Computed Tomography

Image Artifacts: Causes & Reduction Techniques

Sofia Skyttner, MSc
Creative Commons-licens

Why learn about artifacts...?

Can you find the 10 lung nodules…?

310_gorilla.jpg
Trafton Drew and Jeremay Wolfe, Brigham and Women's Hospital

Patient with stiff neck...?

mermaid
Horniman Public Museum & Public Park Trust

Patient positioning issue

mermaid_CT_scan
Horniman Public Museum & Public Park Trust

Patient positioned to low...?

low_centered_CT_thorax
Higher dose to skindeep organs
Photon starvation artifact

Hands up...?

hands up cat

Objects outside SFOV

image_artifacts
Patient out of field artifact

Unruly patient...?

CT_motion_artifacts
Patient movement artifact

Fixation, faster rotation, and/or more detector rows...

Mummy_at_British_Museum.jpg
British Museum
…and gating for internal movements (heart, lungs)

Pierced patient...?

Elaine Davidson Front.jpg

Hip or tooth implant patient...?

CT_metal_topogram

Dark streaks, with surrounding white streaks, and noise

CT_metal_artifactCT_hip_metal_artifact

Metal artifacts

Comparison ordinare X-ray...?

Female navel with piercingUmbilical bar piercing
Only one projection…!

Tattooed patient...?

skull_celtic_backpiece_tat_by_2face_tattoo.jpg

When can CT numbers be false...?

For example:

  • Increased density seen just inside the scull bone
  • Pseudoenhancement of renal cysts

High density seen inside ring of high density

due to bone, intravenous contrast, …

Beam hardening artifact

Noise

fat cat

Obese patient at X-ray...?

x-ray-obese-patient
Radiology Rounds, ”Imaging Obese Patients”, Janet Cochrane Miller

Obese patient at X-ray...?

x-ray-obese-patient
A No action
B Raise kV (of course AEC and grid)

Obese patient at CT...?

normal_pat_vs_fat_pat

Left Normal patient
Right Obese Patient

AAPM ”CT obesity Tips & Tricks”, Mannudeep K.Kalra

Obese patient at CT...?

normal_pat_vs_fat_pat

Left Normal patient
Right Obese Patient? Raise kV!

AAPM ”CT obesity Tips & Tricks”, Mannudeep K.Kalra

Fat inside or outside peritoneum...?

patient_fat_shield
AAPM ”CT obesity Tips & Tricks”, Mannudeep K.Kalra

Malnourished patient...?

sculpture_fat_vs_skinny

Physically fit patient...?

physically_fit

Automatic Exposure Control

ctdi_vol_AEC

Extremely tall or short patient...?

Michael Dunn Richard Kiel Wild Wild West
One and the same…?

Slice thickness at least half object size

partial_volume_effect_not

In-adequate slice thickness...?

partial_volume_effect_yes
Partial volume effect artifact

Symmetric rings...?

CT_detector_failure_artifact

Detector failure/miscalibration

CT_detector_failure_artifactmiscalibrated-dector-element
Ring artifact

Random shadows & streaks...?

tube_arc_artifact

X-ray tube failure

tube_arc_artifact
Tube arcing artifact

Too large pitch value...?

windmill-artifact

Undersampling if pitch too large (p>>1)

windmill-artifact
Spiral/helical (a.k.a windmill) artifacts

Pronounced if high contrast edge between detector rows

Object of interest far from center of field of view...?

…combined with helical, multidetector row CT?

  • Zebra artifacts (alternating high and low noise slices)
  • Streak artifacts
  • Stair-step artifacts (coronal & sagittal)

What about this artifact in coronal & sagittal view...?

no_artifactartifact Left: No artifact (160 mm) Right: Stitching artifact (4*40 mm)

Axial CT head, 140 kV, 42 mGy (left) 52 mGy (right)

Patient related CT artifacts

  • Metal artifacts
  • Patient movement artifacts
  • Patient out of field artifacts

Scanner related CT artifacts

  • Ring artifacts (detector failure/miscalibration)
  • Tube arcing artifacts (X-ray tube failure)
  • Spiral artifacts (undersampling, pitch >> 1), a.ka. helical or windwill artifacts
  • Cone beam artifacts
  • MPR (Multiplanar Reconstruction) artifacts etc…

Physics related CT artifacts

  • Partial volume effect artifacts
  • Photon starvation artifacts
  • Beam hardening artifacts
  • Aliasing artifacts (undersampling)

Summary

CT_artifact_sum

References

Artifacts in CT: Recognition and Avoidance, Julia F Barrett & Nicholas Keat, RadioGraphics, RSNA 2004

Abdominal Multislice CT for Obese Patients: Effect on Image Quality and Radiation Dose in a Phantom Study1, Sebastian T. et al, Acad Radiol 14:486–494, 2007

CT Artifacts: Causes and Reduction Techniques, Boas and Fleischmann, Imaging Med 4(2):229-240, 2012

&

http://www.impactscan.org
http://www.ctug.org.uk/