Sorbonne Université, CNRS UMR 7371, INSERM UMR S 1146, Laboratoire d'Imagerie Biomédicale, Paris, France.
Phys Med Biol. 2020 Nov 27;65(23):235016. doi: 10.1088/1361-6560/abb92c.
This work investigates the feasibility of estimating the parameters of an exact transverse isotropy model in cortical bone. The model describes the anisotropy of the velocity of compressional and shear bulk elastic waves. We propose to achieve this with ultrasound imaging relying on the transmission of unfocused beams and with an autofocus method. The latter is based on the principle that the reconstructed ultrasound image shows optimal quality if the velocity model is correct. The autofocus approach is applied to a composite image of the interface between cortical bone and marrow. It is obtained by incoherent summation of four types of images exploiting four different ray paths in the cortical bone layer, three of them involving mode-converted shear waves. If the parameters of the model are correct, spatial co-localization of the interface appears in the four images. As a result, intensity and sharpness in the composite image are maximal. The five parameters of the model of transverse isotropy are successfully estimated in a tube made of a bone-mimicking material. The estimates are in good agreement with resonant ultrasound spectroscopy (RUS) measurements. The tube thickness is recovered with an error smaller than 0.3%. In vivo results at the forearm of a volunteer are promising, four parameters could be estimated and are in good agreement with ex vivo RUS measurements. Moreover x-ray peripheral computed tomography corroborates the thickness of the cortical bone layer in the ultrasound image. Weak-anisotropy and exact transverse isotropy models provide very close measurements of the thickness of the tube and the radius bone. Thus, we recommend using the model of weak transverse isotropy for real-time anatomical imaging because more computationally efficient. For material characterization however, the model of exact transverse isotropy is preferred because the elastic anisotropy of cortical bone is moderate, rather than weak.
这项工作研究了在皮质骨中估计精确横观各向同性模型参数的可行性。该模型描述了压缩和剪切体弹性波速度的各向异性。我们建议通过依赖于非聚焦波束的超声成像并使用自动聚焦方法来实现这一目标。后者基于这样的原理，即如果速度模型正确，则重建的超声图像具有最佳质量。自动聚焦方法应用于皮质骨和骨髓之间界面的复合图像。它通过在皮质骨层中利用四种不同的射线路径的四种类型的图像的非相干求和来获得，其中三种涉及模式转换剪切波。如果模型的参数正确，则接口的空间共定位出现在四个图像中。因此，复合图像中的强度和锐度最大。成功地在由仿生材料制成的管中估计了横观各向同性模型的五个参数。这些估计值与共振超声光谱（RUS）测量值非常吻合。管的厚度的恢复误差小于 0.3%。在志愿者前臂的体内结果很有前景，可以估计出四个参数，并且与离体 RUS 测量值吻合良好。此外，X 射线外周计算机断层扫描证实了超声图像中皮质骨层的厚度。弱各向异性和精确横观各向同性模型对管的厚度和骨半径的测量非常接近。因此，我们建议在实时解剖成像中使用弱横观各向同性模型，因为它的计算效率更高。然而，对于材料特性化，优选使用精确横观各向同性模型，因为皮质骨的弹性各向异性是适度的，而不是弱的。
