(COR) The center of rotation is the rotation or system axis in SPECT systems.

(Scintillation Camera, Scintillation Gamma Camera, Gamma Scintillation Camera or Anger Gamma Camera) A gamma camera is an imaging device used in nuclear medicine to scan patients who have been injected, inhaled, or ingested with small amounts of radioactive materials emitting gamma rays. The gamma camera records the quantity and distribution of the radionuclide that is attracted to a specific organ or tissue of interest.
The first gamma camera was developed and introduced by Hal O. Anger in 1957/58. The structure hasn't changed by today. A gamma camera consists of:

Through this design the simultaneous registration of gamma ray photons is possible, the computer further allows dynamic imaging.

See also Pinhole, Elution, Center of Rotation, First Pass Scintigraphy, and Anger Hal Oscar.

(FID) The focus to isocenter distance is the distance from the focus of the x-ray tube to the isocenter (the rotation center of the CT gantry).

The isocenter in x-ray imaging is the space through which the central ray of the radiation beams passes.
The CT isocenter is defined by intersection of the gantry axis of rotation. In case of a cylindrical CT type, the isocenter is at the center of the gantry bore.
In radiation oncology the isocenter is defined by the beam centerline of a medical accelerator or cobalt unit. Typically, the isocenter is located 1 m from the radiation source.