The physics of medical diagnosis and treatment through nuclear medicine. In each of the following cases, a medical condition is described. Identify the medical procedure that would best be used in the diagnosis or treatment of the condition. In your answer, choose one of the following: tracers, external beam treatment, gamma knife surgery, brachytherapy, SPECT, and PET.

1. This type of treatment can be used to treat cancer by inserting catheters into the treatment area and then placing tiny radioactive sources into the catheters.
2. These two forms of diagnosis can be used to take an image of a slice through the body in order to examine the metabolism occurring inside an organ.
3. This type of treatment involves directing powerful, highly focused beams of γ rays aimed at the tumor or malformation through holes in a metal helmet.
4. This type of diagnosis involves tagging molecules with radioactive isotopes to determine the overactivity or underactivity of an organ.