A thermoacoustic imaging apparatus comprises an electromagnetic radiation source, an acoustic signal detection probe and a radiation responsive acoustic signal generator outside the sample area. The detection probe arrangement detects both signals generated in the sample area in response to the electromagnetic irradiation and an acoustic signal from the radiation responsive acoustic signal generator that has traveled through the sample area. An acoustic transmission parameter such as a speed of sound or absorption as a function of a position in the sample area is computed from signals due to the radiation responsive acoustic signal generator. The acoustic transmission parameter is used to correct the computation of a thermoacoustic image from detections due to acoustic signals generated in the sampled area.