Robarts Research Institute and Department of Medical Biophysics, Western University, London, Ontario, Canada, N6A 5B7.
Department of Computer Engineering, University of Waterloo, Waterloo, Ontario, Canada.
Med Phys. 2017 Sep;44(9):4525-4535. doi: 10.1002/mp.12420. Epub 2017 Aug 12.
Acquisition of high-quality x-ray images using low patient exposures requires detectors with high detective quantum efficiency (DQE). We describe a novel apodized-aperture pixel (AAP) design that increases high-frequency modulation transfer function (MTF) and DQE values. The AAP design makes a separation of physical sensor elements from image pixels by using very small sensor elements (e.g., 0.010-0.025 mm) to synthesize desired larger image pixels (e.g., 0.1-0.2 mm).
A cascaded systems model of signal and noise propagation is developed to describe the benefits of the AAP approach in terms of the MTF, Wiener noise power spectrum (NPS), and DQE. The theoretical model was validated experimentally using a CMOS/CsI detector with 0.05 mm sensor elements to synthesize 0.20 mm image pixels and a clinical Se detector with 0.07 mm sensor elements to synthesize 0.28 mm pixels. A Monte Carlo study and x-ray images of a star-pattern and rat leg are used to visually compare AAP images.
When used with a high-resolution converter layer and sensor elements one quarter the size of image pixels, the MTF is increased by 53% and the DQE by a factor of 2.3× at the image sampling cut-off frequency. Both simulated and demonstration images show improved detectability of high-frequency content and removal of aliasing artifacts. Evidence of Gibbs ringing is sometimes seen near high-contrast edges.
It is shown that the AAP approach preserves the MTF of the small sensor elements and attenuates frequencies above the image sampling cut-off frequency. This has the double benefit of improving the MTF while reducing both signal and noise aliasing, resulting in an increase of the DQE at high spatial frequencies. For optimal implementation, the converter layer must have very high spatial resolution and the detector must have low readout noise.
为了在低患者曝光下获取高质量的 X 射线图像，需要具有高探测量子效率（DQE）的探测器。我们描述了一种新的变迹孔径像素（AAP）设计，该设计提高了高频调制传递函数（MTF）和 DQE 值。AAP 设计通过使用非常小的传感器元件（例如 0.010-0.025mm）来合成所需的较大图像像素（例如 0.1-0.2mm），从而将物理传感器元件与图像像素分离。
开发了信号和噪声传播的级联系统模型，以根据 MTF、Wiener 噪声功率谱（NPS）和 DQE 来描述 AAP 方法的优势。该理论模型通过使用 0.05mm 传感器元件来合成 0.20mm 图像像素的 CMOS/CsI 探测器和 0.07mm 传感器元件来合成 0.28mm 像素的临床 Se 探测器进行了实验验证。使用蒙特卡罗研究和星型图案和老鼠腿的 X 射线图像进行视觉比较 AAP 图像。
当与高分辨率转换器层和传感器元件一起使用时，这些传感器元件的尺寸为图像像素的四分之一，MTF 增加了 53%，在图像采样截止频率处的 DQE 增加了 2.3 倍。模拟和演示图像都显示出高频内容的检测能力提高，消除了混叠伪影。在高对比度边缘附近有时会看到吉布斯环的证据。
结果表明，AAP 方法保留了小传感器元件的 MTF，并衰减了图像采样截止频率以上的频率。这具有提高 MTF 的双重好处，同时减少信号和噪声混叠，从而在高空间频率处提高 DQE。为了实现最佳效果，转换器层必须具有非常高的空间分辨率，并且探测器必须具有低读出噪声。
