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.
We investigate the possibilities of improving performance of an acousto-optic imaging system operating in reflection mode geometry with linear ultrasound array and optodes in direct contact with a sample by using plane ultrasound waves instead of focused pulses. Differences in acoustic pressure field distributions for various transducer excitation patterns were determined numerically. Acousto-optic images of phantoms with optically absorbing inclusions were obtained experimentally by measuring laser speckle contrast decrease due to the light modulation by plane wave and focused ultrasound pulses. The light fluence rate maps reconstructed using plane wave imaging proved to be clearer and more distinct.
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.
Utilization of the acousto-optic effect by detection of light scattered within tissues and modulated by focused ultrasound pulses could provide diagnostic information impossible to obtain by purely acoustic or optical imaging modalities. It could also support photoacoustic imaging by mapping fluence rate distribution. However, practical implementation of this technique encounters numerous difficulties preventing it from rapid adoption in clinical use. One of the important limitations that has not yet been adequately addressed is that in many practical medical applications the region of interest may be accessed only from one side. In the present study we introduce the results of investigations on acousto-optic detection and localization of optically distinct inclusions inside acoustically homogeneous phantoms using a linear ultrasound array with electronically scanned focus and optical fibers arranged in reflectance geometry. Speckle contrast differences between speckle patterns captured in absence and presence of ultrasound pulses with different focal point coordinates are determined for various samples. The results allow clear distinction between phantoms with and without optically absorbing inclusions, although these are neither visible from the surface nor distinguishable on ultrasound images. It is also shown, that data analysis allows to obtain further cues on localization of absorbing regions. Conditions and limitations in this regard are discussed.
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