PET/CT imaging: Effect of respiratory motion on apparent myocardial uptake

Positron emission tomography (PET) attenuation correction (AC) using computed tomography (CT) can be affected by respiratory motion: hi-speed CT captures 1 point of the respiratory cycle while PET emission data averages many cycles. We quantified the changes in apparent myocardial uptake due to this respiratory-induced CT attenuation mismatch. Twenty-two patients undergoing fluorine-18 fluorodexyglucose (FDG) PET/CT received 3 sequential CT scans at normal resting end-inspiration (CT INSPIR), ending expiration (CT EXPIR), and at midvolume between end-expiration and end-inspiration (CT MIDVOL). A pneumotachometer measured absolute changes in lung volume. Seven subjects also underwent a 3-minute transmission scan with a 68Ge rotating rod source (RRS). The PET emission data set was reconstructed up to 4 times using CT EXPIR, CT INSPIR, CT MIDVOL, and RRS AC maps. Relative heart position and cardiac uptake was measured for each CT attenuation correction. Respiratory motion produced marked changes in global and regional myocardial uptake. Changes were large in the lateral and anterior regions at the lung-soft tissue interface (up to 30% using CT INSPIR compared to CT EXPIR for AC) and smaller in the septal region (10% or less). Data corrected with CT EXPIR agreed best with the RRS. Respiratory effects can introduce large inhomogeneities in apparent myocardial uptake when CT is used for attenuation correction.