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Truncation Artifact on PET/CT: Impact on Measurements of Activity Concentration and Assessment of a Correction Algorithm

¹ Department of Imaging Physics, M. D. Anderson Cancer Center, 1515 Holcombe Blvd., Box 56, Houston, TX 77030.
² Department of Diagnostic Imaging, M. D. Anderson Cancer Center, Houston, TX.
³ GE Healthcare, Waukesha, WI.

OBJECTIVE. Discrepancy between fields of view (FOVs) in a PET/CT scanner causes a truncation artifact when imaging extends beyond the CT FOV. The purposes of this study were to evaluate the impact of this artifact on measurements of ¹⁸F-FDG activity concentrations and to assess a truncation correction algorithm.

MATERIALS AND METHODS. Two phantoms and five patients were used in this study. In the first phantom, three inserts (water, air, bone equivalent) were placed in a water-filled cylinder containing ¹⁸F-FDG. In the second phantom study, a chest phantom and a 2-L bottle fitted with a bone insert were used to simulate a patient's torso and arm. Both phantoms were imaged while positioned centrally (baseline) and at the edge of the CT FOV to induce truncation. PET images were reconstructed using attenuation maps from truncated and truncation-corrected CT images. Regions of interest (ROIs) drawn on the inserts, simulated arm, and background water of the baseline truncated and truncation-corrected PET images were compared. In addition, extremity malignancies of five patients truncated on CT images were reconstructed with and without correction and the maximum standard uptake values (SUVs) of the malignancies were compared.

RESULTS. Truncation artifact manifests as a rim of high activity concentration at the edge of the truncated CT image with an adjacent low-concentration region peripherally. The correction algorithm minimizes these effects. Phantom studies showed a maximum variation of -5.4% in the truncation-corrected background water image compared with the baseline image. Activity concentration in the water insert was 6.3% higher while that of air and bone inserts was similar to baseline. Extremity malignancies showed a consistent increase in the maximum SUV after truncation correction.

CONCLUSION. Truncation affects measurements of ¹⁸F-FDG activity concentrations in PET/CT. A truncation-correction algorithm corrects truncation artifacts with small residual error.

Keywords: attenuation correction • nuclear medicine • oncologic imaging • PET/CT • truncation artifact

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