Department of Radiology, Neuroradiology and Nuclear Medicine, Johannes Wesling University Hospital, Ruhr University Bochum, Minden, Germany.
Medical Physics, University Hospital Schleswig-Holstein, Christian Albrechts University Kiel, Kiel, Germany.
Invest Radiol. 2022 Sep 1;57(9):620-626. doi: 10.1097/RLI.0000000000000873. Epub 2022 Mar 21.
Photon counting computed tomography (PCCT) might offer an effective spatial resolution that is significantly improved compared with conventional state-of-the-art computed tomography (CT) and even provide a microstructural level of detail similar to high-resolution peripheral CT (HR-pQCT). The aim of this study was to evaluate the volumetric effective spatial resolution of clinically approved PCCT as an alternative to HR-pQCT for ex vivo or preclinical high-resolution imaging of bone microstructure.
The experiment contained 5 human vertebrae embedded in epoxy resin, which were scanned 3 times each, and on 3 different clinical CT scanners: a PCCT (Naeotom Alpha), a dual-energy CT (Somatom Force [SF]), and a single-energy CT (Somatom Sensation 40 [S40]), all manufactured by Siemens Healthineers (Erlangen, Germany). Scans were performed with a tube voltage of 120 kVp and, to provide maximum scan performance and minimum noise deterioration, with exposures of 1500 mAs (SF), 2400 mAs (S40), and 4500 mAs (PCCT) and low slice increments of 0.1 (PCCT) and 0.3 mm (SF, S40). Images were reconstructed with sharp and very sharp bone kernels, Br68 and Br76 (PCCT), Br64 (SF), and B65s and B75h (S40). Ground truth information was obtained from an XtremeCT scanner (Scanco, Brüttisellen, Switzerland). Voxel-wise comparison was performed after registration, calibration, and resampling of the volumes to isotropic voxel size of 0.164 mm. Three-dimensional point spread- and modulation-transfer functions were calculated with Wiener's deconvolution in the anatomical trabecular structure, allowing optimum estimation of device- and kernel-specific smoothing properties as well as specimen-related diffraction effects on the measurement.
At high contrast (modulation transfer function [MTF] of 10%), radial effective resolutions of PCCT were 10.5 lp/cm (minimum resolvable object size 476 μm) for kernel Br68 and 16.9 lp/cm (295 μm) for kernel Br76. At low contrast (MTF 5%), radial effective spatial resolutions were 10.8 lp/cm (464 μm) for kernel Br68 and 30.5 lp/cm (164 μm) for kernel Br76. Axial effective resolutions of PCCT for both kernels were between 27.0 (185 μm) and 29.9 lp/cm (167 μm). Spatial resolutions with kernel Br76 might possibly be still higher but were technically limited by the isotropic voxel size of 164 μm. The effective volumetric resolutions of PCCT with kernel Br76 ranged between 61.9 (MTF 10%) and 222.4 (MTF 5%) elements per cubic mm. Photon counting CT improved the effective volumetric resolution by factor 5.5 (MTF 10%) and 18 (MTF 5%) compared with SF and by a factor of 8.7 (MTF 10%) and 20 (MTF 5%) compared with S40. Photon counting CT allowed obtaining similar structural information as HR-pQCT.
The effective spatial resolution of PCCT in trabecular bone imaging was comparable with that of HR-pQCT and more than 5 times higher compared with conventional CT. For ex vivo samples and when patient radiation dose can be neglected, PCCT allows imaging bone microstructure at a preclinical level of detail.
光子计数计算机断层扫描（PCCT）可能提供显著优于传统最先进 CT 的有效空间分辨率，甚至可以提供类似于高分辨率外周 CT（HR-pQCT）的微观结构细节。本研究旨在评估临床认可的 PCCT 的体积有效空间分辨率，作为替代 HR-pQCT 的方法，用于骨微结构的离体或临床前高分辨率成像。
该实验包含 5 个人体椎体，嵌入环氧树脂中，分别在 3 台不同的临床 CT 扫描仪上扫描 3 次，分别为：PCCT（Naeotom Alpha）、双能 CT（Somatom Force [SF]）和单能 CT（Somatom Sensation 40 [S40]），均由西门子医疗（德国埃朗根）制造。扫描采用 120 kVp 管电压，为了提供最大扫描性能和最小噪声恶化，采用 1500 mAs（SF）、2400 mAs（S40）和 4500 mAs（PCCT）的曝光量，以及 0.1（PCCT）和 0.3mm（SF、S40）的低切片增量。图像采用锐利和非常锐利的骨核（PCCT）、Br68 和 Br76、Br64（SF）以及 B65s 和 B75h（S40）进行重建。从 XtremeCT 扫描仪（Scanco，瑞士布鲁泰森）获得真实信息。在对体积进行注册、校准和重采样以达到等方像素尺寸 0.164mm 后，进行体素级比较。在解剖性小梁结构中采用 Wiener 反卷积计算三维点扩展和调制传递函数，允许对设备和核特定的平滑特性以及标本相关的衍射效应进行最佳估计，从而影响测量。
在高对比度（调制传递函数[MTF]为 10%）下，PCCT 的径向有效分辨率为 Br68 核的 10.5 lp/cm（最小可分辨物体尺寸 476 μm）和 Br76 核的 16.9 lp/cm（295 μm）。在低对比度（MTF 5%）下，Br68 核的轴向有效空间分辨率为 10.8 lp/cm（464 μm），Br76 核为 30.5 lp/cm（164 μm）。两种核的 PCCT 轴向有效分辨率均在 27.0（185 μm）和 29.9 lp/cm（167 μm）之间。Br76 核的空间分辨率可能更高，但受到 164μm 等方像素尺寸的技术限制。Br76 核的 PCCT 有效体积分辨率在 MTF 10%时为 61.9（MTF 10%）个/mm3，在 MTF 5%时为 222.4（MTF 5%）个/mm3。与 SF 相比，PCCT 将有效体积分辨率提高了 5.5 倍（MTF 10%）和 18 倍（MTF 5%），与 S40 相比，提高了 8.7 倍（MTF 10%）和 20 倍（MTF 5%）。PCCT 允许获得与 HR-pQCT 相似的结构信息。
PCCT 在小梁骨成像中的有效空间分辨率可与 HR-pQCT 相媲美，比传统 CT 高 5 倍以上。对于离体样本和可以忽略患者辐射剂量的情况，PCCT 允许以临床前的细节水平对骨微结构进行成像。
