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)
Penetration Depth
(mm)
Axial Resolution Lateral Resolution
(um)
(um)
Primary Contrast
Photoacoustic microscopy
50 M
Optical absorption
3
15
45
Photoacoustic microscopy
5M
Optical absorption
50
700
700
Confocal microscopy
Fluorescence, scattering
0.2
20
0.3-3
Two-photon microscopy
Fluorescence
0.5-1.0
10
0.3-3
Optical coherence tomography
50 T
Optical scattering
2
0.5-10
10
Scanning Laser Acoustic Microscopy
300 M
Ultrasonic scattering
2
20
20
Acoustic microscopy
50 M
Ultrasonic scattering
20
20-100
80-160
Ultrasonography
5M
Ultrasonic scattering
60
300
300
[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 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
PC
DAQ
X Y PC
Enclosure Filled
With Distilled Water
Quadrant
Photodiode
Probe Beam
L1
1
2
n5
n4
n3
FP
3
4
V5 V4
L3
n2
FP
V3
Wave Front
P
FP
V2
n1
V1
L2
OPO Laser
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.
 n 

 n 
Vk 1   k  Vk    k  cos  k  cos  k 1  n k if n k  Vk  0
 n

 nk 1 
  k 1 

  n 

 n 
Vk 1   k  Vk     k  cos  k  cos  k 1  n k if n k  Vk  0
 n

 nk 1 
  k 1 

Acoustic Wave Front (H)
Acoustic Wave Front (L)
P
r
Vk 1
Tangent
Plane
 k 1
k
Vk
nk
Visual Example of Ray Trace
Probe Beam Orientations
Quadrant Photodiode Concept
( A  B )  (C  D )
Y
A BC  D
( A  C )  ( B  D)
X
A BC  D
y
tan  
x
Beam Intersection on QPD Surface
(Simulation)
MATLAB Visualization
MATLAB Visualization
QPD Y SIGNAL
QPD X 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-0932339), Drs. Demetris Kazakos and
Richard Smith, project managers.
• PREM Grant # DMR- 0934218.
References
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Questions??