Fig. 1
a Schematic representation of the experiment setup. The focal point location of the HIFU transducer was controlled using electronic steering, i.e., by adjusting the phases of the driving signals for individual elements. The transducer could also be translated using MRI-compatible motors and encoders integrated into the HIFU patient table. The patient table above the HIFU transducer was sealed with a 50 μm thick Mylar membrane to provide an acoustic window. A cylindrical, acrylic water tank (diameter 15 cm, height 30 cm), sealed on the bottom with a polyester film (McMaster-Carr, Atlanta, GA, USA), was placed on top of the acoustic window and filled with degassed, distilled water. The hydrophone was fixed within the water tank, while the HIFU focus was electronically steered around the fiber tip for 3D pressure field mapping. b T2-weighted MRI of the experimental setup. The hydrophone holder location was traced on MRI, and the fiber location (dashed line) was projected based on the distance from the hydrophone tip to the holder as measured before submerging the assembly into the water tank. The nominal HIFU focus was positioned at the fiber tip using the HIFU therapy planning software, as illustrated by the drawing of the HIFU beam outline (the beam outline is shown for illustrative purposes only and may not accurately represent the actual pressure field distribution) and the nominal HIFU focus