Chapters 10 and 11 have laid out the principles behind 90Y posttreatment imaging using bremsstrahlung single-photon emission computed tomo graphy (SPECT)/computed tomography (CT) or 90Y positron emission tomography (PET)/ CT with an emphasis on quantication. However, dosimetric calculation for internal emitters based on SPECT or PET imaging is not a new idea-it was suggested long before “quantitative imaging” was a routine component of clinical vocabulary (Loevinger et al., 1989; Bolch et al., 1999). It is now standard practice to perform compartmentbased dosimetry for radiolabeled agents in vivo using SPECT or PET. Given a single injected bolus of a radioactive agent, pharmacokinetic parameters can describe a complex relationship of uptake followed by clearance into the tissue of interest. Increasing this complexity, the timeactivity curve can vary from patient to patient, especially in cancer therapy necessitating a patient-specic evaluation. As a result, dosimetry typically involves imaging at multiple time points in order to determine the activity concentration, as a function of time, in both the tissue of interest and the surrounding tissues.