The Impulse Response of Negatively Focused Spherical Ultrasound Detectors and its Effect on Tomographic Optoacoustic Reconstruction.

source: © 2019  IEEE Transactions on Medical Imaging. 

In optoacoustic tomography, negatively focused detectors have been shown to improve the tangential image resolution without sacrificing sensitivity. Since no exact inversion formulae exist for optoacoustic image reconstruction with negatively focused detectors, image reconstruction in such cases is based on using the virtual-detector approximation, in which it is assumed that the response of the negatively focused detector is identical, up to a constant time delay, to that of a point-like detector positioned in the detector’s center of curvature. In this work, we analyze the response of negatively focused spherical ultrasound detectors in three dimensions and demonstrate how their properties affect the optoacoustic reconstruction. Our analysis sheds new light on commonly reported experimental reconstruction artifacts in optoacoustic systems that employ negatively focused detectors. Based on our analysis, we introduce a simple correction to the virtual-detector approximation that significantly enhances image contrast and reduces artifacts.  [Read more…]

The Impulse Response

Fig. (a) The geometry of the negative acoustic lens studied with full
acoustic simulations. The speed of sound of the surrounding medium and lens
material were 1500 m/s and 2757 m/s. respectively. (b) The detected acoustic
signals obtained when the lens was acoustically matched to the surrounding
medium (solid-blue curve) and when its acoustic impedance was 1.xx times that
of the surrounding medium (dashed-red curve), leading to internal reflections
in the lens structure. The reflection from the detection surface was 50% of the
pressure signal. 

Gilad Drozdov, Ahiad Levi, Amir Rosenthal.
IEEE Transactions on Medical Imaging.DOI: 10.1109/TMI.2019.2897588