McDannold Nathan, King Randy L, Jolesz Ferenc A, Hynynen Kullervo
Department of Radiology, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts 02115, USA.
Med Phys. 2002 Mar;29(3):356-65. doi: 10.1118/1.1449495.
In this study, we investigated the use of MRI-derived thermal imaging for determining the exposure parameters for focused ultrasound (FUS) surgery. Since the temperature rise induced by a FUS beam scales linearly with power, the temperature maps acquired during subthreshold sonications can be used to determine the power necessary to produce thermal tissue damage with a desired size. Thermal images acquired during multiple sonications delivered at different locations in rabbit thigh muscle and brain tissue in vivo were analyzed to test this hypothesis. First, the linearity of the induced temperature rise with the acoustic power was tested. Next, the temperature maps acquired during preliminary low power sonications were scaled up until the estimated size of the tissue damage was equal to the tissue damage size of subsequent high power sonications. A threshold thermal dose was used to estimate the onset of thermal damage. The predicted power (based on amount of scaling required to reach the target size) was then compared to the true high power value. Overall, the temperature rise varied linearly with power (slope of deltaThigh/deltaTlow vs Power(high)/Power(low) = 0.97, 0.93 for pairs of sonications at each location in brain, muscle). The predicted power matched the true high power in the brain sonications (slope = 1.04). The predicted power underestimated the true high power in the muscle sonications (slope = 0.87). This under-prediction was due to a deviation from linearity in those cases where tissue damage was detected in subsequent MR images (slope of deltaThigh/deltaTlow vs Power(high)/Power(low) = 1.02, 0.84 for no tissue damage, tissue damage). The source of this deviation was not clear from these experiments. Even with this underestimation of the power, this method will be useful because it will allow an estimate of the proper power to use during FUS surgery without exact knowledge of the tissue parameters.
在本研究中，我们调查了利用磁共振成像衍生的热成像来确定聚焦超声（FUS）手术的暴露参数。由于FUS束诱导的温度升高与功率呈线性关系，因此在亚阈值超声处理期间获取的温度图可用于确定产生具有所需大小的热组织损伤所需的功率。分析了在体内兔大腿肌肉和脑组织的不同位置进行多次超声处理期间获取的热图像，以检验这一假设。首先，测试了诱导温度升高与声功率的线性关系。接下来，将在初步低功率超声处理期间获取的温度图按比例放大，直到估计的组织损伤大小等于后续高功率超声处理的组织损伤大小。使用阈值热剂量来估计热损伤的起始点。然后将预测功率（基于达到目标大小所需的缩放量）与实际高功率值进行比较。总体而言，温度升高与功率呈线性变化（大脑、肌肉中每个位置的超声处理对的deltaThigh/deltaTlow与Power(high)/Power(low)的斜率分别为0.97、0.93）。预测功率与大脑超声处理中的实际高功率相匹配（斜率 = 1.04）。预测功率在肌肉超声处理中低估了实际高功率（斜率 = 0.87）。这种预测不足是由于在后续磁共振图像中检测到组织损伤的那些情况下偏离了线性关系（对于无组织损伤、有组织损伤的情况，deltaThigh/deltaTlow与Power(high)/Power(low)的斜率分别为1.02、0.84）。从这些实验中尚不清楚这种偏差的来源。即使存在这种功率低估情况，该方法仍将是有用的，因为它将允许在不精确了解组织参数的情况下估计FUS手术期间使用的合适功率。
