Combined photoacoustic, pulse-echo laser ultrasound, and speed-of-sound imaging using integrating optical detection.

A purely optical setup for the coregistration of photoacoustic (PA), ultrasound (US), and speed-of-sound (SOS) section images is presented. It extends a previously developed method for simultaneous PA and laser-US (LUS) pulse-echo imaging with a LUS transmission imaging setup providing two-dimensional (2-D) SOS maps. For transmission imaging, the sound waves traversing the investigated object are generated instantaneously by illuminating optically absorbing targets that are arranged at various distances in front of the sample. All signals are recorded by an optical beam which is part of a Mach–Zehnder interferometer that integrates the acoustic field along its path. Due to the cascaded arrangement of LUS sources, a single-recorded signal yields information for a projection of the SOS distribution. After collection of data from all directions, an inverse Radon transform is applied to this set of projections to obtain a 2-D SOS image. The setup is characterized and its performance is tested on phantom experiments. In addition to providing additional contrast, it is also shown that the resolution of the coregistered PA and LUS images can be improved by implementing the knowledge of the SOS distribution in the reconstruction.