Abstract
.Significance: Performance of an acousto-optic imaging system is limited by light fluence rate and acoustic pressure field distributions characteristics. In optically scattering media, the former determines the achievable contrast, whereas the latter the imaging resolution. The system parameters can be shaped by changing relative positions of ultrasound (US) transducer array and optodes. However, in the case of many potential clinical applications, optimization possibilities in this regard are limited, as a sample is accessible from one side only and using a water tank for coupling is not feasible.Aim: We investigate the possibilities of improving performance of an acousto-optic imaging system operating in reflection mode geometry with linear US array in direct contact with a sample using plane wave instead of focused US pulses.Approach: Differences in acoustic pressure field distributions for various transducer excitation patterns were determined numerically and experimentally. Acousto-optic images of phantoms with and without optically absorbing inclusions were acquired by measuring laser speckle contrast decrease due to the light modulation by plane wave and focused US pulses with different apodization patterns.Results: The residual acoustic pressure field components occupy relatively large volume and contribute to light modulation. Using nonsteered plane wave US pulses instead of focused ones allows one to mitigate their influence. It also allows one to obtain clear two-dimensional reconstructions of light fluence rate maps by shifting transducer apodization along the lateral direction.Conclusions: Using nonsteered plane wave US pulses allows one to achieve better imaging performance than with focused pulses in the assumed system geometry.
Highlights
We investigate the possibilities of improvements of imaging performance of a reflection-mode acousto optic imaging (AOI) system with linear ultrasound (US) array using nonsteered plane wave acoustic pulses instead of focused ones
Examples of results of numerical simulations of acoustic pressure field distributions obtained for focused and plane wave US pulses with different apodization patterns are shown in Figs. 2(a)– 2(d)
The confinity coefficient is significantly higher for plane wave than for focused US pulses, with relatively low differences related to the apodization numbers
Summary
We investigate the possibilities of improvements of imaging performance of a reflection-mode acousto optic imaging (AOI) system with linear ultrasound (US) array using nonsteered plane wave acoustic pulses instead of focused ones. Due to the induced motion of optical scatterers within the illuminated and insonified volume, the light that is scattered within this region undergoes modulation (the acousto-optic effect).[1,2] By determining the ratio of intensities of modulated to nonmodulated light for various acoustic pressure field distributions, the fluence rate map in the medium can be determined. We consider a reflection mode AOI, i.e., configuration in which both optodes and US transducer are positioned on the same side of an investigated sample Such a configuration was previously investigated by Lev et al.,[7,8,9] Hisaka and Saskura,[10] Kim et al.,[11] and Hong-Bo et al.[12] In all of these investigations, the imaging was performed by physically changing the relative positions of the investigated sample and the probe while keeping the US focus constant. The introduced restrictions are in accordance with practical limitations related to various clinical problems, in which the region of interest is accessibled from one side only, and physical scanning might be not feasible or not desired
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