Reflection-mode acousto-optic imaging using a one-dimensional ultrasound array with electronically scanned focus

Abstract

. Significance: Practical implementation of acousto-optic imaging (AOI) encounters difficulties that prevent it from rapid adoption in clinical use. In many practical medical applications, the region of interest may be accessed only from one side, and using a water tank for coupling is not feasible. The solution might be to use reflection-mode imaging with an electronically scanned ultrasound (US) focus. Such an approach, however, entails considerable challenges. Aim: The possibilities of detecting and localizing light-absorbing inclusions inside turbid media by combining reflection-mode AOI conducted using a one-dimensional US array with electronic scanning of the US focus are investigated experimentally and signal processing algorithms that could be used for this purpose are introduced. Approach: We determine the speckle contrast decrease due to the acousto-optic effect as a function of the US focal point coordinates. Different signal postprocessing techniques are investigated. Results: A significant decrease in the determined speckle contrast difference values is observed due to the presence of light-absorbing inclusions. However, local minima occur in the plots only under specific conditions. Subtracting individual distributions and determining symmetry deviations allow for localizing the inclusions. Conclusions: Detection and localization of optically distinct regions are possible using the introduced approach. Signal postprocessing is required in a general case.

Reflection-mode acousto-optic imaging using plane wave ultrasound pulses

“…This in turn can be-under certain conditions-used to detect and localize optically absorbing inclusions inside the region of interest. 3 There have been many approaches to implementing AOI systems described in the literature. Comprehensive reviews of the related studies were presented by Elson et al, 4 Resink et al, 5 and Gunther and Andersson-Engels.…”

“…This in turn can be—under certain conditions—used to detect and localize optically absorbing inclusions inside the region of interest. 3 …”

“…Such an approach allows one to directly link the observed changes of the received signal with changes of optical properties within the region of interest. 3 Here, we impose a condition that the measurement probe should remain stationary relative to the sample during the whole imaging procedure. In other words, the scanning should be performed by electronic US pulse shaping, instead of physically displacing either the probe or the sample.…”

“…It is then assumed that the acoustic pressure field distribution is entirely concentrated within the desired focal region and by scanning the US focus and measuring AOI signal, the light fluence rate map is reconstructed. 3 The accurateness of such an assumption, however, strongly depends on transducer type and excitation scheme. In general, focusing of US pulses is achieved either by using specially shaped electroacoustic transducers with constant focus and/or by using a matrix of singly driven transducer elements with excitation delays determined in such a way that constructive interference between all the emitted pulses occurs at the desired focal point.…”

“…This influence could explain, for instance, limited decrease of the speckle contrast difference (SCD) values determined for samples with highly optically absorbing inclusions, as presented in our previous publication. 3 To test this hypothesis, we developed a new AOI routine, using plane wave instead of focused US pulses.…”