Fourier Domain Reconstruction Methods in Laser Ultrasonics and Photoacoustic Imaging

Laser-ultrasonics as well as photoacoustic imaging use optically generated acoustic waves detected at the sample surface to image its interior. In laser-ultrasonics, a laser pulse is absorbed at the sample surface generating an ultrasound pulse that propagates into the sample, is reflected at internal structures, and is detected at the surface. In photoacoustic imaging, the source of the ultrasound wave is the investigated structure inside of an optical semitransparent sample itself. The goal in photoacoustic imaging is to recover the spatial distribution of absorbed energy density inside the sample from the acoustic pressure signals measured outside the sample (photoacoustic inverse problem). Fourier reconstruction is based on the decomposition into plane waves and is a fast and efficient method used in photoacoustic imaging. Interpolation is needed when signal Fourier components are mapped to source Fourier components. We have shown that the synthetic aperture focusing technique (SAFT) in frequency domain, which needs no interpolation, and the Fourier reconstruction method are mathematically equivalent if the step size of the spatial discretization goes to zero. Both imaging methods are compared using simulated data and measurement data acquired with our interferometer set-up.