Sato Kazuhiro, Shidahara Miho, Goto Mitsunori, Yanagawa Isao, Homma Noriyasu, Mori Issei
Health Sciences, Tohoku University Graduate School of Medicine, 2-1 Seiryo-machi, Aoba-ku, Sendai, 980-8575, Japan,
Radiol Phys Technol. 2015 Jul;8(2):178-92. doi: 10.1007/s12194-015-0306-5. Epub 2015 Jan 11.
X-ray CT projection data often include components with frequencies that are markedly higher than the pixel Nyquist frequency f PN, which is determined by the pixel size. Noise components higher than f PN are folded back into a region lower than f PN through the backprojection process, thereby creating aliased noise. With clinical CT scanners, we evaluated the aliased noise using an aliasing prevention measure, band-limiting processing (BLP), which suppresses frequency components higher than f PN in the projection data. Indices we used to evaluate improvement by BLP were the noise power spectrum (NPS), modulation transfer function (MTF), signal-to-noise-ratio (SNR) spectrum, matched filter SNR (MF SNR), and two-alternative forced-choice (2-AFC) test. With BLP, the NPS was decreased not only beyond f PN, but also within f PN. The same level of MTF was maintained as that without BLP within f PN. No remarkable reduction in spatial resolution was observed. The SNR spectrum and the MF SNR of the BLP image nearly agreed with those of an ideal state without aliased noise. A notable improvement in the visuoperceptual image quality by BLP was recognized with a reconstruction field of view (FOV) of more than 45 cm. We then applied BLP to clinical data and confirmed that significant aliased noise of a large FOV image was removed without notable side effects. The results showed that at least some CTs suffering from aliased noise can be improved by proper band-limiting.
X射线CT投影数据通常包含频率明显高于像素奈奎斯特频率fPN的成分，该频率由像素大小决定。高于fPN的噪声成分在反投影过程中被折叠回低于fPN的区域，从而产生混叠噪声。使用临床CT扫描仪，我们采用一种防止混叠的措施——带限处理（BLP）来评估混叠噪声，该处理可抑制投影数据中高于fPN的频率成分。我们用于评估BLP改善效果的指标有噪声功率谱（NPS）、调制传递函数（MTF）、信噪比（SNR）谱、匹配滤波器信噪比（MF SNR）以及二选一强制选择（2-AFC）测试。采用BLP时，NPS不仅在fPN以上降低，在fPN以内也降低。在fPN以内，MTF保持与未采用BLP时相同的水平。未观察到空间分辨率有明显降低。BLP图像的SNR谱和MF SNR与无混叠噪声的理想状态几乎一致。当重建视野（FOV）大于45 cm时，可观察到BLP在视觉感知图像质量方面有显著改善。然后我们将BLP应用于临床数据，并证实大FOV图像中的明显混叠噪声被消除，且无明显副作用。结果表明，至少一些存在混叠噪声的CT通过适当的带限处理可以得到改善。
