Department of Radiology, Perelman School of Medicine at the University of Pennsylvania, Philadelphia, PA, 19104-4206, USA.
Med Phys. 2019 Feb;46(2):505-516. doi: 10.1002/mp.13312. Epub 2019 Jan 11.
One limitation of experimental techniques for quantifying resolution and noise in detectors is that the measurement is made in a region-of-interest (ROI). With theoretical modeling, these properties can be measured at a point, allowing for quantification of spatial anisotropy. This paper calculates nonstationary transfer functions for amorphous selenium (a-Se) detectors in breast imaging. We use this model to demonstrate the performance advantage of a "next-generation" tomosynthesis (NGT) system, which is capable of x-ray source motion with more degrees of freedom than a clinical tomosynthesis system.
Using Swank's formulation, the optical transfer function (OTF) and presampled noise power spectra (NPS) are determined based on the point spread function derived in Part 1. The modulation transfer function (MTF) is found from the normalized modulus of the OTF. To take into account the presence of digitization, the presampled NPS is convolved with a two-dimensional comb function, for which the period along each direction is the reciprocal of the detector element size. The detective quantum efficiency (DQE) is then determined from combined knowledge of the OTF and NPS.
First, the model is used to demonstrate the loss of image quality due to oblique x-ray incidence. The MTF is calculated along various polar angles, corresponding to different orientations of the input frequency. The MTF is independent of the incidence angle if the polar angle is perpendicular to the ray incidence direction. However, along other polar angles, oblique incidence results in MTF degradation at high frequencies. The MTF degradation is most substantial along the ray incidence direction. Unlike the MTF, the normalized NPS (NNPS) is independent of the incidence angle. To measure the relative signal-to-noise, the DQE is also calculated. Oblique incidence yields high-frequency DQE degradation, which is more pronounced than the MTF degradation. This arises because the DQE is proportionate with the square of the MTF. Ultimately, this model is used to evaluate how the image quality varies over the detector area. For various projection images, we calculate the variation in the incidence angle over this area. With the NGT system, the source can be positioned in such a way that this variation is minimized, and hence the DQE exhibits less anisotropy. To achieve this improvement in the image quality, the source needs to have a component of motion in the posteroanterior (PA) direction, which is perpendicular to the conventional direction of source motion in tomosynthesis.
In a-Se detectors, the DQE at high frequencies is degraded due to oblique incidence. The DQE degradation is more pronounced than the MTF degradation. This model is used to quantify the spatial variation in DQE over the detector area. The use of PA source motion is a strategy for minimizing this variation and thus improving the image quality.
定量检测探测器分辨率和噪声的实验技术有一个局限性，即只能在感兴趣区域（ROI）进行测量。而通过理论建模，可以在一个点上测量这些特性，从而量化空间各向异性。本文计算了用于乳房成像的非晶硒（a-Se）探测器的非平稳传递函数。我们使用该模型演示了一种“下一代”断层合成（NGT）系统的性能优势，该系统的 X 射线源运动具有比临床断层合成系统更多的自由度。
使用 Swank 的公式，基于第 1 部分推导的点扩散函数，确定光学传递函数（OTF）和预采样噪声功率谱（NPS）。调制传递函数（MTF）是从归一化 OTF 的模中找到的。为了考虑数字化的存在，将预采样 NPS 与二维梳状函数卷积，每个方向的周期是探测器元件尺寸的倒数。然后，结合 OTF 和 NPS 的知识确定检测量子效率（DQE）。
首先，该模型用于演示由于斜入射 X 射线而导致的图像质量损失。沿着各种极角计算 MTF，对应于输入频率的不同方向。如果极角垂直于射线入射方向，则 MTF 与入射角无关。然而，在其他极角下，斜入射会导致高频 MTF 下降。沿射线入射方向，MTF 下降最大。与 MTF 不同，归一化 NPS（NNPS）与入射角无关。为了测量相对信噪比，还计算了 DQE。斜入射会导致高频 DQE 下降，比 MTF 下降更为明显。这是因为 DQE 与 MTF 的平方成正比。最终，该模型用于评估探测器区域内的图像质量如何变化。对于各种投影图像，我们计算了该区域内入射角的变化。在 NGT 系统中，可以将源定位在这样一种方式，使这种变化最小化，从而使 DQE 表现出较小的各向异性。为了提高图像质量，需要使源具有在前后（PA）方向的运动分量，这与断层合成中源运动的常规方向垂直。
在 a-Se 探测器中，由于斜入射，高频 DQE 会下降。与 MTF 下降相比，DQE 下降更为明显。该模型用于量化探测器区域内 DQE 的空间变化。使用 PA 源运动是最小化这种变化从而改善图像质量的一种策略。
