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)是加拿大批准的一种非侵入性治疗方法，用于骨和软组织的热消融。MRgHIFU从体外传输超声波能量，并将该能量集中在体内，产生米粒大小(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来控制的，以覆盖大体积，而不需要重新定位目标。由于能量传递是非电离和非切开的，感染的风险和长期副作用是最小的。利用磁共振测温仪(MRT)通过测量热水分子中的信号变化来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1C。然而，有许多情况下，例如处理移动的目标(传统的MRT方法不再稳定)或治疗骨骼(水MRI信号较弱)，可能会导致温度不确定性大于5摄氏度。还有一些疗法不是由热效应引起的，也不会产生温度变化。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。我的程序通过将磁共振信号分离为温度引起的变化和运动引起的变化来研究改进MRT的方法。这一过程可以将腹部成像目标时不适合治疗的5摄氏度以上的温度不确定性降低到1摄氏度以下。腹部的MRT代表了监测的最坏情况，因为由于肺部的周期性运动和肠道的非周期性运动，温度测量被破坏。我还在开发骨HIFU程序的数学模型，将声波传播的模拟与骨和软组织中热传播的特定组织模型结合在一起。这些模拟包括从MRI扫描中分割内和外骨骼表面，以及整个骨组织中超声波相互作用的声学模拟。这一模拟与骨骼、骨髓和周围组织中达到的温度的特定组织模型相结合。波和温度模型都是以计算高效的方式实现的，能够在一分钟内实现MRgHIFU声学的全3D模拟。这使得模拟能够在术中设置内运行，而无需在计划的MRI扫描和能量输送之间重新定位靶点。最后，我正在开发使用核磁共振现场测量来成像超声压力波产生的声辐射力的方法。这种技术被称为ARFI，能够在MRgHIFU过程中实现非热HIFU能量传递的本地化。

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.