Clement G T, Sun J, Hynynen K
Department of Radiology, Brigham and Women's Hospital, Harvard Medical School, Boston, MA 02115, USA.
Ultrasonics. 2001 Mar;39(2):109-13. doi: 10.1016/s0041-624x(00)00052-4.
Phase distortion due to reflection in transcranial ultrasound propagation is investigated. Understanding of these phase-dependent properties is motivated by efforts to construct a reliable prediction model for noninvasive ultrasound therapy in the brain. The present study measures the phase of an ultrasound wave after propagation through an ex vivo human skull and considers the dependence of this phase on reflections between the transducer and the skull surface in addition to reflections within the skull. Experiments are performed using a human calvarium fragment placed between an underwater ultrasonic transducer and a polyvinylidene difluoride hydrophone. Data are presented indicating the ultrasound phase dependence as a function of burst length and the distance of the transducer element from the skull at a driving frequency of 0.5 MHz. Experimental results are compared with predictions obtained from a propagation model which considers transmission at the skull interfaces as well as multiple reflections within the skull. It is concluded that by using short ultrasound bursts a distance may be indicated that beyond which the contributions of transducer reflections on the phase of the propagating wave may be neglected. Additionally, a comparison of the measurements with simulated data supports the contention that for reasonably small incident angles, reflection within the skull causes minimal phase shift.
研究了经颅超声传播中反射引起的相位畸变。构建可靠的脑部无创超声治疗预测模型的努力促使人们去了解这些与相位相关的特性。本研究测量了超声波在离体人类颅骨中传播后的相位，并考虑了该相位除了依赖于颅骨内的反射外，还依赖于换能器与颅骨表面之间的反射。实验使用放置在水下超声换能器和聚偏二氟乙烯水听器之间的人类颅骨碎片进行。给出的数据表明，在0.5 MHz的驱动频率下，超声相位依赖性是脉冲长度和换能器元件与颅骨距离的函数。将实验结果与从传播模型获得的预测结果进行比较，该传播模型考虑了颅骨界面处的传输以及颅骨内的多次反射。得出的结论是，通过使用短超声脉冲，可以指出一个距离，超过该距离后，换能器反射对传播波相位的贡献可以忽略不计。此外，测量结果与模拟数据的比较支持了这样的观点，即对于合理小的入射角，颅骨内的反射引起的相移最小。
