This paper reports the development of dual-modal all-optical photoacoustic microscopy (PAM) and photoacoustic viscoelasticity testing (PAVT) based on a common setup with tunable annular-beam excitation and wideband laser-vibrometer detection. For PAM, the annular beam is reduced to a small diameter to excite longitudinal bulk acoustic waves (BAWs) from light-absorbing targets buried in the sample, and the imaging quality is improved by a specified synthetic aperture focusing technique (SAFT). For PAVT, circularly converging surface acoustic waves (SAWs) are generated on the sample surface by annular beams with larger and different diameters. Both the induced BAWs and SAWs are detected at the center of the annular beam in a non-contact fashion by a broad-band laser vibrometer. Experiments have been conducted on the tissue-mimicking agar phantoms as well as animal tissues and have successfully demonstrated the initial concept of dual-modal all-optical PAM and PAVT.
This paper reports a new optically transparent focused P(VDF-TrFE) (poly(vinylidene fluoride-co-trifluoroethylene)) transducer for photoacoustic microscopy (PAM), which is fabricated by a new process based on pre-cutting and direct-lamination. Compared with the previous fabrication process, it is simpler, and makes it possible to achieve a high numerical aperture (NA) without stretching the (brittle) piezoelectric film. For demonstration, a prototype transducer has been fabricated with a 10-μm-thick 70/30 P(VDF-TrFE) film laminated onto a plano-concave glass lens with an NA of 0.64. Experimental characterization shows that the transducer has an optical transmittance of 88.6% (@ 532 nm), an acoustic center frequency and -3 dB bandwidth of 24 MHz and 29 MHz, respectively. Using the new optically transparent focused P(VDF-TrFE) transducer, an optical-resolution PAM (OR-PAM) imaging setup has been built and imaging experiments have been conducted on different targets. The experimental results show the optically transparent focused P(VDF-TrFE) transducer could be useful for the development of new PAM systems for different imaging applications.
This paper reports a new photoacoustic (PA) excitation method for evaluating the shear viscoelasticity of soft tissues. By illuminating the target surface with an annular pulsed laser beam, circularly converging surface acoustic waves (SAWs) are generated and detected at the center of the annular beam. The shear elasticity and viscosity of the target are extracted from the dispersive phase velocity of the SAWs based on the Kelvin-Voigt model and non-linear regression fitting. Agar phantoms with different concentrations and animal liver and fat tissue samples have successfully been characterized. Different from previous methods, the self-focusing effect of converging SAWs allows sufficient signal-to-noise ratio (SNR) to be obtained even with low laser energy density, which makes this approach well compatible with soft tissues.
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