Optoacoustic imaging using two-dimensional ultrasonic detection

Optoacoustic imaging uses thermoelastic waves generated by short laser pulses to localize structures with preferential light absorption inside a material. The acoustic waves are directly generated in absorbing structures and are detected outside the sample with a wide-band ultrasonic transducer. Image reconstruction is usually done by backprojection of temporal ultrasound signals that are taken at different positions. As an alternative, we present a method where the acoustic field caused by thermoelastic excitation is captured as a snapshot in a plane, using an optical reflectance based detection principle. Image reconstruction is accomplished by backprojection of the detected two-dimensional stress distributions into the sample volume, using the delay times at which the snapshots were taken after the laser pulse. Two-dimensional stress signals and image reconstruction are demonstrated in simulations and in experiments, where small objects like hairs are irradiated with laser pulses of 6 ns duration. The main advantages of this system are the high spatial resolution that can be achieved with the optical sensing technique and the possibility to irradiate the sample directly through the detector plane. This enables front surface detection of the optoacoustic signals, which is especially important if structures close to the tissue surface are to be imaged.