Photoacoustic signals originating from weak sources can be hard to discriminate from higher intensity signals resulting from the photoacoustic background. In order to reveal these weaker signals we propose a method where the expected background signal is subtracted from the actual signals. In this method an ultrasound image provides the geometry for the pressure distribution used in a photoacoustic wave field simulation. The simulated photoacoustic signals are subtracted from the actual recordings and the residual is used for image reconstruction. This method was successfully validated experimentally with a vessel phantom containing three optical absorption irregularities within the vessel wall.

We demonstrate the feasibility of intravascular ultrasound (IVUS) chirp imaging as well as chirp reversal ultrasound contrast imaging at intravascular ultrasound frequency. Chirp excitations were emitted with a 34 MHz single crystal intravascular transducer and compared to conventional Gaussian-shaped pulses of equal acoustic pressure. The signal to noise ratio of the chirp images was increased by up to 9 dB relative to the conventional images. Imaging of contrast microbubbles was implemented by chirp reversal, achieving a contrast to tissue ratio of 12 dB. The method shows potential for intravascular imaging of structures in and beyond coronary atherosclerotic plaques including vasa vasorum.