Sokka S D, King R, Hynynen K
Brigham and Women's Hospital, Harvard Medical School, 75 Francis Street, Boston, MA 02115, USA.
Phys Med Biol. 2003 Jan 21;48(2):223-41. doi: 10.1088/0031-9155/48/2/306.
In this study, we propose a focused ultrasound surgery protocol that induces and then uses gas bubbles at the focus to enhance the ultrasound absorption and ultimately create larger lesions in vivo. MRI and ultrasound visualization and monitoring methods for this heating method are also investigated. Larger lesions created with a carefully monitored single ultrasound exposure could greatly improve the speed of tumour coagulation with focused ultrasound. All experiments were performed under MRI (clinical, 1.5 T) guidance with one of two eight-sector, spherically curved piezoelectric transducers. The transducer, either a 1.1 or 1.7 MHz array, was driven by a multi-channel RF driving system. The transducer was mounted in an MRI-compatible manual positioning system and the rabbit was situated on top of the system. An ultrasound detector ring was fixed with the therapy transducer to monitor gas bubble activity during treatment. Focused ultrasound surgery exposures were delivered to the thighs of seven New Zealand while rabbits. The experimental, gas-bubble-enhanced heating exposures consisted of a high amplitude 300 acoustic watt, half second pulse followed by a 7 W, 14 W or 21 W continuous wave exposure for 19.5 s. The respective control sonications were 20 s exposures of 14 W, 21 W and 28 W. During the exposures, MR thermometry was obtained from the temperature dependency of the proton resonance frequency shift. MRT2-enhanced imaging was used to evaluate the resulting lesions. Specific metrics were used to evaluate the differences between the gas-bubble-enhanced exposures and their respective control sonications: temperatures with respect to time and space, lesion size and shape, and their agreement with thermal dose predictions. The bubble-enhanced exposures showed a faster temperature rise within the first 4 s and higher overall temperatures than the sonications without bubble formation. The spatial temperature maps and the thermal dose maps derived from the MRI thermometry closely correlated with the resulting lesion as examined by T2-weighted imaging. The lesions created with the gas-bubble-enhanced heating exposures were 2-3 times larger by volume, consistently more spherical in shape and closer to the transducer than the control exposures. The study demonstrates that gas bubbles can reliably be used to create significantly larger lesions in vivo. MRI thermometry techniques were successfully used to monitor the thermal effects mediated by the bubble-enhanced exposures.
在本研究中，我们提出了一种聚焦超声手术方案，该方案可在焦点处诱导并利用气泡来增强超声吸收，并最终在体内产生更大的损伤。我们还研究了用于这种加热方法的磁共振成像（MRI）和超声可视化及监测方法。通过精心监测单次超声照射产生更大的损伤，可极大地提高聚焦超声凝固肿瘤的速度。所有实验均在MRI（临床用，1.5T）引导下，使用两个八扇区球形弯曲压电换能器之一进行。该换能器，频率为1.1或1.7MHz的阵列，由多通道射频驱动系统驱动。换能器安装在与MRI兼容的手动定位系统中，兔子置于该系统上方。一个超声探测器环与治疗换能器固定在一起，以监测治疗期间的气泡活动。聚焦超声手术照射作用于7只新西兰白兔的大腿。实验性的、气泡增强加热照射包括一个300声瓦的高振幅半秒脉冲，随后是19.5秒的7瓦、14瓦或21瓦连续波照射。相应的对照超声处理为14瓦、21瓦和28瓦的20秒照射。在照射期间，通过质子共振频率偏移的温度依赖性获得MR温度测量值。利用MRT2增强成像来评估产生的损伤。使用特定指标来评估气泡增强照射与其各自对照超声处理之间的差异：温度随时间和空间的变化、损伤大小和形状，以及它们与热剂量预测的一致性。与未形成气泡的超声处理相比，气泡增强照射在最初4秒内温度上升更快，总体温度更高。由MRI温度测量得出的空间温度图和热剂量图与通过T2加权成像检查的产生的损伤密切相关。与对照照射相比，气泡增强加热照射产生的损伤体积大2至3倍，形状始终更接近球形且更靠近换能器。该研究表明，气泡可可靠地用于在体内产生明显更大的损伤。MR温度测量技术成功用于监测气泡增强照射介导的热效应。
