A method and apparatus are presented for non-invasively monitoring at least one characteristic of a region of interest in a body. The body is illuminated by pulsed light to induce a photoacoustic effect. Acoustic radiation is detected, and measured data indicative thereof is generated. The photoacoustic effect is controlled by carrying out at least one of the following: providing the pulsed light of at least two different wavelength, analyzing the measured data to determine time variations of at least one predetermined parameter of a time dependent acoustic signal for each of wavelength, and determining oxygen saturation level in the region of interest; and operating a plurality of acoustic elements configured for detection of the acoustic radiation, to thereby define a focal volume for acoustic radiation detection to match dimensions of the region of interest.