Development of simulation tools and enhanced image-guidance for optimized planning and monitoring of high intensity focused ultrasound energy delivery

磁共振引导高强度聚焦超声（MRgHIFU）是一种非侵入性治疗，在加拿大被批准用于骨和软组织的热消融。MRgHIFU从体外传输超声能量，并将能量集中在体内，产生一个小如米粒（8毫米长x 2毫米直径）的热损伤。这种聚焦是通过电子和机械方式控制的，可以覆盖很大的区域，而无需重新定位目标。由于能量输送是非电离的和无切口的，感染的风险和长期的副作用是最小的。通过测量加热水分子的信号变化，利用磁共振测温技术（MRT）监测软组织中的能量传递。在理想条件下，即不移动的软组织目标，MRT以非常快速和可重复的方式监测温度，不确定度小于1℃。然而，在许多情况下，例如对移动目标的处理（传统的MRT方法不再稳定）或对骨骼的处理（水中MRI信号较弱），可能导致温度的不确定性大于5℃。也有一些疗法不是由于热效应，不会产生温度变化。在这些情况下，MRT不是理想的监测技术；需要一种更直接的方法来测量声波与组织相互作用时的压力。

Magnetic resonance guided high intensity focused ultrasound (MRgHIFU) is a noninvasive therapy that is approved in Canada for thermal ablation of bone and soft tissue. MRgHIFU transmits ultrasound energy from outside the body and focuses that energy within the body, producing a thermal lesion as small as a rice kernel (8 mm long x 2 mm diameter). This focus is steered electronically and mechanically to cover a large volume, without repositioning the target. Since energy delivery is nonionizing and incisionless, the risk of infection and long-term side effects are minimal. The energy delivery is monitored in soft tissues with MR thermometry (MRT) by measuring signal changes in heated water molecules. Under ideal conditions, namely non-moving soft-tissue targets, MRT monitors temperature in a very fast and repeatable manner with an uncertainty less than 1C. However, there are many conditions, such as the treatment of moving targets (where conventional MRT methods are no longer stable) or the treatment of bone (where the water MRI signal is weak) that can lead to temperature uncertainties greater than 5C. There are also therapies that are not due to thermal effects and do not produce a temperature change. In these instances, MRT is not the ideal monitoring technique; a more direct method to measure the pressure of the sound waves as they interact with the tissue is required. My program investigates methods to improve MRT by separating the MR signal into changes due to temperature and changes due to motion. This process can reduce temperature uncertainties of over 5C, which is unsuitable for therapy, to under 1C when imaging targets in the abdomen. MRT of the abdomen represents a worst-case scenario for monitoring as temperature measurement is corrupted by periodic motion due to the lungs, and aperiodic motion due to the intestines. I am also developing mathematical models for bone HIFU procedures that incorporate simulations of sound wave propagation with a tissue-specific model of heat propagation in bone and soft tissues. These simulations involve segmentation of inner and outer bone surfaces from MRI scans, and an acoustic simulation of the ultrasound wave interaction throughout the bone tissue. This simulation integrates with a tissue-specific model of the temperature reached in the bone, bone marrow, and surrounding tissue. The wave and temperature models are both implemented in a computationally efficient manner that enables full 3D simulation of an MRgHIFU sonication in under one minute. This enables simulations to run within an intraoperative setting without repositioning the target between the planning MRI scans and the energy delivery. Lastly, I am developing methods to image the acoustic radiation force generated by an ultrasound pressure wave using MRI field measurements. This technique, known as ARFI, enables the localization of non-thermal HIFU energy delivery during MRgHIFU procedures.