School of Medicine, Department of Radiology, Stanford University, Stanford, California, USA.
Department of Electrical Engineering, Stanford University, Stanford, California, USA.
J Magn Reson Imaging. 2019 Nov;50(5):1526-1533. doi: 10.1002/jmri.26726. Epub 2019 Apr 5.
Thermal dosimetry during MR-guided focused ultrasound (MRgFUS) of bone tumors underpredicts ablation zone. Intraprocedural understanding of heat accumulation near bone is needed to prevent undesired treatment of nontargeted tissue.
Temperature decay rates predict prolonged, spatially varying heating during MRgFUS bone treatments.
Prospective case series.
Nine patients with localized painful bone tumors (five bone metastasis, four osteoid osteomas), were compared with five patients with uterine fibroid tumors treated using MRgFUS.
FIELD STRENGTH/SEQUENCE: Proton resonance frequency shift thermometry using 2D-GRE with echo-planar imaging at 3 T.
Tissue response was derived by fitting data from extended thermometry acquisitions to a decay model. Decay rates and time to peak temperature (TTP) were analyzed in segmented zones between the bone target and skin. Decay rates were used to calculate intersonication cooling times required to return to body temperature; these were compared against conventional system-mandated cooling times.
Kolmogorov-Smirnov tests for normality, and Student's t-test was used to compare decay rates. Spatial TTP delay and predicted cooling times used Wilcoxon signed rank tests. P < 0.05 was significant.
Tissue decay rates in bone tumor patients were 3.5 times slower than those in patients with fibroids (τ = 0.037 ± 0.012 vs. τ = 0.131 ± 0.010, P < 0.05). Spatial analysis showed slow decay rates effecting baseline temperature as far as 12 mm away from the bone surface, τ = 0.015 ± 0.026 (median ± interquartile range [IQR]). Tissue within 9 mm of bone experienced delayed TTP (P < 0.01). In the majority of bone tumor treatments, system-predicted intersonication cooling times were insufficient for nearby tissue to return to body temperature (P = 0.03 in zone 4).
MRgFUS near bone is susceptible to long tissue decay rates, and unwanted cumulative heating up to 1.2 cm from the surface of the bone. Knowledge of decay rates may be used to alter treatment planning and intraprocedural thermal monitoring protocols to account for prolonged heating by bone.
4 Technical Efficacy: Stage 4 J. Magn. Reson. Imaging 2019;50:1526-1533.
在 MR 引导聚焦超声（MRgFUS）治疗骨肿瘤过程中，热剂量测定会低估消融区域。为了防止对非靶向组织造成不必要的治疗，需要在术中了解骨附近的热量积累情况。
温度衰减率可预测 MRgFUS 骨治疗过程中长时间、空间变化的加热。
前瞻性病例系列研究。
9 名患有局限性疼痛骨肿瘤（5 例骨转移，4 例成骨细胞瘤）的患者与 5 名接受 MRgFUS 治疗的子宫肌瘤患者进行了比较。
磁场强度/序列：3T 时使用二维 GRE 进行质子共振频率偏移测温，采用回波平面成像。
通过将扩展测温采集的数据拟合到衰减模型中，得出组织反应。在骨靶区与皮肤之间的分段区域分析衰减率和达到峰值温度（TTP）的时间。衰减率用于计算返回体温所需的两次超声治疗之间的冷却时间；将这些冷却时间与常规系统要求的冷却时间进行比较。
使用 Kolmogorov-Smirnov 检验进行正态性检验，使用学生 t 检验比较衰减率。空间 TTP 延迟和预测的冷却时间使用 Wilcoxon 符号秩检验。P<0.05 为差异有统计学意义。
骨肿瘤患者的组织衰减率比子宫肌瘤患者慢 3.5 倍（τ=0.037±0.012 与 τ=0.131±0.010，P<0.05）。空间分析显示，衰减率缓慢，会影响距骨表面最远达 12mm 处的基础温度，τ=0.015±0.026（中位数±四分位距 [IQR]）。距骨 9mm 内的组织 TTP 延迟（P<0.01）。在大多数骨肿瘤治疗中，系统预测的两次超声治疗之间的冷却时间不足以使附近组织恢复到体温（在 4 区 P=0.03）。
MRgFUS 接近骨骼时容易出现较长的组织衰减率，以及从骨表面 1.2cm 范围内的不必要的累积加热。了解衰减率可以用于改变治疗计划和术中热监测方案，以应对骨骼引起的长时间加热。
4 级技术功效：第 4 阶段 J. Magn. Reson. Imaging 2019;50:1526-1533.
