Purpose: Advances in CT and cone-beam CT (CBCT) provide capability for isotropic spatial resolution, but the modulation transfer function (MTF) for such systems is typically reported only in the axial (x-y) plane (2D, MTF_Axial), and rigorous methodology for assessment of fully 3D MTF is lacking. We report an easily-manufactured test tool and a new oversampling method based on an angled-edge for assessment of the MTF in any direction and show how the MTF measured at 45Â° from the axial plane (MTF_45) offers a useful characterization of the 3D MTF in CBCT.
Methods: The angled-edge test tool is an acrylic cylindrical rod with two measurement faces: (1) the circular side face of the cylinder (for MTF_Axial); and (2) an angled edge face (for MTF_45). The 45Â° angle is chosen specifically to avoid the unsampled â€œnull-coneâ€? frequency domain in CBCT (typically < 20Â°) and provides an oversampled edge-spread function (ESF) with a small tilt of the cylinder. The angled-edge tool was tested using a CBCT prototype scanner to evaluate the effects of reconstruction techniques (binning and filtering options) on 3D spatial resolution.
Results: The test tool and oversampling method provided reliable assessment of the CBCT presampled MTF, validated against conventional MTF measurement methods. The CBCT system demonstrated isotropic resolution with a 2D apodization filter, and non-isotropic resolution with a 1D apodization filter (which is common in CBCT systems). Asymmetric pixel binning was reflected in the 3D MTF characteristics, as expected. The MTF_45 provides a practical 1D measure of the underlying 3D MTF characteristics.
Conclusion: A test-tool and oversampling method was demonstrated for quantitative assessment of 3D MTF in CBCT, while avoiding complexities of unsampled â€œnull-coneâ€? spatial frequencies. The experiments validate intuitive aspects of 3D spatial resolution and quantify the extent to which a CBCT system may be considered to have isotropic resolution.