Acousto-optical imaging (AOI) in diffuse media is a hybrid technique that is based on the interaction of multiply scattered laser light with a focused ultrasound beam. A phase-modulated optical field emanates from the interaction region and carries with it information about the local opto-mechanical properties of the insonated media. The goal of AOI is to reveal the optically relevant physiological information while maintaining ultrasonic resolution. Among the state-of-the-art optical detection techniques used for AOI, there is a trade-off between the axial resolution (or ultrasound bandwidth) and the signal-to-noise ratio (SNR). In this paper, a photorefractive-crystal (PRC) based interferometry system is employed to detect acousto-optical (AO) signals in highly diffuse media. This system allows for the use of short pulses of focused ultrasound and is capable of imaging mm-scale inhomogeneities imbedded inside tissue-mimicking phantoms. One-dimensional (1-D) AO image along the transducer axis is obtained from a single, time-averaged time-domain acousto-optical signal, and the axial resolution is determined by the acoustic spatial pulse length, rather than the longer axial dimension of the ultrasonic focal region (as is the case when using a continuous-wave (CW) ultrasound source). Two-dimensional (2-D) images can be constructed by scanning the transducer in one dimension, which results in a reduction in imaging acquisition time and makes fast acousto-optical imaging possible.
Ultrasound modulated optical signals from scattered laser light in biological tissue have been investigated in order to improve optical imaging quality, a technique we call acousto-photonic imaging (API). Recent experiments using a photorefractive crystal based detection system have shown that there is a large DC-offset to the acoustically modulated AC optical signal, an effect that is second order in the large acoustically-induced optical phase shifts. We present numerical calculations for the size of the phase shifts of various photons in an API set-up. The calculations use a Monte Carlo simulation for the propagation of laser photons in an optically diffuse medium with properties similar to tissue and a finite-difference time domain simulation of a realistic focused ultrasound beam in that medium. The phase shifts of photons which passed through different regions around the ultrasound focus were compared. Quantities which characterize the AC and DC signal components were evaluated using the calculated phase shifts. It was found that the DC term dominates as it does in experiments, even though it is second order in the phase shifts. This is due to the fact that the contributions to the DC signal from each photon add coherently. In addition, the significant contributions to the DC signal are limited to those photons that passed near the ultrasound focal region, which has important implications for improving the imaging resolution in API.
Acousto-photonic imaging (API) is a dual-wave sensing technique in which a diffusive photon wave in a turbid medium interacts with an imposed acoustic field that drives scatterers to coherent periodic motion. A phase-modulated photon field emanates from the interaction region and carries with it information about the local opto-mechanical properties of the insonated media. A technological barrier to API has been sensitivity - the flux of phase-modulated photons is very small and the incoherence of the resulting speckle pattern reduces the modulation of the scattered light leading to low sensitivity. We report preliminary results from a new detection scheme in which a photorefractive crystal is used to mix the diffusively scattered laser light with a reference beam. The crystal serves as a dynamic holographic medium where the signal beam interferes with the reference beam, creating a photorefractive grating from which beams diffract. In addition, the phase modulation is converted to an amplitude modulation so that the API signal can be detected. Measurements of the API signal are presented for gel phantoms with polystyrene beads used as scatterers, showing a qualitative agreement with a simple theoretical model developed.