Ct artifacts
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computed tomography AND artifacts
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CT image artifacts causes reduction beam hardening motion metal ring artifact authoritative radiology
CT image artifacts
A CT artifact is any image feature that does not represent the patient’s true anatomy. It can obscure a lesion or imitate disease.
| Artifact | Typical appearance | Main cause | Reduction / prevention |
|---|---|---|---|
| Beam hardening | Dark bands or streaks, especially between dense bones at skull base or shoulders | Low-energy photons are absorbed preferentially, so the beam becomes “harder” | Use beam-hardening correction, higher kVp when appropriate, iterative reconstruction, or dual-energy CT |
| Metal artifact | Severe bright and dark streaks around dental fillings, prostheses, clips, or spinal hardware | Beam hardening, photon starvation, scatter, and edge effects | Remove external metal, optimize positioning, higher kVp/mAs if justified, thin slices, metal-artifact-reduction algorithms, dual-energy virtual monoenergetic images |
| Photon starvation | Very noisy dark streaks through large or highly attenuating regions, such as shoulders or metal | Too few photons reach detector | Increase exposure appropriately, adjust positioning, use iterative reconstruction |
| Motion artifact | Blurring, double outlines, or streaks | Patient breathing, movement, swallowing, cardiac pulsation | Explain breath-hold, immobilize, use faster acquisition, ECG-gate cardiac CT where indicated |
| Partial-volume artifact | Incorrect averaged attenuation or obscured small lesion | Different tissues included in a thick slice | Use thinner collimation/slices and multiplanar reconstructions |
| Ring artifact | One or more concentric rings centered on image | Miscalibrated or faulty detector element | Detector calibration and scanner servicing |
| Helical / windmill artifact | Repeating streaks or distorted edges, often around high-contrast structures | Helical interpolation and inadequate sampling, especially with wide detectors | Optimize pitch, reconstruction interval, and acquisition parameters |
| Cone-beam artifact | Shading or streaks, usually toward image periphery | Wider multidetector geometry and imperfect reconstruction assumptions | Use appropriate acquisition and manufacturer correction algorithms |
| Aliasing artifact | Fine alternating lines or streaks | Insufficient projection sampling | Increase number of projections or use proper reconstruction settings |
| Truncation / out-of-field artifact | Peripheral shading or streaks | Part of patient lies outside scan field of view | Center patient correctly; use larger field of view if available |
| Scatter artifact | Shading and loss of contrast, often in large patients | Scattered photons reach detector | Adequate technique selection and scatter correction |
| Pseudoenhancement | Artificially increased CT number in a small structure near dense contrast, e.g., renal cyst next to enhancing renal parenchyma | Beam hardening and scatter from adjacent iodinated contrast | Interpret with caution; compare phases or use alternative reconstruction/technique |
High-yield pattern recognition
- Dark bands between dense structures: beam hardening
- Streaks adjacent to prosthesis or dental work: metal artifact
- Concentric rings: detector calibration fault
- Blurred/doubled anatomy: motion
- Small lesion averaged with adjacent tissue: partial-volume effect
- Peripheral cutoff or streaking in a large patient: truncation
CT artifacts arise from physics, patient factors, scanner hardware, and acquisition/reconstruction technique. They are clinically important because they may simulate pathology or hide it, as summarized in this CT artifact review.
For metal artifacts specifically, recent systematic reviews support metal-artifact-reduction and virtual monoenergetic approaches, but image quality and diagnostic accuracy still vary by implant, scanner, and algorithm (2025 systematic review).
What is attennuation
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x-ray attenuation computed tomography
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x ray attenuation CT Hounsfield units definition radiology
Attenuation means the reduction in intensity of an X-ray beam as it passes through the body.
It happens because X-ray photons are:
- Absorbed by tissue
- Scattered away from the detector
Different tissues attenuate X-rays by different amounts:
| Tissue/material | Attenuation | CT appearance |
|---|---|---|
| Air | Very low | Black |
| Fat | Low | Dark gray |
| Water / soft tissue | Moderate | Gray |
| Bone | High | White |
| Metal or iodinated contrast | Very high | Very bright white |
In CT, the scanner measures attenuation from many angles and reconstructs the data into cross-sectional images. Greater attenuation means fewer X-rays reach the detector, so the structure appears brighter.
CT attenuation is expressed in Hounsfield units (HU):
- Air: about -1000 HU
- Water: 0 HU
- Fat: about -100 to -50 HU
- Soft tissue: roughly 30 to 70 HU
- Bone: usually +300 to >1000 HU
So, simply: attenuation = how much a tissue weakens the X-ray beam.
What is rectifier
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