Real time magnetic resonance imaging guided high intensity focused ultrasound in the free-breathing liver [electronic resource]
Keywords: Bioengineering.

High Intensity Focused Ultrasound (HIFU) is a promising technique for non-invasively destroying tissue deep inside the body, without damage to the skin or intervening tissue. The technique has been performed under both ultrasound and magnetic resonance imaging (MRI) guidance in multiple organ systems, including the uterus, prostate, and brain. Treating unresectable cancers and metastases in the liver is another potential application for HIFU. However the method is currently limited to breath-hold treatments under general anesthesia. The purpose of this work is to build a system for a less-invasive free-breathing MRI-guided real time liver ablation. The three key elements to the system are 1) real-time temperature imaging suitable for the free breathing liver, 2) rapid focal spot localization, and 3) beam steering during free-breathing. For in vivo temperature imaging in the presence of motion, liver motion in 5 normal volunteers was studied. The maximum velocity of the liver was found to be 10.8 mm/s in the superior-inferior direction and 4.23 mm/sec in the anterior-posterior direction. A pulse sequence that allows for high speed, high resolution MR-thermometry utilizing the proton resonant frequency shift (PRF shift) thermometry technique with referenceless image reconstruction was developed. The sequence utilizes outer volume suppression and readout-segmented EPI trajectories to increase the frame rate while not sacrificing resolution. Ablations in a moving phantom moving at speeds of 13 mm/s are compared to static ablations and found to have good agreement. Additionally, SNR characteristics were measured in the liver of normal volunteers, where the average standard deviation was 1.48 degrees C. A HIFU focus must be visualized prior to performing the actual ablation. In static ablations, a non-lethal thermal test spot is performed to calibrate the focus. However, in the liver, perfusion and the need to steer the beam with the motion makes this technique difficult and still require significant energy. The use of MR acoustic radiation force imaging (MR-ARFI) was investigated as a method for visualizing the focus without the high power of a test spot but with the same location accuracy as the thermometry images. The MR-ARFI sequence is a single shot spin echo sequence with an identical readout trajectory to the thermometry sequence that can localize tissue focus displacement in two acquisitions. MR-ARFI displacement maps were compared to foci in ablation thermometry images in both phantoms and in porcine livers. MR-ARFI was found to be effective in focus targeting calibration, with a standard deviation error of only 1.13 mm between displacement and thermal foci in vivo. Using these two sequences and an MR-tracking sequence for HIFU transducer localization, a technique for steering a HIFU focus to a specific target was developed. A lookup table of transducer location, target location, and respiration value measured by an external respiratory belt is created prior to an ablation. The focus location is varified using the gated MR-ARFI pulse sequence discussed prior. During the ablation, the focus is steered based on the lookup table and the current respiration value. This method is investigated by performing steered HIFU ablations in moving phantom experiments and compared to non-steered moving ablations and static ablations. Utilizing this technique, steered HIFU ablations required 1039 J to reach a temperature of 15 degrees C, compared to 7480 J and 921 J for non-steered and static ablations, respectively.