Effects of nonlinear ultrasound propagation on high intensity brain therapy.
Effects of nonlinear ultrasound propagation on high intensity brain therapy.
复制标题
非线性超声传播对高强度脑治疗的影响。
DOI：
10.1118/1.3531553
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发表时间：
2011
期刊：
影响因子：
3.8
通讯作者：
M. Tanter
中科院分区：
文献类型：
--
作者：
G. Pinton;J. Aubry;M. Fink;M. Tanter
目的
当超声波非线性传播时，能量会转移到更高的频率，并在更高的频率上衰减得更厉害。与软组织相比，头骨具有强烈的异质材料参数。作者通过实验表征并建立了一种数值方法，可以描述颅骨对超声波传播非线性分量的影响，以应用于大脑中的高强度聚焦超声 (HIFU) 治疗。通过将声学模拟与脑组织加热模型耦合，对非线性声学传播对热沉积和热剂量传递的影响进行量化，并与线性假设进行比较。
方法
将脱气干燥的人类头骨放入水箱中，并使用专为 HIFU 治疗设计的定制阵列中的 7 毫米换能器在 1 MPa 下进行声处理。在通过头骨传播之前和之后用校准的水听器进行二维扫描。来自头骨之前的扫描数据被用作三维有限差分时域 (FDTD) 模拟的输入，该模拟通过弛豫机制和二阶非线性计算衍射、密度、衰减的影响以及对频率的线性依赖。头骨的测量表示用于确定头骨的声学特性。经过验证的声学模型用于确定非线性传播造成的损失，然后与距离头骨表面 3 厘米和 7.5 厘米处的两个焦点配置的生物热方程的有限差分模拟相结合。
结果
在通过头骨传播之前，二次谐波分量比基波低 19 dB，三次谐波分量比基波低 37 dB。沿着头骨，二次谐波分量降低了 35 dB，三次谐波分量降低了 55 dB。在所考虑的频率范围内，仿真与测量结果的一致性在 0.5 dB 以内，并且在二维扫描上显示出良好的一致性。然后表明，假设非线性声学时，经过治疗的大脑体积至少是原来的两倍。
结论
作者建立了三维 FDTD 模拟，可以准确模拟通过头骨传播的非线性和衰减的影响。实验验证表明在广泛的频率范围和空间范围内具有良好的一致性。焦点处的非线性热剂量比线性热剂量大一个数量级，并且坏死体积至少大 df 2 倍。这些结果对于治疗计划有特殊的应用。
PURPOSE
As an ultrasound wave propagates nonlinearly, energy is transferred to higher frequencies where it is more strongly attenuated. Compared to soft tissue, the skull has strongly heterogeneous material parameters. The authors characterize with experiments and establish a numerical method that can describe the effects of the skull on the nonlinear components of ultrasonic wave propagation for application to high intensity focused ultrasound (HIFU) therapy in the brain. The impact of nonlinear acoustic propagation on heat deposition and thermal dose delivery is quantified and compared to linear assumptions by coupling an acoustic simulation with a heating model for brain tissue.
METHODS
A degassed dessicated human skull was placed in a water tank and insonified at 1 MPa with 7 mm transducer from a custom array designed for HIFU treatment. Two dimensional scans were performed preceding and following propagation through the skull with a calibrated hydrophone. Data from the scan preceding the skull were used as an input to a three dimensional finite difference time domain (FDTD) simulation that calculates the effects of diffraction, density, attenuation with linear dependence on frequency via relaxation mechanisms, and second order nonlinearity. A measured representation of the skull was used to determine the skull's acoustic properties. The validated acoustic model was used to determine the loss due to nonlinear propagation and then coupled to a finite difference simulation of the bioheat equation for two focal configurations at 3 and 7.5 cm from the skull surface.
RESULTS
Prior to propagation through the skull, the second harmonic component was 19 dB lower than the fundamental, and the third harmonic component was 37 dB lower. Following the skull, the second harmonic component was 35 dB lower and the third harmonic was 55 dB lower. The simulation is in agreement with the measurements to within 0.5 dB across the considered frequency range and shows good agreement across the two dimensional scan. It is then shown that the volume of treated brain is at least twice as large when assuming nonlinear acoustics.
CONCLUSIONS
The authors have established a three dimensional FDTD simulation that accurately models the effects of nonlinearity and attenuation for propagation through the skull. Experimental validation shows good agreement across a broad frequency range and spatial extent. The nonlinear thermal dose was over an order of magnitude larger at the focus than the linear thermal dose and the necrotic volume was larger by at least a factor df 2. These results have particular applications to treatment planning.
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