Department of Radiology, University of California Davis, Sacramento, CA, 95817, USA.
Department of Biomedical Engineering, Johns Hopkins University, Baltimore, MD, 21205, USA.
Med Phys. 2021 Jun;48(6):2760-2771. doi: 10.1002/mp.14789. Epub 2021 Apr 15.
The purpose of this study was to quantify location and direction-dependent variations in the 3D modulation transfer function (MTF) of a high-resolution CT scanner with selectable focal spot sizes and resolution modes.
The Aquilion Precision CT scanner (Canon Medical Systems) has selectable 0.25 mm or 0.5 mm detectors (by binning) in both the axial (x-y) and detector array width (z) directions. For the x-y and z orientations, detectors are configured (x-y) = 0.5 mm/(z) = 0.5 mm for normal resolution (NR), 0.25/0.5 mm for high resolution (HR), and 0.25/0.25 mm for super high resolution (SHR). Six focal spots (FS1-FS6) range in size from 0.4 (x-y) × 0.5 mm (z) for FS1 to 1.6 × 1.4 mm for FS6. Phantoms fabricated from spherical objects were positioned at radial distances of 0, 4.0, 7.5, 11.0, 14.5, and 18.5 cm. Axial and helical acquisitions were utilized and reconstructed using filtered back projection with the FC18 "Body," FC30 "Bone," and FC81 "Bone Sharp" kernels. The reconstructions were used to measure a 1D slice of the 3D MTF by oversampling the 3D ESF in the axial plane [MTF(f ); φ = 0°)], 45° out of the axial plane [MTF(f ); φ = 45°)], in the longitudinal direction [MTF(f ); φ = 80°)], and along the radial and azimuthal directions within the axial plane.
The MTF(f ); φ = 45°) drops to 10% (f ) at 1.20, 1.45, and 2.06 mm for NR, HR, and SHR, respectively, for a helical acquisition with FS1, FC30, and r = 4 cm from the isocenter. The MTF(f ); φ = 45°) includes contributions of both the axial-plane MTF (f = 1.10, 2.04, and 2.01 mm ) and the longitudinal MTF (f = 1.17, 1.18, and 1.82 mm ) for the NR, HR, and SHR modes, respectively. For SHR, the axial scan mode showed a 15-25% improvement over helical mode in the longitudinal resolution. Helical pitch, ranging from 0.569 to 1.381, did not appreciably affect the 3D resolution (<2%). The radial MTFs across the axial field of view (FOV) showed dependencies on the focal spot length in z; for example, for SHR with FS2 (0.6 × 0.6 mm), f at r = 11 cm was within 17% of the value at r = 4 cm, but for SHR with FS3 (0.6 × 1.3), the reduction in f was 46% from 4 to 11 cm from the isocenter. The azimuthal MTF also decreased as r increased but less so for longer gantry rotation times and smaller focal spot dimensions in the axial plane. The longitudinal MTF was minimally affected (<11%) by position in the FOV and was principally affected by the focal spot length in the z-dimension.
The 3D MTF was measured throughout the FOV of a high-resolution CT scanner, quantifying the advantages of different resolution modes and focal spot sizes on the axial-plane and longitudinal MTF. Reconstruction kernels were shown to impact axial-plane resolution, imparting non-isotropic 3D resolution characteristics. Focal spot size (both in x-y and in z) and gantry rotation time play important roles in preserving the high-resolution characteristics throughout the field of view for this new high-resolution CT scanner technology.
本研究旨在量化高分辨率 CT 扫描仪的三维调制传递函数（MTF）在位置和方向上的变化，该扫描仪具有可选的焦点尺寸和分辨率模式。
Aquilion Precision CT 扫描仪（佳能医疗系统）在轴向（x-y）和探测器阵列宽度（z）方向上都具有可选的 0.25mm 或 0.5mm 探测器（通过 binning）。对于 x-y 和 z 方向，探测器配置为（x-y）= 0.5mm/（z）= 0.5mm 用于标准分辨率（NR），0.25/0.5mm 用于高分辨率（HR），0.25/0.25mm 用于超高分辨率（SHR）。六个焦点（FS1-FS6）的大小范围从 0.4（x-y）×0.5mm（z）的 FS1 到 1.6×1.4mm 的 FS6。由球形物体制成的体模放置在距等中心的 0、4.0、7.5、11.0、14.5 和 18.5cm 的径向距离处。利用轴向和螺旋采集，并使用 FC18“Body”、FC30“Bone”和 FC81“Bone Sharp”内核进行滤波反投影重建。重建用于通过在轴向平面[MTF（f）；φ=0°）]、轴向平面外 45°[MTF（f）；φ=45°）]、纵向[MTF（f）；φ=80°）]以及轴向平面内的径向和方位方向对 3D ESF 进行过采样来测量 1D 切片的 3D MTF。
对于螺旋采集，当焦点为 FS1 时，NR、HR 和 SHR 的 MTF（f）；φ=45°）分别在 1.20、1.45 和 2.06mm 处降至 10%，在距等中心 4cm 处的 r 上使用 FC30 和 r=4cm。MTF（f）；φ=45°）分别包含 NR、HR 和 SHR 模式下轴向平面 MTF（f=1.10、2.04 和 2.01mm）和纵向 MTF（f=1.17、1.18 和 1.82mm）的贡献。对于 SHR，轴向扫描模式在纵向分辨率方面比螺旋模式提高了 15-25%。螺旋螺距范围从 0.569 到 1.381，对 3D 分辨率（<2%）没有明显影响。轴向视野（FOV）内的径向 MTF 取决于焦点在 z 方向上的长度；例如，对于 SHR 与 FS2（0.6×0.6mm），在 r=11cm 处的 f 值在 r=4cm 处的 17%以内，但对于 SHR 与 FS3（0.6×1.3），在从等中心到 11cm 的距离上，f 的减少量为 46%。方位 MTF 也随着 r 的增加而降低，但对于较长的机架旋转时间和轴向平面中较小的焦点尺寸，降低幅度较小。纵向 MTF 受 FOV 位置的影响最小（<11%），主要受 z 方向上焦点长度的影响。
在高分辨率 CT 扫描仪的 FOV 中测量了 3D MTF，量化了不同分辨率模式和焦点尺寸在轴向平面和纵向 MTF 上的优势。重建内核被证明会影响轴向平面分辨率，赋予非各向同性的 3D 分辨率特性。焦点尺寸（在 x-y 和 z 方向上）和机架旋转时间在保持该新型高分辨率 CT 扫描仪技术整个视野的高分辨率特性方面起着重要作用。
