Oregon Medical Laser Center NewsEtc., Feb. 1, 1998. Steven Jacques
A noncontact optical transducer based on interferometry is being developed for detection of laser-induced photoacoustic signals in a collaborative effort between the Risoe National Labs of Denmark and the Oregon Medical Laser Center (OMLC). The OMLC has been developing "PhotoAcoustic Imaging" (PAI) as an imaging modality with the use of piezoelectric devices that contact the tissue. A noncontact method of transduction allows PAI in tissues where surface contact is not wanted or possible, for example the retina of the eye or a sterile site during skin surgery.
The report was presented at last week's SPIE Photonics West conference in San Jose, California, entitled: Non-contact detection of laser-induced acoustic waves from buried absorbing objects using a dual-beam common-path interferometer.
PhotoAcoustic Imaging (PAI) uses a Q-switched pulsed laser to generate a slight temperature rise in an absorbing object buried within a tissue, for example a hemorrhage in the brain or the vasculature of a tumor. The temperature rise during the laser pulse elicits thermoelastic expansion on a very short time scale which allows stress confinement, hence maximal pressures are generated in the object. This pressure distribution then propagates to the tissue surface as acoustic pressure waves at the speed of sound. Their time of arrival at a surface site of detection indicates the distance the wave has traveled and hence the detected signal can be backprojected into the tissue. Superposition of such backprojected signals from an array of surface detectors can locate objects and provides a means for image reconstruction. The advantage of PAI is the ability to detect the edges of deep objects with roughly mm resolution rather than the cm resolution of photon migration measurements alone.
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Fig. 1. Experimental setup.
(TOP VIEW) The two polarization components of a linearly polarized HeNe laser beam are split by a Wallaston prism into two angles to yield two beams which are collimated by a lens and turned by a mirror down onto a liquid phantom. The reflected light from the phantom surface returns the two beams back through the Wallaston prism, off the beamsplitter, through a polarizer at 45 degrees to equally sample both components of the two reflected polarized beams, through an interference filter to reject any Nd:YAG laser radiation (532 nm), and focused by a lens through a pinhole to reach a photodiode detector. The two beams interfere at the photodiode yielding a signal which is sensitive to the differential total pathlength of photons after reflection off the phantom surface.
(SIDE VIEW) The two beams are reflected down onto the phantom surface. Pressure waves arriving at the surface reach one beam site before arriving at the second beam site. The differential height of the surface at the two beams affects the round-trip path of the interferometer and generates a change in the interferometric signal at the detector.
(END VIEW) Laser irradiation was delivered via a mirror down through the clear water. The adjustable mirror varied the site of laser irradiation of the absorbing gel.
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Fig. 2. Point spread function for optical transducer.
The response signal from a point source of acoustic energy generated by the pulsed laser. The site of laser irradiation was a bed of collagen and India ink located 18 mm below the water/air surface. The beam was directed by mirror to various sites perpendicular to the midplane of null response between the two HeNe laser beams. As the irradiation site was moved about the response was measured. The signal falls off as 1/r where r is the distance between the site of sound generation and the nearest HeNe laser beam.
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Fig. 3. Edge detection of absorbing object buried 11 mm below air/water surface.
Experimental setup irradiated the absorbing object from below and detected sound at air/water surface. The absorbing object was a gel with India ink (20 mm long x 10 mm wide x 2 mm thick) located 11 mm below the air/water surface which is seen in figure as the black absorber at bottom of phantom. The phantom was translated while the laser source and HeNe beam detector remained fixed.
The edge of the absorbing object was clearly sensed by the photoacoustic imaging with sub-mm resolution of the edge. The transduction of surface movement by the HeNe laser interferometer was not adversely affected by addition of optical scatterer to the water and the signal and image resolution did not degrade.
The report included some criteria of performance. The calibration factor for the device was tentatively about 30-70 mV/bar. For comparison, the calibration factor for a lithium niobate piezoelectric transducer has been reported by Oraevsky et al.  to be 100 nV/Pa, or 10 mV/bar. Therefore, the interferometer is tentatively 3-7-fold more sensitive than the piezoelectric transducer.
The noise threshold was about 1 mV (14-30 mbar). Oraevsky et al. used the third-harmonic Q-switched Nd:YAG laser (355-nm, 3-mm-dia. spot, energy = 5 mJ, exposure = 4.4 mJ/cm2) and K2Cr04 solutions to test the linearity of their lithium niobate transducer. Their lowest measurement was for µa = 6.5 cm-1 which with their laser exposure would yield a pressure of 40 mbar. We assume that their noise threshold was only a little lower than 40 mbar. Hence, the interferometer appears to have a noise threshold comparable to the piezoelectric detector.
The maximum signal that still retained 95% linearity was estimated to be 3276 mV or 46-100 bar. Hence, the interferometer provided a dynamic range of 1-3300 mV. In comparison, the maximum absorption coefficient measured by Oraevsky et al. was µa = 1000 cm-1, which with their laser exposure would correspond to 6 bar. Probably the piezoelectric transducer can detect much larger pressures, however in practical experiments involving laser-induced temperature jumps much less than 100°C it is difficult to attain higher pressures. Higher absorption coefficients have such thin depths of attenuation that it is difficult for a 10-ns laser pulse to attain stress confinement in the object. Also, the high acoustic frequencies associated with thin depths of attenuation are strongly attenuated in most media, especially tissues. So from a practical matter, the upper limit of detector response is not usually utilized in photoacoustic imaging. The interferometer appears to have sufficient upper dynamic range to provide linear responses for photoacoustic imaging applications.
For more information, contact Steven L. Jacques at the Oregon Medical Laser Center in Portland, Oregon, or contact Peter E. Andersen at the Risoe National Labs in Denmark.