Nondestructive photoacoustic subsurface tissue imaging: a feasibility study

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Abstract
Photoacoustic (PA) techniques may be used for imaging of absorbing structures within a light scattering medium. By use of an array of detectors, the macroscopic structure of the absorbers in the medium may be determined, based on differences in light absorption. A physical explanation of the spherical PA profile is presented, from which the signals generated by other source geometries can be derived. The potentialities of PA imaging of blood perfused tissue have been investigated. Experiments were performed with a pulsed frequency-doubled Nd:YAG laser which delivered 10 ns pulses at 532 nm wavelength. Dilutions of India ink, dyed epoxy strands and dyed polystyrene spheres acted as PA sources. Characteristic source dimensions varied between ca. 10 and 250 micrometer. The PA signals were detected with wide band piezoelectric transducers made of 9 and 28 micrometer thick PVdF film with mm and sub-mm lateral dimensions. Detection distances were between 1 and 50 mm. Nonlinear effects have been observed for higher levels of absorbed energy. Dilutions of Liposyn were used as optically scattering media. The calculations suggest the applicability of the method. For imaging of dermal blood vessels up to a depth of 1 mm, piezoelectric signals in the (mu) V range may be expected. The experimental PA signals contain significant frequency components up to 75 MHz, depending on the source characteristics. In principle from such signals the detection distance can be determined with micrometer resolution. The detectors show a forward directivity caused by acoustic interference on the detector surface which depends on the lateral dimensions and the acoustic pulse shape and pulse duration. Optical attenuation coefficients of the Liposyn dilutions have been determined photoacoustically. A spatial resolution of ca. 5-50 micrometer was achieved in the reconstruction of the PA source locations.

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