Optimization of transmission and emission scan duration in 3D whole-body PET

Whole-body PET imaging is being increasingly used to identify and localize malignant disease remote from the site of the primary tumour. Patients are typically scanned at multiple contiguous bed positions over an axial length of 75-100 cm. Ideally, for oncology patients, the total scan duration should not exceed about an hour and therefore only 5-10 minutes of imaging can be performed at each bed position. To minimize the total scan duration, the transmission scan is often omitted and the emission scan reconstructed without attenuation correction. However, whole-body scans reconstructed without attenuation are non-quantitative and can lead to incorrect diagnoses, particularly for tumours located deep within the body. We have performed a series of torso phantom measurements on a 3D PET scanner (ECAT ART) to investigate the optimal partition of scan time between the emission and transmission scans for a fixed total scan duration. We find that 20%-40% of the total scan time should be dedicated to the transmission acquisition to minimize noise in the attenuation-corrected image. The optimal partition depends on the method of calculating the attenuation correction factors and on the reconstruction algorithm.