Patent Number: 050330730
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be made in detail to present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Preferred embodiments of the system for radiographically inspecting a relatively stationary object are shown in FIGS. 1 and 5 and are generally represented by the numerals 10 and 10' respectively. These systems include a radiation source 12 which is operative to transmit radiation along a radiation path in a direction, indicated by line 14, toward a selected location. An object 16 is positioned at the selected location. The radiation transmitted by source 12 is in the form of a radiation beam having a cross-sectional area at least corresponding to a predetermined inspection area of the object 16. Radiation source 12 preferably transmits either X-ray or gamma ray radiation of a predetermined energy spectrum. Accordingly, source 12 may be, for example, a 320 KVP X-ray tube and housing available from Siefert X-Ray Corp. of Fairview Village, Pa. System 10' (FIG. 5) also includes a piece of shielding material 15 that shapes the spectrum of the radiation such that the lower end of the spectrum is more attenuated than the higher end of the spectrum. Systems 10 and 10' further include detection means 18 having a detection area. The detection area of detection means 18 corresponds to the predetermined inspection area of object 16. That is, the predetermined inspection area of object 16 and the detection area of detection means 18 are of substantially the same size and are aligned in the radiation path. In FIG. 5, the area of shielding or filtering material 15 is the same as the radiation beam area. As here embodied, detection means 18 comprises electronic detectors responsive to radiation to generate response signals. Electronic detectors of the type described above include, for example, plastic scintillator-photomultiplier combination electronic radiation detectors available from Bicron Corp. of Newburg, Ohio, each of which generates electronic signals in response to radiation incident upon a given electronic radiation detector. As shown in FIG. 1, detection means 18 is disposed relative to radiation source 12 and object 16 such that object 16 is disposed, at the selected location, between source 12 and detection means 18, in the radiation path. As shown in FIG. 5 filter means 15 is disposed relative to radiation source 12 and object 16 such that filter area 15 is disposed at the selected location between source 12 and object 16. System 10 or 10' also includes a scanning point selection means 20. As here embodied, scanning point selection means 20 includes a first rotating disk 22 and a second rotating disk 24. Both rotating disks 22 and 24 are formed of a material having a high atomic number such as, for example, lead (atomic no. - 82). First rotating disk is disposed in the radiation path of FIG. 1 and FIG. 5 between radiation source 12 and detecting means 18. Accordingly, the first radiation beam from radiation source 12 is transmitted to first rotating disk 22, the cross-sectional area of the first radiation beam being incident upon first rotating disk 22. First rotating disk 22 includes a spirally-shaped aperture 26. As disk 22 is rotated, aperture 26 of disk 22 operates to sequentially select and transmit spirally-shaped portions of the cross-sectional area of the first radiation beam along the radiation path towards detection means 18. The sequential selection and transmission of spirally-shaped portions of the first radiation beam is performed inasmuch as rotating disk 22 blocks radiation from source 12, except for radiation incident at aperture 26 of disk 22. Rotation of disk 22 is described in detail hereinafter. The shape of aperture 26 of rotating disk 22 is also described in detail hereinafter. Rotating disk 24 is also disposed in the radiation path. Rotating disk 24 is disposed between rotating disk 22 and detection means 18 in FIG. 1 and FIG. 5 and is preferably aligned with rotating disk 22, such that an imaginary line from source 12 to detection means 18 includes points coincident with the centers of both rotating disks 22 and 24. Planes defining surfaces of the disks are substantially parallel. Rotating disk 24 includes a spirally-shaped aperture 28. As disk 24 is rotated, aperture 28 operates to sequentially select and transmit from the spirally selected portions from disk 22 areas of radiation each corresponding to a pencil beam of radiation. The pencil beam areas of radiation are transmitted along the radiation path to detection means 18. Rotating disk 24 operates to sequentially select and transmit pencil beam areas of radiation by blocking the spirally-shaped portions of radiation from rotating disks 22 except for the radiation incident at the aperture 28. Rotating disks 22 and 24 are rotated at separate speeds using for example, separate motors and rotating gears (not shown). The rotation of disk 22 is relatively slow in comparison with the rotation of rotating disk 24. Rotating disk 22 is kept stationary during a predetermined time while second rotating disk 24 is rotated through one complete revolution of 360.degree.. The predetermined time period during which the second rotating disk 24 is rotated through a complete revolution may be, for example, 1/30th of a second, but may vary in accordance with the predetermined intensity of radiation from source 12. This predetermined time period may preferably be increased as the intensity of the radiation from source 12 is decreased to provide a larger signal to noise ratio for a weaker source. At the end of the predetermined time, first rotating disk 22 is rotated to a new position. Rotating disk 22 is rotated through a distance corresponding to the width of aperture 26 of disk 22. As disk 22 and 24 rotate, apertures 26 and 28 are themselves rotated, cooperatively, to permit a single pencil beam of radiation to pass from radiation source 12 to detection means 18 in the manner described above. That is, as aperture 26 of rotating disk 22 is rotated to permit spirally-shaped portions of the radiation beam from source 22 to be sequentially transmitted to rotating disk 24, aperture 28 is rotated in a manner such that alignment exists for only a single point from each of apertures 26 and 28. The alignment which exists between points of apertures 26 and 28 is further described below. Preferably, the spirally-shaped apertures 26 and 28 of disks 22 and 24, respectively, are shaped in the form of spiral curves. The spiral curves may have a variety of specific shapes such as parabolic, linear (Archimedian) quadradic or logarithmic (equiangular). Referring to FIG. 2, three specific, exemplary, shapes for corresponding spiral curves for rotating disks 22 and 24 are shown. Also shown are equations for the corresponding spiral curves of rotating disks 22 and 24. Equations in polar coordinates for curved aperture 26 of disk 22 are given below. ##EQU1## EQU R.sub.1 =a.times..theta. (spiral or Archimedes) (2) EQU R.sub.1 =a.times..theta..sup.2 (quadradic spiral) (3) In the foregoing equations, a represents a constant, .theta. represents the polar angle in radian measure from a horizontal axis, and R.sub.1 denotes the radius vector of disk 22. Corresponding equations for curved aperture 28 of disk 24 are given below. ##EQU2## EQU R.sub.2 =a.times.(.pi.-.theta.) (5) EQU R.sub.2 =a.times.(.pi.-.theta.).sup.2 (6) In equations (4), (5) and (6) which respectively correspond to equations (1), (2) and (3), a represents a constant, .theta. represents the polar angle in radian measure from the horizontal axis, and R.sub.2 denotes the radius vector of disk 24. The width of both apertures 26 and 28 is preferably between 0.1 and 1.00 millimeters. In FIGS. 3A, 3B and 3C, the manner in which apertures 26 and 28 are aligned to selectively transmit pencil beams of radiation is depicted. Curved apertures 26 and 28 are shown, superimposed cross-sectionally at times t.sub.1, t.sub.2 and t.sub.3. The time between times t.sub.1, t.sub.2 and t.sub.3 is less than a predetermined time period for rotating second disk 24 through a complete revolution. Accordingly, aperture 26 of disk 22 is shown in the same position during each of times t.sub.1, t.sub.2 and t.sub.3. On the other hand, aperture 28 of disk 24 is shown in different positions corresponding to positions assumed by aperture 28 during rotation. At each time t.sub.1, t.sub.2 and t.sub.3 points of cross-sectional alignment of aperture 26 and 28 exist at cross-sectional points P.sub.1, P.sub.2 and P.sub.3, respectively. These points indicate the location of pencil beams of radiation from source 12 passing through both rotating disks. As aperture 28 rotates, aperture 28 sequentially selects and transmits pencil beams of radiation, from points as shown, to individual point-area portions of the detection area of detection means 18 in system 10 of FIG. 1 and through filter member 15 to detection means 18 of FIG. 5. With the rotation of disk 22, points of alignment for the disks are moved and pencil beams of radiation are sequentially selected and transmitted by disk 24 to the remaining portions of the detection area of detection means 18, the sequential selection and transmission resulting in a spiral sweep of point area portions of detection means 18. Since, as described above in connection with the embodiments of FIG. 1 and FIG. 5, the selected location where the object is positioned is located between source 12 and detection means 18, in the radiation path, the first radiation beam from source 12 is incident upon object 16 or through filter 15 (FIG. 5) and incident upon 16. After interaction with object 16, radiation reaches scanning point selection means 20, and is sequentially transmitted in pencil beam form to detection means 18. Radiation from source 12 incident upon object 16, interacts with object 16 resulting in the production of compton scattered gamma rays, total photoelectric absorption of incident radiation, and production of gamma ray photon pairs at photon energies sufficient to cause radiation to pass through and emerge from object 16. The above-described interactions are unique to and representative of object 16. Accordingly, radiation emerging from object 16 and reaching detection means 18 is characteristic of object 16, or characteristic of the elements constituting object 16, as are the signals generated in response to the radiation reaching detection means 18. These signals are used to obtain an image of the inspection area of object 16. With more particular reference to the system of FIG. 5, generally referred to as 10' wherein like reference numerals refer to like elements, filtering means 15 is disposed between radiation source 12 and object 16. Filtering means 15 is preferably in the form of a plate of a selected thickness that has an area corresponding at least to the area of the radiation beam. Filtering means 15 may be made from different elements that attenuate the X-ray source. Depending upon the element used, the X-ray spectrum will be shaped such that the lower end of the spectrum will be attenuated more than the higher end of the spectrum. This will provide a narrow energy spectrum that will cause X-ray fluorescents from only those elements that have K and L shells within the energy band. Further, the X-ray spectra will not only be varied by filtering the spectra with different elements, but will also be varied by changing the energy of X-ray source 12 by conventional means referred to in block 25. All of the interactions via the photoelectric process result in the emission of fluorescent photons. Thus, the amount of radiation emitted from object 16 depends on the amount of radiation in the spectra at the atomic energy levels for the element of interest. As an electron is ejected from its energy shell, another electron fills its position with the emission of a fluorescent photon. With reference to FIG. 7, curve 31 represents the energy of the fluorescent photons as a function of the atomic number Z of the object 16. The X-ray absorption energies for the individual elements is well known. A table of such energies may be found in the CRC Handbook published by The Chemical Rubber Company, 45th Edition pages E72-E75 thereof and the Review of Scientific Instruments, Volume 23, Nos. 10, at page 523, published in 1952. To excite the various elements requires photons in the primary beam from source 12 at the energy levels of interest. The amount of fluorescent photons given off from an element is directly proportional to the photons in the spectrum at that particular energy level. Referring to FIG. 8, the X-ray spectra for tungsten from an X-ray tube is illustrated for two different energies by curves 33 and 35, respectively. Curve 33 represents a maximum voltage of 50 KEV, and curve 35 a maximum voltage of 100 KEV. The spectra contains photons up to the maximum voltage applied to the X-ray tube. They contain a continuous distribution of photons plus fluorescent radiation characteristic of the X-ray target element. The spectra 33 and 35 can be modified by absorbing a portion of the spectra using various elements of various thickness as shown by the curves in FIG. 9 for the element copper. An unlimited number of combinations of energy and filters may be utilized to shape the beam. With reference to FIG. 10, curves 37 and 39 represent the imaging of silver and tantalum, respectively. With respect to silver, the X-ray beam is generated using a 50 KEV source with light filtration of 0.001 cm. of copper. Only the element silver will show up of in the scanning image object 16 at 50 KEV. In the event the X-ray potential is increased to 100 KEV and 0.04 cm. of copper is applied to filter out the lower portion of the spectrum as shown in FIG. 10, the major source of fluorescent radiation will now come from the tantalum and show up in the scanning image. Referring again to FIGS. 1 and 5, systems 10 and 10' further include a signal conditioning means. As here embodied, the signal conditioning means comprises an analog to digital converter 30 responsive to detection means 18 for converting the analog signals generated by detection means 18 to digital form. Analog to digital converter 30 may be, for example, a model 7612D analog to digital converter available from Tektronix, Inc. of Beaverton, Oreg., which converts received analog signals into digital signals. Systems 10 and 10' further include position encoder means 32. As here embodied, the position encoder means 32 includes means for determining the position of each selected portion of the first radiation beam. That is, position encoder means 32 is responsive to scanning point selection means 20 to determine the point of alignment between disk 22 and 24 to obtain the location of the portion of the first radiation beam transmitted from source 12 through object 16 to detection means 18 at any given time. Position encoder 30 may comprise, for example, means for sensing edge markings on each of disks 26 and 28, and for utilizing information as to the mathematical relationship between the edge marking for each disk and the location of the spiral aperture for the disk relative to the markings. This exemplary position encoder would also comprise a photodetector for receiving light reflections from disk edge markings to determine the present rotational position of a disk. A data processing means of the systems is responsive to position encoder means 32 and detection means 18 for processing the signals generated by detection means 18. As here embodied, the processing means comprises a data processor 34 which establishes a correspondence between a portion of object 16 through which the first radiation beam passes to detection means 18 and response signals generated as radiation is passed through the portion of object 16. Response signals for each pencil beam portion of the detection area of detection means 18 are obtained and processed by data processor 34. Systems 10 and 10' also include an image memory 36 responsive to data processor 34 for storing the signals processed by processor 34, and a display means governed by data processor 34 for generating a radiographic image of the inspection area of object 16 in accordance with the signals stored by image memory 36. Image memory 36 may be, for example, a model ST-100 memory device available from Star Technologies, Inc. of Sterling, Va. The display means, here embodied as display means 38, and which may be, for example, a model NEC Multisync, monitor available from NEC Home Electronics, Inc. of Wood Dale, Ill., provides a visual display of an image of object 16. Referring to FIGS. 4 and 6, the preferred embodiments 50 and 50' of the systems of the present invention are described. In FIG. 4, a system 50 having apparatus and a manner of operation substantially similar to the apparatus and manner of operation of system 10, is shown. In FIG. 6, a system 50' having apparatus and a manner of operation similar to system 10'. Accordingly, the apparatus of systems 50 and 50' are shown with like reference numerals. Since systems 10 and 50, and 10' and 50' are substantially similar, only the differences between such systems are set forth below. In system 50 and system 50', the selected location at which object 16 is positioned is located in the radiation path between scanning point selection means 20 and detection means 18. In connection with the location of object 16 between scanning point selection means 20 and detection means 18, systems 50 and 50' each further comprise a backscatter detection means 40. Backscatter detection means 40 detects radiation interactive with and backscattered by object 16, that is, radiation traveling "backward" from object 16 as a result of either compton scattering or photoelectric fluorescence, and generates signals in response to the radiation detected. Data processor 34 is responsive to the response signals generated by backscatter detection means 40 to process the response signals as described above. An image obtained using both backscatter and through radiation more fully reveals the interactions undergone by object 16 and thus such an image provides a truer representation of the object or the classification of elements in the object. Accordingly, in instances where it is not otherwise undesirable to utilize backscatter detection means 40, the radiographic inspection system of the present invention may include such means. In fact, where for some reason use of detection means 18 proves impractical, only backscatter detection mean 40 may be used in obtaining an image of object 16. With the rotation of disks 22 and 24, as described above, it may occur that an area of detection means 18 and/or 40 detects radiation from a pencil beam of radiation transmitted through scanning point selection means 20 that does not have a uniform cross-section at each point on object 16. This happens in cases where the areas for aperture intersection points vary in size or when the radiation from unit 20 is not uniform in intensity. To compensate for such non-uniformity, data processor 34 corrects the data obtained for image 16 using data obtained from a field flattening scan. Using a uniform calibration object for object 16, spatial variation inspection data is stored in the data memory in a calibration file that will be used to correct all other inspection data. Also, as described above, disks 22 and 24 are rotated by gears. These gears support disks 22 and 24 at their edges, so that no "dead spots" occur in the center. In view of the foregoing, it should be understood that in addition to disclosure of an improved system for radiographically inspecting a relatively stationary object, a related method for doing the same has been disclosed. The method comprises the steps of: transmitting a first radiation beam having a cross-sectional area at least corresponding to the predetermined inspection area along a radiation path in a direction toward the selected location; selecting, sequentially, portions from the cross-sectional area of the first radiation beam and transmitting the selected portions along the radiation path to the selected location, each said selected portion of the cross-sectional area of the first radiation beam comprising a pencil beam of radiation; detecting each of the selected portions of the first radiation beam striking corresponding portions of the predetermined inspection area of an object at the selected locations and generating signals in response to the radiation stiking the corresponding portions; determining the position of each selected portion of the first radiation beam; processing the signals generated in response to the radiation striking the corresponding portions; and generating a radiographic image of the predetermined area of the object. In another aspect the method further comprises the step of filtering each selected portion of the first radiation beam into at least a selected atomic element group for generating a radiographic image of the predetermined area of the object. It should be apparent to those skilled in the art that various modifications may be made to the improved system and method for radiographically inspecting a relatively stationary object without departing from the scope or spirit of the invention. Thus, it is intended that the invention cover modifications and variations of the invention, provided they come within the scope of the appended claims and their legally entitled equivalents.