Abstract:
A hand holdable inspection device for three-dimensional inspection of a volume distal to a surface. The inspection device has a hand-holdable unit including a source of penetrating radiation for providing a beam of specified cross-section and a detector arrangement for detecting penetrating radiation from the beam scattered by the object in the direction of the detector arrangement and for generating a scattered radiation signal. Additionally, the inspection device has a controller for characterizing the volume based at least on the scattered radiation signal. The detector arrangement includes one or more backscatter detectors that may be disposed asymmetrically with respect to the beam and at differing displacements with respect to the surface.

Description:
The present application claims priority from U.S. Provisional Application No. 60/112,102, entitled “Unilateral Hand-Held X-Ray Inspection Apparatus,” filed Dec. 14, 1998, and from U.S. Provisional Application No. 60/113,412, entitled “Separate Lateral Processing of Backscatter Signals,” filed Dec. 22, 1998, both of which applications are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an apparatus for imaging a concealed target by unilateral irradiation and detection of penetrating radiation. 
     BACKGROUND OF THE INVENTION 
     Schulte (U.S. Pat. No. 5,763,886) teaches a two-dimensional imaging backscatter probe for using a source of gamma-rays to illuminate a surface and for generating a two-dimensional image of the backscattered radiation. It is valuable, in many applications, to know the shape and volumetric distribution as well as material characteristics of objects lying behind or beneath the illuminated surface. Schulte, however, fails to suggest that any depth or compositional information may be obtained with respect to objects lying behind or beneath the illuminated surface or to teach any manner in which such information may be obtained. Additionally, Schulte requires that the probe be moved and that the position of the radiation detectors with respect to the target be sensed using means external to the probe in order to map the backscattered radiation. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, in one of its embodiments, there is provided a hand-holdable inspection device for three-dimensional inspection of a volume distal to a surface. The inspection device has a hand-holdable unit including a source of penetrating radiation for providing a beam of specified cross-section and a detector arrangement for detecting penetrating radiation from the beam scattered by the object in the direction of the detector arrangement and for generating a scattered radiation signal. Additionally, the inspection device has a controller for characterizing the volume based at least on the scattered radiation signal. 
     In accordance with alternate embodiments of the invention, the source of penetrating radiation may be an x-ray source, the source may include a scanner, such as an electronic scanner, for scanning a direction of emission of the beam, the detector arrangement may be integral to the hand-holdable unit and may include an array of semiconductor detectors, and the inspection device may additionally have a display screen for displaying an image characterizing the volume distal to the surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing features of the invention will be more readily understood by reference to the following detailed description taken with the accompanying drawings: 
     FIG. 1 provides a side view of an inspection system employing a hand-held source, detector, and display of penetrating radiation in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is a side view of an inspection system employing a hand-held source and detector assembly, with an external display and control unit in accordance with an embodiment of the present invention; 
     FIG. 3 a  is a cross-sectional view of the inspection system of FIG. 1, employed in a “Probe” mode, in accordance with an embodiment of the present invention; 
     FIG. 3 b  is a cross-sectional view of a further embodiment of an inspection system, employed in a “Probe” mode, wherein extended backscatter detectors are employed; 
     FIG. 4 a  provides a schematic representation of an x-ray system employing backscatter detectors asymmetrically disposed with respect to an illuminating beam in accordance with a preferred embodiment of the present invention; 
     FIG. 4 b  provides a schematic representation of an x-ray system employing backscatter detectors asymmetrically disposed with respect to an illuminating beam in accordance with a further embodiment of the present invention; and 
     FIG. 5 is a cross-sectional view of an embodiment of an inspection system, employed in an “Image” mode, wherein the direction of the incident x-ray beam is scanned with respect to a detector arrangement fixed with respect to a beam-forming snout associated with the x-ray source. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a side view of the elements of an x-ray inspection device, designated generally by numeral  10 . A source  12  emits penetrating radiation in a beam  14 . Beam  14  of penetrating radiation, may be, for example, a beam of x-rays such as a polychromatic x-ray beam. The cross-sectional shape of beam  14  may be determined by the geometry of source  12  and a collimating arrangement such as an aperture  28  provided in shielding  16 . Source  12  of penetrating radiation is preferably an cathode ray tube x-ray generator with a sweeping electron beam, as shown. Electron gun  18  emits an electron beam  20  that is accelerated toward anode  22  by virtue of the large positive electrical potential applied to anode  22  with respect to electron gun  18  as well known in the art of x-ray generation. Since electrons comprising electron beam  20  bear electrical charge, their path may be modified by electrical or magnetic deflector means well known in the art. A deflector (otherwise referred to herein also as a “steerer”)  24  employing one or both of electrical or magnetic deflector means is disposed along the path of electron beam  20  between electron gun  18  and anode  20 . By application of an electrical potential or magnetic field to deflector  24 , electron beam  20  may be scanned across anode  22  in a controlled manner such as a raster scanning pattern. X-rays are generated as electron beam  20  impinges upon anode  20 . The emitted x-rays  26  are coupled through aperture  28  in shielding  16  such that, as electron beam  20  is scanned across anode  22 , the orientation of the emergent x-ray beam  14  is swept correspondingly. A high-voltage potential and electron beam control potentials may be derived using on-board power supplies and a local battery  50 . 
     X-ray inspection device  10  may readily be held by an operator who can grip handle  30  and activate the emission of x-ray beam  14  by operation of trigger  32 . The x-ray inspection device is advantageously employed for detecting the presence of an object  36  or cavity or other anomaly in a region posterior to the front surface  38  of a wall  40 , under circumstances where only front surface  38  is accessible to the operator. Inspection of the surface and of the volume disposed distally to the surface may thus be accomplished with only unilateral access to wall  40 . Additionally, object  36  may be imaged, as described below. 
     X-rays  14  impinging on object  36  and on surrounding medium  42  are scattered by the respective matter through the process of Compton scattering. Backscattered radiation is designated by dashed rays labeled  44 . Scattered radiation is detected by one or more backscatter detectors (not shown) which constitute detector arrangement  46  disposed on device  10  on the side of shielding  16  facing outward from the device and toward the object or surface undergoing inspection. In a preferred embodiment of the invention, detector arrangement  46  is an array of x-ray detectors arranged in a planar configuration. Within the scope of the invention, any x-ray detection technology known in the art may be employed for scatter detector arrangement  46 . The detectors may be segmented scintillators or other solid state detectors, for example, or photomultipliers or liquid scintillators which may be doped with tin or other metal. The use of cesium-iodine on PIN diodes and of room-temperature CdZnTe semiconductors are examples of detector technologies which may be employed. Energy resolution of detector arrangement  46  is within the scope of the present invention and advantageously allows a determination of material characteristics of the object according to algorithms well-known in the art. 
     Output signals from the scatter detectors are transmitted to a processor  48 , and processed to obtain images of object  36  and its surrounding medium  42 , or to obtain other characteristics such, for example, as mass, mass density, mass distribution, mean atomic number, or likelihood of containing targeted threat material, all as known to persons skilled in the art of x-ray inspection. The distribution of detected radiation as beam  14  is swept in orientation allows a two-dimensional image of the concealed volume to be obtained. The term “image” refers to a mapping of raw or processed detector signals to positions in the plane, and may be stored in an internal or external memory, or, alternatively, may be displayed visually on a display  52  such as a video monitor. 
     Referring now to FIG. 2, portions of x-ray inspection system  10  may be separated from hand-held unit  54  and may be included in base unit  56  connected to hand-held unit  54  via cable  58  or via any other appropriate means of communication with hand-held unit  54 . Base unit  56  may contain, for example, a controller  60  for processing the signals provided by detector assembly  16  and/or battery supply  50  and power conditioning electronics  62  to provide power for electron gun filament  18  of source  12  and the electrical potentials applied to control steerer  24  and anode  22 . Additionally, base unit  56  may contain display  52  for displaying an image of target  36  posterior to wall  40 . An ambient radiation monitor  64  may be provided, in accordance with a preferred embodiment of the invention, so that x-ray beam intensity may be regulated to provide for safe operation of the unit. The use of ambient radiation monitors in the context of x-ray inspection equipment is described in detail in a co-pending U.S. patent application filed Dec. 1, 1998. 
     Various modes of operation of x-ray inspection system  10  are described with reference to FIGS. 3 a-b  and  4 , wherein like numerals designate identical or similar structural elements of the system. More particularly, operation in a “Probe” mode is described first with reference to FIG. 3 a . In the Probe mode, electron beam  20  is maintained in a substantially fixed orientation relative to anode  22  such that, from among x-rays  78  generated at anode  22 , x-ray beam  14  is emitted through beam forming snout  66  in a substantially forward direction toward, and incident normally upon, wall  40 . Scattered x-ray radiation  44  from object  36  is detected by backscatter detectors  68 . Shielding  70  may be provided to discriminate against radiation  72  scattered by any source near to the inspection apparatus, such as in the wall facing itself. Further shielding  74  may be provided to protect personnel from scattered radiation. 
     Any method of determining the absolute or relative position in space of hand-held unit  10  in order to infer the position behind wall  40  of target  36  is within the scope of the present invention. As examples, and without limitation, optical or other electromagnetic locators  76 , or acoustic locators, may be employed to allow mapping via triangulation of the position of unit  10  and thereby to image and/or display backscatter signals vs. position. 
     In accordance with another embodiment of “Probe” mode operation, backscatter detectors  68  and  80  may be displaced in a direction either tangential to or perpendicular to wall  40 , either symmetrically with respect to beam  14  or asymmetrically. By separately processing the backscatter signals derived respectively at distinct laterally displaced backscatter detectors, the depth of object  36  may be determined in accordance with the detailed teachings of copending U.S. Provisional Patent Application 60/113,412, filed Dec. 22, 1998, and entitled “Separate Lateral Processing of Backscatter Signals.” 
     Referring now to FIG. 4 a , the position and relative sizes of backscatter detectors  68  and  80  may be chosen, in accordance with preferred embodiments of the invention, to optimize the efficiency of the system in discriminating among x-rays scattered from various selected regions of the space penetrated by beam  14 , and to obtain images that enhance scattering features located at different depths behind wall  30 . Comparison of the scattered radiation flux detected at detectors  68  and  80  disposed with lateral asymmetry with respect to beam  14  may advantageously provide a quantitative measure of the distance from the plane of the detectors to scattering object  36  making reasonable assumptions regarding the isotropy of any medium ambient to object  36  through which scattered radiation  140  and  142  propagates to the respective detectors. Scattered radiation  144  scattered from a nearby scattering source  112  may be shielded from detection by one or more of the backscatter detectors  68  and  80 . 
     As shown in FIG. 4 a , backscatter detectors  68  and  80  are disposed asymmetrically with respect to beam  14 . Detector  68  subtends an angle, in the plane shown, of 6.1° with respect to object  36 , whereas detector  80  subtends an angle, in the plane shown, of 5.2° with respect to object  36 . The further detector  80  gets a fraction less than 1 of the counts recorded by near counter  3 . The ratio of counts detected by the respective counters approaches unity as the distance to object  36  increases (as measured with respect to the separation between detectors  68  and  80 ). In this discussion, it is assumed, for simplicity, that propagation effects with respect to scattered beams  140  and  142  may be neglected. Knowledge of the orientation of beam  14  may allow the location of object  36  to be derived using straightforward algorithms. 
     An embodiment of source  10  of beam  14  of penetrating radiation is shown. Beam  20  of electrons emitted by cathode  18  is accelerated toward anode  22 . Electron beam  20  may be scanned with respect to anode  22  such that the orientation of beam  14  may be varied. 
     Referring now to FIG. 4 b , an alternate embodiment of the invention is depicted in which backscatter detectors  68  and  80  are disposed at different distances with respect to concealing surface  30 . As discussed with reference to FIG. 4 a , the difference in counts received by detectors  68  and  80  may be used to determine the distance between concealing surface  30  and scattering object  36 . Again, the ratio of scatter flux detected by the respective detectors approaches unity as the distance to scattering object  36  increases. 
     FIG. 5 depicts operation of x-ray inspection device  10  in an “Image” mode. In this mode, electron beam  20  is swept by deflector  24  across anode  22 , such as between positions A and B, indicated for illustrative purposes only. The resultant x-ray beam  14  emitted through beam forming snout  66  is thus scanned as well, impinging at different time intervals on object A′ and object B′. Since the orientation of the beam  14  is known at each instant, a map may be derived of the orientation of scattering objects A′ and B′. Additionally, since multiple backscatter detectors  68  and  80  are employed, tomographic information may also be derived and the position in space of the scattering objects may be inferred and displayed. 
     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.