EFFECT OF SMOOTHING DURING TRANSMISSION PROCESSING ON QUANTITATIVE CARDIAC PET SCANS

The effects of attenuation in cardiac PET are large and are produced b y varied and inhomogeneous attenuating media. Although a measured atte nuation correction can potentially provide an exact correction for att enuation, it introduces noise into the attenuation-corrected emission scan, Transmission smoothing reduces this noise but can introduce erro rs of its own, This study investigates these errors in absolute and re lative quantitation and estimates their magnitude in a clinical settin g. Methods: Fluorodeoxyglucose cardiac PET scans of 24 subjects were p rocessed using measured attenuation correction with different levels o f transmission smoothing. Mean activity concentrations were determined in septal, anterior and lateral regions of the left ventricle at each level of transmission smoothing. A theoretical derivation of the effe cts of transmission smoothing is presented, so that the observed effec ts could be compared with theory-based predictions. Results: In additi on to the reduction of noise, transmission smoothing produced two furt her effects: (a) a previously unreported reduction in noise-induced bi as, which is beneficial acid (b) introduction of errors due to bad est imates of attenuation correction factors resulting from smoothing over regions where attenuation changes. The first effect was observed over all regions of the left ventricle, whereas the second reduced counts primarily in the lateral wall. Twenty-millimeter smoothing reduced noi se-induced bias by an average of 4% (compared with 6-mm smoothing). Th is same smoothing caused an additional 9% decrease in the lateral wall as a result of the adjacent lung-lateral wall boundary. Conclusion: T ransmission smoothing reduces both noise and noise-induced bias, but n ear transitions between differently attenuating media (e.g., lung-myoc ardial borders) may produce errors in absolute acid relative quantitat ion, The data presented here document the magnitudes of these effects, permitting one to ensure that artifactually introduced inhomogeneitie s are kept small.