Evaluation of CT Field of View Restoration for PET-CT Attenuation Correction

The Discovery ST PET-CT scanner corrects the attenuation of the 511 keV PET emission using an attenuation map derived from a CT scan which is acquired with a 50 cm detector field of view. The PET scanner uses a 70 cm reconstruction field of view, and to support attenuation correction over the 70 cm diameter, the CT data are extended using a previously published method. The restored CT data have some errors in the extrapolated outline and the density in the extrapolated region, but the integral attenuation along the PET lines of response are restored with an acceptable error for the purpose of emission attenuation correction. The effect of the truncation on the reconstructed PET activity and the reduction in these effects with the restoration algorithm was measured using an active PET phantom. The use of the truncated attenuation map resulted in a loss of activity outside the truncated field of view and also had an effect on the activity within the 50 cm field of view. The use of the extrapolated CT images for attenuation correction results in a considerable improvement in the accuracy of the reconstructed activity.