Low-Cost Position-Sensitive X-Ray Detector

The present application claims priority from US Provisional Application No.

61/348,161, filed May 25, 2010, which is incorporated herein by reference.

Technical Field

The present invention relates to systems and methods for producing x-ray images, and, more particularly, for producing images of x-rays transmitted through an inspected object.

Background Art

Spatial resolution is typically obtained in x-ray transmission images either by means of a segmented detector, where pixels are illuminated in parallel by means of a fan- or cone- shaped x-ray beam, or else by means of a pencil beam that is scanned across an inspected object in a raster fashion. In some applications, transmission images are still obtained by exposure of an x-ray sensitive film. Segmented detectors are costly, whereas the mechanical structure required for collimating and scanning a pencil beam is not only costly but also heavy and cumbersome.

Summary of Certain Embodiments of the Invention

In various embodiments of the present invention, an imaging apparatus is provided for imaging transmission of penetrating radiation through an object. The apparatus has a scintillator region, adapted for translation along a path within the illuminated cross-section of a beam of penetrating radiation. A photodetector detects light emitted by the scintillator region as a function of time, and generates a detection signal corresponding to each position of the scintillator region within the beam cross-section, which is received by a processor, and serves, in turn, to generate an image signal. Finally, the apparatus has a display for depicting an image of the transmitted penetrating radiation based on the image signal. Additionally, the apparatus may have a source for generating the beam of penetrating radiation, with respect to which the scintillator region moves in a plane distal to the object with respect to the source.

In accordance with alternate embodiments of the invention, the source of penetrating radiation may be an x-ray fan beam source. The scintillator region may be a phosphor dot or strip, and may be translated over a portion, up to an entirety, of the beam cross-section, on a belt, or otherwise. A sensor may determine an instantaneous disposition of the scintillator region with respect to the beam of penetrating radiation. The scintillator region may be a portion of a pattern progressively exposed to the beam. The phosphor dot may be exposed to the beam at a glancing angle with respect to a direction of motion of the dot.

In accordance with further embodiments of the invention, a method is provided for imaging transmission of penetrating radiation through an object. The method has steps of:

a. translating a scintillator region along a path within a beam cross-section of a beam of penetrating radiation in a plane distal to the object with respect to the source;

b. detecting light emitted by the scintillator region and generating a detection signal; and

c. receiving the detection signal and generating an image signal; and d. depicting an image of the transmitted penetrating radiation based on the image signal.

Generating the beam of penetrating radiation may be an additional component step of the invented method.

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 reference to the accompanying drawings, in which:

Fig. 1 is a perspective depiction of an apparatus for imaging contents of an object in x-ray transmission, in accordance with an embodiment of the present invention; Fig. 2 shows a relative orientation of a scintillation dot relative to an incident beam, in accordance with an embodiment of the present invention;

Fig. 3 is an image of a circuit board acquired using an apparatus and method of the present invention;

Fig. 4 is a perspective depiction of an apparatus for imaging contents of an object in x-ray transmission, in accordance with an alternate embodiment of the present invention; and

Fig. 5 shows segmented optical readout of multiple pixilated scintillation regions in accordance with an embodiment of the present invention.

Detailed Description of Specific Embodiments

As used in this description and in the appended claims, a "vehicle" includes any conveyance that may be driven, pushed, or pulled from one place to another, whether over the surface of land or otherwise. The term "vehicle," as used herein, further includes the structures, components and contents that are conveyed together with the vehicle.

The invention described herein serves to characterize materials which may be contained within a vehicle and thus not readily susceptible to visual scrutiny. The

characteristics of a material which might be the object of non-invasive inspection and which lend themselves to detection using the device and method taught by the invention include, but are not limited to, electron density, atomic number, mass density, linear dimensions and shape. These characteristics are unveiled by taking advantage of the various physical processes by which penetrating radiation interacts with matter. Penetrating radiation refers to electromagnetic radiation of sufficient energy per photon to penetrate materials of interest to a substantial and useful degree and include x-rays and more energetic forms of radiation. The interaction of such radiation with matter can generally be categorized as either scattering or absorption processes. Both types of process remove x-ray photons from a collimated (i.e., directional) beam; scattering processes do so by deflecting photons into new directions (usually with loss of energy), while absorption processes simply remove photons from the beam.

Conventional transmission imaging measures the total beam attenuation as a function of position on the image plane, without discriminating between absorption and scattering processes. The total beam attenuation is described by a parameter called the mass attenuation coefficient, as commonly employed by persons skilled in the art of x-ray inspection. The mass attenuation coefficient is a characteristic of a particular material at a specific x-ray photon energy, and is independent of the imaging geometry. As such, it is the sum of individual coefficients (or "cross sections") for each relevant physical process, each of which varies differently with x-ray energy and with the atomic number (Z) of the interacting material.

In the range of photon energies useful for penetrating and screening vehicles, the scattering contribution is dominated by incoherent, or Compton scattering, and the absorption contribution is dominated by the photoelectric effect at lower energies, and by pair production at higher energies. The cross sections for Compton scattering and photoelectric absorption vary with both the atomic number of the material and with the energy of the x-ray photon, but in very different ways. The photoelectric absorption decreases very rapidly with increasing photon energy, and increases very rapidly with increasing Z of the material. The Compton scattering cross section changes very slowly with energy and is only weakly dependent on atomic number. The pair production cross section can be ignored for sources with an energy below about 4 MeV, and increases with increasing Z of the material. Such differences in scattering and absorption characteristics between low Z materials,

characteristic of organic materials, and high Z materials, characteristic of most metals and their alloys, are typical and provide the means to differentiate between these two classes of materials.

Transmission x-ray images, taken alone, provide a map of the attenuation

characteristics of the inspected object for the full spectrum of the x-ray beam. It should be noted that images may be directly displayed in graphic format for the visual inspection of human operators, but need not be so displayed. As used in this description and in the appended claims, the term "image" refers to any multidimensional representation, whether in tangible or otherwise perceptible form or otherwise, whereby a value of some characteristic is associated with each of a plurality of locations corresponding to dimensional coordinates of an object in physical space, though not necessarily mapped one-to-one thereonto. Thus, for example, the graphic display of the spatial distribution of some feature, such as atomic number, in one or more colors constitutes an image. So, also, does an array of numbers in a computer memory or holographic medium. Similarly, "imaging" refers to the rendering of a stated physical characteristic in terms of one or more images.

Backscatter imaging, in which the x-rays scattered by a material in a generally backward direction are employed, offers several unique inspection capabilities and operational features. (1) Taken alone, it is a one-sided imaging modality: images can be obtained even when the object is accessible from only one side, or, the object is too thick to be penetrated radio graphically. (2) Because the scatter signal falls off quite rapidly with increasing depth into the object, backscatter images effectively represent a "slice" of the object characteristic of the side nearest to the x-ray source; this image is frequently useful even when a transmission image representing the same scanned area is hopelessly confused by image clutter. (3) The underlying physical phenomenon that leads to scattered radiation is the Compton effect. Low atomic number (low Z) materials, which encompass organic materials, interact with x-rays principally by Compton scattering. Narcotic drugs, being among the densest of organic materials, tend to produce the brightest signatures in a backscatter image, as do organic explosives, making backscatter imaging a useful imaging modality for bomb or drug detection. (4) Alignment requirements of the x-ray beam with detectors or collimation devices are less exacting than for transmission imaging thereby enabling rapid deployment in a wide range of inspection scenarios.

It is known to persons skilled in the art of x-ray inspection that high-Z and low-Z materials may be separately identified by measuring total attenuation at two different photon energies. This is the basis for dual-energy systems. Another method to image low-Z materials is backscatter imaging. The technique relies upon the direct detection of photons which have

As used in this description and in the appended claims, the term "image" refers to any representation, in one or more dimensions, whether in tangible or otherwise perceptible form or otherwise, whereby a value of some characteristic is associated with each of a plurality of locations corresponding to dimensional coordinates of an object in physical space, though not necessarily mapped one-to-one thereonto. Similarly, "imaging" refers to the rendering of a stated physical characteristic in terms of one or more images.

In accordance with preferred embodiments of the present invention, a simple and low-cost method is provided for replacing an expensive segmented detector array for acquiring transmission images with a beam of penetrating radiation, such as x-rays. One embodiment of the invention is now described with reference to Fig. 1. A small piece of scintillating material 10 (such as a phosphor "dot" or strip) is driven along a path within a beam 14 of x-rays, generated by a source 36, and having a specified cross section such as that of a fan beam, for example. The cross-section is taken in a plane distal to the object with respect to source 36. In the embodiment shown in Fig. 1, phosphor dot 10 is coupled to a moving belt 12 and, by virtue of its motion with the belt, samples the intensity of beam 14, as transmitted through an object 18, as the dot 10 moves rapidly along the length of the beam cross section. Belt 12 is driven by drive motor 13. Other means for moving dot 10 within the region illuminated by beam 14 are within the scope of the present invention. For every integration time period (for example, every 16 microseconds), the intensity of emission of scintillation light from the piece of scintillator 10 is measured and recorded by a photo- detector 16, such as a Photomultiplier Tube (PMT). As the integration time period corresponds, roughly, to a resolution element, if the resolution is to be on the order of 1 mm, then the speed of moving dot 10 would be on the order of 60 m/s, although the scope of the present invention is limited in no way to any particular range of resolution or speed.

In the present description and in any claims appended hereto, the terms "dot" or "phosphor dot" may be used interchangeably with the term "piece of scintillator," "pixilated scintillation region," or the like. As the phosphor dot 10 moves along the entire length of the cross section of fan beam 14, the intensity of transmitted x-rays is measured as a function of position, allowing a one-dimensional attenuation profile of an illuminated object 18 to be acquired. Such a one-dimensional attenuation profile may be encompassed, herein, within the term "image," although it is generally intended that the examined object be translated in a direction transverse to the fan beam, thereby allowing a full two-dimensional image of the object to be formed. In a typical embodiment of the invention, belt 12 is polyurethane, or other elastomer, and is on the order of 2-5 mm wide.

By moving the object 18 through the fan beam 14, many such profiles can be combined to form a two-dimensional transmission image 20 (shown in Fig. 3) of an object. An optical sensor 22 may be used to read off the position of the phosphor dot 10 in order to ensure that the profiles are "stitched" together accurately into the final 2D image 20.

Alternatively, a Hall sensor 24 may be used to create a reference signal when a piece of conductor (such as aluminum foil) 25 attached to the belt 12 passes the sensor 24, so that the position of the phosphor dot 10 relative to the fan beam 14 can always be determined.

In the embodiment of the invention depicted in Fig. 1, two phosphor dots 10 and 30 are used so that one of the dots is always being illuminated by the x-ray beam 14 at any time. In addition, each of the dots 10 and 30 can use a different type of phosphor, which preferentially absorbs different ranges of x-ray energies. This allows dual-energy

transmission x-ray images to be acquired, where alternating scan lines in the image correspond to the intensity of different energy ranges of transmitted x-rays. Thus, a dual- energy analysis may be performed on adjacent image lines, yielding the effective atomic number of the attenuating material using known algorithms.

In order to increase the effectiveness of the phosphor dot 10 to absorb the x-rays of beam 14, the belt 12 may be oriented as now described with reference to Fig. 2. In this case, the dot 10 consists of a thin strip 32 of scintillating material, with the long dimension of the strip 32 oriented substantially along the beam direction 34 of x-ray beam 14 from x-ray source 36 through inspected object 18. The short dimension of thin strip 32 is substantially parallel to the direction of belt motion. The belt 12, driven by pulley 33, is tilted relative to the fan beam 14 so that the phosphor strip 32 is illuminated at a grazing incidence. The normal to the belt direction of motion is designated by numeral 38. Tilting the phosphor dot increases the stopping power of the phosphor by a factor of l/sin0, where Θ is the grazing incidence angle of the x-rays with the phosphor.

For example, if the grazing incidence angle is 5 degrees, the stopping power of the phosphor is increased by about a factor of 12 without significantly affecting the imaging resolution of the detector. An example of an image of a circuit board acquired in this manner is shown in Figure 3, for a 1mm x 8mm strip of phosphor at 140kV.

A further embodiment of the invention is now described with reference to Fig. 4. In accordance with a transmission imaging system designated generally by numeral 40, motion of a detection region (or "spot") 42 that defines the size of an imaging resolution pixel is achieved by rotating a shaft 43 having a helical scintillation band 44 applied thereon. The helical scintillation band may be painted onto shaft 43 in a low-attenuation binder matrix, or injected into pores microdrilled into the shaft in an amorphous slurry, of phosphor power in a polymeric matrix, as taught, for example, by Kleinmann et al, "An x-ray imaging pixel detector based on scintillator filled pores in a silicon matrix," Nuclear Instruments and Methods in Physics Research, vol. 460, pp. 15-19 (2001), and US Patent No. 7,265,357 (to Khanh, et al), both of which are incorporated herein by reference. A collimator slit 45 may be interposed between inspected object 18 and detection region 42 to define pixel resolution size. Optical detector 16 detects scintillation, typically in the visible portion of the electromagnetic spectrum, and generates a signal proportional to the x-ray intensity transmitted through object 18 at a particular pixel and giving rise to scintillation at detection region 42. Shaft 43 is rotated by motor 46, and, over the course of a single rotation, detection region 42 (defined by the scintillator material that is visible through the collimation slit when viewed from the source) moves along the entirety, or a specified portion, of the vertical extent of fan beam 14. A rotational encoder 47 may be used to determine the position of the detection region 42 within the cross section of beam 14.

In accordance with other embodiments of the invention, described, now, with reference to Fig. 5, an imaging x-ray detector, designated generally by numeral 50, may be segmented, such that each segment subtends a portion of the cross section of an incident beam 14. Each segment of detector 50 is enclosed within a light-tight enclosure 51, the wall 52 of which that faces the incident beam 14 of penetrating radiation is substantially transparent to the penetrating radiation. The interior of each light-tight enclosure 51 is reflective such that visible light emitted by detection region 42 upon incidence of x-rays is effectively captured by a photodetector 54. Detection signals generated by photodetectors 54 are processed by processor 56 to produce an image signal. The image signal may be further processed to derive characteristic features of the inspected object, and may be displayed as an image 58 of transmission through the inspected object on display 59, or otherwise.

In accordance with other embodiments of the invention, multiple detectors may be combined to form L-shaped or U-shaped detectors. Alternating phosphor dots (or regions) may have different sizes in addition to a different phosphor material, yielding for example, alternating low and high resolution image lines. Multiple phosphor dots can be illuminated simultaneously using phosphors that emit at different wavelengths. By combining this with photo-detectors that only detect scintillation light in certain frequency ranges (with the use of filters, for example), the signals coming from each of the dots can be determined and separately measured. This would allow for more efficient use of the flux in the x-ray fan beam, or for a higher resolution detector.

In addition to the one-dimensional detector described above that requires translation of the object being imaged through the fan beam, a two-dimensional detector can be created for use with a cone beam that does not require translation of the object. In this embodiment, the belt and pulleys (or rotating shafts) are mounted inside a box on a translation mechanism that moves them in a perpendicular direction to the direction of motion of the belt. In this way, the phosphor dot samples the transmitted x-rays at all points in a two-dimensional plane, allowing a two-dimensional image of a stationary object to be created.

Applications of the invention include high resolution imaging for nondestructive testing (NDT) applications using a micro-focus x-ray source without requiring costly detector arrays. The size of the phosphor dot can be made arbitrarily small to increase image resolution - albeit with an increase in scan time. Another application is the manufacture of lower cost x-ray systems which could have one or more costly detector arrays replaced with the much cheaper detector described in this disclosure. A further application is to allow for the imaging of very large objects, for which the cost of a segmented detector array might be prohibitive.

All of the heretofore 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 described by the appended claims.

01945/B40WO 145441 1.1