Representative photos of the rinsed surface of the kidneys after sonication. The control samples (a) exhibited a homogenous light blue hue. Blue circles could be observed in the tissue after 2-min pulsed-HIFU exposure of (b) 940 W/cm2, 4% DF, 1 Hz PRF, (c) 940 W/cm2, 8% DF, 2 Hz PRF, and (d) 940 W/cm2, 8% DF, 0.125 Hz PRF. Scale bar represents 3 mm. DF: duty factor; PRF: pulse repetition frequency.