A system and method for determining fractional fat content of tissue comprises registering thermoacoustic image coordinates to an acquired ultrasound image, the acquired ultrasound image at least comprising target tissue within a region of interest; defining a thermoacoustic voxel grid coincident with the region of interest; obtaining thermoacoustic image measurement values from tissue within the region of interest corresponding to the voxels within the defined thermoecoustic voxel grid to yield a thermoacoustic measurement matrix; normalizing the thermoacoustic image measurement values within the thermoacoustic measurement matrix; calculating a fractional fat content map for the target tissue within the region of interest based on the normalized thermoacoustic image measurement values within the thermoacoustic measurement matrix and a reference thermoacoustic measurement value; and correcting the fractional fat content map based on tissue speed-of-sound data to yield a final fractional fat content map for the target tissue within the region of interest.