Jacobs E M, Thijssen J M
Biophysics Laboratory of the Institute of Ophthalmology, University Hospital, Hb Nijmegen, The Netherlands.
Ultrason Imaging. 1991 Oct;13(4):316-33. doi: 10.1177/016173469101300402.
Realistic simulations of echographic image formation were performed. The simulations were based on a focussed single element transducer of 3.5 MHz and linear scanning. The tissue model scanned was composed of a homogeneous, nonattenuating, medium containing point-like scatterers. The scatterers were distributed in space in two different arrangements: randomly with a fixed number density of 7500 per cubic centimeter and regularly in a cubic matrix with a characteristic distance of 1 mm. These two populations of scatterers yielded the diffuse and the structural scattering component, respectively. The B-mode texture was assessed by first and second order statistical parameters. The effects of various modifications of the matrix, i.e., the relative scattering strength, the orientation and the position uncertainty of the scatterers, on the statistical parameters were systematically investigated. Increasing the relative scattering strength yields a monotonic increase of the mean grey level, the ratio of structural over diffuse scattering intensity and a decrease of the signal-to-noise ratio and the autocorrelation lengths. Increasing the position uncertainty to 20 percent reduces these effects practically to zero, except for the lateral ACF's. Rotation of the matrix with respect to the beam axis induces a periodicity in all statistical parameters, which is symmetric around 45 degrees. Again, the effects become statistically insignificant at a position uncertainty of 20 percent, with the exception of length of the lateral ACF. While assuming that in a single clinical B-mode echogram of the liver many orientations of the matrix will be simultaneously present, the B-mode lines of images obtained for a range of orientations were taken together and the overall statistical parameters calculated. The mean, the signal-to-noise ratio of the echo amplitude, SNRA, and the axial and lateral lengths of the ACF for the multirotation condition are significantly different from both the diffuse and the zero degree orientation. The structural backscattering intensity cannot be estimated any more with a reasonable precision. The structural separation distance, however, can still be assessed at a 10 percent level of the position uncertainty, even in the multirotation case and is therefore a robust clinical parameter.
进行了超声图像形成的逼真模拟。模拟基于一个3.5MHz的聚焦单元素换能器和线性扫描。扫描的组织模型由一种均匀、无衰减的介质组成，其中包含点状散射体。散射体在空间中以两种不同的排列方式分布：一种是随机分布，固定数密度为每立方厘米7500个；另一种是规则地分布在边长为1mm的立方矩阵中。这两种散射体群体分别产生了漫散射和结构散射分量。通过一阶和二阶统计参数评估B模式纹理。系统地研究了矩阵的各种变化，即散射体的相对散射强度、取向和位置不确定性对统计参数的影响。增加相对散射强度会导致平均灰度级单调增加、结构散射强度与漫散射强度之比增加，以及信噪比和自相关长度降低。将位置不确定性增加到20%实际上会将这些影响降低到零，但横向自相关函数除外。矩阵相对于波束轴的旋转会在所有统计参数中引起周期性，该周期性在45度左右对称。同样，在位置不确定性为20%时，这些影响在统计上变得不显著，但横向自相关函数的长度除外。虽然假设在肝脏的单个临床B模式超声图中会同时存在矩阵的许多取向，但将一系列取向获得的图像的B模式线合并在一起，并计算总体统计参数。多旋转条件下的平均灰度级、回波幅度的信噪比SNRA以及自相关函数的轴向和横向长度与漫散射和零度取向均有显著差异。结构后向散射强度不再能以合理的精度估计。然而，即使在多旋转情况下，结构分离距离仍可在位置不确定性的10%水平上进行评估，因此是一个可靠的临床参数。
