Modeling, Simulation and Application of the Probe Beam
Deflection Technique for Photoacoustic and Ultrasound Imaging
Implementing the probe beam deflection technique for acoustic
sensing in photoacoustic and ultrasound imagingRonald 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.OutlineIntroductionBackgroundModeling (MATLAB)Acoustic Wave
PropagationRay TracingSimulation (MATLAB)Optimum Sensor
TopologyOptimum Beam TopologyQuadrant Photodiode SimulationAcoustic
Wave Directionality MeasurementSensor Frequency
ResponseVisualization (ParaView and
MATLAB)ConclusionIntroductionWhat 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 ApplicationsMelanoma detectionPhotoacoustic
tomography of gene expression.Doppler photoacoustic tomography for
flow measurement.Photoacoustic and thermoacoustic tomography of the
brainLow-background thermoacoustic molecular imaging.[2]. Prospects
of photoacoustic tomography, Lihong V. WangPhotoacoustic vs. Other
Contrast MethodsContrast MethodBandwidth (Hz)Primary
ContrastPenetration Depth (mm)Axial Resolution (um)Lateral
Resolution (um)Photoacoustic microscopy 50 M Optical absorption
31545Photoacoustic microscopy 5 M Optical absorption
50700700Confocal microscopy Fluorescence, scattering 0.2200.3-3
Two-photon microscopy Fluorescence 0.5-1.0 100.3-3 Optical
coherence tomography 50 T Optical scattering 20.5-10 10Scanning
Laser Acoustic Microscopy 300 M Ultrasonic scattering 22020Acoustic
microscopy 50 M Ultrasonic scattering 2020-100 80-160
Ultrasonography 5 M Ultrasonic scattering 60300300[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 ModelStep 1: Produce a model of acoustic wave
propagation in homogeneous and heterogeneous mediums based on the
2nd 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
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
MATLAB Visualization
QPD X SIGNALQPD Y SIGNALExperimental Implementation
Probe Beam Orientations(Experiment)
Experimental ResultsABC
Future WorkDefine 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.
AcknowledgmentsNSF grant (HRD-0932339), Drs. Demetris Kazakos and
Richard Smith, project managers.PREM Grant # DMR- 0934218.
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Questions??r
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