As an acoustic wave propagates through the tissue, part of it is absorbed and converted to heat. With focused beams, a very small focus can be achieved deep in tissues (usually on the order of milimeters, with the beam having a characteristic “cigar” shape in the focal zone, where the beam is longer than it is wide along the transducer axis). Tissue damage occurs as a function of both the temperature (nearly 195 ºF = 90 ºC) to which the tissue is heated and how long (3 seconds) the tissue is exposed to this heat level in a metric referred to as “thermal dose”. By focusing at more than one place or by scanning the focus, a volume can be thermally ablated. At high enough acoustic intensities, cavitation (microbubbles forming and interacting with the ultrasound field) can occur.