A Quick and Reliable Acoustic Calibration Method for a Clinical Magnetic Resonance Guided High-Intensity Focused Ultrasound System

With the expanding use and applications of MR-HIFU in both thermal-based and pressure-based therapies, there is an urgent need to develop acoustic field characterization and quality assurance (QA) tools for MR-HIFU systems. We developed a method for quick and reliable acoustic field assessment inside the magnet bore of a clinical MRI system. A fiber-optic hydrophone with a 2-m long fiber was fixed inside a water tank that was placed on the HIFU table above the acoustic window. The long fiber allowed the MRI-incompatible hydrophone control unit to be located outside the MRI suite. MR images of the fiber were used to position the HIFU focus approximately at the tip of the fiber. The HIFU focus was electronically steered within a 5×5×5mm3 volume in synchronization with hydrophone measurements. The HIFU focus location was then identified based on the 3D field scans. Peak positive and negative pressures were measured at the focus at various nominal acoustic powers. Furthermore, focus dimensions and spatial peak pulse average intensities were assessed. The results were compared to and were consistent with standard hydrophone measurements outside the MRI suite. This method provides a useful tool for field characterization and QA of MR-HIFU systems.