Relationship between CT Numbers and Artifacts Obtained Using CT-based Attenuation Correction of PET/CT

Abstract

The aim of this study was to clarify the artifacts that occurred in the non-activity signal with computed tomography (CT)-based attenuation correction (CTAC) error due to image misregistration. We used a cylindrical phantom containing a test tube with a diameter of 15 mm as the non-activity signal part. Positron emission tomography (PET) images were acquired for 30 minutes using the phantom with water in the non-activity signal part and 18F-fluoro-2-deoxy-d-glucose (18F-FDG) (5.3 kBq/ml) in the background area. CT scanning was performed by replacing the water with contrast agents at different dilutions to obtain arbitrary CT numbers (-1000 to 1000). The PET images were attenuation-corrected individually by the CT images in which the CT number of the non-activity signal part had changed. The relationship between the CT numbers and the CTAC artifact was determined by measuring the PET value in the non-activity signal part of the PET images and comparing Ci. As the CT number of the CT images increased, Ci of the artifact increased. The CT number and Ci had a correlation of y=1.48x+2.86×103 (R2 =0.99) when CTAC was performed in units of CT numbers above 0 for PET data of water (0 HU) and a correlation of y=3.15x+6.26×103 (R2 =0.97) when CTAC was performed in units of CT numbers below 0 for PET data of air (-1000 HU). Although the original CT image was air, the artifacts due to CTAC errors with different Hounsfield units showed larger changes. In particular, positive artifacts were recognized in the PET images after CTAC depending on the Hounsfield units. When the CT number was different from the original in CTAC, the PET value was different. CTAC should be performed with caution as there may be image misregistration.