Purpose: After a recent software upgrade and subsequent exam cards reconfiguration on Philips BigBore computed tomography (CT) scanners, concentric rings artifacts were observed on 4-dimensional CT (4DCT) acquired at multiple scanners of this type, at the authors’ institution. Notably, the artifact in clinical acquisitions was observed only near the diaphragm region on 4DCT. Due to presence of the artifact on multiple scanners instead of a single one, the authors ruled out detector hardware defect or calibration defect as the typical cause of concentric rings artifact. The authors sought to identify the true cause of such artifacts.
Methods: A literature review was performed to identify some possible hypotheses, which were tested by subsequent phantom studies. Hypotheses include: (1) “photon starvation” according to the vendor; (2) Helical scan interpolation error; (3) Fan-beam assumption for multi-detector scanner; (4) “Z-sloped” sudden longitudinal change of attenuation. Phantom study was performed in both axial scans, helical scans, and 4DCT. Multiple factors were varied: 200 mAs vs 2,000 mAs, slice thickness 1-3 mm, helical pitch 0.813 vs 0.563, collimation of 16x1.5, 16x0.75, 4x1.5, and 4x0.75. Some scans were also performed on a different vendor’s scanner.
Results: Phantom study showed that the concentric rings appear near the longitudinal edges of the phantom, with the following findings: (A) increase of mAs reduces the appearance of artifact but do not eliminate them. (B) Axial scan carries the same artifact. (C) Decreasing slice thickness and collimation reduce the longitudinal extent of the artifact, but do not eliminate them. (D) Helical pitch has no impact. (E) Similar artifact was observed on phantom’s longitudinal edge scanned on a different vendor’s scanner.
Conclusion: Findings (A)-(E) above ruled out Hypotheses (1)-(3). Hypothesis (4) is most consistent with phantom study, and with clinical 4DCT observation: due to sudden longitudinal change of attenuation across diaphragm.