Fig 1.
Two-ball phantom for CT calibration.
(A) 3D rendering and (B) schematic image of a two-ball phantom. (C) For calibration purposes, the distance between the centre of the balls is used, as the outer edge may appear smaller or larger depending on the windowing in the image reconstruction (black arrows). The distance between the two spheres can be measured in 2D (D) or in 3D using matching spheres (E).
Table 1.
Overview of the used CT systems.
Table 2.
Scanning and reconstruction parameters of the used CT systems. The dynamic range refers to the mapping of attenuation values to the 8-bit output scale during reconstruction of micro-CT scans.
Table 3.
Results of the measured scan accuracy.
Fig 2.
Measurement of phantom distance.
CT scans of a calibrated 19.89221 mm (±0.0008 mm) two-ball phantom (blue line) were analysed in (A) 2D cross-sections and (B) 3D volume renderings and shown as violin plots (median, quartiles and distribution). Scans taken with the oversize micro-CT were below the correct phantom length and scans taken with the high-resolution micro-CT were above the correct phantom length. Dual-source CT scans were analysed in the sagittal plane in addition to the coronal plane.
Fig 3.
Corrected measurements of phantom distance after calibration.
Measurement results after adjustment of the mean values to the size of the calibrated two-ball phantom of 19.89221 mm (blue line) in (A) 2D cross-sections and (B) 3D volume renderings. The corrected results show a good representation of the actual phantom size. Measurement of scans taken with the SOMATOM Force showed a greater scatter of data compared to the micro-CTs.
Fig 4.
Visual comparison of uncalibrated and calibrated measurements.
An overlay on a 2D slice from the oversize micro-CT shows the uncalibrated measured distance (red line and overlay), which is visibly shorter than the actual center-to-center distance. After applying the correction factor derived from the phantom, the calibrated measurement (blue line) accurately aligns with the centers of the two ruby spheres, demonstrating the effectiveness of the calibration.
Fig 5.
A profile line of 50 pixels (25 pixels for SOMATOM Force scans) was placed over the outer edge of the ruby ball in each CT scan in the 2D view to determine the gradient of the grey values. (A) The slope of the fit curves (Rodbard fit) with a grey value between 60 and 140 (grey box) was determined for the profile lines and shown as an example for a scan in the high-resolution micro-CT (11 µm voxel size, central position) in red and the oversize micro-CT (20 µm voxel size, central position) in green. (B) Comparison of the slope of the profile lines (B). In the micro-CTs, scans with smaller voxel sizes have steeper profile lines. The steepest profile lines were measured in the high-resolution micro-CT scans.
Fig 6.
Exemplary CT scans cropped to one ruby ball.
A cross-sectional view of the top ruby sphere of the phantom is presented in the 2D view with differing scan parameters (left) and the corresponding FFT conversion (right). Positions and voxel sizes were varied. For better interpretation, a computer-generated virtual phantom was added as an idealized reference case (bottom right image pair). This virtual image was created with a defined, slight blur at the edge to serve as a benchmark. The FFT of this virtual phantom produces a characteristic checkered pattern, which serves as a reference for how a blurred edge is represented in the frequency domain. In comparison, the FFTs of the real scans with larger voxel sizes (e.g., 23 µm) and consequently lower edge sharpness show ring-shaped gradations, indicating greater image blur. Scale: 3 mm.