Characterization of integrating ultrasound detectors for photoacoustic tomography

Photoacoustic tomography is based on generation of sound waves in a semitransparent medium by illumination with short light pulses. In standard methods, measurements of the acoustic waves around the sample with point like ultrasound detectors are used for reconstruction of the distribution of absorbed energy, which contains information on light-absorbing structures such as blood vessels in tissue. Integrating ultrasound detectors are planes or lines larger than the imaged object and measure temporal signals that are given by spatial integrals over the sound field. It can be shown that such integrated signals give exact reconstructions with constant, high resolution throughout the imaging zone. The goal of the present study was to investigate with the help of simulations and experiments how far real implementations of integrating detectors based on piezoelectric films or optical interferometry have characteristics approximating those of ideal planes or lines. It is shown that the directive sensitivity of piezoelectric films tends to distort signals, mainly in the case of large area detectors. This distortion can, on the other hand, be used to directly measure a part of the directivity that is caused by distribution of stress components in the detector. Optical beams as part of an interferometer have omnidirectional response, but need focusing in order to achieve high temporal and spatial resolution. For example, with a beam focused to a diameter of 38 mu m a spatial image resolution of 52 mu m could be observed. Because of the beam waist, this resolution can only be achieved for acoustic sources lying within a range corresponding to the focal depth of the beam. It is concluded that line detectors made of piezoelectric thin films yield almost ideal performance for acoustic waves at normal incidence. Even better suited for photoacoustic tomography are focused optical beams as line detectors due to their omnidirectional response and higher signal to noise ratio, but only for objects with a size smaller than the focal depth.