WO2007054837A2 - Gantry for housing an x-ray source and method of examining an item for examination by means of x-radiation - Google Patents

Gantry for housing an x-ray source and method of examining an item for examination by means of x-radiation Download PDF

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Publication number
WO2007054837A2
WO2007054837A2 PCT/IB2006/003926 IB2006003926W WO2007054837A2 WO 2007054837 A2 WO2007054837 A2 WO 2007054837A2 IB 2006003926 W IB2006003926 W IB 2006003926W WO 2007054837 A2 WO2007054837 A2 WO 2007054837A2
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WIPO (PCT)
Prior art keywords
gantry
detector
rotation
axis
examination
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Application number
PCT/IB2006/003926
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French (fr)
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WO2007054837A3 (en
Inventor
Geoffrey Harding
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Ge Homeland Proctection, Inc.
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Application filed by Ge Homeland Proctection, Inc. filed Critical Ge Homeland Proctection, Inc.
Publication of WO2007054837A2 publication Critical patent/WO2007054837A2/en
Publication of WO2007054837A3 publication Critical patent/WO2007054837A3/en

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    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/48—Diagnostic techniques
    • A61B6/483—Diagnostic techniques involving scattered radiation
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/02—Arrangements for diagnosis sequentially in different planes; Stereoscopic radiation diagnosis
    • A61B6/03—Computed tomography [CT]
    • A61B6/032—Transmission computed tomography [CT]
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N23/00—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00
    • G01N23/02—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by transmitting the radiation through the material
    • G01N23/04—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by transmitting the radiation through the material and forming images of the material
    • G01N23/046—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by transmitting the radiation through the material and forming images of the material using tomography, e.g. computed tomography [CT]
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N23/00—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00
    • G01N23/20—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by using diffraction of the radiation by the materials, e.g. for investigating crystal structure; by using scattering of the radiation by the materials, e.g. for investigating non-crystalline materials; by using reflection of the radiation by the materials
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2223/00—Investigating materials by wave or particle radiation
    • G01N2223/40—Imaging
    • G01N2223/419—Imaging computed tomograph

Definitions

  • the invention relates to a gantry for housing an X-ray source, with an axis of rotation, with a transmission detector which extends along the inner surface of the gantry, and with a spatially resolving coherent scatter detector which is arranged laterally in the direction of the axis of rotation alongside the transmission detector.
  • the invention also relates to an X-ray computed tomograph with an X-ray source and a gantry with the features described above.
  • the invention relates to a method of examining an item for examination by means of X-radiation for striking features using such an X-ray computed tomograph.
  • a computed tomograph for ascertaining the momentum transfer spectrum in an examination area is known from DE 100 09 285 Al.
  • an X-ray source with a primary collimator is arranged on a gantry rotatable about an axis, with which a fan beam is produced.
  • the X-ray source is a detector array, likewise attached to the gantry, for the detection of the X-ray beams penetrating an examination area.
  • a secondary collimator, arranged between the examination area and the detector array, allows only X-radiation from a specific scatter voxel from the examination area to pass into an allocated column of the detector array.
  • ART iterative algebraic reconstruction technique
  • the momentum transfer spectrum is characteristic of the matter in the scatter voxel concerned and information about the physical composition is thus also obtained.
  • ART iterative algebraic reconstruction technique
  • Such a computed tomograph and the method implemented with it suffer from a substantial disadvantage.
  • the entire item for examination located in the examination area is always examined, which requires a large detector for the coherently scattered X-radiation. Because such detectors are very expensive, such a computed tomograph is also very expensive.
  • the object of the invention is to provide a gantry, an X-ray computed tomograph and a method for operating an X-ray computed tomograph which are more economical, in particular with regard to the required coherent scatter detector.
  • a coherent scatter detector substantially smaller in spatial terms, is used.
  • this smaller coherent scatter detector is always to be brought, by means of the method according to the invention described below, into such a position that all the information from a previously specified region where a problem has been found in an item for examination (hereinafter called ROI - Region of Interest), is completely preserved and thus the diffraction profiles can be determined clearly from this ROI.
  • the coherent scatter detector is arranged on a carriage which is preferably driven by a motor and travels on a rail. It is thus possible to move the coherent scatter detector on a preset path in very simple and economical manner. In particular it is also possible to move the coherent scatter detector in computer-controlled manner.
  • a further advantageous development of the invention provides that the coherent scatter detector is also movable parallel to the axis of rotation of the gantry. It is thereby possible that in wide (in the direction of the axis of rotation of the gantry) transmission detectors there is no blocking of the movement of the coherent scatter detector. This is the case in particular with transmission detectors over 40 mm wide. This is also possible because the coherent scatter detector travels in an orbit about the axis of rotation which lies closer to the axis of rotation than the transmission detector. This version is to be preferred because no additional complicated movement of the coherent scatter detector is necessary.
  • a further advantageous development of the invention provides that the plane of the coherent scatter detector is inclined vis-a-vis the plane of the transmission detector - in each case in the cross-section parallel to the axis of rotation of the gantry - and the two perpendiculars intersect in an area in which an item for examination which is to be examined, in particular an item of luggage, can be introduced.
  • the coherent scatter detector consists of individual pixels. It is preferred in this case that the pixels are arranged in several rows in the direction of the axis of rotation of the gantry. A spatially resolving detector is thus created which is very easy to produce, which in the case of several rows of pixels also has an even greater sensitivity because it records several scatter quanta from the same scatter area inside the examination area (through the rows of pixels more remote from the transmission detector).
  • a further advantageous development of the invention provides that the coherent scatter detector is energy-resolving. It is thereby possible to also use a polychromatic X-ray source instead of a monochromatic X-ray source which has only low intensity. Through such an energy-resolving coherent scatter detector an energy-sensitive CSCT (coherent scattering computed tomography) can also be used which is already known from the state of the art.
  • CSCT coherent scattering computed tomography
  • a further advantageous development of the invention provides that an X-ray source is inserted into the gantry, and it has an examination area for an item for examination, in particular for an item of luggage, wherein the X-ray beam of the X-ray source covers the entire examination area in a fan shape — seen perpendicular to the axis of rotation of the gantry - and with an anti-scatter collimator between the examination area and the transmission detector which allows through only X-ray beams directly penetrating the item for examination.
  • the term fan describes a geometric shape which is much greater in its longitudinal extension than in its thickness.
  • the CSCT already briefly discussed above can be carried out.
  • an anti-scatter collimator known from the state of the art which allows through only X-radiation directly penetrating the item of luggage is arranged between the examination area and the transmission detector. The method according to the invention described in more detail below can be carried out with such an X-ray computed tomograph.
  • a further advantageous development of the invention provides that a primary collimator for producing a fan beam is arranged between the X-ray source and the examination area. It is thereby possible to also use an X-ray source which a fan beam does not itself have to produce (for example a conical beam), this happening rather by means of the primary collimator.
  • a further advantageous development of the invention provides that the shape and position of the aperture of the primary collimator can be changed.
  • the primary collimator can thereby be used to produce a beam from the whole X-ray beam which, when examining an ROI, penetrates only this. Only information from the ROI is thereby obtained in the coherent scatter detector.
  • the primary collimator is preferably rotatable about the axis of rotation and its aperture can be changed both in its width parallel to the axis of rotation and its length tangential to the axis of rotation.
  • the width of the aperture of the primary collimator parallel to the axis of rotation can be changed between 0.2 and 50 mm and the length tangential to the axis of rotation between 25 and 750 mm.
  • a further advantageous development of the invention provides that there is arranged between the examination area and the coherent scatter detector a secondary collimator, the plates of which are directed towards the X-ray source. It is thereby guaranteed that the coherent scatter detector only ever “sees” a strip of the examination area, so that there is a fixed relationship between the position of the strip inside an item of luggage to be examined and the location of the scattered radiation at the detector. If no such secondary collimator is present a more costly design of the overall arrangement of the X-ray computed tomograph must be used. However, this is also possible in principle with a so-called "self-collimated CSCT". The fact is exploited that coherently scattered X-ray quanta are bundled in a narrowly forwardly directed cone. It is thereby superfluous to introduce a scatter collimator into the path of the beam.
  • the detector used must have a very good spatial resolution and an energy resolution if no monoenergetic X-ray source is used.
  • a further advantageous development of the invention provides that the X-ray tube is monoenergetic. Although it is thereby possible - as already stated above - to carry out the examination of the item of luggage at only low intensity, no energy-resolving coherent scatter detectors are required, so that the costs of such an X-ray computed tomograph prove to be lower.
  • the object is achieved by a method according to the invention with the features of claim 16.
  • a method according to the invention in the above-described gantry, in a first step the whole item for examination is recorded and examined by means of the transmission X-ray image obtained in the transmission detector, hi this CT process, under certain circumstances an area (or even several areas) is found where striking features are to be seen in the transmission X-ray image. These may be for example potentially dangerous objects inside an item of luggage, such as weapons or explosives. Such an area is known as an ROI. Its coordinates are precisely determined. The coordinates of the ROI are used in a second step to carry out an examination of the item for examination by means of a CSCT process.
  • the whole item for examination - only the ROI (or where necessary several ROIs) is subjected to the CSCT process.
  • a smaller coherent scatter detector described above, can also be used. To carry out the CSCT process this is moved along the gantry so that its centre lies on a straight line with the centre of the ROI and the X-ray source in a projection perpendicular to the axis of rotation of the gantry. It is thereby guaranteed that all the scatter information from the ROI strike the coherent scatter detector and thus a good and reliable statement can be made about the diffraction structure inside the ROI.
  • the position of coherent scatter detector must take place uncoupled from the movement of the gantry in order - irrespective of the state of the rotation in which the gantry finds itself about its axis of rotation - to always guarantee the above-named relationship, so that the whole of the ROFs scattered radiation falls into the coherent scatter detector.
  • the position of the scatter detector relative to the X-ray source repeats periodically if the X-ray source has completed a full rotation about the axis of rotation. Using the obtained diffraction images false alarms can be excluded and dangerous materials or striking features specified and classified in further detail.
  • the distance between the centre of the coherent scatter detector and the centre of the transmission detector in a projection perpendicular to the axis of rotation of the gantry is set equal to the product of the distance between the transmission detector and the axis of rotation of the gantry and the angle between the centre of the transmission detector and the centre of the coherent scatter detector.
  • An advantageous development of the invention provides that the coherent scatter detector is brought into a position outside the X-ray beam while recording the whole item for examination by means of the transmission detector. It is thereby guaranteed that the coherent scatter detector does not cover the transmission detector during the examination of the whole item of luggage.
  • a further advantageous development of the invention provides that the X-ray beam for the examination of the ROI of the item for examination is collimated so that only the ROI is penetrated. As already stated above, only information from the ROI solely of interest is thereby obtained.
  • the primary collimator is preferably set so that the X- ray beam penetrates only the ROI.
  • Figure 1 a schematic view along the axis of rotation of the gantry of an X-ray computed tomograph according to the invention
  • Figure 2 a longitudinal section perpendicular to the plane represented in Figure
  • Figure 3 a schematic view relating to the positioning of the coherent scatter detector
  • Figure 4 the relationship between a Shepp-Logan filter and the number of detectors.
  • FIG. 1 a side view of an X-ray computed tomograph according to the invention with a gantry 1 according to the invention is schematically represented.
  • the plane of the drawing stands perpendicular to the axis of rotation, not shown, of the gantry 1.
  • An X-ray source 4 which emits an X-ray beam 6 downwards is arranged at the gantry 1.
  • the X-ray beam 6 passes through an item for examination 7 (hereinafter an item of luggage 7 is representatively assumed) which lies on a conveyor belt 9.
  • the X-ray beam 6 passing through the item of luggage 7 strikes a transmission detector 2 on its inner wall in the lower area of the gantry 1.
  • Such a structure is best known from the state of the art in relation to the CT processes, so that it is not necessary to go into further detail below with regard to the precise design of the individual elements and their mode of operation.
  • the transmission detector 2 must be so long along the arc of the gantry 1 that it detects in transmission all the radiation which penetrates the item of luggage (in the present case the two extreme points are the left and right-hand top corners of the item of luggage 7). This is true of every angular position of the gantry 1 during its rotation about the item of luggage 7. Depending on the geometry of the computed tomograph, lengths of over one metre are definitely common for this.
  • the whole item of luggage 7 is examined by means of the known CT transillumination method.
  • the fluoroscopic image which is obtained in the transmission detector 2 frequently has striking areas which are called ROIs 8.
  • ROIs 8 ROIs 8.
  • material-selective analysis by means of X-ray diffraction in particular comes into consideration here. This is called coherent-scatter computed tomography (CSCT) in the literature.
  • CSCT coherent-scatter computed tomography
  • the photon energy of the X-radiation which is used for the coherent scatter must be determined. This involves either using a monochromatic X-ray source 4 or using energy-resolving detectors. As the emission from monochromatic X-ray tubes 4 is very weak in comparison with the capacities of conventional X-ray tubes 4, the use of energy- resolving detectors is given preference.
  • Such an energy-sensitive CSCT is known in the literature. However, the problem with this energy-sensitive CSCT is that energy- resolving detectors are as a rule produced from an expensive room temperature semiconductor material, for example from cadmium zinc telluride (CZT). Moreover, a spatial resolution must take place when determining the coherent scatter. Accordingly it is necessary that the energy-resolving detector field must be so large that the X-ray quanta coherently scattered from every point of the item of luggage 7 are recorded. However, this leads to high costs, as such large detector fields are very expensive.
  • the detectors for recording the coherently scattered X-ray quanta are hereinafter called coherent scatter detectors 3. These are arranged - as can be best seen in Figure 2 —laterally alongside the transmission detector 2.
  • the representation in Figure 2 is a schematic sectional drawing perpendicular to the plane represented in Figure 1.
  • the fan beam required is achieved here by introducing a primary collimator 5 with a longitudinal slit perpendicular to the plane of the drawing of Figure 1 in the beam path between X-ray source 4 and item of luggage 7.
  • a conventional X-ray source 4 can thereby be used which normally produces a conical beam.
  • a conical beam is not suitable for CSCT.
  • the coherent scatter detector 3 which is energy resolving and spatially resolving is tilted out from the plane of the transmission detector 2 and points towards the area in the item of luggage 7 which is penetrated by the X-ray beam 6 (in Figure 2 the thin layer of the fan beam). It can be readily seen that that the coherent scatter detector 3 is arranged on a carriage 10. This carriage 10 can be moved perpendicular to the plane of the drawing, thus parallel to the fan beam and to the transmission detector 2, by means of a motor, not shown, on rails, not shown. The movement is independent of the movement of the gantry 1, but in principle such that the coherent scatter detector 3 is always located next to the transmission detector 2.
  • the whole item of luggage 7 is no longer examined and its coherent scatter quanta analyzed, but only the ROIs 8 obtained in the first step by means of the transmission CT.
  • the necessary coherent scatter detector 3 also no longer has to cover the whole volume of the item of luggage 7, but can remain limited to the volume of the ROI 8.
  • the coherent scatter detector 3 is moved to the correct position so that the information from the ROI 8 also actually strikes the coherent scatter detector 3. Unlike a secondary scatter detector 3 required over the whole spatial area, according to the invention a reduction to only 10 % of the detector channels can thus be achieved.
  • the mechanical requirements when using a small coherent scatter detector 3 according to the invention are firstly that the ROI 8 which was ascertained in the first method step by means of the transmission CT are, in the second step, still penetrated by the coherent scatter quanta so that these strike in the coherent scatter detector 3. This means that the beam from the X-ray source 4 always strikes through the centre of the ROI 8 in the centre 12 of the coherent scatter detector 3. This must be true of any angle of rotation of the gantry 1. This means that the coherent scatter detector 3 must constantly be moved along its path by means of the motor. In order to constantly arrive at the proper position this is monitored by a computer.
  • Figure 3 represents how the relationships between the individual beam angles must turn out for a singled-out case of an ROI 8.
  • the arc distance t between the centre 12 of the coherent scatter detector 3 and the centre 11 of the transmission detector 2 changes sinusoidally with the angle of projection if the generally satisfactorily correct approximation is accepted that the X-ray source 4 and the two detectors are much further apart than the typical dimensions of the item of luggage 7.
  • the coherent scatter detector 3 covers at least 19 detector elements which are energy resolving.
  • a 2-D detector field can also be formed so that the total counting rate is increased.
  • a secondary collimator (not shown) is arranged between the coherent scatter detector 3 and the item of luggage 7.
  • This consists of thin plates of an X-ray beam-absorbing material, for example a suitable metal. The plates are directed towards the X-ray source 4 and serve to ensure that the coherent scatter detector 3 "sees" only a narrow strip of the ROI 8.
  • These plates of the secondary collimator can also be part of an anti- scatter collimator (not shown) which is arranged between the item of luggage 7 and the transmission detector 2.
  • the "self-collimated CSCT" technique already described above can also be applied.
  • the path of the coherent scatter detector 3 can theoretically be blocked by the transmission detector 2 if the coherent scatter detector 3 is moved on a parallel plane to the gantry 1 and tangential to the axis of rotation.
  • this entails very complicated movements and guidings of the carriage 10 on which the coherent scatter detector 3 is arranged, this should be avoided if possible.
  • the transmission detector 2 It is also possible to remove the transmission detector 2 from the X-ray beam 6 and instead to move the coherent scatter detector 3 to the provided position. However, this is also laborious, as during the rotation of the gantry 1, taking place at approx. 1 Hz, the transmission detector 2 must be rotated towards same. At its simplest the arrangement can be realized by having the coherent scatter detector 3 travel in an orbit about the axis of rotation which lies closer to the axis of rotation than the transmission detector 2.
  • the position of the coherent scatter detector 3 is set as follows: A cartesian coordinates system is used - as represented in Figure 3. In this the position of the ROI 8 is defined as P(x,y). Also, the position of the X-ray source is defined as (R 3 , ⁇ ) and the centre 11 of the transmission detector 2 as (R D , ⁇ ) . ⁇ is the angle of projection for the central beam 13. The result for arc distance t is that it is then the product of R D with the angle ⁇ . The angle ⁇ can easily be obtained using elementary geometric considerations.

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Abstract

The invention relates to a gantry 1 for housing an X-ray source 4, with an axis of rotation, with a transmission detector 2 which extends along the inner surface of the gantry 1, and with a spatially resolving coherent scatter detector 3 which is arranged laterally in the direction of the axis of rotation alongside the transmission detector 2. According to the invention it is provided that the coherent scatter detector 3 has a smaller longitudinal extension in the direction of the arc of the gantry 1 and is movable parallel to the plane of the gantry 1 independently of the rotation of the gantry 1. The invention also relates to a method of examining an item for examination 7 by means of X-radiation for striking features using an X-ray tomograph according to the invention with the following steps: recording and examining of the whole item for examination 7 by means of the transmission X-ray image obtained in the transmission detector 2; determining an ROI 8 in which striking features have been recognized in the transmission X-ray image; moving the coherent scatter detector 3 so that its centre 12 lies on a straight line with the centre of the ROI 8 and the X-ray source 4 in a projection perpendicular to the axis of rotation of the gantry 1; examining the ROI 8 by means of the diffraction profiles obtained in the coherent scatter detector 3.

Description

GANTRY FOR HOUSING AN X-RAY SOURCE AND METHOD OF EXAMINING AN ITEM FOR EXAMINATION BY MEANS OF X-RADIATION
The invention relates to a gantry for housing an X-ray source, with an axis of rotation, with a transmission detector which extends along the inner surface of the gantry, and with a spatially resolving coherent scatter detector which is arranged laterally in the direction of the axis of rotation alongside the transmission detector. The invention also relates to an X-ray computed tomograph with an X-ray source and a gantry with the features described above. Finally the invention relates to a method of examining an item for examination by means of X-radiation for striking features using such an X-ray computed tomograph.
A computed tomograph for ascertaining the momentum transfer spectrum in an examination area is known from DE 100 09 285 Al. There, an X-ray source with a primary collimator is arranged on a gantry rotatable about an axis, with which a fan beam is produced. Opposite the X-ray source is a detector array, likewise attached to the gantry, for the detection of the X-ray beams penetrating an examination area. A secondary collimator, arranged between the examination area and the detector array, allows only X-radiation from a specific scatter voxel from the examination area to pass into an allocated column of the detector array. From the obtained scatter data and the measured primary radiation in the plane of the fan beam, a reconstruction is carried out by means of an iterative algebraic reconstruction technique (ART), using the momentum transfer spectrum for each scatter voxel in the examination area through which a primary beam passes. The momentum transfer spectrum is characteristic of the matter in the scatter voxel concerned and information about the physical composition is thus also obtained. However, such a computed tomograph and the method implemented with it suffer from a substantial disadvantage. The entire item for examination located in the examination area is always examined, which requires a large detector for the coherently scattered X-radiation. Because such detectors are very expensive, such a computed tomograph is also very expensive. Thus the object of the invention is to provide a gantry, an X-ray computed tomograph and a method for operating an X-ray computed tomograph which are more economical, in particular with regard to the required coherent scatter detector.
The object is achieved by a gantry with the features of claim 1. Because, unlike gantries known until now - in which the spatially resolving coherent scatter detector has the same longitudinal extension along the inner surface of the gantry as the transmission detector - the coherent scatter detector has a small longitudinal extension in the direction of the arc of the gantry, a coherent scatter detector, substantially smaller in spatial terms, is used. Because of the possibility that it is movable parallel to the plane of the gantry independently of the rotation of the gantry, this smaller coherent scatter detector is always to be brought, by means of the method according to the invention described below, into such a position that all the information from a previously specified region where a problem has been found in an item for examination (hereinafter called ROI - Region of Interest), is completely preserved and thus the diffraction profiles can be determined clearly from this ROI.
An advantageous development of the invention provides that the coherent scatter detector is arranged on a carriage which is preferably driven by a motor and travels on a rail. It is thus possible to move the coherent scatter detector on a preset path in very simple and economical manner. In particular it is also possible to move the coherent scatter detector in computer-controlled manner.
A further advantageous development of the invention provides that the coherent scatter detector is also movable parallel to the axis of rotation of the gantry. It is thereby possible that in wide (in the direction of the axis of rotation of the gantry) transmission detectors there is no blocking of the movement of the coherent scatter detector. This is the case in particular with transmission detectors over 40 mm wide. This is also possible because the coherent scatter detector travels in an orbit about the axis of rotation which lies closer to the axis of rotation than the transmission detector. This version is to be preferred because no additional complicated movement of the coherent scatter detector is necessary. A further advantageous development of the invention provides that the plane of the coherent scatter detector is inclined vis-a-vis the plane of the transmission detector - in each case in the cross-section parallel to the axis of rotation of the gantry - and the two perpendiculars intersect in an area in which an item for examination which is to be examined, in particular an item of luggage, can be introduced. By inclining the plane of the coherent scatter detector this, although it is rotated out of the transmission plane, is substantially perpendicular to the coherent scatter quanta striking it, which strike it from the examination area.
A further advantageous development of the invention provides that the coherent scatter detector consists of individual pixels. It is preferred in this case that the pixels are arranged in several rows in the direction of the axis of rotation of the gantry. A spatially resolving detector is thus created which is very easy to produce, which in the case of several rows of pixels also has an even greater sensitivity because it records several scatter quanta from the same scatter area inside the examination area (through the rows of pixels more remote from the transmission detector).
A further advantageous development of the invention provides that the coherent scatter detector is energy-resolving. It is thereby possible to also use a polychromatic X-ray source instead of a monochromatic X-ray source which has only low intensity. Through such an energy-resolving coherent scatter detector an energy-sensitive CSCT (coherent scattering computed tomography) can also be used which is already known from the state of the art.
A further advantageous development of the invention provides that an X-ray source is inserted into the gantry, and it has an examination area for an item for examination, in particular for an item of luggage, wherein the X-ray beam of the X-ray source covers the entire examination area in a fan shape — seen perpendicular to the axis of rotation of the gantry - and with an anti-scatter collimator between the examination area and the transmission detector which allows through only X-ray beams directly penetrating the item for examination. Here, the term fan describes a geometric shape which is much greater in its longitudinal extension than in its thickness. Because the X-ray beam of the X-ray source is fan-shaped in the area of the examination area (seen perpendicular to the axis of rotation of the gantry), the CSCT already briefly discussed above can be carried out. In order not to record any disruptive scatter quanta in the transmission detector, an anti-scatter collimator known from the state of the art which allows through only X-radiation directly penetrating the item of luggage is arranged between the examination area and the transmission detector. The method according to the invention described in more detail below can be carried out with such an X-ray computed tomograph.
A further advantageous development of the invention provides that a primary collimator for producing a fan beam is arranged between the X-ray source and the examination area. It is thereby possible to also use an X-ray source which a fan beam does not itself have to produce (for example a conical beam), this happening rather by means of the primary collimator.
A further advantageous development of the invention provides that the shape and position of the aperture of the primary collimator can be changed. The primary collimator can thereby be used to produce a beam from the whole X-ray beam which, when examining an ROI, penetrates only this. Only information from the ROI is thereby obtained in the coherent scatter detector. The primary collimator is preferably rotatable about the axis of rotation and its aperture can be changed both in its width parallel to the axis of rotation and its length tangential to the axis of rotation. In particular the width of the aperture of the primary collimator parallel to the axis of rotation can be changed between 0.2 and 50 mm and the length tangential to the axis of rotation between 25 and 750 mm.
A further advantageous development of the invention provides that there is arranged between the examination area and the coherent scatter detector a secondary collimator, the plates of which are directed towards the X-ray source. It is thereby guaranteed that the coherent scatter detector only ever "sees" a strip of the examination area, so that there is a fixed relationship between the position of the strip inside an item of luggage to be examined and the location of the scattered radiation at the detector. If no such secondary collimator is present a more costly design of the overall arrangement of the X-ray computed tomograph must be used. However, this is also possible in principle with a so-called "self-collimated CSCT". The fact is exploited that coherently scattered X-ray quanta are bundled in a narrowly forwardly directed cone. It is thereby superfluous to introduce a scatter collimator into the path of the beam. The detector used must have a very good spatial resolution and an energy resolution if no monoenergetic X-ray source is used.
A further advantageous development of the invention provides that the X-ray tube is monoenergetic. Although it is thereby possible - as already stated above - to carry out the examination of the item of luggage at only low intensity, no energy-resolving coherent scatter detectors are required, so that the costs of such an X-ray computed tomograph prove to be lower.
Furthermore the object is achieved by a method according to the invention with the features of claim 16. According to the invention, in the above-described gantry, in a first step the whole item for examination is recorded and examined by means of the transmission X-ray image obtained in the transmission detector, hi this CT process, under certain circumstances an area (or even several areas) is found where striking features are to be seen in the transmission X-ray image. These may be for example potentially dangerous objects inside an item of luggage, such as weapons or explosives. Such an area is known as an ROI. Its coordinates are precisely determined. The coordinates of the ROI are used in a second step to carry out an examination of the item for examination by means of a CSCT process. So as not to have to once again examine the whole examination area - thus the whole item for examination - only the ROI (or where necessary several ROIs) is subjected to the CSCT process. As only a very small partial section of the whole item for examination is to be verified, a smaller coherent scatter detector, described above, can also be used. To carry out the CSCT process this is moved along the gantry so that its centre lies on a straight line with the centre of the ROI and the X-ray source in a projection perpendicular to the axis of rotation of the gantry. It is thereby guaranteed that all the scatter information from the ROI strike the coherent scatter detector and thus a good and reliable statement can be made about the diffraction structure inside the ROI. The position of coherent scatter detector must take place uncoupled from the movement of the gantry in order - irrespective of the state of the rotation in which the gantry finds itself about its axis of rotation - to always guarantee the above-named relationship, so that the whole of the ROFs scattered radiation falls into the coherent scatter detector. The position of the scatter detector relative to the X-ray source repeats periodically if the X-ray source has completed a full rotation about the axis of rotation. Using the obtained diffraction images false alarms can be excluded and dangerous materials or striking features specified and classified in further detail. In the process it is preferred that the distance between the centre of the coherent scatter detector and the centre of the transmission detector in a projection perpendicular to the axis of rotation of the gantry is set equal to the product of the distance between the transmission detector and the axis of rotation of the gantry and the angle between the centre of the transmission detector and the centre of the coherent scatter detector.
An advantageous development of the invention provides that the coherent scatter detector is brought into a position outside the X-ray beam while recording the whole item for examination by means of the transmission detector. It is thereby guaranteed that the coherent scatter detector does not cover the transmission detector during the examination of the whole item of luggage.
A further advantageous development of the invention provides that the X-ray beam for the examination of the ROI of the item for examination is collimated so that only the ROI is penetrated. As already stated above, only information from the ROI solely of interest is thereby obtained. The primary collimator is preferably set so that the X- ray beam penetrates only the ROI.
Further details and advantages of the invention are explained in more detail with the help of the embodiment represented in the Figures. There are shown in:
Figure 1 a schematic view along the axis of rotation of the gantry of an X-ray computed tomograph according to the invention,
Figure 2 a longitudinal section perpendicular to the plane represented in Figure
1, Figure 3 a schematic view relating to the positioning of the coherent scatter detector and
Figure 4 the relationship between a Shepp-Logan filter and the number of detectors.
In Figure 1 a side view of an X-ray computed tomograph according to the invention with a gantry 1 according to the invention is schematically represented. The plane of the drawing stands perpendicular to the axis of rotation, not shown, of the gantry 1. An X-ray source 4 which emits an X-ray beam 6 downwards is arranged at the gantry 1. The X-ray beam 6 passes through an item for examination 7 (hereinafter an item of luggage 7 is representatively assumed) which lies on a conveyor belt 9. The X-ray beam 6 passing through the item of luggage 7 strikes a transmission detector 2 on its inner wall in the lower area of the gantry 1. Such a structure is best known from the state of the art in relation to the CT processes, so that it is not necessary to go into further detail below with regard to the precise design of the individual elements and their mode of operation.
The transmission detector 2 must be so long along the arc of the gantry 1 that it detects in transmission all the radiation which penetrates the item of luggage (in the present case the two extreme points are the left and right-hand top corners of the item of luggage 7). This is true of every angular position of the gantry 1 during its rotation about the item of luggage 7. Depending on the geometry of the computed tomograph, lengths of over one metre are definitely common for this.
As the first step in carrying out a method according to the invention, the whole item of luggage 7 is examined by means of the known CT transillumination method. The fluoroscopic image which is obtained in the transmission detector 2 frequently has striking areas which are called ROIs 8. However, it is well known from the state of the art that transmission CT frequently produces false alarms. To resolve such false alarms - thus either to verify them or discard them - it is necessary to apply another technique. Material-selective analysis by means of X-ray diffraction in particular comes into consideration here. This is called coherent-scatter computed tomography (CSCT) in the literature. In CSCT the X-ray diffraction profiles are reconstructed from the coherent scatter projections.
In order to measure the coherent scatter with the necessary precision, the photon energy of the X-radiation which is used for the coherent scatter must be determined. This involves either using a monochromatic X-ray source 4 or using energy-resolving detectors. As the emission from monochromatic X-ray tubes 4 is very weak in comparison with the capacities of conventional X-ray tubes 4, the use of energy- resolving detectors is given preference. Such an energy-sensitive CSCT is known in the literature. However, the problem with this energy-sensitive CSCT is that energy- resolving detectors are as a rule produced from an expensive room temperature semiconductor material, for example from cadmium zinc telluride (CZT). Moreover, a spatial resolution must take place when determining the coherent scatter. Accordingly it is necessary that the energy-resolving detector field must be so large that the X-ray quanta coherently scattered from every point of the item of luggage 7 are recorded. However, this leads to high costs, as such large detector fields are very expensive.
The detectors for recording the coherently scattered X-ray quanta are hereinafter called coherent scatter detectors 3. These are arranged - as can be best seen in Figure 2 —laterally alongside the transmission detector 2. The representation in Figure 2 is a schematic sectional drawing perpendicular to the plane represented in Figure 1.
The fan beam required is achieved here by introducing a primary collimator 5 with a longitudinal slit perpendicular to the plane of the drawing of Figure 1 in the beam path between X-ray source 4 and item of luggage 7. A conventional X-ray source 4 can thereby be used which normally produces a conical beam. However, a conical beam is not suitable for CSCT.
The coherent scatter detector 3 which is energy resolving and spatially resolving is tilted out from the plane of the transmission detector 2 and points towards the area in the item of luggage 7 which is penetrated by the X-ray beam 6 (in Figure 2 the thin layer of the fan beam). It can be readily seen that that the coherent scatter detector 3 is arranged on a carriage 10. This carriage 10 can be moved perpendicular to the plane of the drawing, thus parallel to the fan beam and to the transmission detector 2, by means of a motor, not shown, on rails, not shown. The movement is independent of the movement of the gantry 1, but in principle such that the coherent scatter detector 3 is always located next to the transmission detector 2.
In principle it suffices to record only a small area of the penetrated item of luggage - namely only that identified as ROI 8 previously in the CT transillumination method- in the coherent scatter detector 3. This is because, upon a CT reconstruction by a filtered back projection (FBP), the spatial extension of the "convolution kernel" - with which the projections are filtered before the back projection step takes place, drops outwards very rapidly from its central pack (see Figure 4). Here, the number in the X-axis gives the number of detector elements, starting with the detector element designated "0". Thus it is not necessary to have all the projection data available in order to have a reconstruction of only a small area. It is quite sufficient to have only one section of the whole scatter data. This is in particular the case if only a qualitative reconstruction of the peaks in the diffraction profiles is necessary, such as for example when detecting explosives in items of luggage 7. The position of the peak can already be obtained from strongly pruned projection data for a small area from the item of luggage 7.
Therefore in a second step according to the invention the whole item of luggage 7 is no longer examined and its coherent scatter quanta analyzed, but only the ROIs 8 obtained in the first step by means of the transmission CT. As only small volumes are still involved compared with the large volume of the complete item of luggage 7, the necessary coherent scatter detector 3 also no longer has to cover the whole volume of the item of luggage 7, but can remain limited to the volume of the ROI 8.
Then it is only necessary that the coherent scatter detector 3 is moved to the correct position so that the information from the ROI 8 also actually strikes the coherent scatter detector 3. Unlike a secondary scatter detector 3 required over the whole spatial area, according to the invention a reduction to only 10 % of the detector channels can thus be achieved. The mechanical requirements when using a small coherent scatter detector 3 according to the invention are firstly that the ROI 8 which was ascertained in the first method step by means of the transmission CT are, in the second step, still penetrated by the coherent scatter quanta so that these strike in the coherent scatter detector 3. This means that the beam from the X-ray source 4 always strikes through the centre of the ROI 8 in the centre 12 of the coherent scatter detector 3. This must be true of any angle of rotation of the gantry 1. This means that the coherent scatter detector 3 must constantly be moved along its path by means of the motor. In order to constantly arrive at the proper position this is monitored by a computer.
Figure 3 represents how the relationships between the individual beam angles must turn out for a singled-out case of an ROI 8. The arc distance t between the centre 12 of the coherent scatter detector 3 and the centre 11 of the transmission detector 2 changes sinusoidally with the angle of projection if the generally satisfactorily correct approximation is accepted that the X-ray source 4 and the two detectors are much further apart than the typical dimensions of the item of luggage 7.
Secondly, the above described requirement of the presence of a fan beam which is produced by the primary collimator 5 in Figure 2 obtains. The coherent scatter detector 3 covers at least 19 detector elements which are energy resolving. A 2-D detector field can also be formed so that the total counting rate is increased.
A secondary collimator (not shown) is arranged between the coherent scatter detector 3 and the item of luggage 7. This consists of thin plates of an X-ray beam-absorbing material, for example a suitable metal. The plates are directed towards the X-ray source 4 and serve to ensure that the coherent scatter detector 3 "sees" only a narrow strip of the ROI 8. These plates of the secondary collimator can also be part of an anti- scatter collimator (not shown) which is arranged between the item of luggage 7 and the transmission detector 2. Instead of a secondary collimator the "self-collimated CSCT" technique already described above can also be applied.
Because of the sinusoidal arc distance which was described above, the path of the coherent scatter detector 3 can theoretically be blocked by the transmission detector 2 if the coherent scatter detector 3 is moved on a parallel plane to the gantry 1 and tangential to the axis of rotation. In order to avoid such a blocking it is either necessary to limit the width of the transmission detector 2 to a maximum 40 mm or to enable a movement of the coherent scatter detector 3 not only in the prescribed plane but additionally also perpendicular to this plane, i.e. parallel to the axis of rotation of the gantry 1. However, as this entails very complicated movements and guidings of the carriage 10 on which the coherent scatter detector 3 is arranged, this should be avoided if possible. It is also possible to remove the transmission detector 2 from the X-ray beam 6 and instead to move the coherent scatter detector 3 to the provided position. However, this is also laborious, as during the rotation of the gantry 1, taking place at approx. 1 Hz, the transmission detector 2 must be rotated towards same. At its simplest the arrangement can be realized by having the coherent scatter detector 3 travel in an orbit about the axis of rotation which lies closer to the axis of rotation than the transmission detector 2.
To carry out the method according to the invention, the position of the coherent scatter detector 3 is set as follows: A cartesian coordinates system is used - as represented in Figure 3. In this the position of the ROI 8 is defined as P(x,y). Also, the position of the X-ray source is defined as (R3 , φ) and the centre 11 of the transmission detector 2 as (RD, φ) . φ is the angle of projection for the central beam 13. The result for arc distance t is that it is then the product of RD with the angle γ. The angle γ can easily be obtained using elementary geometric considerations.

Claims

Claims
1. Gantry (1) for housing an X-ray source (4),
with an axis of rotation,
with a transmission detector (2) which extends along the inner surface of the gantry (1),
and with a spatially resolving coherent scatter detector (3) which is arranged in the direction of the axis of rotation laterally alongside the transmission detector (2),
characterized in that
the coherent scatter detector (3) has a smaller longitudinal extension in the direction of the arc of the gantry (1) and is movable parallel to the plane of the gantry (1) independently of the rotation of the gantry (1).
2. Gantry (1) according to claim 1, characterized in that the coherent scatter detector (3) is arranged on a carriage (10) which is preferably driven by a motor and travels on a rail.
3. Gantry (1) according to claim 1 or 2, characterized in that the coherent scatter detector (3) is also movable parallel to the axis of rotation of the gantry (1).
4. Gantry (1) according to one of the previous claims, characterized in that the coherent scatter detector (3) travels in an orbit about the axis of rotation which lies closer to the axis of rotation than the transmission detector (2).
5. Gantry (1) according to one of the previous claims, characterized in that the plane of the coherent scatter detector (3) is inclined vis-a-vis the plane of the transmission detector (2) - in each case in the cross-section parallel to the axis of rotation of the gantry (1) - and the two perpendiculars intersect in an area in which an item for examination (7) which is to be examined, in particular an item of luggage, can be introduced.
6. Gantry (1) according to one of the previous claims, characterized in that the coherent scatter detector (3) consists of individual pixels.
7. Gantry (1) according to claim 6, characterized in that the pixels are arranged in several rows in the direction of the axis of rotation of the gantry (1).
8. Gantry (1) according to one of the previous claims, characterized in that the coherent scatter detector (3) is energy-resolving.
9. Gantry (1) according to one of the previous claims, characterized in that an X- ray source (4) is inserted into it, and it has an examination area for an item for examination (7), in particular for an item of luggage, wherein the X-ray beam (6) of the X-ray source (4) covers the entire examination area in a fan-shape - seen perpendicular to the axis of rotation of the gantry (1) - and with an anti-scatter collimator between the examination area and the transmission detector (2) which allows through only X-ray beams directly penetrating the item for examination (7).
10. Gantry (1) according to one of the previous claims, characterized in that a primary collimator (5) for producing a fan beam is arranged between the X-ray source
(4) and the examination area.
11. Gantry (1) according to claim 10, characterized in that the shape and position of the aperture of the primary collimator (5) can be changed.
12. Gantry (1) according to claim 11, characterized in that the primary collimator
(5) is rotatable about the axis of rotation and its aperture can be changed both in its width parallel to the axis of rotation and its length tangential to the axis of rotation.
13. Gantry (1) according to claim 12, characterized in that the width of the aperture of the primary collimator (5) parallel to the axis of rotation can be changed between 0.2 and 50 mm and the length tangential to the axis of rotation between 25 and 750 mm.
14. Gantry (1) according to one of the previous claims, characterized in that there is arranged between the examination area and the coherent scatter detector (3) a secondary collimator, the plates of which are directed towards the X-ray source (4).
15. Gantry (1) according to one of the previous claims, characterized in that the X- ray source (4) is monoenergetic.
16. Method of examining an item for examination (7) by means of X-radiation for striking features using a gantry (1) according to one of the previous claims with the following steps:
- Recording and examining of the whole item for examination (7) by means of the transmission X-ray image obtained in the transmission detector (2);
- Determining an ROI (8) in which striking features have been recognized in the transmission X-ray image;
- Moving the coherent scatter detector (3) so that its centre (12) lies on a straight line with the centre of the ROI (8) and the X-ray source (4) in a projection perpendicular to the axis of rotation of the gantry (1);
- Examining the ROI (8) by means of the diffraction profiles obtained in the coherent scatter detector (3).
17. Method according to claim 16, characterized in that the distance between the centre (12) of the coherent scatter detector (3) and the centre (11) of the transmission detector (2) in a projection perpendicular to the axis of rotation of the gantry (1) is set equal to the product of the distance between the transmission detector (2) and the axis of rotation of the gantry (1) and the angle between the centre (11) of the transmission detector (2) and the centre (12) of the coherent scatter detector (3).
18. Method according to claim 16 or 17, characterized in that the coherent scatter detector (3) is brought into a position outside the X-ray beam (6) while recording the whole item for examination (7) by means of the transmission detector (2).
19. Method according to one of claims 16 to 18, characterized in that the X-ray beam (6) for the examination of the ROI (8) of the item for examination (7) is collimated so that only the ROI (8) is penetrated.
20. Method according to claim 19, characterized in that the primary collimator (5) is set so that the X-ray beam (6) penetrates only the ROI (8).
PCT/IB2006/003926 2005-05-31 2006-05-31 Gantry for housing an x-ray source and method of examining an item for examination by means of x-radiation WO2007054837A2 (en)

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