A scanning electron beam CT system generates an electron beam along a beam source axis offset from the scanner axis, or axis of symmetry, thereby permitting the X-ray subject to pass completely through the stationary gantry. The electron beam is produced with the first drift tube region of an evacuated housing chamber, and is directed downstream toward a second region that includes a gantry. A scan target and a tuning target, each concentric with and defining a plane normal to the system axis of symmetry, are located in the gantry. A beam optics system, through which the electron beam passes, is located within the housing intermediate the electron gun and gantry. A control system focusses and scans the electron beam upon the scan target, maintaining a beam spot of desired quality. Upon impingement by the scanning beam spot, the scan target emits a fan beam of X-rays. A detector array, concentric with and defining a plane normal to the system axis of symmetry, is located opposite the scan target within the gantry and provides output signals that are computer processed to reconstruct a CT image of a subject placed within the gantry. The scanner axis is preferably above the beam source axis, and preferably the drift tube is straight, defining a drift tube axis that is inclined relative to the scanner axis, or is parallel to the scanner axis but not co-axial therewith. Alternatively, the drift tube may be kinked.