and HRF reported velocities were highly correlated, with r = 0.967 (P < 0.001) from 0.0 mm/sec to 2.7 mm/sec mean velocity using a light background, and r = 0.900 (P < 0.001) from 2.7 mm/sec to 8.8 mm/sec using a darker background. However, a large change in the y-intercept occurred in the calibration curve with the background change. CONCLUSIONS. The HRF may report velocities inaccurately because of varying brightness in the fundus. In the present experiment, a darker background pr oduced an overreporting of velocities. An offset, possibly introduced by a noise correction routine, apparently contributed to the inaccuracies of the HRF measurements. Such offsets vary with local and global brightness. Ther efore, HRF measurements may be error prone when comparing eyes. When used t o track perfusion in a single eye over time, meaningful comparison may be p ossible if meticulous care is taken to align vessels and intensity controls to achieve a similar level of noise correction between measurements.