Hernandez Andrew M, Shin Daniel W, Abbey Craig K, Seibert J Anthony, Akino Naruomi, Goto Takahiro, Vaishnav Jay Y, Boedeker Kirsten L, Boone John M
Department of Radiology, University of California Davis, Sacramento, CA, USA.
Canon Medical Systems Inc, Otawara-shi, Japan.
Med Phys. 2020 Oct;47(10):4775-4785. doi: 10.1002/mp.14395. Epub 2020 Aug 5.
To validate a normal-resolution (NR) simulation (NRsim) algorithm that uses high-resolution (HR) or super-high resolution (SHR) acquisitions on a commercial HR computed tomography (CT) scanner by comparing image quality between NRsim-generated images and actual NR images. NRsim is intended to allow direct comparison between normal-resolution CT and HR/SHR reconstructions in clinical investigations, without repeating exams.
The Aquilion Precision CT (Canon Medical Systems Corporation) HR CT scanner has three resolution modes resulting from detector binning in the channel (x-y) and row (z) directions. For NR, each detector element is 0.5 mm × 0.5 mm along the channel and row directions, 0.25 mm × 0.5 mm for HR, and 0.25 mm × 0.25 mm for SHR. The NRsim algorithm simulates NR acquisitions from HR or SHR acquisitions (termed NR and NR , respectively) by downsampling the pre-log raw data in the channel direction for the HR acquisitions and in the channel and row direction for the SHR acquisition. The downsampled data are then reconstructed using the same process as NR. The axial modulation transfer function (MTF), slice sensitivity profile (SSP), and CT number accuracy were measured using the Catphan 600 phantom, and the three-dimensional noise power spectrum (NPS) was measured in water-equivalent phantoms for standard protocols across a range of size-specific dose estimates (SSDE): head (6.2-29.8 mGy), lung (2.2-18.2 mGy), and body (5.6-19.4 mGy). The MTF and NPS measurements were combined to estimate low-contrast detectability (LCD) using a non-prewhitening model observer with an eye filter for a 5-mm disk with 10 HU contrast. All metrics were compared for NR, NR , and NR images reconstructed using filtered back projection (FBP) and an iterative reconstruction algorithm (AIDR3D). We chose a 15% error threshold as a reasonable definition of success for NRsim when compared against actual NR based on published studies showing that a just-noticeable difference in image noise level for human observers is typically <15%.
The axial MTF and SSPs for NRsim were in good agreement with NR demonstrated by a maximum difference of 5.1% for the MTF at 10% and 50% across materials (air, Teflon, LDPE, and polystyrene) and a maximum SSP difference of 2.2%. Noise magnitude differences were within 15% across the SSDE levels with the exception of below 4.5 mGy for the lung protocol with FBP. The relative RMSE of normalized NPS comparisons were all <15%. Differences in CT numbers for NRsim reconstructions were within 2 HU of NR. LCD for NRsim was within 15% of NR with the exception of NR for the lung protocol SSDE levels below 3.7 mGy with FBP.
NRsim, an algorithm for simulating NR acquisitions using HR and SHR raw data, was introduced and shown to generate images with spatial resolution, noise, HU accuracy, and LCD largely equivalent to scans acquired using an actual NR acquisition. At SSDE levels below ~5 mGy for the lung protocol, differences in noise magnitude and LCD for NR were >15% which defines a region where NRsim degrades due to contributions from electronic noise.
通过比较正常分辨率模拟（NRsim）算法生成的图像与实际正常分辨率（NR）图像的质量，验证一种在商用高分辨率计算机断层扫描（CT）扫描仪上使用高分辨率（HR）或超高分辨率（SHR）采集数据的正常分辨率模拟（NRsim）算法。NRsim旨在允许在临床研究中直接比较正常分辨率CT与HR/SHR重建图像，而无需重复检查。
Aquilion Precision CT（佳能医疗系统公司）HR CT扫描仪具有三种分辨率模式，这是通过探测器在通道（x - y）和行（z）方向上的合并实现的。对于NR，每个探测器元件在通道和行方向上均为0.5 mm×0.5 mm，HR为0.25 mm×0.5 mm，SHR为0.25 mm×0.25 mm。NRsim算法通过对HR采集数据在通道方向上以及SHR采集数据在通道和行方向上对对数前原始数据进行下采样，来模拟从HR或SHR采集数据中获取NR采集数据（分别称为NR和NR ）。然后使用与NR相同的过程对下采样后的数据进行重建。使用Catphan 600体模测量轴向调制传递函数（MTF）、层厚灵敏度曲线（SSP）和CT值准确性，并在一系列特定尺寸剂量估计（SSDE）的水等效体模中测量三维噪声功率谱（NPS）：头部（6.2 - 29.8 mGy）、肺部（2.2 - 18.2 mGy）和身体（5.6 - 19.4 mGy）。将MTF和NPS测量结果结合起来，使用带有眼滤波器的非预白化模型观察者对对比度为10 HU的5 mm圆盘估计低对比度可探测性（LCD）。对使用滤波反投影（FBP）和迭代重建算法（AIDR3D）重建的NR、NR 和NR图像的所有指标进行比较。基于已发表的研究表明人类观察者可察觉的图像噪声水平差异通常<15%，我们选择15%的误差阈值作为NRsim与实际NR相比成功的合理定义。
NRsim的轴向MTF和SSP与NR高度一致，在各种材料（空气、聚四氟乙烯、低密度聚乙烯和聚苯乙烯）上，10%和50%处MTF的最大差异为5.1%，SSP的最大差异为2.2%。除了肺部协议FBP在低于4.5 mGy时，噪声幅度差异在所有SSDE水平上均在15%以内。归一化NPS比较的相对均方根误差均<15%。NRsim重建的CT值差异在NR的2 HU以内。NRsim的LCD在NR的15%以内，但肺部协议SSDE水平低于3.7 mGy且使用FBP时的NR除外。
引入了NRsim算法，该算法使用HR和SHR原始数据模拟NR采集，结果表明其生成的图像在空间分辨率、噪声、HU准确性和LCD方面与使用实际NR采集获得的扫描图像基本等效。对于肺部协议，在SSDE水平低于约5 mGy时，NR的噪声幅度和LCD差异>15%，这定义了一个由于电子噪声的影响NRsim性能下降的区域。
