Moy J P
Trixell, Zone Industrielle Centr'Alp, Moirans, France.
Med Phys. 2000 Jan;27(1):86-93. doi: 10.1118/1.598859.
In most imaging detectors, the modulation transfer function (MTF) is regarded as a good parameter to describe spatial resolution. This is undoubtedly valid for visual observation. However, the detectability of a detail is essentially a matter of signal-to-noise ratio, which is not accounted for by the MTF. In x-ray imaging, signal-to-noise ratio in the image is generally limited by incident photons statistics, often larger than readout noises. Therefore, the MTF of the detector applies to both signal and noise, and does not impair the image content. Contrast can easily be restored by image processing without altering the signal-to-noise ratio. However, a number of effects may alter very differently noise and signal: (i) If the MTF significantly extends beyond half the sampling frequency, the aliasing introduced by spatial sampling can severely enhance the noise and cancel the benefit of the good signal transfer. This is illustrated by synthetic images which simulate the response of imagers with different MTFs to the same test pattern in the presence of quantum noise. (ii) Parallax and blurring by the x-ray spot size or motion are shown to degrade the transfer properties of signal, but do not affect the quantum noise; they must be treated separately. Contrary to the x-ray converter MTF, parallax directly impacts the detective quantum efficiency (DQE). Finally, it is shown that only the detective quantum efficiency can reliably describe the spatial resolution of an x-ray imaging detector in the presence of noise, parallax and blurring.
在大多数成像探测器中，调制传递函数（MTF）被视为描述空间分辨率的一个良好参数。这对于视觉观察无疑是有效的。然而，细节的可探测性本质上是一个信噪比的问题，而MTF并未考虑这一点。在X射线成像中，图像中的信噪比通常受入射光子统计的限制，往往大于读出噪声。因此，探测器的MTF对信号和噪声都适用，并且不会损害图像内容。通过图像处理可以很容易地恢复对比度，而不会改变信噪比。然而，一些效应可能会对噪声和信号产生非常不同的影响：（i）如果MTF显著延伸超过采样频率的一半，空间采样引入的混叠会严重增强噪声并抵消良好信号传递的益处。这通过合成图像得到了说明，这些合成图像模拟了在存在量子噪声的情况下具有不同MTF的成像器对相同测试图案的响应。（ii）X射线光斑尺寸或运动引起的视差和模糊会降低信号的传递特性，但不会影响量子噪声；必须对它们进行单独处理。与X射线转换器的MTF相反，视差直接影响探测量子效率（DQE）。最后，结果表明，在存在噪声、视差和模糊的情况下，只有探测量子效率能够可靠地描述X射线成像探测器的空间分辨率。
