Lei Sui

Detection of ultrasound-modulated photons in diffuse media using the photorefractive effect.

Ultrasound-modulated optical tomography is a dual-wave sensing technique in which diffusive light in a turbid medium interacts with an imposed acoustic field. A phase-modulated photon field emanates from the interaction region and carries with it information about the optomechanical properties of the medium. We present a technique for detection of ultrasound-induced optical phase modulation using an adaptive, photorefractive-crystal-based interferometry system. Experimental results are presented demonstrating detection of ultrasound-modulated signals in highly scattering media by use of pulsed ultrasound insonation.

Imaging in diffuse media with pulsed-ultrasound-modulated light and the photorefractive effect

Acousto-optic imaging in diffuse media is a dual wave-sensing technique in which an acoustic field interacts with multiply scattered laser light. The acoustic field causes a phase modulation in the optical field emanating from the interaction region, and this phase-modulated optical field carries with it information about the local optomechanical properties of the media. We report on the use of a pulsed ultrasound transducer to modulate the optical field and the use of a photorefractive-crystal-based interferometry system to detect ultrasound-modulated light. The use of short pulses of focused ultrasound allows for a one-dimensional acousto-optic image to be obtained along the transducer axis from a single, time-averaged acousto-optic signal. The axial and lateral resolutions of the system are controlled by the spatial pulse length and width of the ultrasound beam, respectively. In addition, scanning the ultrasound transducer in one dimension yields two-dimensional images of optical inhomogeneities buried in turbid media.

Computations of the acoustically induced phase shifts of optical paths in acoustophotonic imaging with photorefractive-based detection.

Acoustophotonic imaging uses ultrasound-modulated scattered light to improve the quality of optical imaging in diffusive media. Experiments that use photorefractive-crystal-based detection have shown that there is a large dc shift in the acoustically modulated or ac optical signal, which could be utilized to further improve optical imaging resolution. We report that photon paths in a diffusive medium were generated by a Monte Carlo simulation, and the optical phase shifts of the various photons induced by the presence of a realistic focused ultrasound beam were calculated. Quantities that characterize the ac and dc signal components were evaluated by use of the calculated phase shifts. It was confirmed that the dc component dominates owing to coherent summation of the contributions from all the photons.

Enhanced detection of acousto-photonic scattering using a photorefractive crystal

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.

Shedding light on sound: the fusion of acousto-optic and B-mode ultrasound imaging

Ultrasound modulation of light can be used to image optical properties in highly diffusive media such as biological tissues. Much of the previous work in this area employed continuous wave (cw) ultrasound to pump the acousto-optic response. We argue in favor of using pulsed ultrasound, based on regulatory considerations related to bioeffects and enhanced axial resolution along a scan line. We describe a system in which a commercial ultrasound scanner was combined with a photorefractive crystal (PRC) based optical detection scheme to generate simultaneous 3D images of acoustic and optical properties of optically absorbing inclusions embedded in excised biological tissue. Representative images are presented that demonstrate the utility of this dual mode imaging technique. Technological limitations are discussed, as are plans for future work

The combination of pulsed acousto-optic imaging and B-mode diagnostic ultrasound for three-dimensional imaging in ex vivo biological tissue

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.

Modeling of optoacoustic signal generation for high resolution near-surface imaging with experimental verification

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.

Investigation of the photorefractive crystal based detection system for acousto-optical imaging (AOI) in highly diffuse media

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.

Optoacoustic Systems for Subsurface Materials Characterization

Optoacoustic imaging is a promising technique for the detection of subsurface optical inhomogeneities in optically turbid media. A pulsed laser is used to irradiate a sample volume where it is absorbed and causes rapid heating. Ultrasonic waves are launched through the thermoelastic effect and are detected at the sample surface. The detected ultrasonic transients are related to the position dependent absorption coefficient allowing for imaging of the absorbed electromagnetic energy distribution. In this work, the use of optical interferometry for the detection of optoacoustic signals is explored using three detection schemes: a photorefractive crystal based interferometer, a Fabry‐Perot sensor head which is coupled directly to the specimen and a Michelson interferometer. Experimental results are presented demonstrating optical detection of optoacoustic transients in both homogeneous absorbing tissue phantoms and optically turbid tissue phantoms. The measured optoacoustic signal profiles are compared to th...

Combining acousto‐optical imaging with diagnostic ultrasound: resolution, contrast and speed

Ultrasound modulation of light can be used to image optical properties in highly diffusive media such as biological tissues. In a previous study [Bossy et al., J. Acoust. Soc. Am. 115, 2523 (2004)], a commercial ultrasound scanner was combined with a photorefractive crystal (PRC)‐based optical detection scheme to generate simultaneous images of acoustic and optical properties of inclusions embedded in diffusive tissue phantoms. In this paper, we quantitatively investigate the performance of the system in terms of resolution, contrast, and acquisition time. Measurements are performed on gel‐based highly diffusive tissue phantoms, with embedded targets possessing optical contrast relative to the surrounding medium. The resolution of the acousto‐optical images turns out to be given by the ultrasound beamwidth for the lateral resolution, and by the pulse duration for the axial resolution. Phantoms and targets with different optical contrasts are imaged to assess the feasibility using this technique to delinea...