Location and direction dependence in the 3D MTF for a high‐resolution CT system

Abstract

Purpose 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. Methods 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(fr); φ = 0°)], 45° out of the axial plane [MTF(fr); φ = 45°)], in the longitudinal direction [MTF(fr); φ = 80°)], and along the radial and azimuthal directions within the axial plane. Results The MTF(fr); φ = 45°) drops to 10% (f10) at 1.20, 1.45, and 2.06 mm−1 for NR, HR, and SHR, respectively, for a helical acquisition with FS1, FC30, and r = 4 cm from the isocenter. The MTF(fr); φ = 45°) includes contributions of both the axial‐plane MTF (f10 = 1.10, 2.04, and 2.01 mm−1) and the longitudinal MTF (f10 = 1.17, 1.18, and 1.82 mm−1) 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), f10 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 f10 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. Conclusions 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.

Theory, method, and test tools for determination of 3D MTF characteristics in cone‐beam CT

“…While the experiments in this work were limited to CBCT, the methods and test tools are also applicable to helical MDCT as presented in related work 32 . In addition to the caveats discussed above for CBCT, helical MDCT invites a variety of distinct considerations.…”

“…While the experiments in this work were limited to CBCT, the methods and test tools are also applicable to helical MDCT as presented in related work. 32 In addition to the caveats discussed above for CBCT, helical MDCT invites a variety of distinct considerations. For axial mode MDCT, the null cone as described above should be recognized, although the measurement direction (φ 0 ) may extend to larger angles (e.g., up to ~88°for a scanner with SAD = 1085 mm covering 32 mm in z) and remain within the fully sampled region of the Fourier domain.…”

“…Other factors not studied directly in the current work also contribute to the extent to which 3D spatial resolution is isotropic and/or stationary, such as the number of projection views and effective focal spot size. 32 While it is certainly important to understand the fully 3D MTF [MTF f x , f y , f z À Á ], it is often more practical (e.g., in routine quality assurance) to report a single MTF curve measured in a particular direction from which the characteristics of the full 3D MTF can be appreciated. The MTFð f r ;φ ¼ 45°) was shown to be a useful 1D representation, since it reflects almost equal contributions from both As mentioned above, the MTF analysis considered in this work is understood to apply within the context of LSI systems, with assumptions of linearity and shift-invariance common in Fourier-based analysis of system response.…”

“…Because conventional test tools (e.g., a wire phantom or cylindrical edge‐spread phantom for measurement of the axial plane MTF) can be blind to such basic effects, the importance of practical test tools such as the wedge or sphere is clear. Other factors not studied directly in the current work also contribute to the extent to which 3D spatial resolution is isotropic and/or stationary, such as the number of projection views and effective focal spot size 32 …”

“…The methods are fairly general so as to support application to other CBCT systems. For MDCT, analysis of the 3D MTF characteristics are reported in related work, 32 focusing on use of the sphere phantom for assessment of high‐resolution imaging protocols (Aquilion Precision, Canon Medical Systems Corporation, Otawara, Japan).…”