神经动态超声弹性成像的三维有限元分析

Three-dimensional finite element analysis of dynamic ultrasound elastography of human nerves

目的探讨神经周围复杂力学环境对动态超声弹性成像定量表征神经剪切模量的影响。方法针对不同直径的神经,通过建立三维有限元模型分析神经周围力学环境与沿神经轴向传播的弹性波群速度的关系。结果神经截面半径为1.0 mm时,轴向导波群速度较基于神经材料用经典体波理论给出的剪切波波速约减低20%;而当神经的截面半径为1.5 mm和2.0 mm时,根据上述波速分析方法得到的轴向导波群速度趋近于神经材料沿纤维方向的剪切波波速。神经的截面半径为2.0 mm时,当肌腱的轴向剪切模量μL T在较大范围内变化时,其对神经内轴向导波群速度影响较小,并且导波群速度趋近于体波理论给出的神经材料沿神经纤维方向剪切波的理论波速;而其他周边软组织的剪切模量μ为2 kPa时,神经内轴向导波群速度较体波理论预测的神经材料沿纤维方向的剪切波速度约提高17%;当剪切模量μ为5.88 kPa和10 kPa时导波群速度和剪切波波速接近。结论对平均直径3 mm以上的神经,且神经横截面内弹性波速度不远大于周围组织内弹性波速度的前提下,采用各向异性介质中的体波理论可以较为准确地反演沿神经轴向的剪切模量;否则采用仪器植入的经典公式得出的剪切模量误差可达到40%。

Objective To investigates the effect of surrounding soft tissues on the mechanical characterization of human nerves by dynamic ultrasound elastography.Methods A three-dimensional finite element model was built to explore the correlation between the group velocities of elastic waves along the nerve fibers and the mechanical properties of surrounding soft tissues for nerves with different diameters.Results When the radius of the nerve was 1.0 mm,the axially guided wave group velocity was about 20%lower than the shear wave velocity based on the classical body wave theory of neural materials;when the radius of the nerve was 1.5 mm and 2.0 mm,the axial guided wave group velocity obtained by the above wave velocity analysis method was close to the shear wave velocity of the nerve along the fiber direction.When the cross-sectional radius of the nerve was 2.0 mm,if the axial shear modulus of the tendonμL T varied in a large range,it had little effect on the axial guided wave group velocity of the nerve,and the guided wave group velocity approached the shear wave velocity of the nerve material along the fiber direction;when the shear modulus of other peripheral soft tissuesμwas 2 kPa,the axial guided wave group velocity in the nerve was about 17%higher than the shear wave velocity of the nerve material along the fiber direction predicted by the body wave theory;when the shear modulusμwas 5.88 kPa and 10 kPa,the guided wave group velocity was close to the shear wave velocity.Conclusions Our results show that the body wave theory for the shear wave propagating in an transversely isotropic solid can be used to infer the shear modulus along axial direction of a nerve when the diameter of a nerve d is greater than 3 mm and the wave velocities in the cross-section of a nerve are not much greater than those in surrounding soft tissues;otherwise,the error in the identified shear modulus of a nerve with the method involved in the common-used instrument can be up to 40%.