Combining PET and CT: effect on volume measurements

Anatomic and metabolic imaging is important in cancer patients. "Mechanical" fusion with CT and PET is in-vogue because of improved lesion localization. Tumor volume imaging with CT and PET is more complex. Spheres varying from 1-20 ml were used to simulate tumors. For CT. spheres were filled with: 1) air, or 2) iodine contrast 6 mg/ml. with water as background. CT collimation was I mm and pitch 1-2. For PET, spheres were filled with C-11 and the background with F-18. yielding varying contrasts during the 5-hr acquisition. PET acquisition was 2D and 3D, reconstruction FBP and OSEM. CT volume was determined with a 50% tumor/background threshold. and PET volume with 30-80% isocontours. Contrast difference for CT was 60-80 HU. CT volume was significantly different for spheres with air than contrast. related to the wall thickness. PET contrast ratios decreased from 250 to 0.5 Generally. volumes by PET were larger than CT for small lesions. PET volume was unreliable for lesions less than 10 mm. Lesion contrast had to be more than 1.5 to obtain accurate results. Volume can be accurately measured with CT and PET. Both modalities can be calibrated to provide the correct volume. PET produces significant deviations for contrast ratios <1.5 and lesions <10 mm.