Department of Radiology, Stanford University, Stanford, CA, USA.
Med Phys. 2011 Sep;38(9):5081-9. doi: 10.1118/1.3622610.
High intensity focused ultrasound (HIFU) in the abdomen can be sensitive to acoustic aberrations that can exist in the beam path of a single sonication. Having an accurate method to quickly visualize the transducer focus without damaging tissue could assist with executing the treatment plan accurately and predicting these changes and obstacles. By identifying these obstacles, MR acoustic radiation force imaging (MR-ARFI) provides a reliable method for visualizing the transducer focus quickly without damaging tissue and allows accurate execution of the treatment plan.
MR-ARFI was used to view the HIFU focus, using a gated spin echo flyback readout-segmented echo-planar imaging sequence. HIFU spots in a phantom and in the livers of five live pigs under general anesthesia were created with a 550 kHz extracorporeal phased array transducer initially localized with a phase-dithered MR-tracking sequence to locate microcoils embedded in the transducer. MR-ARFI spots were visualized, observing the change of focal displacement and ease of steering. Finally, MR-ARFI was implemented as the principle liver HIFU calibration system, and MR-ARFI measurements of the focal location relative to the thermal ablation location in breath-hold and breathing experiments were performed.
Measuring focal displacement with MR-ARFI was achieved in the phantom and in vivo liver. In one in vivo experiment, where MR-ARFI images were acquired repeatedly at the same location with different powers, the displacement had a linear relationship with power [y = 0.04x + 0.83 μm (R(2) = 0.96)]. In another experiment, the displacement images depicted the electronic steering of the focus inside the liver. With the new calibration system, the target focal location before thermal ablation was successfully verified. The entire calibration protocol delivered 20.2 J of energy to the animal (compared to greater than 800 J for a test thermal ablation). ARFI displacement maps were compared with thermal ablations during seven breath-hold ablations. The error was 0.83 ± 0.38 mm in the S/I direction and 0.99 ± 0.45 mm in the L/R direction. For six spots in breathing ablations, the mean error in the nonrespiration direction was 1.02 ± 0.89 mm.
MR-ARFI has the potential to improve free-breathing plan execution accuracy compared to current calibration and acoustic beam adjustment practices. Gating the acquisition allows for visualization of the focal spot over the course of respiratory motion, while also being insensitive to motion effects that can complicate a thermal test spot. That MR-ARFI measures a mechanical property at the focus also makes it insensitive to high perfusion, of particular importance to highly perfused organs such as the liver.
腹部高强度聚焦超声（HIFU）对可能存在于单次超声束路径中的声像差较为敏感。如果有一种准确的方法可以在不损伤组织的情况下快速可视化换能器焦点，那么这将有助于准确执行治疗计划，并预测这些变化和障碍。通过识别这些障碍，磁共振声辐射力成像（MR-ARFI）为快速可视化换能器焦点提供了一种可靠的方法，而无需损伤组织，并允许准确执行治疗计划。
使用门控自旋回波折返读出分段回波平面成像序列，利用 MR-ARFI 观察 HIFU 焦点。在一个体模和五只全麻下的活猪肝脏中创建 HIFU 点，使用初始位置由相移 MR 跟踪序列定位的 550 kHz 体外相控阵换能器，以定位嵌入换能器中的微线圈。观察焦点位移和转向的难易程度，从而可视化 MR-ARFI 点。最后，将 MR-ARFI 用作肝脏 HIFU 校准的主要系统，并在呼吸暂停和呼吸实验中进行了相对于热消融位置的焦点位置的 MR-ARFI 测量。
在体模和体内肝脏中都实现了用 MR-ARFI 测量焦点位移。在一个重复在同一位置使用不同功率获取 MR-ARFI 图像的体内实验中，位移与功率呈线性关系[y=0.04x+0.83 μm（R²=0.96）]。在另一个实验中，位移图像描绘了肝脏内焦点的电子转向。使用新的校准系统，成功验证了热消融前的目标焦点位置。整个校准协议向动物输送了 20.2 J 的能量（相比之下，用于测试热消融的能量大于 800 J）。在 7 次呼吸暂停消融过程中，将 ARFI 位移图与热消融进行了比较。在 S/I 方向的误差为 0.83±0.38 mm，在 L/R 方向的误差为 0.99±0.45 mm。对于呼吸消融中的 6 个点，在非呼吸方向的平均误差为 1.02±0.89 mm。
与当前的校准和声束调整实践相比，MR-ARFI 有可能提高自由呼吸计划执行的准确性。采集门控允许在呼吸运动过程中对焦点光斑进行可视化，同时对可能使热测试光斑复杂化的运动效应不敏感。MR-ARFI 测量焦点处的机械特性也使其对高灌注不敏感，这对肝脏等灌注较高的器官尤为重要。
