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Opto-Acoustic Imaging
Peter E. Andersen
Optics and Fluid Dynamics Department
Risø National Laboratory
Roskilde, Denmark
E-mail [email protected]
P.E. Andersen [BIOP], Feb. 2, 2000
Outline
Tissue optics
– optical properties,
– light propagation in highly scattering media.
Photoacoustic imaging
– generation, propagation, and detection of stress
–
waves,
imaging systems and clinical potential.
Optics and Fluid
Dynamics
P.E. Andersen [BIOP], Feb. 2, 2000
Tissue optics
Optically tissue may be characterized by its
– scattering, refractive index, and absorption.
The scattering arises from
– cell membranes, cell nuclei, capillary walls, hair
follicles, etc.
The absorption arises from
– visible and NIR wavelengths (400 nm - 800 nm);
hemoglobin and melanin,
– IR wavelengths;
water and molecular vibrational/rotational states.
Optics and Fluid
Dynamics
P.E. Andersen [BIOP], Feb. 2, 2000
Tissue optics
Optics and Fluid
Dynamics
P.E. Andersen [BIOP], Feb. 2, 2000
Tissue optics
Optics and Fluid
Dynamics
P.E. Andersen [BIOP], Feb. 2, 2000
Tissue optics
Single particle
– light scattering by a single particle is
–
characterized by its scattering cross section [m2]
and phase function p(),
using Mie theory the scattering may be determined knowing;
the size parameter (perimeter compared to wavelength),
refractive index ratio between particle and media.
Optics and Fluid
Dynamics
P.E. Andersen [BIOP], Feb. 2, 2000
Tissue optics
Turbid media
– tissue is a (huge) collection of scattering particles;
various sizes and shapes,
– light propagation cannot be described as single
–
scattering,
models taking into account multiple scattering
must be applied.
Optics and Fluid
Dynamics
P.E. Andersen [BIOP], Feb. 2, 2000
Tissue optics
Modeling light propagation in tissue
– transport theory (or the diffusion approximation);
known from heat transfer (Boltzman’s equation),
– extended Huygens-Fresnel principle,
– Monte Carlo simulations.
Optical properties (macroscopic)
– absorption coefficient a [m-1],
– scattering coefficient s [m-1],
– asymmetry parameter g or phase function p(),
– refractive indices.
Optics and Fluid
Dynamics
P.E. Andersen [BIOP], Feb. 2, 2000
Tissue optics
Light propagation (Monte Carlo simulation)
Absorption
“Snake” component
Incident
light
Diffuse
reflectance
Ballistic component
Diffuse
transmittance
Optics and Fluid
Dynamics
P.E. Andersen [BIOP], Feb. 2, 2000
Tissue optics
References
– Light scattering;
C. Bohren and D. Huffman, Absorption and scattering of
light by small particles, J. Wiley & Sons, New York, 1983,
– Multiple scattering;
A. Ishimaru, Wave propagation and scattering in random
media I & II, Academic Press, New York, 1978,
R. F. Lutomirski and H. T. Yura, Appl. Opt. 7, 1652
(1971),
– Tissue optics;
A. J. Welch and M. J. C. van Gemert (eds.), OpticalThermal response of laser-irradiated tissue, Plenum
Press, New York, 1995.
Optics and Fluid
Dynamics
P.E. Andersen [BIOP], Feb. 2, 2000
Photoacoustic imaging
Thermoelastic stress and generation of stress
waves
Short laser
pulse
Stress wave
(acoustic wave)
Thermoelastic
stress
Absorber
Optics and Fluid
Dynamics
P.E. Andersen [BIOP], Feb. 2, 2000
Photoacoustic imaging
Stress waves
– thermoelastic stress is generated due to the
–
–
–
absorption of a short laser pulse,
knowing the optical, mechanical, and thermal
properties of the absorber, the amplitude and
shape of the stress wave may be calculated,
vice versa, measuring the amplitude and shape of
the stress wave may provide e.g. the optical
properties of the absorber,
stress confinement;
duration of the irradiating laser pulse must be smaller
than the time for the acoustic wave to traverse the
optically heated volume.
Optics and Fluid
Dynamics
P.E. Andersen [BIOP], Feb. 2, 2000
Photoacoustic imaging
Stress waves (cont’d)
– stress confinement (mathematically);
t pulse
D

c
c:
D:
speed of sound
Min{optical penetration, laser
beam diameter, slab}
– the stress building up inside the absorbing target is
p  r   a   r 
:
:
 a:
Grüneisen parameter (0.11 for water at room
temperature)
radiant exposure (from laser)
optical absorption coefficient
Optics and Fluid
Dynamics
P.E. Andersen [BIOP], Feb. 2, 2000
Photoacoustic imaging
Example: estimation of T and P
– Grüneisen parameter:
– absorption coefficient a:
– radiant exposure :
16 mJ/( 0.22 cm2) =
0.11 @ room temp.
20 cm-1
127 mJ/ cm2
beam diameter 4 mm
pulse energy 16 mJ
– temperature change:
(a )/( cv) =
0.63 °C
density  1 g/cm3 and specific heat cv 4 J/(g K)
– Pressure change:
2.6 bar
Optics and Fluid
Dynamics
P.E. Andersen [BIOP], Feb. 2, 2000
Photoacoustic imaging
Stress waves (cont’d)
– the radiant exposure  depends on the optical
–
–
–
properties of the tissue being probed, and found
using “tissue optics”,
the thermoelastic stress couples into the
surrounding medium,
the resulting stress wave may then be calculated
from the acoustic wave equation,
diffraction and rarefaction effects may have to be
included.
Optics and Fluid
Dynamics
P.E. Andersen [BIOP], Feb. 2, 2000
Photoacoustic imaging
Detection
– microphone (hydrophone),
– piezoelectric transducers,
– all-optical method(s) based on interferometry.
Optics and Fluid
Dynamics
P.E. Andersen [BIOP], Feb. 2, 2000
Photoacoustic imaging
Suggested reading
– Stress waves in liquids and gases (review);
M. W. Sigrist, J. Appl. Phys. 60, R83 (1986),
– Determination of optical properties from stress
waves;
A. A. Oraevsky et al., Proc. SPIE 1882, 86 (1993),
– Optical transducer;
G. Paltauf and H. Schmidt-Kloiber, J. Appl. Phys. 82,
1525 (1997),
– All-optical detection;
S. L. Jacques et al., Proc. SPIE 3254, 307 (1998),
P. E. Andersen et al., Proc. SPIE 3601 (1999).
Optics and Fluid
Dynamics
P.E. Andersen [BIOP], Feb. 2, 2000
Photoacoustic imaging
Three-dimensional
Key figures:
imaging
– system built at Dept. of
Applied Optics,
University of Twente, NL;
–
 C. G. Hoelen et al., Opt.
Lett. 23, 648 (1998).
–
–
laser; 8 ns pulses, 10 Hz
rep. rate,
spatial resolution 10
m,
acquisition time: >2
hours(!).
Optics and Fluid
Dynamics
P.E. Andersen [BIOP], Feb. 2, 2000
Photoacoustic imaging
Imaging tissue (in vitro)
–
–
many source-detector pairs,
back-propagation algorithm.
Experiment
–
–
–
–
–
6 mm chicken breast tissue,
two nylon capillaries (inner
diameter 280 m) filled
with whole blood,
placed at 2 and 4 mm
depth,
spatial resolution 10 m,
acquisition time: from
minutes to hours.
Optics and Fluid
Dynamics
P.E. Andersen [BIOP], Feb. 2, 2000
All-optical detection scheme
Motivation for the study
– to investigate the photoacoustic imaging method
–
with respect to the all-optical detection scheme,
the all-optical detection scheme facilitates noncontact compact, highly sensitive probing of the
stress wave.
Optics and Fluid
Dynamics
P.E. Andersen [BIOP], Feb. 2, 2000
All-optical detection scheme
The setup
– a HeNe laser as the source,
– a beam splitter,
– a Wollaston prism and a lens;
to form two co-aligned beams,
these two components determine the beam separation,
– the focus of the lens should be as close as
possible to the object (surface) of investigation to
insure optimum system performance.
The reflected light
– collected through the lens and sent to the detector
by passing the beam splitter.
Optics and Fluid
Dynamics
P.E. Andersen [BIOP], Feb. 2, 2000
All-optical detection scheme
The all-optical detection scheme (top view)
Optics and Fluid
Dynamics
P.E. Andersen [BIOP], Feb. 2, 2000
All-optical detection scheme
The setup may be operated in
– transmission mode,
– reflection mode.
The irradiating laser is a pulsed Nd:YAG source
– pulse duration 5 ns,
– pulse energy 16 mJ @ 532 nm or 30 mJ @ 1064
–
–
nm,
10 Hz pulse repetition rate,
spot size 4 mm at the object.
Optical detection
– beam separation of 9 mm.
Optics and Fluid
Dynamics
P.E. Andersen [BIOP], Feb. 2, 2000
All-optical detection scheme
Figures-of-merit
– minimum signal: 10-30 mbar (measured, not optimized),
– linear dynamic range: 0.03 - 33 bar (measured).
Advantages
– high common-mode rejection ratio,
– non-contact procedure,
– compact and robust, when integrated into a single
HOE.
Disadvantages
– high performance requires a free, smooth surface,
e.g. water.
Optics and Fluid
Dynamics
P.E. Andersen [BIOP], Feb. 2, 2000
All-optical detection scheme
The tissue phantom
–
–
–
the tissue sample
is chicken breast
samples of
various thickness,
the absorbing
object is silicon
rubber dyed with
India ink,
various shapes;
 circular disk,
 rectangular box.
HeNe beams
632 nm
Water
Tissue
Absorber
Translation
532 nm
Nd:YAG
Optics and Fluid
Dynamics
P.E. Andersen [BIOP], Feb. 2, 2000
All-optical detection scheme
The “peak” at edge
–
–
–
depends on sample
thickness,
pronounced with thin
sample,
primarily due to changes
in the stress wave shape.
Broadening of the image
profile
– due to a combination of
scattering of the
illuminating beam and
attenuation of the stress
wave.
Optics and Fluid
Dynamics
P.E. Andersen [BIOP], Feb. 2, 2000
All-optical detection scheme
Comparison
– all-optical method (not optimized);
minimum signal level: 10-30 mbar,
linear dynamic range: 0.03-33 bar,
– piezo-electric transducers;
minimum signal level: 20-40 mbar,
linear dynamic range: 0.04-6* bar,
[from Oraevsky et al., Appl. Opt. 36, 402 (1997)].
* probably larger
Optics and Fluid
Dynamics
P.E. Andersen [BIOP], Feb. 2, 2000
Summary
Opto-acoustic is a feasible method for imaging
in human tissue
All-optical detection is advantageous due to
– high sensitivity,
– non-contact procedure.
Applications
– imaging of breast cancers,
– in vivo concentration measurements.
Optics and Fluid
Dynamics