Accurate SUV determination for small solitary pulmonary nodules using CT-based lesion volume estimates

Evaluation and follow up of small pulmonary nodules has become commonplace with advancing PET and CT technology. Limits in system resolution, recovery coefficient estimations and transaxial ROI determinations often make obtaining useful SUV data difficult. We demonstrate a method which may be more accurate for making these determinations. It is assumed that the nodule can be distinguished clearly on CT and that it resides in a region of homogenous background PET uptake. By integrating the PET activity over a large region including the small nodule and determining the volume of the lesion (V/sub lesion/) from thin slice CT images the activity concentration in the lesion (AC/sub lesion/) can be determined if the background activity concentration (AC/sub background/) is determined away from the lesion. The total integrated activity (IA) in the ROI is: IA=AC/sub lesion//spl middot/V/sub lesion/+AC/sub background/$ 8SV/sub background/. Alternatively the ratio of the activity concentration in the lesion to the background is: [(IA/AC/sub background/)-V/sub background/]/V/sub l /e/sub sion/. Results for activity ratios using a hollow spheres phantom indicate this method gives the correct value for the smallest sphere (activity volume=0.6cc) whereas the ratio using usual SUV determinations is spuriously low by a factor of up to /spl sim/2. Clinical results for a 1cc volume SPN indicate a tumor to background ratio of /spl sim/20 compared with a ratio of 15 using the maximum SUV, similar to the behavior seen in phantom measurements.