Yu Lifeng, Vrieze Thomas J, Leng Shuai, Fletcher Joel G, McCollough Cynthia H
Department of Radiology, Mayo Clinic, Rochester, Minnesota 55905.
Med Phys. 2015 May;42(5):2261-7. doi: 10.1118/1.4916802.
The spatial resolution of iterative reconstruction (IR) in computed tomography (CT) is contrast- and noise-dependent because of the nonlinear regularization. Due to the severe noise contamination, it is challenging to perform precise spatial-resolution measurements at very low-contrast levels. The purpose of this study was to measure the spatial resolution of a commercially available IR method using ensemble-averaged images acquired from repeated scans.
A low-contrast phantom containing three rods (7, 14, and 21 HU below background) was scanned on a 128-slice CT scanner at three dose levels (CTDIvol = 16, 8, and 4 mGy). Images were reconstructed using two filtered-backprojection (FBP) kernels (B40 and B20) and a commercial IR method (sinogram affirmed iterative reconstruction, SAFIRE, Siemens Healthcare) with two strength settings (I40-3 and I40-5). The same scan was repeated 100 times at each dose level. The modulation transfer function (MTF) was calculated based on the edge profile measured on the ensemble-averaged images.
The spatial resolution of the two FBP kernels, B40 and B20, remained relatively constant across contrast and dose levels. However, the spatial resolution of the two IR kernels degraded relative to FBP as contrast or dose level decreased. For a given dose level at 16 mGy, the MTF50% value normalized to the B40 kernel decreased from 98.4% at 21 HU to 88.5% at 7 HU for I40-3 and from 97.6% to 82.1% for I40-5. At 21 HU, the relative MTF50% value decreased from 98.4% at 16 mGy to 90.7% at 4 mGy for I40-3 and from 97.6% to 85.6% for I40-5.
A simple technique using ensemble averaging from repeated CT scans can be used to measure the spatial resolution of IR techniques in CT at very low contrast levels. The evaluated IR method degraded the spatial resolution at low contrast and high noise levels.
由于非线性正则化，计算机断层扫描（CT）中迭代重建（IR）的空间分辨率取决于对比度和噪声。由于严重的噪声污染，在非常低的对比度水平下进行精确的空间分辨率测量具有挑战性。本研究的目的是使用从重复扫描中获取的总体平均图像来测量一种商用IR方法的空间分辨率。
在128层CT扫描仪上，对一个包含三根棒（比背景低7、14和21HU）的低对比度体模在三个剂量水平（CTDIvol = 16、8和4mGy）下进行扫描。使用两种滤波反投影（FBP）核（B40和B20）以及一种商用IR方法（正弦图确认迭代重建，SAFIRE，西门子医疗）和两种强度设置（I40 - 3和I40 - 5）重建图像。在每个剂量水平下，相同的扫描重复100次。基于在总体平均图像上测量的边缘轮廓计算调制传递函数（MTF）。
两种FBP核B40和B20的空间分辨率在对比度和剂量水平上保持相对恒定。然而，随着对比度或剂量水平降低，两种IR核的空间分辨率相对于FBP有所下降。对于16mGy的给定剂量水平，相对于B40核归一化的MTF50%值，I40 - 3从21HU时的98.4%降至7HU时的88.5%，I40 - 5从97.6%降至82.1%。在21HU时，I40 - 3的相对MTF50%值从16mGy时的98.4%降至4mGy时的90.7%，I40 - 5从97.6%降至85.6%。
一种使用重复CT扫描总体平均的简单技术可用于在非常低的对比度水平下测量CT中IR技术的空间分辨率。所评估的IR方法在低对比度和高噪声水平下会降低空间分辨率。
