Time-shifting correction in optoacoustic tomographic imaging for media with non-uniform speed of sound

An analysis of the time-shifting correction in optoacoustic tomographic reconstructions for media with an a priori known speed of sound distribution is presented. We describe a modification of the filtered back-projection algorithm, for which the absorbed optical energy at a given point is estimated from the value of the measured signals at the instant corresponding to the time-of-flight between such point and the measuring points. In the case that a non-uniform speed of sound distribution does exist, we estimate the time-of-flight with the straight acoustic rays model, for which acoustic waves are assumed not to change direction as they propagate. The validity of this model is analysed for small speed of sound variations by comparing the predicted values of the time-of-flight with the ones estimated considering the refraction of the waves. Experimental results with tissuemimicking agar phantoms with a higher speed of sound than water showcase the effects of the time-shifting of the optoacoustic signals caused by the acoustic mismatch. The performance of the time-shifting correction relates to the optoacoustic imaging of biological tissues, for which the speed of sound variations are usually lower than 10%.