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A 2D Quantitative Retinal Blood Flow Measurement Method in Humans with Multimodal Adaptive Optics Imaging

Catalog of Regulatory Science Tools to Help Assess New Medical Devices 

This regulatory science tool (RST) is a lab method for absolute vessel caliber and retinal blood flow velocity measurements in humans using a high-resolution (spatial and temporal) multimodal adaptive optics system with scanning laser ophthalmoscopy (SLO) and optical coherence tomography (OCT).

Technical Description

This tool is intended to be a quantitative method for measurement of retinal blood flow rate (RBF) in human retinal vessels using a multimodal adaptive optics (AO) system, which includes both SLO and OCT channels. The tool includes a detailed image acquisition protocol and a description of the post-processing analysis pipeline. AO-SLO operated in line-scan mode for vessel imaging, with an innovative interleaved frame approach enables better image registration despite eye motion. Both AO-OCT and AO-SLO acquisitions targeting the same retinal region allows precise blood flow analysis. 

The tool is also comprised of a semi-automated MATLAB algorithm for AO-SLO image analysis using an enhanced Radon transform method for human imaging. The pipeline includes automatic blink removal, motion correction through a two-step image registration process, and filtering of static features. RBC velocity is quantified by measuring the angle of bright streaks in space-time images, with velocity calculations optimized for vessel orientation and physiological limits. A signal-to-noise ratio threshold excludes spurious measurements, and mean velocity is determined over multiple cardiac cycles for accuracy, incorporating adjustments for fixation stability and eye motion. Further, the tool contains an AO-OCT analysis method for measuring vessel morphology and inner diameter. Eye motion artifacts are corrected using a custom 3D registration algorithm, with the most stable volume selected as a reference. Averaged AO-OCT volumes enhance SNR and generate en face images for vessel analysis. A custom ImageJ macro, adapted from VasoMetrics, applies a full-width half-max approach to measure inner and outer diameters. The wall-to-lumen ratio is calculated from these measurements. AO-SLO analysis measures pulsatile flow velocity (mm/s), while AO-OCT analysis on the same vessel provides vessel diameter (µm). Assuming a circular cross-section of the vessel, RBF (µL/min) can then be calculated for the vessel of interest. Further information is provided in the journal article that describes the methodology and initial validation in human subjects [1].

Intended Purpose 

This tool is designed for retinal imaging device developers and manufacturers and is intended to assist in establishing a reliable RBF measurement method and normative data in humans. Further this tool is intended to aid in assessment of impaired autoregulation.

Testing

Repeatability and reproducibility measurements are initially demonstrated in a small cohort of five healthy volunteers [1]. This study involved collecting and analyzing both AO-SLO and AO-OCT images from the same set of retinal vessels on three different days (with a mean interval of 47.9 days between sessions) to assess reproducibility (inter-session variance) and three consecutive measurements on the same day to assess repeatability (intra-session variance).

Demonstration of the use of this tool in evaluating retinal autoregulation is also provided.

Limitations

This method measures the blood flow velocity component parallel to the en face plane, thereby limiting access to retinal vessels less than 5° from the center of the optic nerve head due to significant errors induced by axial orientation. However retinal architecture is predominantly planar, making this method appropriate for retinal vascular assessment. The resonant scanner frequency, which sets the line rate of the AO-SLO line scan technique, sets an upper range on the measurable velocity. On the lower end, measurement of single-file flow of a few mm/s in retinal capillaries is limited by the sensitivity with which we can detect very short streak angles. Target vessels selection criteria of a shallow intersection angle (<45°), restricts the number of accessible vessels for imaging due to the fixed orientation of the fast axis scanner in the imaging system. Another current limitation is that the technique does not typically measure flow in every vessel in the field owing to angular considerations.

Supporting Documentation

  1. Raghavendra, A. J., Damani, A., Oechsli, S., Magder, L. S., Liu, Z., Hammer, D. X., & Saeedi, O. J. (2024). Measurement of retinal blood flow precision in the human eye with multimodal adaptive optics imaging. Biomedical Optics Express, 15(8), 4625-4641.
    https://doi.org/10.1364/BOE.524944

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