Jacobs Journal of Molecular and Translational Medicine

Functional Ultrasound Imaging of Cerebral Capillaries in Rodents and Humans

*Alan Urban
Department Of Case Reports, Université Paris Descartes Sorbonne Paris Cité, France

*Corresponding Author:
Alan Urban
Department Of Case Reports, Université Paris Descartes Sorbonne Paris Cité, France
Email:alan.urban@alan-urban.fr

Published on: 2018-05-12

Abstract

Monitoring capillary blood flow is of great clinical value since microcirculation is crucial for proper delivery of oxygen and nutrients to the biological tissue, particularly in the brain. Functional ultrasound imaging is a novel method to measure hemodynamics in small vessels at a high resolution providing new insight into brain activity. Nevertheless, a drawback of this modality is the need for clutter filtering to suppress signals originating from slowly moving tissue that may hinder not only the detection of low blood flow velocity in micro vessels but also significantly underestimate the power spectrum of the Doppler signal. Here, we demonstrate how a spatiotemporal filtering approach based on the Singular Value Decomposition (SVD) can efficiently remove the clutter signal while preserving the blood flow signal even at low frequency. This strategy was applied to image brain capillaries in rodents and to visualize the cortical microvasculature in the human brain during neurosurgery.

Keywords

Cerebral Blood Flow; Cerebral Blood Volume (CBV); functional ultrasound; Singular Value Decomposition (SVD)

Introduction

Doppler Ultrasound imaging is a widely used technique for making non-invasive velocity measurements of blood flow. In this technique, the signal scattered from blood is added to signals scattered from stationary or slowly moving tissue that is typically 40 to 60 dB stronger than the signal from blood cells [1]. The echo signal of moving blood cells has a larger Doppler shift than the echoes reflected from slowly moving tissue and therefore it is possible to separate both signals using various static high-pass filters [2-5]. Nevertheless, the clutter from tissue often change through space and time due to change in physiology that can be overcome using adaptive filters based on principal component analysis [6-8].In addition, it should be noted that none of these filtering strategies could effectively suppress the clutter without affecting the flow signal of interest. For example, a direct consequence of the clutter filter is the inability to detect all particles with a Doppler shift lower than the filter cut-off frequency. Then, all micro vessels with slow blood velocities below few mm/s including capillaries, arterioles and venules are basically not detected using standard ultrasound scanners.