Troubleshooting Common Artifacts in Cone-Beam Computed Tomography

Cone-beam computed tomography (CBCT) is valuable for image-guided radiotherapy, dental imaging, interventional procedures and surgical planning. Its wide field of view and flexible geometry provide useful three-dimensional information, yet the images can contain artefacts that resemble disease, obscure anatomy or compromise dose calculations.

A practical troubleshooting method begins by separating patient-related effects from equipment, acquisition and reconstruction problems. Motion, metal, scatter, beam hardening, detector lag and calibration errors each leave recognisable patterns. Knowing when an artefact is expected, correctable or serious enough to stop scanning protects both image quality and clinical workflow.

For Australian departments, the issue also has an operational dimension. A CBCT unit in a busy Sydney cancer centre, a regional Queensland hospital or a private Melbourne dental practice may have different service access, staffing and regulatory arrangements. The core physics remains consistent, but the response must fit local procedures, state legislation and the realities of an Australian clinical day.

Recognising The Main Artefact Patterns

Motion commonly produces double edges, blurred interfaces or repeated anatomy in the projection data. It may arise when a patient shifts during a lengthy rotational acquisition, when a breathing cycle is not controlled, or when a treatment couch vibrates. Review the raw projections if available, compare the affected anatomy with immobilisation images and check whether the distortion follows the direction of gantry rotation.

Metal-related streaks usually radiate from dental fillings, hip prostheses, fixation screws or treatment accessories. High-density objects cause photon starvation, beam hardening and severe inconsistencies between projections. A metal artefact reduction algorithm can help, but it may also create new shading or texture changes. Images should therefore be assessed before and after correction rather than accepting the processed result automatically.

The following visual clues can narrow the search quickly:

• Broad cupping or darkening through a uniform phantom suggests beam hardening or scatter
• Concentric rings usually indicate detector-element calibration problems
• Repeated streaks aligned with metal often reflect photon starvation
• Blurred edges and duplicated structures are typical of patient or couch motion
• Localised bands near the field boundary may indicate truncation or limited coverage

Checking Acquisition And Reconstruction Settings

CBCT artefacts often begin with an inappropriate protocol rather than a failed component. A small field of view may truncate the patient, while a large field can increase scatter and reduce contrast. Excessive exposure can hide noise but increase dose; insufficient exposure makes photon starvation and streaking more prominent. Confirm kVp, mAs, pulse width, projection number, collimation and scan trajectory against the intended clinical task.

Reconstruction settings can alter the appearance of the same acquisition. A sharp kernel may accentuate edges and noise, whereas a smoother kernel can conceal fine detail. Voxel size, interpolation, ring correction, scatter correction and metal reduction should be recorded when an image is exported for planning. A mismatch between the reconstructed volume and the treatment-planning system can produce geometric or density errors that are not obvious on a workstation.

Before escalating a fault, review these practical checks:

• Confirm the correct patient position, immobilisation device and scan field
• Compare protocol values with the department’s approved clinical preset
• Check whether the selected voxel size suits the anatomy and task
• Inspect for truncated anatomy at the edge of the reconstruction volume
• Verify that the planning system received the correct orientation and data set

In Australia, a protocol change should be documented within the department’s quality system rather than treated as an informal workstation preference. State and territory radiation requirements differ, while national guidance from ARPANSA provides a common safety framework. A medical physicist should assess changes affecting dose, image quality or treatment planning, with local authorisation and records maintained under the relevant jurisdiction.

Isolating Equipment And Calibration Faults

Ring artefacts are strongly associated with detector gain or offset inconsistencies. Run the manufacturer’s air scan or uniform phantom test and inspect whether the rings remain fixed relative to the detector. If they rotate with the reconstructed image or vary between acquisitions, the problem may involve calibration, detector temperature, data transfer or reconstruction software rather than patient anatomy.

Non-uniform shading can result from scatter, a misaligned x-ray source, detector calibration drift or an obstructed beam path. A warm-up sequence, tube output check and uniformity measurement can distinguish a temporary start-up condition from a persistent hardware issue. Record the date, protocol, reconstruction version and phantom results so a service engineer receives useful evidence instead of a general report that the images “look wrong.”

A structured test sequence saves time:

• Repeat the scan with a uniform phantom and no patient accessories
• Compare current images with a baseline acceptance or constancy image
• Inspect tube output, exposure indicators and detector temperature
• Check cables, gantry covers and accessories for objects in the beam
• Preserve raw data and screenshots before applying post-processing

Departments outside capital cities may have longer waits for vendor engineers and fewer replacement systems. A site in Newcastle, Hobart or regional Western Australia may need a clear escalation threshold and a documented contingency plan. If an artefact affects anatomical localisation, dose calculation or image guidance, the equipment should be restricted until the responsible physicist and service provider determine whether clinical use is safe.

Managing Scatter, Noise And Patient Factors

Scatter is a defining limitation of wide-cone imaging. It can cause cupping, streaks and inaccurate Hounsfield values, particularly when the field includes shoulders, dental restorations or large immobilisation devices. Anti-scatter grids, bow-tie filters and correction algorithms reduce the effect, but none is perfect. A uniform phantom and a water-equivalent test object help establish whether the correction is stable across the field of view.

Noise increases when exposure is low, the patient is large, the scan covers a broad volume or the reconstruction uses a small voxel. Australian services must balance image quality with radiation protection under local regulations and approved protocols. Increasing exposure should not be the first response to every noisy image; repositioning, selecting a suitable field, improving immobilisation or using validated iterative reconstruction may be more appropriate.

Everyday workflow can influence artefacts. A patient arriving late from a busy outpatient clinic may be anxious or unable to remain still, while a long afternoon list can tempt staff to bypass positioning checks. Clear instructions, comfortable supports and a short rehearsal of breath-holding can prevent repeat scans. In Brisbane or Darwin, clothing, perspiration and removable accessories may also affect setup comfort, although they are rarely the primary cause of a reconstruction defect.

Documenting Findings And Restoring Confidence

A useful incident record describes the artefact, affected protocols, patient impact and immediate action. Include representative images, phantom results, scan parameters, software versions and whether the problem is reproducible. This creates a defensible history for internal review and helps identify gradual detector deterioration before it becomes a clinical incident.

Australian equipment procurement can add complexity because many systems, detectors and service contracts are supplied through overseas manufacturers or national distributors. Replacement parts may take time to arrive, and smaller private practices may share a physicist across several locations. A documented baseline, scheduled constancy testing and an agreed vendor escalation pathway are therefore valuable commercial as well as clinical safeguards.

Archived conference material can also help delegates understand the practical setting surrounding a professional meeting. Venue and travel planning information remains available through the accommodation details, while older bookmarks may use the alternate accommodation page. Such records illustrate how scientific sessions, business meetings and informal discussions support the exchange of troubleshooting experience.

A final review should confirm that the corrective action worked under the original clinical protocol, not only under an easier test condition. Compare image quality, geometric accuracy, uniformity and relevant dose indicators with the department’s acceptance criteria. Keep the result with the quality records, brief affected staff and update the protocol or maintenance schedule if the investigation exposed a recurring weakness.

Use a repeatable response whenever an unexpected CBCT pattern appears: identify its geometry, protect the patient from unnecessary repeat exposure, test with a phantom, check acquisition and reconstruction settings, and escalate equipment faults with evidence. Physicists, radiographers, radiation oncologists, dentists and service engineers all contribute different observations. Applying that shared process turns a distracting image defect into a controlled quality-assurance event and supports safer, more reliable clinical imaging.