Patent Number: 
Section: description

One method for generating a scanned beam of specified cross section employs a chopper wheel, as shown in FIG. 1 and designated there generally by numeral 10. Electromagnetic radiation is produced by source 12. For purposes of the present description, the electromagnetic radiation will be described as penetrating radiation, and, more particularly, as x-ray radiation, though it need not be so limited in the practice of the invention. Thus, source 12 may be an x-ray tube of any description. In accordance with preferred embodiments of the invention, in order to produce a beam 16 of specified cross-section, chopper wheel 10 may be in the form of an offset hoop 18. X-ray tube 12 is mounted off-axis inside rotating drum or hoop 18, where xe2x80x98off-axisxe2x80x99 refers to the position of a x-ray emitting target relative to the axis 20 about which hoop 18 rotates. Referring to FIG. 2, in accordance with alternate embodiments of the invention, chopper wheel 10 may also be a rotating disc 30 with slit apertures 32, or a rotating wheel with x-ray tube mounted at its center, as shown and described in U.S. Pat. No. 5,764,683, for example. Referring again to FIG. 1, collimated fan beam 14 of x-rays is emitted from x-ray tube 12 and is incident on the inner surface 6 of the drum or hoop 18, in a direction substantially perpendicular to the axis 20 of rotational symmetry of the hoop. Hoop 18 is opaque to the impinging electromagnetic radiation in the energy range of the radiation. Thus, for x-ray radiation, hoop 18 may contain shielding by a heavy element such as lead. Hoop 18 also includes a number of apertures 22, 24 that are typically regularly spaced about its circumference. There is at least one such aperture 22 and the one or more apertures are referred to as a set of apertures herein and in any appended claims. Apertures of differing size, shape, and spectral transmission characteristics are within the scope of the invention. As well, the apertures may include filters selected to attenuate a specified range of electromagnetic energies. At any given time, only one of apertures 24 on the hoop is illuminated by the x-rays 14. All the x-rays 14 are absorbed in the lead shielding in hoop 18 except for the x-rays that can directly escape through illuminated aperture 24. These escaping x-rays form a pencil beam 16 whose direction is defined by the line between the center of the focal spot on the x-ray source target and the center of the illuminated aperture 24. Hoop 18 is rotated about its central axis 20 (called the xe2x80x9crotationxe2x80x9d axis) by means of a rotary actuator 8 which may include a motor, for example. By virtue of rotation of hoop 18 about axis 20; the location of aperture 24 changes, and a scanning beam 16 of x-rays is created that follows the rotation of the hoop. The hoop rotation therefore creates a beam that scans along one dimension, i.e., in the plane that is perpendicular to axis 20. It is to be understood that the emission of multiple beams 16 simultaneously from a plurality of apertures 24 is also within the scope of the present invention as described and claimed herein. To create a beam that raster scans over a two-dimensional area, hoop 18 is also rotated about a second axis 26 (called the xe2x80x9cscanxe2x80x9d axis) that may pass through the wheel center, but that is not parallel to the rotation axis 20. In a preferred embodiment, the scan axis is perpendicular to the rotation axis. The scan axis, however, need not be perpendicular to the rotation axis as the term xe2x80x9cscan axisxe2x80x9d is used herein and in any appended claims. Rotation about the scan axis 26 is effectuated by a rotary actuator 28 as known in the art, and the rotation may be complete or partial, within the scope of the present invention. The rate of rotation about the scan axis 26 is typically slower than the rate of rotation about rotation axis 20, so that successive scan lines are created as the wheel is slowly rotated about the scan axis. A scanning pencil beam that raster-scans over two dimensions in a manner periodic in time is therefore created by simultaneously rotating about the rotation axis and the scan axis. Embodiments of the present invention may advantageously be employed, for example, in an x-ray backscatter system that scans the walls and ceiling of a room. The system is placed in the center of the room, with hoop 18 (called the xe2x80x9cchopper wheelxe2x80x9d) spinning about a horizontal rotation axis at about 80 rpm. As the wheel rotates, pencil beam 16 scans in the vertical direction. To scan the entire room, the wheel and x-ray tube assembly is rotated very slowly about the scan axis, completing one revolution in about 5-10 minutes. The x-ray energy of such a system is preferably between 120 and 225 kV. This system may also be used to covertly detect weapons such as guns and knives concealed: on a person at distances of up to 10 feet. In accordance with further alternate embodiments of the invention, an X-ray backscatter system is used for scanning people. The system is placed in front of a person at a distance of about 3 feet, with the wheel spinning about a horizontal rotation axis at about 100 rpm. As the wheel rotates, the pencil beam scans across the person in the vertical direction. To create a two-dimensional image, the wheel is rotated very slowly about the scan axis over an angular range of about 35 degrees, with the scan being completed in about 5-10 seconds. The x-ray energy of such a system is preferably between 80 and 140 kV. In accordance with yet other embodiments of the invention, chopper wheel 18 rotates about the scan axis 26 but X-ray tube 12 remains stationary. In this case, X-ray tube 12 emits a wide fan beam 14 of X-rays (rather than a narrow, highly collimated fan beam), and the wide fan beam 14 is incident on the inside surface of a wide chopper wheel 18. An advantage of this embodiment is that fewer components need to be rotated about the scan axis. A major disadvantage of this embodiment is that the chopper wheel must be considerably wider, and contain a lot more lead shielding. This leads to a more expensive wheel, with a higher moment of inertia. The described embodiments of the invention are intended to be merely exemplary and numerous variations and modifications will be apparent to those skilled in the art. All such variations and modifications are intended to be within the scope of the present invention as defined in the appended claims.