In-Line Holography is Used for In-Vitro Validation of Microbubble Tracks Recorded by Contrast Enhanced Ultrasound Imaging

Contrast Enhanced Ultrasound (CEUS) imaging, combined tracking of the microbubbles velocity using ultrasound localization microscopy (ULM), is a widely used tool for assessing blood flows in various organs. Owing to the spatial variations in the size and shape of the (2-3um diameter) bubble images, their tracks should be calibrated. Here, we introduce a microcirculation phantom designed for simultaneous ultrasonic and optical tracking of the same bubbles in physiologically relevant microchannels. The optical bubble tracking is performed using microscopic in-line holography. The phantom is made of transparent polyacrylamide hydrogel, which replicates the acoustic properties of human tissues, including the speed of sound and acoustic impedance. Unobstructed optical access is achieved by matching the refractive index of the gel to that of the water. Microchannels with diameters from 10 to 400 mm are formed by installing and then extracting tungsten wires coated with Parylene C inside the hydrogel. The resulting parallel micro-channels have minimum spacings of 30 to 60 um at realistic depths varying from 7 to 54 mm from the ultrasonic probes. The axis of the microscopic holography system is perpendicular to the CEUS scanning plane, enabling observations at any depth. The presentation will include results of direct comparisons between the 2D CEUS-ULM and the 3D optical trajectories under relevant pulsatile flow conditions and will discuss several algorithms for detecting the bubble tracks.