A thermoacoustic imaging apparatus comprises an electromagnetic radiation source configured to irradiate a sample area and an acoustic signal detection probe arrangement for detecting acoustic signals. A radiation responsive acoustic signal generator is added outside the sample area. The detection probe arrangement detects both signals generated in a sample area in response to the irradiation and an acoustic signal from the radiation responsive acoustic signal generator that has travelled through the sample area. A computing system distinguishes first acoustic signal detections attributed to the acoustic signal generated by the radiation responsive acoustic signal generator and second acoustic signal detections attributed to the acoustic signals generated by the sample area. The computing system performs a tomographic computation of an acoustic transmission parameter as a function of a position in the sample area from the first acoustic signal detections. The acoustic transmission parameter may be a speed of sound or absorption. The acoustic transmission parameter may be used to correct the computation of a thermoacoustic image.
