Magnetic resonance imaging of boiling induced by high intensity focused ultrasound

Bubble activity in high intensity focused ultrasound (HIFU) medical therapy is commonly but not rigorously divided between mechanically induced cavitation (μ size gas bubbles) and thermally induced boiling (mm size vapor bubbles). Our goal was to confirm that boiling occurred at 100 °C. A 2 MHz focused transducer (42 mm aperture, 44 mm focal length) was used to heat tissue phantoms in a 4.7 Tesla magnet. Temperature was measured by magnetic resonance imaging (MRI) proton resonance frequency shift and calculated from acoustic absorption. The MRI voxel was 0.3×0.5×2 mm, and acquisition time was 1.3 s. Boiling was observed as a dark spot in MRI images and fluctuation in the transducer drive voltage. At 30 MPa peak pressure, boiling occurred in 7 s. Calculations yielded 100 circC in 7 s and a temperature half maximum width of 1 mm. Averaging the calculated temperature field over the MRI voxel yielded a maximum of 73 circC, which was the peak temperature measured in the last MRI slice before boiling. In conclusion, boiling appeared when the peak temperature reached 100 °C, and the results warn that MRI monitoring alone may underestimate the peak temperatures. [Work supported by NIH DK43881, NIBIB R21EB005250, and NSBRI SMS00402.]