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CT Attenuation-Correction Artefacts on PET/CT

Artefacts · 2 min read

Metal, contrast, breathing and truncation, and why the non-AC images settle most of them

PET/CT converts CT numbers into 511 keV attenuation values. When the CT is wrong for the PET, so is the correction: tissue that looks too dense on CT is over-corrected and appears hot; a CT that does not match the PET in position puts activity in the wrong place. The non-attenuation-corrected (NAC) images carry none of this error.

Metal

  • Dental work, hip prostheses and other metallic implants have very high CT numbers. These map to high attenuation coefficients and overestimate activity next to the metal.
  • Near dental metal, CT-corrected activity was about 1.2 times the germanium-corrected value, against about 1.06 away from the artefact.
  • In phantoms, hip prostheses produced foci that mimicked increased FDG uptake. They were worse when the object moved between the CT and PET.

Contrast

  • An undiluted intravenous bolus can create a hot focus exactly over an enhanced vessel. In one series this occurred in 4 of 30 patients (13%); their veins measured about 2260 HU against about 1060 HU without the artefact.
  • Ordinary venous-phase contrast matters much less. SUVmax rose by 27% in the subclavian vein on the injection side, 17% in the heart and 8% in tumours, which the authors judged clinically insignificant.
  • Dense residual oral barium over-corrects and can mimic bowel uptake.

Breathing

  • A CT taken in deep inspiration against PET averaged over free breathing gives curvilinear cold bands at the lung–diaphragm interface. EANM advises against deep-inspiration CT for attenuation correction when quantification matters.
  • Misregistration can also move lesions. In 300 patients, liver-dome lesions appeared in the right lung base in 6 (2%). None was mislocalised on the NAC images.

Truncation

The CT field of view is often smaller than the PET field. Arms by the sides or a large body fall outside it. The result is a rim of spuriously high activity at the edge of the truncated CT, with low activity beyond it, and unreliable SUVs there. Arms up, extended CT field of view and truncation correction reduce it.

Checking the NAC images

EANM recommends reconstructing with and without attenuation correction, and checking lesions next to metal or contrast on the NAC images. Use NAC to ask one question: is there a focus? Do not read intensity or SUV from it.

Pearl

Hot on AC, absent on NAC and sitting on metal or dense contrast: artefact until proven otherwise. Hot on both: real uptake, even beside a prosthesis.

Take home
  • Anything that inflates CT numbers, such as metal, a dense contrast bolus or barium, can create a false hot spot on attenuation-corrected PET.
  • Breathing mismatch causes cold bands at the diaphragm and can place liver-dome lesions in the lung; truncation creates a hot rim at the CT edge.
  • Always have the NAC images: they confirm or dismiss a suspect focus, but they are not for quantification.
Sources
  1. Sureshbabu W, Mawlawi O. PET/CT imaging artifacts. J Nucl Med Technol. 2005;33(3):156-61.
  2. Boellaard R, Delgado-Bolton R, Oyen WJ, et al. FDG PET/CT: EANM procedure guidelines for tumour imaging: version 2.0. Eur J Nucl Med Mol Imaging. 2015;42(2):328-54.
  3. Kamel EM, Burger C, Buck A, et al. Impact of metallic dental implants on CT-based attenuation correction in a combined PET/CT scanner. Eur Radiol. 2003;13(4):724-8.
  4. Goerres GW, Ziegler SI, Burger C, et al. Artifacts at PET and PET/CT caused by metallic hip prosthetic material. Radiology. 2003;226(2):577-84.
  5. Antoch G, Freudenberg LS, Egelhof T, et al. Focal tracer uptake: a potential artifact in contrast-enhanced dual-modality PET/CT scans. J Nucl Med. 2002;43(10):1339-42.
  6. Mawlawi O, Erasmus JJ, Munden RF, et al. Quantifying the effect of IV contrast media on integrated PET/CT: clinical evaluation. AJR Am J Roentgenol. 2006;186(2):308-19.
  7. Goerres GW, Burger C, Kamel E, et al. Respiration-induced attenuation artifact at PET/CT: technical considerations. Radiology. 2003;226(3):906-10.
  8. Osman MM, Cohade C, Nakamoto Y, et al. Clinically significant inaccurate localization of lesions with PET/CT: frequency in 300 patients. J Nucl Med. 2003;44(2):240-3.
  9. Mawlawi O, Erasmus JJ, Pan T, et al. Truncation artifact on PET/CT: impact on measurements of activity concentration and assessment of a correction algorithm. AJR Am J Roentgenol. 2006;186(5):1458-67.
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