Characterization and Ex Vivo evaluation of an extracorporeal high-intensity focused ultrasound (HIFU) system

High-intensity focused ultrasound (HIFU) has been in clinical use for a variety of solid tumors and cancers. Accurate and reliable calibration is in a great need for clinical applications. An extracorporeal clinical HIFU system applied for the investigational device exemption (IDE) to the Food and Drug Administration (FDA) so that evaluation of its characteristics, performance, and safety was required.

The acoustic pressure and power output was characterized by a fiber optic probe and a radiation force balance, respectively, with the electrical power up to 2000 W. An in situ acoustic energy was established as the clinical protocol at the electrical power up to 500 W. Temperature elevation inside the tissue sample was measured by a thermocouple array. Generated lesion volume at different in situ acoustic energies and pathological examination of the lesions was evaluated ex vivo.

Acoustic pressure mapping showed the insignificant presence of side/grating lobes and pre- or post-focal peaks (≤-12 dB). Although distorted acoustic pressure waveform was found in the free field, the nonlinearity was reduced significantly after the beam propagating through tissue samples (i.e., the second harmonic of -11.8 dB at 500 W). Temperature elevation was <10°C at a distance of 10 mm away from a 20-mm target, which suggests the well-controlled HIFU energy deposition and no damage to the surrounding tissue. An acoustic energy in the range of 750-1250 J resulted in discrete lesions with an interval space of 5 mm between the treatment spots. Histology confirmed that the lesions represented a region of permanently damaged cells by heat fixation, without causing cell lysis by either cavitation or boiling.

Our characterization and ex vivo evaluation protocol met the IDE requirement. The in-situ acoustic energy model will be used in clinical trials to deliver almost consistent energy to the various targets.