9.4 Equipment Artifacts — Rings, Streaks, Tube Arcing & Cone Beam

Why This Topic Matters

The final sub-item under Artifact Recognition and Reduction (2.C.7) is equipment artifacts, broken into four named types: rings, streaks, tube arcing, and cone beam. These differ fundamentally from the patient-related artifacts covered in the previous section because their cause lives inside the scanner hardware, not the patient's anatomy or position. The ARRT CT exam tests whether a technologist can tell the difference, because the correct response is different: patient-related artifacts are addressed with technique or positioning changes made by the technologist, while equipment artifacts require notifying service engineering or a medical physicist — repeating the scan with different patient technique will not fix a hardware problem.

Ring Artifact

A ring artifact is caused by a single detector element (or a small group of elements) that has drifted out of calibration or is malfunctioning. To understand why this produces a ring specifically, recall the geometry of a rotate-rotate (third-generation) CT system: the x-ray tube and detector array rotate together around the patient, and a given detector element samples the same radial distance from the isocenter at every projection angle throughout the full rotation. If that one element has a consistent measurement error, the reconstruction places that error at the same radial position on every projection — which reconstructs as a complete (or partial) circular ring, centered on the axis of rotation, rather than a random or localized artifact.

Appearance: one or more sharp, complete or near-complete circular/annular patterns, typically centered on the scan isocenter, and — critically — visible across images from multiple different patients scanned on that unit.

Correction: this is a hardware issue. It is resolved through detector recalibration (an air/reference calibration procedure performed by service or physics), not by changing the current patient's scan parameters. A technologist who recognizes a ring artifact should notify service/physics rather than attempt to "fix" it by rescanning with different technique.

Streak Artifact (Equipment-Related)

While streaking can come from many patient-related sources (motion, metal, beam hardening — covered in the previous section), equipment-related streak artifact specifically arises from data inconsistency between adjacent detector channels or projection views — for example, from detector malfunction, electronic noise, or a calibration error somewhere in the acquisition chain.

Appearance: linear streaking patterns that do not correlate with any specific patient anatomy or dense object, and that reproduce regardless of who or what is being scanned.

Correction: system calibration, detector service, or a software/firmware update, depending on the root cause identified by engineering. The key diagnostic clue for the technologist is reproducibility: a streak pattern that appears consistently across different patients and different studies on the same scanner points to equipment, not the patient in front of you. Reproducible artifact patterns should be documented and reported.

Tube Arcing

Tube arcing is a sudden, unwanted electrical discharge (an arc) occurring inside the x-ray tube. Common causes include tube aging, internal contamination or outgassing within the tube envelope, or wear of the filament or anode. Beyond the image artifact itself, tube arcing can also present clinically as an aborted exposure, an error message, or an irregular exposure — not just a visual artifact on the reconstructed image.

Appearance: severe, often transient streak artifacts coinciding with the discharge event.

Correction: tube arcing is an equipment malfunction requiring biomedical or service engineering evaluation, and may require tube reseasoning or outright tube replacement. It is not correctable through technologist technique adjustment. Recurrent arcing is both an image-quality and a safety concern significant enough that the scanner should be taken out of clinical service until evaluated.

Cone Beam Artifact

As multidetector CT (MDCT) scanners have expanded to wide-area detector arrays — modern systems range from roughly 64-detector-row up to 320-detector-row configurations, providing several centimeters of z-axis (longitudinal) coverage per single rotation — the x-ray beam geometry is no longer a simple flat, two-dimensional fan. Instead, it forms a three-dimensional cone. The detector rows located farthest from the central plane of that cone are struck by x-rays at an increasingly oblique angle relative to the patient's long axis, rather than perpendicular to it, which the simpler fan-beam reconstruction model does not fully account for.

Appearance: shading, streaking, or generally reduced image quality that is most pronounced in the outer/peripheral detector rows — that is, the edges of a wide-volume acquisition — and that becomes more significant as the total number of detector rows and z-axis coverage increases.

Correction: dedicated cone-beam reconstruction algorithms (such as Feldkamp-type or iterative cone-beam correction methods) built into modern reconstruction software specifically to account for the oblique ray geometry, rather than assuming a simplified fan-beam model.

Exam Scenario

A technologist performs a wide-volume 320-detector-row cardiac CT and notices a subtle shading artifact confined to the outermost slices of the acquired volume, while the central slices are essentially artifact-free. This pattern — worse at the periphery of a wide-coverage acquisition, absent centrally — is characteristic of a cone beam artifact tied to the oblique ray geometry striking the outer detector rows, and is addressed through the scanner's cone-beam reconstruction algorithm rather than by repeating the scan with different mAs.

Contrast that with a second scenario: a technologist notices the same complete circular artifact appearing on head CT images from several different patients scanned on the same unit over the course of a week. Because the pattern is identical regardless of which patient is scanned, this is a ring artifact from a miscalibrated detector element, and the correct response is to report it to service/physics for detector recalibration — not to keep scanning additional patients on the assumption the artifact is patient-related.