What equipment is needed?
Steven L. Jacques, Guenther Paltauf
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The equipment required for optoacoustic imaging are:
We use a Q-switched 2nd-harmonic Nd:YAG laser that delivers up to 100 mJ pulse energy as 5-ns pulses at 532-nm wavelength. Sometimes we couple the laser into a 600-um-diameter optical fiber. The optical fiber is mechanically strained by coiling to achieve mixing of the optical modes of the beam within the fiber, hence yielding a uniform field of radiant exposure which can be imaged by a lens onto any surface.
The theoretical optimal spatial resolution is d = tp*cs where tp is the laser pulse duration (5 ns) and cs is the speed of sound (1.5 μm/ns), or 7.5 μm. However the resolution is degraded by viscoelastic attenuation of the higher frequencies. Practical resolutions are about 50 μm if the detector is within a few mm of the object, and about 1 mm if the detector is a few cm from the object. So longer laser pulse durations will work without losing spatial resolution. Q-switched laser just happen to be available and usually have about 10-ns pulses.
The choice of the wavelength and pulse energy depend on the absorption of the object being imaged. We are concerned with blood vessels and so use a wavelength strongly absorbed by hemoglobin (hemoglobin spectrum).
We've used three types of pressure transducers:
(1) PVDF film: We use a 25-μm-thick PVDF film with aluminum conductor on both surfaces. We etch away the aluminum in a pattern to create electrodes that overlap in a 1mmx1mm square in the center of the PVDF. This square is the active pressure-sensitive area. The response is typically 10 mV/bar. The advantage of the PVDF film is its sensitivity, however its disadvantage is that it averages the pressure waves over its relatively large area. Hence, there can be a problem if the film behaves as an area detector rather than a point detector. (J.A. Viator, S.L. Jacques, S.A. Prahl, "Depth profiling of absorbing soft materials using photoacoustic methods", IEEE J. Selected Topics Q.E. 5, 989-996 (1999))
(2) Optical reflectance probe: We focus a laser beam through a trapezoidal prism to obliquely strike the glass/tissue interface. The angle is close to the critical angle so there is nearly maximal amount of total internal reflection of the beam. The amount of reflection is proportional to the refractive index mismatch between the glass and the tissues. A fast photodiode detects the reflected light. The detected light is recorded as a function of time following the laser pulse at time zero. The response is perhaps about 10-fold less sensitive than a piezoelectric transducer but has the advantage of focusing its sensitive to a small spot. Therefore, the optical probe samples the pressure at one point and avoids the "area detector" problem. (G. Paltauf, H. Schmidt-Kloiber, "Measurement of laser-induced acoustic waves with a calibrated optical transducer" J. Appl. Phys. 82, 1525-153 (1997))
(3) Optical interferometer probe: We split a laser beam into two parallel beams using a Wallaston prism. The two beams strike the tissue surface at two sites about 3 mm apart. Two reflected light beams return along the same two incident paths and are recombined by the prism into a single beam that is incident on a pinhole in front of a photodiode. The interference of the two reflected beams at the photodiode yields a signal whose amplitude is related to the difference in the position of the two surface sites illuminated by the two probe beams. One beam acts as the reference for the other. A pressure wave arrives at one be first before the other, except for sources on the midplane between the two surface sites In this special case, the probe is unresponsive. The spatial point spread function for this detector needs to be considered when used for imaging. The sensitivity is about 10 mV/bar and avoids the "area detector" problem. (S. L. Jacques, P. E. Andersen, S. G. Hanson, L. R. Lindvold: Non-contact detection of laser-induced acoustic waves from buried absorbing objects using a dual-beam common-path interferometer. SPIE 3254-40, 1998.)
Data acquisition system
We use a digital oscilloscope to acquire the pressure signals. The bandwidth of the oscilloscope is 500 MHz, but the useful signals are usually not more than 20 MHz. We sometimes use a preamplifier with a bandwidth of 42 MHz. The laser is operated at 10 Hz and 10-30 signal traces are averaged, although single pulse traces are adequate. Signals are acquired by the oscilloscope and transfered to a computer for analysis.
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