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_lesion) from thin slice CT images the activity concentration in the lesion (AC_lesion) can be determined if the background activity concentration (AC_background) is determined away from the lesion. The total integrated activity (IA) in the ROI is: IA=AC_lesion·V_lesion+AC_background$ 8SV_background. Alternatively the ratio of the activity concentration in the lesion to the background is: [(IA/AC_background)-V_background]/V_l e_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 ∼2. Clinical results for a 1cc volume SPN indicate a tumor to background ratio of ∼20 compared with a ratio of 15 using the maximum SUV, similar to the behavior seen in phantom measurements.