Mis-registration between emission and transmission data in cardiac PET/CT

The availability of PET/CT has made CT a popular option for attenuation correction (AC) as it can obtain nearly noise-free transmission (Tx) maps faster and with higher resolution than the standard Ge Tx scan. Due to a large difference in the emission (Em) and CT scan times, organ motion can lead to mis-registration and consequently inaccurate AC resulting in erroneous reconstructed images. This study compares the effects of CT to Ge AC methods on the activity distribution of the myocardial wall in PET images. Simulations, containing respiratory motion, were conducted using the NCAT digital phantom linked to the SimSET Monte Carlo program. Gated Em and Tx data were generated over one respiratory cycle, divided into 20 gates. Three approaches to AC were considered. 1) The average Em map with the average Tx map (Ge scan). 2) Gated Em data with gated Tx data (best correction). 3) The average Em map with 3 Tx maps, representing 3 points in the respiratory cycle (CT scans). Circumferential profiles of transaxial slices through the mid-ventricular heart region were plotted for each AC method. Most of the image profiles matched closely, but one CT map showed deviations as large as 25%. This CT map corresponded to full exhalation in the respiratory cycle. Inconsistent heart profiles indicate that CT corrected images can differ from the standard Ge corrected images. These inhomogeneities are a concern as they may lead to misdiagnosis of cardiac viability.