Department of Radiology, Brigham and Women's Hospital, Harvard Medical School, 221 Longwood Avenue, Boston, MA 02115, United States of America.
Phys Med Biol. 2018 Mar 15;63(6):065008. doi: 10.1088/1361-6560/aab0aa.
Previous work has demonstrated that passive acoustic imaging may be used alongside MRI for monitoring of focused ultrasound therapy. However, past implementations have generally made use of either linear arrays originally designed for diagnostic imaging or custom narrowband arrays specific to in-house therapeutic transducer designs, neither of which is fully compatible with clinical MR-guided focused ultrasound (MRgFUS) devices. Here we have designed an array which is suitable for use within an FDA-approved MR-guided transcranial focused ultrasound device, within the bore of a 3 Tesla clinical MRI scanner. The array is constructed from 5 × 0.4 mm piezoceramic disc elements arranged in pseudorandom fashion on a low-profile laser-cut acrylic frame designed to fit between the therapeutic elements of a 230 kHz InSightec ExAblate 4000 transducer. By exploiting thickness and radial resonance modes of the piezo discs the array is capable of both B-mode imaging at 5 MHz for skull localization, as well as passive reception at the second harmonic of the therapy array for detection of cavitation and 3D passive acoustic imaging. In active mode, the array was able to perform B-mode imaging of a human skull, showing the outer skull surface with good qualitative agreement with MR imaging. Extension to 3D showed the array was able to locate the skull within ±2 mm/2° of reference points derived from MRI, which could potentially allow registration of a patient to the therapy system without the expense of real-time MRI. In passive mode, the array was able to resolve a point source in 3D within a ±10 mm region about each axis from the focus, detect cavitation (SNR ~ 12 dB) at burst lengths from 10 cycles to continuous wave, and produce 3D acoustic maps in a flow phantom. Finally, the array was used to detect and map cavitation associated with microbubble activity in the brain in nonhuman primates.
先前的工作已经证明，被动声学成像是可以与 MRI 一起用于监测聚焦超声治疗的。然而，过去的实现方式通常使用的是原本用于诊断成像的线性阵列，或者是特定于内部治疗换能器设计的定制窄带阵列，这两种阵列都不完全与临床磁共振引导聚焦超声（MRgFUS）设备兼容。在这里，我们设计了一种适用于经美国食品和药物管理局批准的磁共振引导颅内置换焦点超声设备的阵列，可在 3T 临床磁共振成像扫描仪的磁体孔内使用。该阵列由 5×0.4mm 压电陶瓷圆盘元件组成，以随机方式排列在一个低轮廓的激光切割亚克力框架上，该框架设计用于贴合 230kHz InSightec ExAblate 4000 换能器的治疗元件之间。通过利用压电圆盘的厚度和径向共振模式，该阵列能够在 5MHz 下进行 B 模式成像，用于颅骨定位，以及在治疗阵列的二次谐波下进行被动接收，用于检测空化和 3D 被动声学成像。在主动模式下，该阵列能够对人类颅骨进行 B 模式成像，显示颅骨外表面，与磁共振成像具有良好的定性一致性。扩展到 3D 显示，该阵列能够在参考点来自 MRI 的±2mm/2°的范围内定位颅骨，这可能允许在不花费实时 MRI 的情况下将患者与治疗系统进行配准。在被动模式下，该阵列能够在距焦点每个轴±10mm 的区域内以 3D 分辨率分辨出一个点源，在 10 个周期到连续波的爆震长度范围内检测到空化（信噪比~12dB），并在流动体模中生成 3D 声学图。最后，该阵列用于在非人类灵长类动物的大脑中检测和绘制与微泡活动相关的空化相关图谱。
