Ultrasonic Atomization: A Mechanism of Tissue Fractionation

High intensity focused ultrasound (HIFU) can be used to atomize liquid by creating a fountain on the surface exposed to air. The mechanism of atomization can be most accurately described by the cavitation-wave hypothesis wherein a combination of capillary waves excited on the liquid surface with cavitation beneath the surface produces a fine spray. Here, we show experimentally that a free tissue surface can also be atomized resulting in erosion of tissue from the surface. A 2-MHz spherically focused transducer operating at linearly predicted in situ intensities up to 14,000 W/cm² was focused at ex vivo bovine liver and in vivo porcine liver tissue surfaces without the capsule. The end result for both in vivo and ex vivo tissues was erosion from the surface. In bovine liver at the maximum intensity, the erosion volume reached 25.7±10.9 mm³ using 300 10-ms pulses repeated at 1 Hz. Jet velocities for all tissues tested here were on the order of 10 m/s. Besides providing a mechanism for how HIFU can mechanically disrupt tissue, atomization may also explain how tissue is fractionated in boiling histotripsy.