Liu Chenhui, Zhou Yufeng
School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore.
Med Phys. 2018 Jul 2. doi: 10.1002/mp.13075.
High-intensity focused ultrasound (HIFU) is becoming an effective and noninvasive treatment modality for cancer and solid tumors. In order to avoid the cancer relapse and guarantee the success of ablation, there should be no gaps left among all HIFU-generated lesions. However, there are few imaging approaches available for detecting the HIFU lesion gaps in real time during ablation.
Transient axial shear strain elastograms (ASSEs) were proposed and evaluated both numerically and experimentally to detect the lesion gaps immediately after the cessation of therapeutic HIFU exposure. Acoustic intensity and subsequent acoustic radiation force were first calculated by solving the nonlinear Khokhlov-Zabolotskaya-Kuznetzov (KZK) equation. Motion of being- and already-treated lesions during and after HIFU exposure was simulated using the transient dynamic analysis module of finite element method (FEM). The corresponding B-mode sonography of tissue-mimicking phantom with two HIFU lesions inside was simulated by FIELD II, and then axial strain elastograms (ASEs) under static compression and transient ASSEs were reconstructed. An ultrasound imaging probe was integrated with the HIFU transducer and used to obtain radio frequency (RF) echo signals at high frame rate using plane wave imaging (PWI). The resulting strains were mapped using the correlation-based method and block search strategy.
Acoustic radiation force from the therapeutic HIFU burst is sufficiently strong to produce significant displacement. As a result, large and highly localized axial shear strain appears in the gap zone between two HIFU-generated lesions and then disappears after sufficient HIFU ablation (no gap between them). Such capability of detecting the lesion gap is validated at the varied acoustic radiation force density, gap width, and the size of the lesion. In contrast, conventional ASEs using the static compression cannot distinguish whether a gap exists between lesions. Static ASEs and transient ASSEs reconstructed using both high-speed photography and sonography in the gel phantom show the same conclusion as that in the simulation. Ex vivo tissue experiments further confirmed that the presence of large axial shear strain in the gap zone. The ratios of axial shear strain in the porcine kidney and liver samples had statistical differences for two HIFU-generated lesions without and with a gap (P < 0.05).
Large axial shear strain induced by the acoustic radiation force from therapeutic HIFU burst only appears between two HIFU-generated lesions with a gap between them. Transient ASSEs reconstructed immediately after the cession of HIFU exposure can easily, reliably, and sensitively detect the gap between produced lesions, which would provide real-time feedback to enhance the success of HIFU ablation.
高强度聚焦超声（HIFU）正成为一种治疗癌症和实体瘤的有效且无创的治疗方式。为避免癌症复发并确保消融成功，HIFU产生的所有病灶之间不应留有间隙。然而，在消融过程中实时检测HIFU病灶间隙的成像方法很少。
提出了瞬态轴向剪切应变弹性图（ASSE），并通过数值模拟和实验进行评估，以在治疗性HIFU暴露停止后立即检测病灶间隙。首先通过求解非线性Khokhlov-Zabolotskaya-Kuznetzov（KZK）方程来计算声强和随后的声辐射力。使用有限元方法（FEM）的瞬态动力学分析模块模拟HIFU暴露期间及之后已治疗和正在治疗的病灶的运动。通过FIELD II模拟了内部有两个HIFU病灶的组织仿体的相应B模式超声成像，然后重建了静态压缩下的轴向应变弹性图（ASE）和瞬态ASSE。将超声成像探头与HIFU换能器集成在一起，并使用平面波成像（PWI）以高帧率获取射频（RF）回波信号。使用基于相关性的方法和块搜索策略对所得应变进行映射。
治疗性HIFU脉冲产生的声辐射力足够强，足以产生明显的位移。结果，在两个HIFU产生的病灶之间的间隙区域出现大且高度局部化的轴向剪切应变，并且在充分的HIFU消融后（它们之间没有间隙）消失。在不同的声辐射力密度、间隙宽度和病灶大小下，这种检测病灶间隙的能力得到了验证。相比之下，使用静态压缩的传统ASE无法区分病灶之间是否存在间隙。在凝胶仿体中使用高速摄影和超声成像重建的静态ASE和瞬态ASSE显示出与模拟相同的结论。离体组织实验进一步证实了间隙区域中存在大的轴向剪切应变。对于两个有无间隙的HIFU产生的病灶，猪肾和肝样本中的轴向剪切应变比率具有统计学差异（P < 0.05）。
治疗性HIFU脉冲产生的声辐射力引起的大轴向剪切应变仅出现在两个之间有间隙的HIFU产生的病灶之间。HIFU暴露停止后立即重建的瞬态ASSE可以轻松、可靠且灵敏地检测所产生病灶之间的间隙，这将提供实时反馈以提高HIFU消融的成功率。
