The Effects of Attenuation and Scatter Correction on Positron Emission Tomography Quantitation

Abstract

X-ray computed tomography (CT) forms the basis for attenuation corrected positron emission tomography (PET) using combined PET/CT scanners. With concerns of high radiation exposure to patients through widespread use of CT, the lowest photon flux that will provide uniform attenuation correction for PET to within 5% over a range of body sizes was investigated. Additionally, clinical uniformity measurements are performed on a uniform phantom, but their results may not be applicable as an estimate of error of hot lesions. PET simulations of variability and localized error were performed with and without hot lesions using a tapering phantom. Images were reconstructed using a variety of fixed and modulated tube-current CT scans and various levels of scatter correction. A physical phantom was designed and scanned to augment the simulation results. Attenuation correction of uniform images was within 5% error when using 120 kVp using a noise index of 50 and 140 kVp using a noise index of 50 for all phantom sizes. Variability with hot lesions was within 5% for scans using 120 kVp and greater than 24 mAs for 21.9 cm and 31.7 cm effective diameters and greater than 48 mAs for 38.5 cm effective diameter. Variability was worse in the background than on hot lesions for poor attenuation correction and poor scatter correction cases. Background error overestimates the error in hot lesions when attenuation correction is biased. Variability was within 5% when estimation of scatter magnitude was within 20% of its true value both with and without hot lesions. Errors in background due to under and overcorrected scatter lead to an over and underestimate of hot lesion errors, respectively. Physical phantom uniformity was within 5% when using 120 kVp and 10 mAs, albeit with a much smaller phantom size. The background error and its underestimation of lesion error was also measured in the physical phantom.