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Opto-Acoustic Imaging

Opto-Acoustic Imaging. Peter E. Andersen Optics and Fluid Dynamics Department Risø National Laboratory Roskilde, Denmark E-mail peter.andersen@risoe.dk. Outline. Tissue optics optical properties, light propagation in highly scattering media. Photoacoustic imaging

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Opto-Acoustic Imaging

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  1. Opto-Acoustic Imaging Peter E. Andersen Optics and Fluid Dynamics Department Risø National Laboratory Roskilde, Denmark E-mail peter.andersen@risoe.dk

  2. 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.

  3. 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.

  4. P.E. Andersen [BIOP], Feb. 2, 2000 Tissue optics

  5. P.E. Andersen [BIOP], Feb. 2, 2000 Tissue optics

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

  7. 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.

  8. 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.

  9. P.E. Andersen [BIOP], Feb. 2, 2000 Tissue optics • Light propagation (Monte Carlo simulation) Absorption “Snake” component Incident light Ballistic component Diffuse reflectance Diffuse transmittance

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

  11. P.E. Andersen [BIOP], Feb. 2, 2000 Photoacoustic imaging • Thermoelastic stress and generation of stress waves Stress wave (acoustic wave) Thermoelastic stress Short laser pulse Absorber

  12. 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.

  13. P.E. Andersen [BIOP], Feb. 2, 2000 Photoacoustic imaging • Stress waves (cont’d) • stress confinement (mathematically); c: speed of soundD: Min{optical penetration, laser beam diameter, slab} • the stress building up inside the absorbing target is: Grüneisen parameter (0.11 for water at room temperature) : radiant exposure (from laser)a: optical absorption coefficient

  14. P.E. Andersen [BIOP], Feb. 2, 2000 Photoacoustic imaging • 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

  15. 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.

  16. P.E. Andersen [BIOP], Feb. 2, 2000 Photoacoustic imaging • Detection • microphone (hydrophone), • piezoelectric transducers, • all-optical method(s) based on interferometry.

  17. 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).

  18. 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(!). P.E. Andersen [BIOP], Feb. 2, 2000 Photoacoustic imaging

  19. 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.

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

  21. 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.

  22. P.E. Andersen [BIOP], Feb. 2, 2000 All-optical detection scheme • The all-optical detection scheme (top view)

  23. 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.

  24. 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.

  25. 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. P.E. Andersen [BIOP], Feb. 2, 2000 All-optical detection scheme HeNe beams632 nm Water Tissue Translation Absorber 532 nm Nd:YAG

  26. 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. P.E. Andersen [BIOP], Feb. 2, 2000 All-optical detection scheme

  27. 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

  28. 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.

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