Impact of simulated light scatter on scanning laser Doppler flowmetry

Abstract

Aim: To determine the impact of simulated light scatter on scanning laser Doppler flowmetry (SLDF) assessment of retinal capillary blood flow and retinal image quality.

Methods: One eye of 10 normal subjects (mean (SD) age 24 (1.7) years, range 22-27) was randomly selected. Varying concentrations of polystyrene microspheres were suspended in optically clear cells to simulate light scatter. The microsphere concentrations used were 0.05%, 0.03%, 0.02%, 0.01%, and a cell containing only water. LogMAR visual acuity and contrast sensitivity were measured both with and without cells. Optimal focus and alignment was established by acquiring three SLDF images each of the optic nerve head (ONH) and of the macula using the Heidelberg retina flowmeter (HRF) with no cell in place. SLDF images were subsequently acquired with each of the light scatter cells mounted in front of the HRF. The group mean retinal capillary blood flow was compared using repeated measures analysis of variance (reANOVA) as a function of microsphere concentration.

Results: Retinal capillary blood flow increased significantly in the ONH, nasal macula, fovea, and temporal macula with increasing microsphere concentration (p<0.0001). Using Dunnett's post hoc test, retinal capillary blood flow was found to be significantly increased relative to the no cell condition for the 0.03% and 0.05% cell concentrations.

Conclusions: Simulated light scatter produces an artifactual increase in retinal capillary blood flow. The impact of cataract on SLDF measurements has yet to be determined.

Figure 1

Schematic representation of the light scatter cell mounted in front of the objective of the Heidelberg retina flowmeter (HRF) using a custom designed adaptor to produce a standard 20° tilt. 1, Subject’s eye; 2, light scatter cell; 3, custom made adaptor used to tilt the cell by 20°; 4, Heidelberg retina flowmeter.

Figure 2

Group mean (SD) capillary blood flow with no cell, water only, 0.01%, 0.02%, 0.03%, and 0.05% microsphere concentrations using SLDF (light shaded bars) and HRF custom analysis (dark shaded bars) in (A) the optic nerve head (ONH), (B) the nasal macula, (C) the fovea, and (D) the temporal macula.

Figure 3

Group mean DC values (indicating the brightness index of the image) for all the measured regions with no cell, water only, and 0.01%, 0.02%, 0.03%, 0.05% microsphere concentrations. Vertical bars denote 95% confidence intervals (*p<0.05).

Figure 4

Fundus image of a normal subject acquired with various cell microsphere concentrations placed in front of a digital fundus camera. (A) No cell. (B) Cell with water only. (C) Cell with microsphere concentration of 0.01%. (D) Cell with microsphere concentration of 0.02%.