CT artifacts can be caused by a variety of factors related to the physics of CT imaging, the patient, and hardware issues. Physics-based artifacts include beam hardening, which causes cupping and streak artifacts, as well as partial volume averaging and noise. Patient motion can also cause artifacts. Hardware issues like ring artifacts may occur from problems with the x-ray tube. Proper use of filters and reconstruction techniques can help reduce artifacts like beam hardening, while keeping the patient still can minimize motion artifacts. Artifacts need to be understood as they can obscure anatomy or be mistaken for pathology.

1. CT ARTIFACTS
DR. Pradeep Patil
Prof. Department of Radio-diagnosis,
DY Patil medical college, hospital & research institute
Kolhapur

2. Definition
• The definition of an image artifact is not as clearly defined as one
might expect.
• Theoretically, an image artifact can be defined as any discrepancy
between the reconstructed values in an image and the true attenuation
coefficients of the object

6. Physics based artifacts
• beam hardening - cupping artifact / streak artifact / metal artifact
• photon starvation
• partial volume averaging
• noise
• aliasing
• truncation artifact

7. BEAM HARDENING
• X-ray photons coming from a tube are made up of a full spectrum of
X-rays energies (POLYCHROMATIC X-ray beam).
• When an X-ray beam begins to penetrate a sample material the lower
energy X-rays preferentially attenuate, resulting in an overall higher
energy of the X-ray beam – “beam hardening”.
• Beam hardening artefacts appear as dark bands/ streaks or cupping
artefacts.

9. CUPPING ARTEFACT
Beam passing through the middle part of the object is hardened more as compared to edge causing
cupping artefact.

10. BEAM HARDENING ARTEFACTS
Beam-hardening artifacts. A- Postoperative CT image of the brain in an adult. B- Comparison of reductions in
beam-hardening artifacts with use of special reconstruction techniques.

14. Solutions to beam hardening
• Bow tie filter – to pre-harden the X-ray beam.
• Beam hardening correction software.
• Calibration correction.

19. Partial volume artifact occurs
when tissues of widely different
absorption are encompassed on
the same CT voxel producing a
beam attenuation proportional
to the average value of these
tissues.

24. PARTIAL VOLUME ARTEFACT
Liver scan showing partial-volume artifact. A, A 2.5-mm slice thickness shows good visibility of small liver lesions. B, A 10-mm slice
thickness shows reduced lesion detectability with partial-volume averaging. (Images courtesy Dr. John Haaga.)

25. Noise
• Noise in computed tomography is an unwanted change in pixel values
in an otherwise homogeneous image.
• Noise in CT is measured via the signal to noise ratio (SNR)
• Noise in a cross-sectional image will equal a decrease in the picture
quality and inadvertently will hinder the contrast resolution.

28. Factors affecting noise
• Slice thickness - The thicker the slice, the more photons available; and
the more photons available, the better the SNR. However, this isn't
without a trade-off because increasing the slice thickness will
decrease the spatial resolution in the z-axis.
• Reconstruction algorithm- Non-linear reconstruction algorithms can
cause noise non-uniformity, meaning the intensity of noise varies
across the image depending on regional structure. Uniform regions of
the image will generally have lower noise levels than highly structured
regions.

29. Aliasing artifact
• Aliasing artifact, otherwise known as undersampling, in CT refers to an error in
the accuracy of analog to digital converter (ADC) during image digitization.
• Image digitization has three distinct steps: scanning, sampling, and
quantization.
• When sampling, the brightness of each pixel in the image is measured, and via
a photomultiplier, creates an output analog signal that is then due to undergo
quantization.
• The more samples that are taken the more accurate the representation of the
signal will be, hence if a lack of sampling has occurred the computer will
process an inaccurate image resulting in an aliasing artifact.

33. Truncation artifact
• apparently increased curvilinear band
of attenuation along the edge of the
image.
• This artifact is encountered when parts
of the imaged body part remain outside
the field of view (e.g. due to patient
body habitus), which results in
inaccurate measurement of attenuation
along the edge of the image. The
artifact can be reduced - if possible - by
using an extended FOV reconstruction
of the affected region.

34. Patient based artifacts
• Motion artifact
• Metal artifact

36. MOTION ARTEFACT
Streak artifact (open arrow) caused by motion of bowel gas in a slow abdominal scan on a conventional single-
slice CT scanner. Machine-caused streaks are also seen. Streak artifact (solid arrow) is caused by an unstable
rotor in the x-ray tube, producing unstable focal spot motion during the scan.

44. Hardware based artifact
• ring artifact
• tube arcing
• helical and multisection artifact - windmill artifact / conebeam artifact
• multiplanar reconstruction artifact - zebra artifact / stair step artifact

53. CONE BEAM ARTEFACT
Phantom study demonstrating cone-beam artifacts. A- Four-slice CT scanner study. The patient's ribs are positioned at a steep angle along
the z-axis to demonstrate cone-beam artifact at pitch 1.25. B- Enhanced contribution of multislice CT cone-beam artifacts caused by
divergence of the x-ray beam in the z-axis when the number of detector rings used to acquire a CT image is increased from four to eight.

60. Thank you