A method for quantifying an amount of fat contained in a liver or other tissue of a subject in vivo includes varying the temperature of a target area in a subject, imaging thermal strain of the target area using an ultrasound scanner, and quantifying the amount of fat in the targeted area based on the thermal strain imaging. In some embodiments, the thermal strain imaging is performed using high-resolution, phase-sensitive speckle tracking to differentiate between fat-based tissue and water-based tissue.