Opto-Acoustic Imaging Peter E. Andersen Optics and Fluid Dynamics Department Risø National...

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

Optics and FluidDynamics

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.



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.



Tissue optics



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 deter-mined knowing; the size parameter (perimeter compared to wavelength), refractive index ratio between particle and media.



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.



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.



Tissue optics

Light propagation (Monte Carlo simulation)

Incident light

Ballistic component

“Snake” component

Diffuse reflectance

Absorption

Diffuse transmittance



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.), Optical-

Thermal response of laser-irradiated tissue, Plenum Press, New York, 1995.




Thermoelastic stress and generation of stress waves

Absorber

Short laser pulse

Stress wave(acoustic wave)

Thermoelastic stress




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.



– the stress building up inside the absorbing target is

: Grüneisen parameter (0.11 for water at roomtemperature)

: radiant exposure (from laser)a: optical absorption coefficient


Stress waves (cont’d)

– stress confinement (mathematically);

pulse

Dt

c

ap r r

c: speed of soundD: Min{optical penetration, laser

beam diameter, slab}




Example: estimation of T and P– Grüneisen parameter: 0.11 @ room temp.

– absorption coefficient a: 20 cm-1

– radiant exposure :16 mJ/( 0.22 cm2) = 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




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.




Detection

– microphone (hydrophone),

– piezoelectric transducers,

– all-optical method(s) based on interferometry.




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).




Three-dimensional imaging

– system built at Dept. of Applied Optics, University of Twente, NL; C. G. Hoelen et al., Opt. Lett.

23, 648 (1998).

Key figures:

– laser; 8 ns pulses, 10 Hz rep. rate,

– spatial resolution 10 m,

– acquisition time: >2 hours(!).




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.



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 non-contact compact, highly sensitive probing of the stress wave.




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.




The all-optical detection scheme (top view)




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.




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.




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. Nd:YAG

Absorber

Tissue

Water

HeNe beams632 nm

532 nmTranslation




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.




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



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.

Opto-Acoustic Imaging Peter E. Andersen Optics and Fluid Dynamics Department Risø National...

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