Technical note: Can photon‐counting CT improve PET/CT's quantitative accuracy?

Abstract

BackgroundLinear attenuation coefficients (LACs) in positron emission tomography combined with computed tomography (PET/CT) are derived from CT scans that utilize energy‐integrating detectors (EID‐CT). These LACs are inaccurate when iodine contrast has been injected. Photon counting detector CT (PCD‐CT) may be able to improve the accuracy.PurposeTo investigate whether PCD‐CT can improve PET/CT quantitative accuracy.MethodsTwo experiments were performed: one with CT only and one that combined PET and CT. The first experiment used an electron density phantom whose inserts were imaged with EID‐CT and PCD‐CT. The inserts simulated normal human tissues, including bone and iodinated blood. In the case of PCD‐CT, virtual‐monoenergetic images at 190 keV were created. LACs were derived in each case and compared against known values. For inserts with iodine, more accurate LACs were expected with PCD‐CT. The second experiment involved a custom PET phantom with various materials simulating human tissues (blood, iodinated blood, and bone) and 18F radioactivity. Data were first acquired with an EID‐CT‐based PET/CT system and then separately in a PCD‐CT system without PET. PET images were reconstructed using LAC from EID‐CT and PCD‐CT. PET image values were compared against known activity values using recovery coefficients (RC).ResultsIn the first experiment, LAC based on EID‐CT were in error by as much as 18%, whereas the corresponding PCD‐CT based measurements were within 3%. In the second experiment, minimum, maximum, and mean RC were (96.1%, 115.4%, and 103.8%) for the EID‐CT method, and (95.8%, 105.5%, and 101.0%) for the PCD‐CT method. The consistency of PET images in body and head orientations was improved.ConclusionsPCD‐CT can acquire the information needed for accurate LAC for PET reconstruction in a single spiral acquisition.