Inter-day repeatability assessment of human retinal blood flow using clinical laser speckle contrast imaging

Abstract

Laser speckle contrast imaging (LSCI) can generate retinal blood flow maps inexpensively and non-invasively. These flow maps can be used to identify various eye disorders associated with reduced blood flow. Despite early success, one of the major obstacles to clinical adoption of LSCI is poor repeatability of the modality. Here, we propose an LSCI registration pipeline that registers contrast maps to correct for rigid movements. Post-registration, intra(same)-day and inter(next)-day repeatability are studied using various quantitative metrics. We have studied LSCI repeatability intra-day by using the coefficient of variation. Using the processing pipelines and custom hardware developed, similar repeatability was observed when compared to previously reported values in the literature. Inter-day repeatability analysis indicates no statistical evidence (p = 0.09) of a difference between flow measurements performed on two independent days. Further improvements to hardware, environmental controls, and participant control must be made to provide higher confidence in the repeatability of blood flow. However, this is the first time that repeatability across two different days (inter-day) using multiple exposure speckle imaging (MESI) has been analyzed and reported.

Fig. 1.

(A) Modified Kowa VX-10α fundus camera with Andor Neo sCMOS camera used to capture the laser speckle pattern projected onto the retina. (B) Laser module attached externally to the fundus camera coupled to an optical cage system.

Fig. 2.

Complete process from the acquisition of raw speckle images to the formation of flow maps.

Fig. 3.

(A) Raw speckle image at 32 ms (uint16) upscaled by 10x for visualization. (B) Contrast map at 32 ms upscaled by 10x. (C) Preprocessed contrast map at 32 ms. (D) First fixed image: all 104 preprocessed contrast maps averaged. (E) Second fixed image: averaged image of all successfully registered preprocessed contrast maps after one iteration. (F) Averaged image of all successfully registered preprocessed contrast maps after two iterations. (G) Averaged image of all 32 ms contrast maps after registration upscaled by 10x. (H) Decorrelation time map after curve fitting. (I) Flow map after inverting decorrelation times.

Fig. 4.

Process of computing a structural similarity value for estimation of registration quality. (A) Each scan has six averaged contrast maps for the 2ms, 4ms, 8ms, 16ms, 32ms, and 64ms exposure times. For the five scans of the same ID on each day, there are 30 averaged contrast maps in total. (B) Registration between scans is necessary as SSIM comparisons require the images to be at the same location. (C) SSIM computes the difference between two images, so each pair of the five scans must be compared, leading to 10 SSIM comparisons in total for each exposure time. Comparisons are only made within the same exposure time due to similar contrast values between images. (D) For each pair, 2-dimensional SSIM maps are created using MATLAB’s ssim() function. (E) Averaging all values within each of the 10 masked SSIM maps leads to 10 SSIM values for each exposure time. (F) A total of 60 SSIM values for all six exposure times are averaged to give a single average SSIM for each ID on each day.

Fig. 5.

(A) Manually selected vein and artery ROIs. The average coefficient of variation values were computed for both vessels across five independent scans. (B) CV map, lower values indicate regions of high repeatability and higher values indicate regions of lower repeatability. N.B. the mask at the center of the flow map and CV map is due to an optical reflection artifact and is excluded from analyses.

Fig. 6.

Day 1 flow maps generated from five independent repeated scans of all participants in the study. There is a clear indication of intra-day repeatability across independent scans. 1/τ has been used as an approximation of flow, so flow values have been given in instrument-dependent arbitrary units.

Fig. 7.

Averaged Day 1 and Day 2 flow maps for all participants. ID 6 did not show up to the study. Only the left eye data was available for ID7.

Fig. 8.

(A) Flow map (187 × 227) of ID 2 Scan 2 where flow values along all 6 rows (pixel position 1-227) are sampled and plotted. (B) Flow values plotted for Row 30 of all 10 scans of ID 2. (C) Average of all five scans of a particular day in panel B but also including the other six rows to give a visualization of flow difference in day 1 compared to day 2. (D) Flow values along all six rows for all scans of day 1 are plotted against that of day 2.