Reflection mode acousto-optic imaging using a 1-D ultrasound array

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.