SUV: from silly useless value to smart uptake value.

One of the early promises of PET was that it would be quantitative (1), as opposed to imaging modalities such as x-ray CT, MRI, or SPECT, in which quantification is more of a challenge than in PET. By quantitative is meant that a raw PET signal can be transferred into absolute activity concentrations of the radiopharmaceutical in all tissues in units of, for example, kBq/cm3. In this sense, PET can be classified as quantitative molecular imaging, since it provides the absolute number of radioactive molecules in each desired volume, whether this be a single image voxel or a larger volume of interest. Once the specific activity of the radiopharmaceutical is known,