This lecture covers the techniques to discriminate scattered photons in SPECT imaging, focusing on the identification of severely scattered photons using a Tc-99m spherical phantom. It also discusses the processes contributing to the scintillation light spectrum, emphasizing the scintillation signal's dependence on x-ray energy. Additionally, the lecture provides a summary of Single Photon Emission Computed Tomography (SPECT), detailing the measurement of single photon emitters injected into the subject, the importance of collimation for x-ray directionality, and the undesirable effects of absorption. The lecture concludes by highlighting the use of SPECT-CT for imaging, combining the benefits of both techniques.