Apparatus and methods are provided for monitoring a parameter of a region of interest (ROI) within a first tissue type within a body. A control unit drives a measurement unit to provide an operating condition such that acoustic waves overlap with a region within the ROI that is illuminated by illuminating light such as to generate a first set of tagged photons, and such that the acoustic waves overlap with a region of tissue within a second tissue type that is illuminated by illuminating light such as to generate a second set of tagged photons, the first and second sets of tagged photons being distinguishable from one another. The control unit determines the parameter of the ROI by extracting data portions associated with, respectively, the light response of the ROI and the light response of the second tissue type, by distinguishing between the sets of tagged photons.