Implementing the probe beam deflection technique for acoustic sensing in photoacoustic and ultrasound imaging Ronald A. Barnes Jr. The University of Texas at San Antonio This work is a collaboration between The University of Texas at San Antonio and The University of Texas Health Science Center.

Outline Introduction Background Modeling (MATLAB) –Acoustic Wave Propagation –Ray Tracing Simulation (MATLAB) –Optimum Sensor Topology –Optimum Beam Topology –Quadrant Photodiode Simulation –Acoustic Wave Directionality Measurement –Sensor Frequency Response Visualization (ParaView and MATLAB) Conclusion

Introduction What is Photoacoustic Tomography? Photoacoustic Tomography (PAT) is accomplished by measuring the propagating acoustic energy radiated from a sample of tissue whose thermal expansion is invoked by a pulse laser. An image of the tissue composition is reconstructed based on the measurement of the of this acoustic energy. What is the Probe Beam Deflection Technique? The Probe Beam Deflection Technique (PBDT) is sensing topology that uses probe beam lasers and there deflection and refraction to measure the properties of the propagating acoustic wave, through the implementation of a Quadrant Photodiode (QPD). Why is Modeling and Simulation important for this project? To develop an efficient algorithm for reconstruction of a tissue composition image, one must understand the interaction between probe beam and propagating acoustic wave front. A ray tracing simulation in combination with an acoustic wave simulation will allow for the prediction of beam deflection or refraction for various experimental topologies and implementations.

Background (PAT) Light enters a scattering medium (Ex. Tissue Phantom) where a portion of the energy is absorbed by the tissue in the form of heat, this produces thermal expansion. If the temperature increase inside the phantom occurs at a faster rate then the thermal relaxation time of the tissue, an acoustic wave will propagate as a result of the photo-acoustic effect. This acoustic wave produced is a wideband ultrasonic transmission and to date is measured with piezoelectric transducers.

PAT Applications Melanoma detection Photoacoustic tomography of gene expression. Doppler photoacoustic tomography for flow measurement. Photoacoustic and thermoacoustic tomography of the brain Low-background thermoacoustic molecular imaging. [2]. Prospects of photoacoustic tomography, Lihong V. Wang

Photoacoustic vs. Other Contrast Methods Contrast Method Bandwidth (Hz)Primary Contrast Penetration Depth (mm) Axial Resolution (um) Lateral Resolution (um) Photoacoustic microscopy50 MOptical absorption31545 Photoacoustic microscopy5 MOptical absorption50700 Confocal microscopyFluorescence, scattering Two-photon microscopyFluorescence Optical coherence tomography50 TOptical scattering Scanning Laser Acoustic Microscopy300 MUltrasonic scattering220 Acoustic microscopy50 MUltrasonic scattering Ultrasonography5 MUltrasonic scattering60300 [1] Optical Imaging Laboratory, Department of Biomedical Engineering, Washington University in St. Louis.

Background (PBDT) PBDT is implemented by focusing probe beams through an enclosure filled with a propagation medium. As an acoustic wave travels through the medium the refractive index is changed relative to the pressure gradient produced by the wave. The probe beam deflects and refracts as it interacts with the refractive index profile along its beam path. The probe beam deflection technique offers various advantages when compared to transducers, these include: Wave front directionality measurement, passive sensing, and low implementation cost.

Development of a Model Step 1: Produce a model of acoustic wave propagation in homogeneous and heterogeneous mediums based on the 2 nd order PDE governing acoustic wave propagation. Step 2: Modify this model in such a way that all parameters are adjustable. This includes: Initial acoustic wave magnitude, propagation medium properties, acoustic wave frequency, etc. Step 3: Convert the pressure values in the four dimensional dataset (3 dim. for space and 1 for time) to refractive index using the lorentz-lorenz relation. Step 4: Develop a ray tracing simulation to trace a bundle of rays through the previously created dataset using the vector form of Snells law. This simulation should have adjustable parameters which include: initial ray origin (for all rays that make up beam), initial ray intensity, and initial ray direction.

Model Setup

MATLAB Visualization

Method for Ray Trace Simulation (PBDT) The nature of Snells law allows the PBDT method to determine the propagation direction of the wavefront in relation to the probe beam. This is a distinct advantage over piezoelectrics whose measurement ability is limited to distance from transducer to acoustic wave source.

Visual Example of Ray Trace

Probe Beam Orientations

Quadrant Photodiode Concept

Beam Intersection on QPD Surface (Simulation)

MATLAB Visualization

QPD X SIGNAL QPD Y SIGNAL

Experimental Implementation

Probe Beam Orientations (Experiment)

Experimental Results A B C

Future Work Define optimum beam and sensor topologies for experimental implementation of PBDT derived from simulation. Define the frequency response of PBDT and compare to the frequency response to commercially available transducers. Develop reconstruction algorithm based on integrating line detectors as proposed by G. Paltauf but with added angular information.

Acknowledgments NSF grant (HRD ), Drs. Demetris Kazakos and Richard Smith, project managers. PREM Grant # DMR

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