Detailed investigation of transmission and emission data smoothing protocols and their effects on emission images

Measured attenuation correction in PET is routinely performed using transmission scans. Acquisition time and noise considerations necessitate low-pass filtering of the transmission data before generating the attenuation correction matrix. This smoothing operation reduces noise propagation from transmission into emission data but also introduces image artifacts that are mostly pronounced around areas of strongly varying attenuation coefficients. The source of these artifacts, which lies in the mismatch of the spatial resolutions of emission and transmission data, was investigated in this study. The effect of different transmission and emission sinogram smoothing protocols on the emission images was also investigated. A method is proposed that addresses the problem, in conjunction with the filtering step during reconstruction. Instead of the standard in-plane low-pass filtering of the emission data during reconstruction, emission and transmission sinograms can be volumetrically filtered to the desired image resolution prior to attenuation correction while reconstruction is performed with no smoothing (Ramp filter). This operation reduces or eliminates resolution mismatch and consequent image artifacts, especially in cardiac studies. The proposed method improves the accuracy of the activity distribution in emission images with minimal computational and signal-to-noise ratio (SNR) cost.