In Nuclear Medicine we inject a radioactive isotope in to the body of the subject to get diagnostic information for various diseases. This radionuclide which emits gamma photons or positrons are also called radiopharmaceuticals. When this radionuclide decays, there is an emission of gamma rays or high-energy photons. These high energy photons can exit the body with less scattering or attenuation. An external detector such as a gamma camera which is position sensitive can be used to capture these photons and then produce an image of distribution of the radioactivity in the subject’s body. Emission computed tomography is a method in which multiple cross sectional images of tissues and organs can be produced. This technique thus provides better image contrast and improvised detection of abnormalities. ECT is divided into two modalities: SPECT and PET.