A multimode imaging system, producing conventional ultrasound (US) and acousto-optic (AO) images, has been developed and used to detect optical absorbers buried in excised biological tissue. A commercially-available diagnostic ultrasound imaging transducer is used to both generate B-mode ultrasound images and as a pump for AO imaging. Due to the fact that the steered and focused beam used for US imaging and the US source for pumping the AO image are generated from the same ultrasound probe, the acoustical and optical images are intrinsically co-registered. AO imaging is performed using short ultrasound pulse trains at a frequency of 5 MHz. The phase-modulated light emitted from the interaction region is detected using a photorefractive-crystal based interferometry system. Experimental results have previously been presented for the two-dimensional imaging in tissue-mimicking phantoms. In this paper, we report further experimental developments demonstrating three-dimensional fusion of B-mode ultrasound imaging and pulsed acousto-optic imaging in excised biological tissue (~2 cm thick). By mechanically scanning the ultrasound transducer array in a direction perpendicular to its imaging plane, both the acoustical and optical properties of an embedded target are obtained in three dimensions. The results suggest that AO imaging could be used to supplement conventional B-mode ultrasound imaging with optical contrast, and the multimode imaging system may find application in the detection and diagnosis of cancer.
Optoacoustic systems making use of optical detection probes are potentially advantageous over contact transducers for noncontact, noninvasive high-resolution near surface imaging applications. In this work, an interferometer is used for high-frequency optoacoustic microscopy. The limitations of this system in terms of both sensitivity and resolution are discussed. A theoretical model has been developed for two-dimensional excitation source geometries, which can be used to predict the optoacoustic signal from a target material with an arbitrary through-thickness optical absorption distribution. The model incorporates the temporal and spatial profile of the excitation laser pulse, and is used to predict the actual out-of-plane displacement at the target surface. An adaptive, photorefractive crystal-based interferometry system has been used to measure the optically induced displacement on the surface of target materials, and the results show reasonable quantitative agreement with theory. The detection system has a 200 MHz bandwidth allowing for high-resolution imaging, and the use of optical probes for both generation and detection allows for the probes to be easily co-aligned on the sample surface. Preliminary experimental results are presented demonstrating the feasibility of using all-optical optoacoustic microscopy for near surface imaging of small-scale spatial variations in optical absorption.
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.
The acousto-optical sensing (AOS) of a turbid medium is based on the interaction of multiply-scattered coherent laser light with an ultrasonic field. A phase-modulated photon field emanates from the interaction region and carries with it information about the acousto-optical properties of the media. Using a novel technique based on a photorefractive crystal interferometer, it is possible to detect the ultrasound-modulated optical signals generated by short ultrasound pulses. As opposed to continuous-wave (CW) ultrasound, pulsed ultrasound directly provides resolution along the ultrasonic propagation axis. In this work, a commercial ultrasound scanner (Analogic AN2300) was used in pulse mode (5 MHz central frequency) to generate both conventional ultrasound and AO images. Gel-based highly diffusive (μs'=10 cm-1) tissue-mimicking phantoms were fabricated, with embedded targets possessing acoustical and/or optical contrast. AO images of 26-mm thick phantoms were generated from optical signals averaged in the time-domain, without further signal processing, and were superimposed on the top of the ultrasound images. Good quality AO images of optical absorbers, intrinsically co-registered with the ultrasound images, were obtained within minutes. The axial resolution of the AO images was given by the spatial length of the ultrasound pulse, typically on the order of one mm in the MHz range. These results show that AO signals can be excited in pulse mode using a commercial scanner, and combined to conventional ultrasound images to provide more information related to the optical properties of the medium.
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.
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