Thijssen J M, Oosterveld B J
Biophysics Laboratory, Institute of Ophthalmology, Academic Hospital, Nijmegen, The Netherlands.
J Ultrasound Med. 1990 Apr;9(4):215-29. doi: 10.7863/jum.1990.9.4.215.
Models of biological tissues are described in terms of acoustic parameters and of structure. Beam formation is discussed for continuous wave and pulsed modes of transducer operation and the concept of the point spread function (PSF) is introduced. The PSF is equivalent to the resolution cell, or the sampling volume, of echographic equipment. The generation of echograms from parenchymal tissues is described in terms of speckle formation due to interference at reception on the transducer. The speckle dimensions are quantitatively compared to the sampling volume of the employed transducer. It is shown that for fully developed speckle the tissue characteristics are exclusively reflected in the mean echolevel and not in the speckle size. The speckle size is, however, greatly dependent on the bandwidth, the frequency, and the geometry of the employed transducer. The attenuation by the insonated tissue yields a depth-dependent increase of mainly the lateral speckle size, in addition to the depth dependence caused by the beam formation. If the number density of scattering sites within the tissue is relatively low, the speckle characteristics are dependent on this density and, hence, tissue characterization is feasible if these characteristics are analyzed by statistical methods. These methods are gray level histogram analysis and the estimation of the autocorrelation function, ie, first and second order statistics, respectively. Structural order in tissues can be quantified by autocorrelation analysis and clinical studies on diffuse liver diseases support this conclusion. The effects of pre- and postprocessing on the detectability of focal lesions are outlined. The impact of multifocus systems and of the acquisition of radio frequency echograms on further developments of clinical echography is discussed.
生物组织模型是根据声学参数和结构来描述的。讨论了换能器在连续波和脉冲模式下的波束形成，并引入了点扩散函数（PSF）的概念。PSF等同于超声设备的分辨单元或采样体积。从实质组织生成超声图是根据换能器接收时由于干扰而形成的散斑来描述的。将散斑尺寸与所采用换能器的采样体积进行定量比较。结果表明，对于充分发展的散斑，组织特征仅反映在平均回声水平上，而不是散斑大小上。然而，散斑大小在很大程度上取决于所采用换能器的带宽、频率和几何形状。除了波束形成引起的深度依赖性外，被照射组织的衰减还会导致主要是横向散斑大小随深度增加。如果组织内散射位点的数量密度相对较低，散斑特征就取决于该密度，因此，如果通过统计方法分析这些特征，组织表征是可行的。这些方法分别是灰度直方图分析和自相关函数估计，即一阶和二阶统计量。组织中的结构顺序可以通过自相关分析来量化，关于弥漫性肝病的临床研究支持这一结论。概述了预处理和后处理对焦点病变可检测性的影响。讨论了多焦点系统以及射频超声图采集对临床超声进一步发展的影响。
