Dobbins J T
Department of Radiology, Duke University Medical Center, Durham, North Carolina 27710, USA.
Med Phys. 1995 Feb;22(2):171-81. doi: 10.1118/1.597600.
The proper understanding of modulation transfer function (MTF), noise power spectra (NPS), and noise equivalent quanta (NEQ) in digital systems is significantly hampered when the systems are undersampled. Undersampling leads to three significant complications: (1) MTF and NPS do not behave as transfer amplitude and variance, respectively, of a single sinusoid, (2) the response of a digital system to a delta function is not spatially invariant and therefore does not fulfill certain technical requirements of classical analysis, and (3) NEQ loses its common meaning as maximum available SNR2 (signal-to-noise) at a particular frequency. These three complications cause the comparisons of MTF and NEQ between undersampled digital systems to depend on the frequency content of the images being evaluated. A tutorial of MTF, NPS, and NEQ concepts for digital systems is presented, along with a complete theoretical treatment of the above-mentioned complications from undersampling.
当数字系统欠采样时，对调制传递函数（MTF）、噪声功率谱（NPS）和噪声等效量子（NEQ）的正确理解会受到严重阻碍。欠采样会导致三个重大问题：（1）MTF和NPS分别不像单个正弦波的传递幅度和方差那样表现，（2）数字系统对δ函数的响应在空间上不是不变的，因此不符合经典分析的某些技术要求，（3）NEQ失去了其作为特定频率下最大可用信噪比（SNR2，信号与噪声比）的通常意义。这三个问题导致欠采样数字系统之间MTF和NEQ的比较取决于所评估图像的频率内容。本文介绍了数字系统的MTF、NPS和NEQ概念教程，并对欠采样引起的上述问题进行了完整的理论处理。
