Characteristics of PET/CT images of the ACR PET phantom

A high-resolution PET/CT scanner was used to make images of a PET phantom that has been recommended by the American College of Radiology. Contrast recovery of 90% is realized for this phantom´s hot inserts of diameter 25, 16, and 12 mm, and 70 to 80% recovery is achieved for the 8 mm hot insert. In the case of the phantom´s cold inserts filled with air or water, the background is reduced to the level of 1 to 2% in the case of filtered-backprojection reconstruction, and below 15% in the case of iterative reconstruction. CT-based attenuation correction does not work correctly in the case of the cold Teflon insert, since Teflon is a dense material not normally found in the human body. In this case, filtered backprojection results in negative pixel values. In images from this tomograph, one can see a negative image of the completely cold walls surrounding the hot inserts. This edge effect means that complete contrast recovery requires measurement in a region that avoids the inner walls of hot inserts by at least 6 mm, which in turn means that cylinders smaller than 12 mm present unavoidable difficulties.