1. A radiant energy imaging apparatus for examining a human body by means of penetrating radiation, said apparatus comprising support means for supporting a human body to be examined and for rotating the human body about a vertically oriented axis of rotation, an X-ray system movable translationally as a unit in a vertical direction parallel to said axis of rotation; said X-ray system comprising a source of X-rays located entirely on one side of said support means, elongated radiant energy detector means located on the opposite side of said support means and extending in a horizontal direction transverse to said vertical axis of rotation, and a mechanical scanning device located entirely between said X-ray source and said support means for producing a single pencil beam of X-rays and for scanning said single pencil beam in a horizontal plane through a human body on said support means and along the horizontal direction of elongation of said detector; means the length of said elongated detector means and the positions of said detector means and mechanical scanning device being selected to cause said pencil beam of X-rays to subtend an angle which embraces a complete cross section of a human body on said support means as said pencil beam of X-rays is scanned horizontally along said detector means; drive means for selectively rotating said support means about its vertical axis of rotation and for selectively effecting vertical translational movement of said X-ray source, said mechanical scanning device and said detector means as a unit in a direction parallel to said vertical axis of rotation and relative to a human body to be examined; and means responsive to the signals which are produced by said elongated horizontal detector means as it is scanned by said pencil beam for generating images of the vertically oriented human body being examined on said support means; said drive means including means operative in a first mode to dispose said unit at a desired angle relative to said support means and to move said unit in translation generally parallel to the elongated vertical axis of rotation of said support means while said unit is maintained at said angle relative to said support means and while said support means is in a nonrotating condition, thereby to produce a localization image representative of a vertical axial section of the human body being examined, and said drive means including means operative in a second mode to position said unit translationally relative to said support means to a predetermined position adjacent a particular cross-sectional portion of interest of the human body being examined, selected by reference to said localization image, and to rotate said support means and the human body thereon smoothly and continuously about said vertically oriented axis of rotation while said unit is held stationary at said predetermined position thereby to produce in said second mode of operation an image representative of a horizontal slice of the human body being examined at the cross-sectional portion of interest selected by reference to the localization image produced during said first mode of operation.