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.

(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).

(SFOV) The scan field of view is the area being scanned. This selectable scan factor is measured from a CT system isocenter to the most distant located edge of the patient.

See also Reconstruction Field Of View.