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

磁共振引导的高强度聚焦超声（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，并将该能量聚焦在体内，产生小至米粒（8 mm长x 2 mm直径）的热损伤。这种聚焦通过电子和机械方式来控制，以覆盖较大的体积，而无需重新定位目标。由于能量输送是非电离和无切口的，因此感染和长期副作用的风险很小。通过测量加热的水分子中的信号变化，利用MR温度测量法（MRT）监测软组织中的能量输送。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，不确定性小于1C。然而，有许多情况，例如移动目标的治疗（传统的MRT方法不再稳定）或骨骼的治疗（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于热效应，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。 我的程序研究通过将MR信号分离为温度变化和运动变化来改善MRT的方法。这个过程可以将不适合治疗的超过5 ℃的温度不确定性降低到1 ℃以下，从而对腹部中的目标进行成像。腹部的MRT代表了监测的最差情况，因为温度测量结果被肺部的周期性运动和肠道的非周期性运动破坏。 我还开发了骨HIFU手术的数学模型，该模型将声波传播的模拟与骨和软组织中热传播的组织特异性模型相结合。这些模拟涉及从MRI扫描分割内部和外部骨表面，以及在整个骨组织中的超声波相互作用的声学模拟。该模拟与骨骼、骨髓和周围组织中达到的温度的特定组织模型集成。波和温度模型都以计算高效的方式实现，使得能够在一分钟内对MRgHIFU超声处理进行完整的3D模拟。这使得模拟能够在术中设置内运行，而无需在计划MRI扫描和能量输送之间重新定位靶点。 最后，我正在开发的方法，图像的超声压力波使用MRI场测量产生的声辐射力。这种技术称为ARFI，能够在MRgHIFU手术期间定位非热HIFU能量输送。

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.