As astronauts venture farther from Earth, the risks of medical crises increase. Ultrasound is a well-known imaging modality that has been successfully used on the International Space Station (ISS) to help mitigate these risks. In addition, there is potential to leverage advances in Earth-based clinical applications of high intensity focused ultrasound (HIFU) to enable non-invasive surgical treatments during exploration missions. HIFU therapies have the potential to treat a wide range of indications that could occur in future exploration missions, including stopping internal bleeding with acoustic hemostasis, treating cancerous tumors that may arise quickly from exposure to radiation, palliation of pain, or even acting as an acoustic scalpel to treat serious internal injuries noninvasively, through the skin.
To realize the potential of using HIFU with flexible ultrasound system (FUS) guidance in space, a suitable test-bed phantom is needed to provide a training apparatus for astronauts and test the delivery of therapy to targeted anatomical sites. The goal of this project was to develop such phantom with anatomical and acoustical features relevant to clinical HIFU such as skin, ribs, inhomogeneous body wall, and tissue targets to simulate thermal and mechanical HIFU treatments.
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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 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]