WO1992002937A1 - Systeme de detecttion a rayons x retrodiffuses - Google Patents

Systeme de detecttion a rayons x retrodiffuses Download PDF

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Publication number
WO1992002937A1
WO1992002937A1 PCT/US1991/005558 US9105558W WO9202937A1 WO 1992002937 A1 WO1992002937 A1 WO 1992002937A1 US 9105558 W US9105558 W US 9105558W WO 9202937 A1 WO9202937 A1 WO 9202937A1
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Prior art keywords
ray
rays
subject
searching
image
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PCT/US1991/005558
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English (en)
Inventor
Steven W. Smith
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Irt Corporation
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Priority claimed from US07704292 external-priority patent/US5181234B1/en
Application filed by Irt Corporation filed Critical Irt Corporation
Priority to CA002083064A priority Critical patent/CA2083064C/fr
Publication of WO1992002937A1 publication Critical patent/WO1992002937A1/fr

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V5/00—Prospecting or detecting by the use of ionising radiation, e.g. of natural or induced radioactivity
    • G01V5/20—Detecting prohibited goods, e.g. weapons, explosives, hazardous substances, contraband or smuggled objects
    • G01V5/22—Active interrogation, i.e. by irradiating objects or goods using external radiation sources, e.g. using gamma rays or cosmic rays
    • G01V5/222—Active interrogation, i.e. by irradiating objects or goods using external radiation sources, e.g. using gamma rays or cosmic rays measuring scattered radiation
    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21K—TECHNIQUES FOR HANDLING PARTICLES OR IONISING RADIATION NOT OTHERWISE PROVIDED FOR; IRRADIATION DEVICES; GAMMA RAY OR X-RAY MICROSCOPES
    • G21K4/00—Conversion screens for the conversion of the spatial distribution of X-rays or particle radiation into visible images, e.g. fluoroscopic screens
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05G—X-RAY TECHNIQUE
    • H05G1/00—X-ray apparatus involving X-ray tubes; Circuits therefor
    • H05G1/08—Electrical details
    • H05G1/60—Circuit arrangements for obtaining a series of X-ray photographs or for X-ray cinematography

Definitions

  • the invention relates to the general field of radiant energy imaging systems, and specifically to systems and techniques for detecting concealed objects, including plastics and ceramics, carried on the body of a person.
  • Security systems are presently limited in their ability to detect contraband, weapons, explosives, and other dangerous objects concealed under clothing.
  • Metal detectors and chemical sniffers are commonly used for the detection of large metal objects and some kinds of explosives, however, a wide range of dangerous objects exist that cannot be detected with these devices.
  • Plastic and ceramic weapons developed by modern technology increase the types of non-metallic objects that security personnel are required to detect.
  • the alternative of manual searching of subjects is slow, inconvenient, and would not be well tolerated by the general public, especially as a standard procedure in, for example, airports.
  • NCRP National Council on Radiation Protection
  • the X-ray tube potential is set at up to 150 Kilovolts and is specifically chosen to transmit X-rays through the person being examined. Operation of the X-ray tube at 150 Kilovolts or even at 100 Kilovolts would negate benefits of imaging by backscatter detection, e.g., low dose scanning. This technique requires the subject to be exposed to a substantial radiation dosage, especially if the subject is scanned often, e.g., a frequent flyer.
  • U.S. Patent No. 4,839,913 of Annis et al. teaches a flying spot scanning system for baggage inspection which uses X-ray-induced fluorescence to permit detection of concealed objects. Since fluorescence is dependent upon atomic number, each object emits a fluorescent radiation line which is unique to its atomic number Z. Scattered, transmitted and fluorescence signals are generated to distinguish objects from the background. The energy level of the source must be sufficient to excite a selected fluorescence radiation line so that the fluorescent radiation line has sufficient energy to escape the object. Thus, it may be necessary to expose the object to relatively high X-ray energy in order to detect certain materials, which would be unacceptable for personnel inspection systems.
  • Systems of the foregoing prior art that apply to body scanning are designed to detect radiant energy that has been transmitted through the body, scattered from the body, or emitted from the body.
  • images are produced by characteristics of the subject's body and any object concealed under the subject's clothing. The system operator then inspects each image for evidence of concealed objects.
  • Low Z materials present a special problem in personnel inspection because of the difficulty in distinguishing the low z object from the background of the subject's body which also has low Z. Detection of low Z materials is addressed in the Annis '247 patent, but only for baggage. As mentioned above, this technique exposes a live subject to radiation levels which are excessive.
  • the present invention is a concealed object detection system in which a pencil beam of X-rays is scanned over the surface of the body of a person being examined. X-rays that are scattered or reflected from the subject's body are detected by a detector such as, for example, a scintillator and photomultiplier tube combination. The signal produced by this scattered X-ray detector is then used to modulate an image display device to produce an image of the subject and any concealed objects carried by the subject.
  • a detector such as, for example, a scintillator and photomultiplier tube combination.
  • the scattered X-ray intensity is related to the atomic number of the material scattering the X-rays.
  • the intensity is related to the atomic number of the material the X-rays pass through before and after being scattered.
  • the intensity of X-ray backscatter, or X-ray reflectance decreases with increasing atomic number.
  • the images are primarily modulated by variations in the atomic number of the subject's body.
  • Objects are potentially visible in X- ray reflection images of persons due to the difference in X-ray reflectances between the objects and human tissue.
  • Soft human tissue scatters a significant amount of X-rays due to the relatively low atomic number of oxygen which predominates its composition. Bones near the surface of the body produce much less scatter due to the higher atomic number of calcium which makes up their structure.
  • Concealed objects, especially metals can be easily visualized in the images due to their significant difference in atomic composition from the background of human tissue.
  • Non-metallic objects are commonly composed of low atomic number elements similar to human tissue, i.e. hydrogen, carbon, nitrogen, and oxygen.
  • an operator is required to identify very low contrast objects in the presence of image clutter resulting from the imaging of internal human anatomy.
  • the difficulty of this task results in poor detection capability for a wide range of dangerous objects composed of low atomic number elements, such as plastics or ceramics.
  • the present invention provides a system and method providing a scattered X-ray imaging system for detecting low atomic number concealed objects.
  • the system includes apparatus for automatically and uniformly enhancing the image edges of low atomic number concealed objects to facilitate their detection.
  • Edge enhancement is provided while simultaneously suppressing the edges of internal anatomy that produce confusion in image interpretation.
  • the preferred embodiment provides a storage means by which previously acquired images can be compared with the present image for analyzing variances in the present image.
  • the present invention also provides means for creating a non-human representation of the body being examined to thereby permit faster inspection with less invasion of privacy.
  • the present invention improves detection efficiency and decreases the time required to inspect a subject.
  • One embodiment of the present invention does not require the operator to view an actual image of the subject's body to obtain an indication of objects desired to be detected; a generic body outline or template can be used to indicate relative location of concealed objects. This eases concerns related to a possibly objectionable invasion of privacy.
  • Figure 1 is a diagrammatic view of the backscatter detection system of the present invention
  • Figure 2 is a diagrammatic perspective view of the present invention
  • Figure 3 is a block diagram illustrating a series of scanning steps
  • Figure 4 is a series of graphs showing relative signal level versus scan position;
  • Figure 5 shows the mechanical features and movement of the inventive system;
  • Figure 6 is a diagrammatic view of a detector used in the system
  • Figure 7 is a graph of atomic number versus relative reflectance
  • Figure 8 is a graph of X-ray energy versus relative SNR
  • Figure 9 is a graph of X-ray tube potential versus average X-ray energy
  • Figure 10 shows several example feature detection kernels used in the inventive system
  • Figure 11 shows the mechanical means of generating a sweeping pencil beam of X-rays in the preferred embodiment
  • Figure 12 illustrates a detector configuration of a second embodiment.
  • the imaging system 10 provides a pencil beam 11 of X-rays directed at the body 12 of the person being examined.
  • an X-ray tube 18, a mechanical chopper wheel 14 and slit 15 combine to create an X-ray pencil beam source 30 as is known in the art for scanning the pencil beam 11 in a horizontal motion across the body 12, illustrated in Figure 11.
  • X-rays 16 that are scattered or reflected from the body 12 are detected by X-ray sensitive detectors 17 positioned on the same side of the subject as is the X-ray pencil beam source 30.
  • the detectors 17 are positioned for substantially uniform X-ray detection on all sides of the incident X-ray beam.
  • the electronic signal 20 produced from the detectors 17 and synchronization signals 21 from the X-ray source 30 are routed into digital computer 24.
  • the computer 24 generates an image display 25 on a monitor (screen) 36 wherein the intensity at each point in the display corresponds to the relative intensity of the detected scattered X-rays.
  • the detectors 17 contain an opening 26 through which the pencil beam 11 of X-rays pass before striking the body 12 being examined.
  • the detectors 17 are mounted on two vertical shafts 28 which are in turn mounted on a base 29 to guide the motion of the detectors 17 as they are moved in a vertical direction.
  • the X-ray pencil beam source 30 is mounted on a carriage 31, which in turn in supported by two pivot joints 32 and 33; one connected to the detector, and the other to a vertical support 35. As the detectors 17 are moved in a vertical direction, the X-ray pencil beam 11 is moved in an arc, such that it always passes through the opening 26 in the detector.
  • FIG 12. An alternate configuration for the detectors is illustrated in Figure 12.
  • the need to move the weight of the detectors is eliminated by making the detectors stationary and scanning only the X-ray source in a manner as described above to facilitate a faster scan rate.
  • Eight detectors 17' are positioned in two stacks with the X-ray source 30' and its scanner centered between the stacks.
  • X-ray source 30* travels up and down on vertical shafts 28' with pivot joints provided so the X-ray pencil beam is scanned in an arc.
  • X-rays backscattered from the subject are collected by detectors 17' as the source 30' is scanned.
  • the intersection of the pencil beam and the body of the person defines an image picture element (pixel) of specified area.
  • the X-ray tube operates in the range of 35 to 70 Kilovolts with a pixel size of 12 to 120 square millimeters.
  • the backscatter X-ray detector 17 comprises an X-ray sensitive area of 150 to 1500 square inches and is positioned 5 to 15 inches from the body of the person being examined. In another embodiment, the backscatter detector may be positioned farther from the person being examined with a corresponding increase in detector area. It has been empirically determined that these parameters are approximately optimized at the values of 50 Kilovolts, 40 square millimeters pixel area, 952 square inches X-ray sensitive area, and an 8 inch subject-to-detector distance.
  • the signal 20 provided by X-ray backscatter detector 17 is averaged and converted to provide a single digital number representative of the number of X-rays striking the detector during the pixel illumination period. This may be accomplished by methods known in the art, for example, counting electronic pulses produced by individual X-rays or integrating the detector signal for the period of the 50 microseconds.
  • a scanning cycle of the apparatus is defined as the time required to scan the pencil beam of X-rays over each pixel in the image to provide a complete image of the person and objects positioned in the scanning window.
  • the X-ray source is composed of an X-ray tube operating at 50 Kilovolts and 5 milliamps, with an inherent filtration equivalent to 1 millimeter aluminum.
  • the X-ray source itself is located approximately 30 inches from the person being examined.
  • Standard references for example: "Catalogue of Spectral Data for Diagnostic X-rays" by Birch, Marshall and Ardran, Published by The Hospital Physicists' Association, London, 1979.”, provide data on X-ray tube output. From this reference, under the conditions created according to the preferred embodiment and the here before stated geometry, the subject receives a radiation dose of approximately three (3) microRem with a corresponding X-ray photon flux of about 11,500 X-rays in each pixel. Each of these 11,500 X-rays per pixel will either (1) pass through the body unaffected, (2) be absorbed in the body by the photoelectric effect, or (3) exit the body by Compton scattering.
  • each detector 17 is composed of a light tight enclosure 40 having its interior walls 41 arranged to have light reflective surfaces, such as by being painted white.
  • An X-ray intensifying screen 42 forms a front cover or wall of enclosure 40. Screen 42 is of the type used in medical radiography for increasing the exposure rate of X-ray film.
  • a large area photomultiplier tube 43 is mounted at the opposite end of the interior of the enclosure. X-ray photons indicated as 16 strike the X-ray intensifying screen 42 producing light photons 47 in the interior of the light tight enclosure 40. A portion of the light photons 47 reflect off of the white, or otherwise highly reflective, interior walls 41 of the enclosure and are detected by the photomultiplier tube 43. In this manner, the output signal of photomultiplier tube 43 is linearly related to the incident X-ray flux.
  • the detected quantum efficiency of each detector 17 is determined by many factors: the fraction of X-rays interacting in the screen 42, the optical pulse-height distribution of light photons leaving the screen, the average number of photoelectrons generated in the photomultiplier tube for each X-ray interacting in the screen, the uniformity of light transfer from the screen 42 to the photomultiplier tube 43, and statistical noise within the photomultiplier tube itself.
  • the preferred embodiment of the detectors 17 comprise components with the following specifications.
  • the input scintillator or phosphor is a calcium tungstate screen approximately 14 x 17 inches (35.5 x 43 cm) in size with an approximate phosphor areal density of 80 milligrams per square centimeter.
  • Four separate detectors 17 with this size of screen are placed around the X-ray pencil beam 11 to form a detector array, as shown in Figure 1.
  • the outputs of the four detectors 17 are processed in the computer 24 to form a single detector signal.
  • a five inch diameter bialkali cathode photomultiplier tube 43 is placed about eight inches from the screen 42.
  • This geometry produces an average of approximately seven photoelectrons at the photo cathode for each X-ray striking the center of the screen 42. Each X-ray striking the corner of the screen generates an average of approximately 5 photoelectrons. Moving the photomultiplier tube 43 farther from the screen 42 increases the uniformity of response over the area of the screen. This, however, decreases the average number of photoelectrons generated. The above noise sources combine resulting in an overall detected quantum efficiency of approximately 50%.
  • the screen phosphor of the detectors 17 may be changed to barium lead sulfate to decrease the decay time at the expense of light output and a resulting decrease in detector quantum efficiency.
  • Figure 6 shows the face of the photomultiplier tube 43 parallel to the screen to maximize light transfer uniformity, however, other orientations may be provided to change the shape of the detector enclosure.
  • the present invention detectors 17 are unique in that they provides a much larger active area than the prior art scintillators attached directly to photomultiplier tubes. Construction limitations of photomultiplier tubes limit the active light detection area of these devices. This, in turn, limits the X-ray sensitive area of the prior art systems. Each of the detectors 17 allows for an X-ray sensitive area of many times larger than the light sensitive area of the associated photomultiplier tube 43, while still maintaining a high detector quantum efficiency. A large sensitive area in desirable for greatly improved system performance, providing lower subject radiation dose, lower required X-ray source output, and automatic edge modification for improved object detection.
  • the present invention provides a system and method for selectively setting various parameters for providing a rapid and safe method of detecting dangerous concealed objects. Otherwise the system might exhibit unacceptably high subject radiation exposure, excessively long image acquisition times, and poor image quality.
  • a first parameter to consider is the X-ray tube potential.
  • the average energy of X-rays produced by an X-ray tube is directly related to the potential at which the X-ray tube is operated.
  • the energy of these X-rays determines the characteristics of the acquired image.
  • Figure 7 shows the relative X-ray reflectance, as sensed at the detectors 17, as a function of the incident X-ray energy and the atomic number of the material scattering the X-rays.
  • the shape of these curves is known through empirical measurement and computer modeling, for example: "A Signal Processing Model of Diagnostic X-ray Scatter" by Smith and Kruger, Med. Phy ⁇ . 13(6) Nov. 1986 and "Backscatter X-ray Imaging", by Bossi et al..
  • Oxygen is the dominant atom in water and, therefore, in human tissue.
  • One of the most difficult tasks required of the inventive detection system is to discriminate between human tissue and objects of similar atomic number. This includes a wide range of plastics, explosives, and organic matter composed mainly of carbon, nitrogen and oxygen, with atomic numbers of 6, 7 and 8, respectively.
  • the optimum X-ray energy can therefore be defined as the energy that provides the highest signal-to-noise ratio (SNR) for discriminating carbon from oxygen.
  • SNR signal-to-noise ratio
  • the signal for distinguishing between carbon and oxygen is the difference in reflectance between these two elements. This can also be expressed as the average slope of the reflectance curve between the two elements. In mathematical terms, this is the first derivative of reflectance with respect to atomic number evaluated at the midpoint between the two elements, or atomic number 7:
  • S(E) is the signal for distinguishing carbon from oxygen as a function of energy
  • the image noise in a well designed X-ray imaging system is determined by Poisson statistics based on the average number of X-rays detected in each image pixel. This results in the coefficient of variation (CV) , or the relative image noise, being inversely proportional to the square root of the number of detected X-rays:
  • CV is the coefficient of variation between pixels and N is the average number of X-rays detected per pixel. This can be most easily calculated for a specific image by, for example, localizing several hundred adjacent image pixels located in an anatomically flat area near the center of the subject's body. The mean and standard deviation are calculated by standard statistical methods. The image CV is then 100% times the standard deviation divided by the mean. The average number of X-rays detected per pixel is directly proportional to the magnitude of the reflectance shown in Figure 7. Rewriting equation (2) with this information:
  • CV(E) is the coefficient of variation as a function of X-ray energy
  • k is a constant of proportionality depending on the incident X-ray flux and detector efficiency.
  • the SNR for distinguishing carbon from oxygen as a function of X-ray energy can be found by dividing S(E) from Equation (1) by CV(E) from Equation (3) :
  • the above function is illustrated in Figure 8.
  • the SNR can be seen to be very dependent on X-ray energy and is optimized in the range of 22 to 37 Kiloelectron-volts (KeV) , as shown by the dotted lines. At energies greater than this range, the SNR is reduced because carbon and oxygen reflect approximately equally, and, thus, the signal is relatively low. At energies lower than this range, the SNR is reduced because the low reflectance results in low detected X-ray flux and, thus, relatively high image noise.
  • KeV Kiloelectron-volts
  • X-ray tubes produce polyenergetic X-ray beams wherein the highest energy X-rays are determined by the X-ray tube potential and the lowest energy X-rays are determined by the beam filtration material.
  • the average X-ray energy produced from an X-ray tube operating at a specific potential and beam filtration can be found from, for example, the aforementioned "Catalogue of Spectral Data for Diagnostic X-rays". Data from this source are shown in Figure 9 comprising a plot of the average X-ray energy produced by an X-ray tube as a function of X-ray tube potential. Data in Figure 9 are for a beam filtration of 1 millimeter aluminum for potentials less than 60 Kilovolts (KV) and 1.5 millimeter aluminum otherwise.
  • KV Kilovolts
  • operation of the X-ray tube with a potential of 35 to 70 KV produces an average X-ray energy of 22 to 37 KeV, the previously discussed optimum. Operation within this optimum potential range results in the highest SNR, and therefore the lowest subject radiation exposure, fastest image acquisition time, lowest X-ray tube power requirements, and best image quality.
  • the optimum dose of 1 to 10 microRem is determined as the range that satisfies each of the conflicting requirements of image quality and health risk. (As previously discussed, higher radiation doses pose a non- trivial health risk to persons being exposed.
  • the optimum X-ray flux and size of the scanning X-ray beam can be derived.
  • this defines the optimum beam width as the square root of 12 to 120 square millimeters, or approximately 3.5 to 11 millimeters, in both the vertical and horizontal dimension.
  • the above dimensions are measured at the point at which the beam intensity is reduced to one-half of its maximum intensity, and refer to the average dimension of the beam over the scan cycle.
  • N p A The total number of X-rays detected during a scan is given by: N p A;
  • N is the total number of X-rays detected during a scan
  • N p is the average number of X-rays detected in each pixel
  • A is the area of the entire scan
  • d is the linear dimension of the square pixels, i.e., the X-ray beam size.
  • I is the X-ray tube current and t is the total scan time.
  • the size of the X-ray pencil beam, d is much more constant and limited in range.
  • the beam size varies as only the one-fourth power of any of the parameters.
  • state-of-the-art stationary target X-ray tubes can operate with a sustained maximum current, I, of 2 to 10 milliamps, a range of 5 to 1.
  • a total scan time, t, of 1 to 5 seconds is acceptable for most security applications, which is also a range of 5 to 1. Multiplying these ranges shows that within the principles of the invention, the X-ray tube current and scan time can together vary by as much as a factor of 25.
  • the dimension of the pencil X-ray beam, d will vary by only twenty-five to the one-fourth power, or about 2.25. This shows that throughout the range of variation of the present invention, the beam size is relatively fixed. It has been empirically determined that for the above range of X-ray tube current and scan time, the X-ray beam size is optimized in the range of 4 to 9 millimeter in both the vertical and horizontal dimensions.
  • the preferred embodiment of the system includes an X-ray tube operating at a potential of approximately 50 KV and 5 milliamps.
  • the vertical and horizontal dimension of the X-ray beam is approximately six millimeters (6 mm) where it strikes the person being examined.
  • the detector system 17, discussed below, has a scan acquisition time of approximately 2 seconds.
  • the empirically measured subject dose with these parameters is approximately three microRem.
  • scintillators vary widely in term of chemical composition, thickness and energy response. The above determination of the optimum energy spectrum allows the identification of the most convenient scintillator that shows acceptable performance in the system.
  • the preferred scintillator must detect a large fraction of the incident radiation, produce significant light output to the photomultiplier tube, and exhibit a temporal decay time which is short compared to the pixel to pixel scanning rate of the pencil beam. It has been empirically determined that these parameters are satisfied by a scintillator screen commonly used in medical radiography composed of Calcium Tungstate (CaW0 4 ) with an exemplary approximate areal density of 80 milligrams per square centimeter. One version of this screen is marketed under the trade name Cronex Lightning Plus by the Dupont Corporation.
  • the relatively fast decay time of 10 microseconds allows rapid scanning of the pencil beam with little image degradation. Approximately 70% of the incident radiation is detected by this screen which in turn produces approximately 250 usable light photons for each 30 KeV X-ray. Thicker screens detect more of the incident radiation at the expense of lower light output.
  • An alternative screen contains approximately 40 milligrams per square centimeter of Lanthanum oxybromide: thulium activated (LaOBr:Tm) and is marketed under the trade name of Cronex Quanta III by Dupont Corporation.
  • these screens When used with the optimum 50 KV X-ray spectrum, these screens show a detector quantum efficiency in the range of 50 to 70 percent. In addition, these scintillators produce a level of light output that is detectable by photomultiplier tubes, and have short temporal decay times. If the optimum X-ray energy were higher, these screens would provide unacceptably poor detective quantum efficiency (DQE) because they are of insufficient areal density to absorb higher energy X-rays. If the optimum X-ray energy were lower, the DQE would also drop because low energy X-rays generate too few light photons to overcome the optical attenuation of the scintillator. This results in insufficient light reaching the photomultiplier and the reduced DQE.
  • DQE detective quantum efficiency
  • Empirical measurements on the present invention show the preferred operating parameters are capable of simultaneously providing a CV of 5% and a subject radiation dose of 3 microRem.
  • the apparatus of the present invention was operated in accordance with the above-described preferred technique factors.
  • the image CV provided by the apparatus was measured by acquiring images of persons and determining the CV by calculating the pixel mean and standard deviation. The determined CV of many images is then averaged to provide the empirically measured CV of 5%.
  • Subject radiation dose provided by the apparatus was measured by positioning thermoluminescent dosimeter (TLD) chips at the subject position in the imaging window and operating the apparatus for 400 scan cycles. Radiation dose readings from the TLD chips are then divided by 400 resulting in the empirically measured subject dose of 3 microRem.
  • TLD thermoluminescent dosimeter
  • the present invention allows the use of hundreds of times less subject radiation dose than required by prior art systems to provide equivalent image quality. As will now be explained, several significant factors contribute to the low radiation dose provided by the present invention:
  • Prior art systems are designed and optimized to operate by detecting X-rays that have been transmitted through the body. Even at high X-ray tube potentials, a very low percentage of X-rays can penetrate through the body. Data on X-ray penetration can be readily found in the literature, for example the previously referenced "Catalogue of Spectral Data". Data from this reference shows that at 140 KV only about 1.4 percent of incident X— rays penetrate through an average person's body thickness of 20 centimeters and are detectable to form an image. Only about 0.19 percent of incident X-rays penetrate through a large person's body thickness of 30 centimeters, and are detectable to form an image.
  • the present invention enables seven percent (7%) of the X-rays to be detected.
  • the greatly increased X-ray flux utilization of the present invention backscatter geometry can therefore be seen to provide a reduction of a factor of five to thirty-five in the required radiation dose compared to prior art X-ray transmission type systems, for equal numbers of detected X- rays.
  • the X-ray potential in prior art systems is selected to optimize the image quality for transmission radiography. This requires high energy X-rays to achieve sufficient penetration through the body, and thus achieve a larger number of detected X-rays.
  • the system disclosed in U.S. Patent Re. 28,544 operates with a peak voltage of 150 Kilovolts. From the above referenced "Catalog of Spectral Data", (extrapolating from Figure 9) this potential produces X-rays with an average energy of approximately sixty (60) Kiloelectron-volts (Kev) .
  • the preferred embodiment of the present invention operates with an empirically and theoretically determined optimum X-ray tube potential of 50 Kilovolts, resulting in X-rays of approximately thirty (30) KeV.
  • Standard references can be used to obtain the values for the X-ray interaction with matter at these energies, for example, "Radiation Detection” by Tait, published by The Butterworth Group, London, 1979.
  • the attenuation coefficients of carbon and oxygen can be used to show characteristics of the two energies.
  • the attenuation coefficients of carbon and oxygen are .1765 and .1945 grams per centimeter squared, respectively, or a difference of about ten percent.
  • the attenuation coefficients are 0.2520 and 0.3733, respectively, a difference of forty-eight percent (48%) .
  • the pixel size is 1 millimeter square.
  • the pixel size is selectable between 0.1 millimeters square and 2 millimeters square.
  • the radiation dose is then proportional to the fixed number of X-rays striking each pixel divided by the area of the pixel. This, in turn, results in the radiation dose being inversely proportional to the pixel area.
  • the preferred embodiment of the present invention uses pixels of area 40 square millimeters, resulting in a factor of 40 reduction in dose compared to 1 square millimeter pixels, a factor of 400 reduction compared to 0.1 millimeter square pixels, and a factor of 10 compared to 2 millimeter square pixels.
  • the X-ray detector in the preferred embodiment of the present system consists of an area sensitive to X-rays of 952 square inches positioned 8 inches from the person being searched.
  • the actual detector area is 1904 square inches positioned at the same distance from the subject, however, not all detectors 17' will be receiving backscattered radiation during a scan.
  • the effective detection area at any given point in the scan is approximately the same as for the scanning detector, i.e., one-half the total detector area of the stationary detector configuration.
  • Empirical measurements show that this size and positioning result in the detector receiving approximately seven percent (7%) of incident X-rays as backscatter. As indicated above, this is approximately a factor of five to thirty-five times greater than for transmission imaging.
  • Prior art systems do not disclose backscatter geometries with sufficient detector collection area to achieve the required low radiation dose. For example, in U.S. Patent Re.
  • the detector consists of an active area 1 inch by 24 inches, or a total X-ray collection area of 24 square inches. This is a factor of approximately 40 lower than the 952 square inches of X-ray sensitive area provided in the present invention.
  • the increased collection area of the present invention is absolutely necessary to provide detection of an increased number of scattered X-rays allowing for a 40 fold reduction in dose compared with the backscatter system of the prior art. It has been determined by empirical measurement that the detector must consist of an active area in the range of 150 to 1500 square inches and placed in the range of 5 to 15 inches from the person being searched so that at least 2% of the scattered radiation is detected.
  • the subject is first scanned with the imaging system 10 to obtain a digitally represented image.
  • This image is then processed by the digital computer 24 to identify key features in the image.
  • certain anatomical features may be suppressed or screened out to avoid the display of these features to the system operator.
  • the key features in the newly acquired image are then compared with a stored library containing previously obtained human images.
  • This library of human images is used to identify common anatomical features in the present subject so those anatomical features can be suppressed.
  • the library may also contain within its images certain common benign objects which can also be suppressed to permit more accurate detection of dangerous or illegal concealed objects. These common anatomical and benign features are not of concern to the system operator and are suppressed in the processed image signal.
  • the remaining features in the image are regarded as unusual and potentially indicative of objects that the system operator is attempting to detect.
  • the location of detected features can be referenced to the absolute location in the image, or in relation to the body of the person being examined.
  • the latter method has the advantage of being insensitive to subject positioning within the imaging window and differing subject size.
  • the library of image features can consist of threshold or coefficient values that have been derived from previously acquired images. For example, in a particular imaging system, it may be desirable to identify the location of the left side of the subject by convolving the image with a left edge detection kernel. The left edge of the subject's body can then be found as the first pixel from the left side of the convolved image that exceeds a threshold value.
  • Various threshold values and kernel coefficients values can be tested using the previously acquired images. The most effective values are identified and stored in the memory of the computer 24.
  • the suppression of anatomical and benign image features can consist of selectively displaying the features at reduced intensity, in a different color, or removing them entirely from the image.
  • the outline of the subject's body is removed from the image and a humanoid outline is substituted for location reference.
  • the display may consist of alpha-numeric information, such as: "No abnormal features detected” or "Diagonal edge detected on upper chest”. Threshold levels can be changed according to the level of sensitivity required and the tolerable level of false positives.
  • Figure 1 shows a block diagram of the imaging system 10 which produces an image signal 20 that is modulated by characteristics of the body 12 of the person being examined.
  • the image signal is routed into a digital computer system 24 such as an IBM PC that is capable of implementing preprogrammed instructions for image analysis.
  • the computer system comprises a memory in which is stored the library of previous image signals of persons not concealing dangerous objects.
  • the computer system 24 uses this information to generate a processed image on monitor 36 in which image features that correspond to common benign objects and normal human anatomy are suppressed.
  • the operator viewable image that may be inspected for evidence of dangerous concealed objects.
  • the scanning movement of the X-ray source may introduce some amplitude distortion in the displayed image where the torso is brighter than the extremities.
  • a corrected signal can be generated by generating an approximation of the distortion.
  • a pixel by pixel comparison of the acquired image is made to a selected threshold which is taken to be the background.
  • Each pixel in the distortion image equals the average of the non- background pixels surrounding the corresponding pixel in the acquired image, thereby creating a 2-dimensional low pass filter.
  • the corrected image is the result of dividing a pixel in the acquired image by the corresponding pixel in the distortion image.
  • Edge enhancement is a common method of processing digitally represented images to increase the detectability or objects.
  • the present invention uses extensive edge detection in visual processing. In order to enable edge enhancement for detection, radiation scattered off at different angles is detected by using a large size X-ray detector, and a unique placement of the detector assembly to provide spatially uniform edge enhancement of concealed objects having low atomic number.
  • the system also provides concurrent suppression of the image edges of the internal anatomy of the person being examined. These edge modifications provide a significantly more informative image for the detection of concealed objects.
  • an X-ray source 30 is used to generate a narrow pencil beam 11 of X-rays directed toward the object being examined.
  • the object is a one-half (%) inch thick bar 50 of plastic in front of the subject's chest.
  • the plastic bar 50 is used to simulate, for example, a plastic explosive.
  • the X-ray beam strikes only the person's body as shown in Figure 3a. Many of the X-rays penetrate a few centimeters into the body, interact by Compton scattering, and exit the body through the same surface that they entered.
  • X-ray sensitive detectors 17 are placed symmetrically around the incident X-ray pencil beam to detect the scattered X-rays and provide an electronic signal characteristic of the X-ray reflectance.
  • the pencil beam 11 of X-rays is scanned over the surface of the plastic bar 11 (and all other objects) , as shown in Figures
  • Figure 4a shows the results of an actual signal generated by the operation of the configuration in Figure 3.
  • the signal level from the array of detectors 17 is plotted as a function of pixel number, as the pencil beam in scanned across the body and plastic.
  • This difference in atomic number accounts for the signal level increasing markedly when the X-ray beam strikes the plastic (region b) compared to when it strikes the body (regions a and c) .
  • the indicated CV of 2% is sufficient for the system, permitting the operator to conclude that region b has distinct characteristics from regions a and c.
  • the transition region between the body and the plastic shows a principle feature of the invention.
  • the edge has been enhanced by lowering the signal level in the body near the interface.
  • Figure 3b shows how the signal level is lowered. If the pencil beam enters the body near the interface, a significant portion of the scattered X-rays 16 must pass through the plastic 50 to reach the detector 17, and are, thus, attenuated. This additional X-ray attenuation reduces the number of detected X-rays, reducing the signal level.
  • the level increases when the pencil beam strikes the plastic near the interface. In this case, much of the X-ray scattering takes place in the plastic. This increases the detected scatter because X-rays can exit the side of the plastic bar with correspondingly less attenuation. Together these two processes result in sharp, well defined edges in the image signal.
  • Figure 4d shows the same graph as in Figure 4a, however, the relative image noise has been increased to a CV of 10%. As is readily apparent, the increased relative image noise interferes with detection of the concealed plastic bar and only a minimal distinction appears between signal areas a, b, and c. It will be appreciated from Figure 4b that the indicated CV of 10% is barely sufficient to detect the plastic bar.
  • Figure 4c shows the same graph as in Figures 4a and 4b with the relative image noise increased to a CV of 20%.
  • Figure 4c there is no discernible distinction between signal areas a, b, and c, and a system operator would clearly be unable to detect the plastic bar based on the image. It can thus be appreciated from a comparison of Figures 4a-c that a CV of less than 10%, as utilized in Figure 4b, is required to detect concealed objects of composition and thickness similar to the indicated sample test object.
  • Figure 4b shows the X-ray scatter profile wherein a 1" thick plastic bar in immersed in water simulating an organ contained within the body of person being examined.
  • the signal level is again higher in the plastic; however, the edges are much less distinct than in the case of Figure 4a. This edge suppression is a result of removing the conditions that enhanced the edges in the previous case, as well as a second mechanism described below.
  • the second mechanism can be explained by examining the X-ray path.
  • the X-rays initially follow the path of the pencil beam into the object. After scattering, the X-rays exit the object along paths directed toward the detector. Depending on the detector size and location, this may encompass passing through material several centimeters from the pencil beam.
  • the detected X-ray scatter will thus be determined by the atomic number of the material around the pencil beam, rather than in the pencil beam alone.
  • This spacial averaging smooths the edges of embedded objects as shown in Figure 4d.
  • the edges of concealed objects and internal anatomy are likely to appear equally at all angles. Uniform enhancement of the edges at these various angles requires the detector to be approximately uniformly responsive to X-rays scattered at equal angles with respect to the pencil beam.
  • the scattered X-ray detector In order for automatic edge detection to be effective, the scattered X-ray detector must be of a certain s and placement. X-rays at 30 Kev typically scatter within about one inch after entering the body 12 of the subject. While an X-ray is equally likely to be scattered in all directions, only those X-rays that are scattered toward the active area of the detectors 17 contribute to the acquired image.
  • the detectors 17 must be of sufficient size and placement to collect a substantial portion of the backscattered X-rays which exit the body 12 greater than a certain angle from the point at which the X-ray beam entered the body.
  • the offset distance is at least a few tenths of an inch, for X-rays scattered at angles on the order of 10° or more.
  • This requires the vertical and horizontal dimensions of the detector to be greater than or equal to approximately one fifth of the distance between the human body and the detector (tan 10° .18) . Detectors smaller than this value will not exhibit significant edge modification because the scatter angle is too small (less than 10°) .
  • the subject is positioned eight inches from the detector of 34 inches by 34 inches, collecting X-rays scattered up to approximately 70° from the incident pencil beam.
  • the detector In practical applications the detector must be spaced greater than 5 inches from the subject to allow room for scanning the subject with the pencil X-ray beam. In addition to providing excellent edge modification, this detector size collects a large percentage of the scattered X-rays. This reduces the radiation dose given the subject, as well as reducing the output required of the X-ray source.
  • Prior art systems have not provided for spatially uniform edge enhancement resulting from the present invention's detector size and placement.
  • Re. 28,544 discloses using a 1 inch by 24 inch active area as a backscatter detector. If this detector were placed horizontally 8 inches from the human body being scanned, edge enhancement would occur only for vertical edges in the image. The vertical height of the detector, i.e., 1 inch at an 8-inch distance, is insufficient to enhance horizontal image edges. Similarly, if the detector were mounted vertically, vertical image edges would not be enhanced.
  • the present invention provides for edge enhancement of low Z objects in both the vertical and horizontal direction by providing a detector of sufficient size in both the vertical and horizontal direction.
  • the digital computer 24 and memory of the invention provide feature storage and recognition.
  • the library of image features is formed by first acquiring a large number of digitally represented images of humans with no dangerous concealed objects which are then stored in a library of images. Each of these scans is then processed, as is known in the art, to extract the key features of the image. Feature extraction in performed on each image using various feature extraction kernels.
  • kernels a, b, and c of Figure 10 When convolved with an image being evaluated, kernels a, b, and c of Figure 10 produce images with enhanced vertical, horizontal, and diagonal edges, respectively. Kernels d, e, and f are capable of enhancing rod shaped objects with vertical, horizontal, and diagonal orientation respectively. Each pixel of each enhanced image is then compared with a threshold value to detect the occurrence of these image features. The location, type, and intensity of each feature is then stored in the digital memory of the computer 24.
  • the system provides for varying kernel size, and provides threshold levels that vary as a function of image parameters.
  • the system includes provisions for selectively sorting the data according to feature location, feature type and statistical frequency of occurrence in several images.
  • prior art systems provide unacceptable images at comparable radiation dose levels compared to the present invention.
  • these prior art systems require much larger amounts of radiation exposure to produce similar quality images as the present invention.
  • a low level of relative image noise is required to provide an operable system for detecting concealed objects under persons' clothing.
  • a radiation dose in the range of 1 to 10 microRem is acceptable for the routine examination of persons entering security areas.
  • No known operable prior art system uses these low levels of radiation for detecting concealed objects.
  • Detailed measurement conducted on the inventive system shows that the present invention is capable of producing images with a CV of five percent while exposing the person being examined to a radiation dose of three microRem.
  • No known prior art system is capable of simultaneously providing both of these operational parameters.
  • the following analysis shows the limitations of an X-ray transmission system, such as disclosed in the prior art. From the above referenced "Catalog of Spectral Data" for an X-ray tube operating at 140 Kilovolts, a three microRem dose corresponds to an X-ray flux of 250 X-rays per square millimeter striking the skin of the person.
  • human bodies in the range of 20 to 30 centimeters thickness will transmit only 0.19 to 1.4 percent of these X-rays. For example, using the average of these two values, or 0.8 percent, results in an average of only two X-rays being detected per millimeter square pixel and a corresponding CV of about 70 percent.
  • the incident exposure of this system is adjusted to provide 400 transmitted X-rays to obtain the corresponding CV of five percent, the dose will be increased by a factor of 400/2.0 or more than 600 microRem.
  • a backscatter system such as mentioned in the prior art can likewise be shown to be incapable of simultaneously providing a CV of five percent and a dose of three microRem.
  • the dose of three microRem corresponds to an X-ray flux of 250 X-rays per square millimeter striking the body of the person being examined. Also as referenced above, approximately seven percent (7%) of these X-rays are detected in the present invention as backscatter.
  • the prior art discloses an X-ray sensitive area approximately forty times smaller than the present invention and therefore the detected X-ray fraction of such smaller detector area will be approximately 7%/40 or 0.175 percent. This results in an average of only 0.44 X-rays being detected per pixel (250 X-rays x 0.175%), and a corresponding CV of 150 percent.
  • the incident exposure of this system is adjusted to provide 400 backscattered X-rays to obtain the corresponding CV of five percent, the dose will be increased by a factor of 400/0.44 or more than 1000 microRem.

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Abstract

Un faisceau filiforme de rayons X (11) balaie la surface du corps (12) d'une personne examinée. Les rayons X (16) qui sont rétrodiffusés ou réfléchis par le corps du sujet (12) sont détectés par un détecteur. Le signal produit par ce détecteur de rayons X rétrodiffusés (17) est ensuite utilisé pour moduler un dispositif d'affichage d'images (36) en vue de produire une image (25) du sujet et de tout objet caché porté par le sujet. L'ensemble détecteur (17) est construit selon une configuration telle qu'il accentue automatiquement et uniformément les contours d'image d'objets cachés de faible numéro atomique (de Z faible) pour faciliter leur détection. Ledit système est pourvu d'un dispositif de mise en mémoire grâce auquel des images précédemment obtenues peuvent être comparées à l'image présente afin d'analyser les différences et les similarites avec l'image présente. Il fournit en outre un dispositif destiné à créer une représentation générique du corps examiné tout en éliminant les caractéristiques anatomiques du sujet de manière à réduire l'intervention dans la sphère privée dudit sujet.
PCT/US1991/005558 1990-08-06 1991-08-05 Systeme de detecttion a rayons x retrodiffuses WO1992002937A1 (fr)

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US07704292 US5181234B1 (en) 1990-08-06 1991-05-22 X-ray backscatter detection system
US704,292 1991-05-22

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WO1998020366A1 (fr) * 1996-11-08 1998-05-14 American Science And Engineering, Inc. Systeme d'imagerie aux rayons x a ouverture codee
US6950495B2 (en) 2003-12-01 2005-09-27 The Boeing Company Backscatter imaging using Hadamard transform masking
EP2165188A2 (fr) * 2007-06-21 2010-03-24 Rapiscan Security Products, Inc. Systèmes et procédés pour améliorer le criblage dirigé de gens
CN103630947A (zh) * 2012-08-21 2014-03-12 同方威视技术股份有限公司 可监测放射性物质的背散射人体安检系统及其扫描方法
US9891314B2 (en) 2014-03-07 2018-02-13 Rapiscan Systems, Inc. Ultra wide band detectors
US10134254B2 (en) 2014-11-25 2018-11-20 Rapiscan Systems, Inc. Intelligent security management system
CN114166874A (zh) * 2020-09-11 2022-03-11 同方威视技术股份有限公司 背散射检查系统和方法
US11280898B2 (en) 2014-03-07 2022-03-22 Rapiscan Systems, Inc. Radar-based baggage and parcel inspection systems

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CN111612020B (zh) * 2019-02-22 2024-04-26 杭州海康威视数字技术股份有限公司 一种异常被检物的定位方法以及安检分析设备、系统

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US3462601A (en) * 1965-10-14 1969-08-19 Westinghouse Electric Corp Gamma ray,x-ray image converter utilizing a scintillation camera system
US4342914A (en) * 1980-09-29 1982-08-03 American Science And Engineering, Inc. Flying spot scanner having arbitrarily shaped field size
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Cited By (11)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1998020366A1 (fr) * 1996-11-08 1998-05-14 American Science And Engineering, Inc. Systeme d'imagerie aux rayons x a ouverture codee
US5940468A (en) * 1996-11-08 1999-08-17 American Science And Engineering, Inc. Coded aperture X-ray imaging system
US6950495B2 (en) 2003-12-01 2005-09-27 The Boeing Company Backscatter imaging using Hadamard transform masking
EP2165188A2 (fr) * 2007-06-21 2010-03-24 Rapiscan Security Products, Inc. Systèmes et procédés pour améliorer le criblage dirigé de gens
EP2165188A4 (fr) * 2007-06-21 2014-01-22 Rapiscan Systems Inc Systèmes et procédés pour améliorer le criblage dirigé de gens
CN103630947A (zh) * 2012-08-21 2014-03-12 同方威视技术股份有限公司 可监测放射性物质的背散射人体安检系统及其扫描方法
US9891314B2 (en) 2014-03-07 2018-02-13 Rapiscan Systems, Inc. Ultra wide band detectors
US11280898B2 (en) 2014-03-07 2022-03-22 Rapiscan Systems, Inc. Radar-based baggage and parcel inspection systems
US10134254B2 (en) 2014-11-25 2018-11-20 Rapiscan Systems, Inc. Intelligent security management system
US10713914B2 (en) 2014-11-25 2020-07-14 Rapiscan Systems, Inc. Intelligent security management system
CN114166874A (zh) * 2020-09-11 2022-03-11 同方威视技术股份有限公司 背散射检查系统和方法

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IL99098A (en) 1996-06-18
CA2083064C (fr) 2001-12-18
IL99098A0 (en) 1992-07-15
AU8437391A (en) 1992-03-02

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