APPROACH:
A HIFU transducer containing an imaging probe in its central opening was developed. A power source was built with the ability to deliver either short pulses at high powers for mechanical treatments or continuous output at moderate powers up to 700 W for thermal treatments. To represent fat and muscle in body wall, phantom layers were made from polyvinyl alcohol with irregular-shaped walls; the skin was represented by a silicone sheet; ribs were fabricated by 3D-printing based on a human anatomical model relevant to liver or kidney treatments. HIFU targets comprised optically transparent alginate or polyacrylamide gels that allows visualization of the lesion. These components were assembled in a water tank to provide coupling between layers and allow shifting of their relative positions. Free-field hydrophone measurements were performed for transducer characterization and shocked waveforms with peak positive/negative pressures of +100 / -20 MPa were measured; in situ fields behind ribs and body wall were then characterized both by hydrophone measurements and observations of lesion formation. When ribs were present, shocks formed at about half amplitude at the same power, and higher pressures were measured with ribs positioned closer to the transducer. The presence of a uniform body wall structure attenuated shock amplitudes by a smaller amount and was insensitive to axial position. Mechanically ablated lesions were visualized in target gels.

RESULTS:
Results showed that the presence of ribs and absorptive tissue-mimicking layers did not prevent shock formation at the focus and each layer simulated the impact on the beam focusing and shock formation similarly to the observations from in vivo and ex vivo experiments. Overall, with real-time lesion visualization, the developed low-maintenance HIFU phantom system compatible with FUS can serve as a training tool for researchers, engineers, and clinicians to facilitate faster acceptance of HIFU therapies for a number of applications to be used both in Earth-based and space-based conditions.

Khokhlova TD, Haider YA, Maxwell AD, Kreider W, Bailey MR, and Khokhlova VA. Dependence of boiling histotripsy treatment efficiency on HIFU frequency and focal pressure levels. Ultrasound in Medicine and Biology. 2017. September; 43(9):1975-85. [DOI]

Ultrasonography

Technology assessment

Frequency intensity