Patent Description:
Various restricted areas, such as airports, sensitive facilities of national infrastructure and some public spaces often require a screening system. Traditional X-ray-absorption security screening systems are an effective tool for automatically identifying threats and other contraband that may be present in some luggage. However, the high detection rate of these systems often leads to a high false alarm rate, in which benign items are wrongly labelled as a threat. This results in additional security screening measures, increased touchpoints between security staff and passenger luggage to resolve the alarm and subsequent increased waiting times.

To reduce instances of false alarm, some screening systems may implement a first stage in the form of an X-ray absorption pre-screener, and a second stage that uses scattering/diffraction information to confirm or overturn an alarm signal generated by the first stage. Such systems are relatively complex and increase the overall scanning time.

It is an object of the disclosure to address one or more of the above-mentioned limitations.

<CIT> describes a metrology system for use during semiconductor manufacturing process. The system is used to detect defects on wafers. The location of incidence of the illumination beam on the surface of the wafer is determined based on occlusion of the illumination beam by two or more occlusion elements. The center of the illumination beam is determined based on measured values of transmitted flux and a model of the interaction of the beam with each occlusion element. The position of the axis of rotation orienting a wafer over a range of angles incidence is adjusted to align with the surface of wafer and intersect the illumination beam at the measurement location. <CIT> describes a scanning system which uses a combination of computed tomography (CT) and coherent X-ray scatter (CXS) for screening liquids, aerosols, and gels (LAGS). In a primary scan, a bag is scanned using dual energy CT technique with fan beam radiation. In case of an alarm, the alarming LAG container is scanned again using CXS technique with cone beam radiation.

According to a first aspect of the invention, there is provided a screening system for screening an item, as defined by claim <NUM>, the screening system comprising a detection apparatus comprising an emitter portion to generate a primary beam of ionising radiation and a detector portion to detect an absorption signal and at least one of a diffraction signal and a scattering signal; the screening system further comprising a rotatable platform adapted to receive the item; and a mechanical arrangement adapted to translate the detection apparatus along a translation axis to scan the item with the primary beam.

For instance, the item may be some luggage containing various articles. It may also be a letter or a parcel.

Still according to the invention, the translation axis is substantially parallel to a surface of the rotatable platform.

Optionally, upon rotation of the rotatable platform and translation of the detection apparatus, the detection apparatus collects data along a curved trajectory in a reference frame of the rotational platform.

For instance, the curved trajectory may form a two-dimensional spiral. Optionally, the rotatable platform is rotatable around a rotational axis and wherein the translation axis is substantially perpendicular to the rotational axis and intersects with the rotational axis.

For instance, the rotational axis may be located at the geometric centre of the platform.

Optionally, the emitter portion comprises a source of ionizing radiation and a beam former adapted to generate the primary beam of ionizing radiation.

For instance, the source of ionizing radiation may be an X-ray source such as an X-ray point source.

Optionally, the source of ionizing radiation is a polychromatic source.

Optionally, the detector portion comprises an absorption sensor, an energy resolving detector and a collimator, the collimator comprising a plurality of channels, each channel being adapted to receive diffracted or scattered radiation.

For instance, the absorption sensor may be an X-ray absorption detector such as a dual-energy ring detector. The absorption sensor may be provided before the collimator, or between the collimator and the energy resolving detector or after the energy resolving detector.

Optionally, the primary beam has a characteristic propagation axis associated with it, and wherein the collimator is provided along the characteristic propagation axis of the primary beam.

Optionally, the energy resolving detector is spatially resolved. For instance, the energy resolving detector may be a pixelated energy resolving detector.

According to the invention, the primary beam is a conical shell beam of ionising radiation.

Optionally, the screening system comprises a processor configured to receive a first data set from the detection apparatus to form a first image and a second data set from the detection apparatus to form a second image, wherein the first and second images overlap spatially.

Optionally, the first image is an absorption image and the second image is a diffraction or a scattering image of the item.

Optionally, the energy resolving detector is a pixelated energy resolving detector and the processor is configured to process the second data set to correct for the angular dependence of each pixel of the energy resolving detector.

Optionally, the absorption sensor comprises a plurality of absorption detector elements and the processor is configured to process the first data set to correct for the angular dependence of each absorption detector element.

For instance, the processor may be configured to apply an orientation correction factor.

Optionally, the processor is configured to execute a reconstruction algorithm to obtain a plurality of absorption images from different depth planes, to select an absorption image from one plane and to overlay the selected absorption image with a corresponding diffraction or scattering image from the same plane.

For example, the reconstruction algorithm may be a tomosynthesis reconstruction algorithm.

Optionally, the processor is configured to perform image segmentation of at least one of the first image and the second image to identify one or more regions of interest and to perform image classification of the said one or more regions of interest.

For instance, the processor may be configured to execute a first algorithm to perform image segmentation and a second algorithm to perform image classification.

Optionally, the processor is configured to calculate a sample parameter based on the second set of data to identify the nature of a sample to be identified, wherein the sample parameter comprises a lattice spacing.

For instance, the sample may be an article present in the item or a portion of an article.

Optionally, the screening system comprises a controller adapted to control the motion of the detection apparatus and of the rotatable platform.

For instance, the mechanical arrangement may comprise at least one guide member, and the controller may be configured to translate the detection apparatus along the said at least one guide member.

For instance, the controller may be adapted to operate the screening system in a first mode in which the rotatable platform rotates with constant frequency and the detector apparatus is translated with a constant linear velocity, or in a second mode in which the angular velocity of the rotatable platform and the linear velocity of the detection apparatus are adjusted over time so that the velocity of the detection apparatus along the curved trajectory remains constant.

According to a second aspect of the invention, there is provided a method for screening an item, as defined in claim <NUM>, the method comprising providing a rotatable platform adapted to receive the item; providing a detection apparatus comprising an emitter portion to generate a primary beam of ionising radiation and a detector portion to detect an absorption signal and at least one of a diffraction and a scattering signal; rotating the rotatable platform and translating the detection apparatus along a translation axis to scan the item with the primary beam.

Optionally, the method comprises detecting the absorption signal and the said at least one of a diffraction signal and a scattering signal simultaneously.

The options described with respect to the first aspect of the disclosure are also common to the second aspect of the disclosure.

The disclosure is described in further detail below by way of example and with reference to the accompanying drawings, in which:.

<FIG> is a diagram of a detection apparatus <NUM> for measuring both high energy absorption and high energy diffraction or scattering. The detection apparatus includes an emitter portion to generate a primary beam of ionising radiation and a detector portion to detect an absorption signal and at least one of a diffraction signal and a scattering signal. The primary beam <NUM> may also be referred to as probe or probe beam.

The emitter portion is formed of a source of ionizing radiations <NUM> such as a source of X-ray or gamma-ray radiations, and a mask or beam former <NUM>. The source of ionizing radiation <NUM> is aligned with the mask <NUM> for forming the primary beam <NUM> of electromagnetic radiation.

The source of ionizing radiation <NUM> may be adapted to provide high-energy electromagnetic EM radiations. For example, the high-energy EM radiations may have photons of energy greater than about <NUM> keV. The high-energy EM radiations may be hard X-ray radiations having photons of energy greater than about <NUM> keV. For example, the source <NUM> may include an X-ray source for providing X-rays having an energy in the region of about <NUM> keV to about <NUM> keV. The source of ionizing radiation <NUM> may be a polychromatic source such as a polychromatic X-ray source.

The mask <NUM> may be formed by a solid body made of a radiopaque material provided with a shaped slit. Example of radiopaque materials that can block X-rays include tungsten or alloys made of steel and lead or from combinations of these materials. The mask <NUM> may be provided with an annular-shaped slit for forming a primary beam <NUM> having a conical shell profile and referred to as conical shell beam. Alternatively, the mask <NUM> may be designed to form a primary beam having other hollow beam shapes. In the present example the source is a polychromatic X-ray source and the primary beam a polychromatic annular X-ray beam.

The detector portion is formed of an ionizing radiation absorption sensor <NUM>, a collimator <NUM> and an energy resolving detector <NUM> for detecting the energy of scattered or diffracted photons from a sample material.

The ionizing radiation absorption sensor <NUM> is provided on the propagation axis <NUM> at a distance L from the mask <NUM>. In this example, the absorption sensor is provided between the mask <NUM> and the collimator, however in other embodiments the absorption sensor <NUM> may be provided between the collimator <NUM> and the energy resolving detector <NUM> or after the energy resolving detector <NUM>. Therefore the sensor <NUM> may be placed at various location on the propagation axis <NUM>, with its surface substantially parallel to the mask <NUM>. The size of the absorption sensor may be selected depending on the chosen arrangement to catch the radiation from the primary beam. The absorption sensor <NUM> has a hollow shape. The absorption sensor <NUM> has a sensing area provided with a plurality of absorption detectors elements distributed along its lengths (not shown), and a hollow centre to let the scattering/diffracting photons travel towards the collimator <NUM>. In this example the absorption sensor <NUM> is a ring X-ray absorption sensor. The space provided between the mask <NUM> and the absorption sensor <NUM> forms an inspection volume for receiving a sample material <NUM>. A ring sensor facilitates a supplementary measurement of the number of rays absorbed by the sample material and does not impede the scattering/diffracting photons. Both scattering and absorption signals can be captured simultaneously.

The collimator <NUM>, also referred to as grid structure, is provided on the propagation axis <NUM> between the absorption sensor <NUM> and the energy resolving detector <NUM>. The collimator <NUM> may be positioned such that the input surface of the collimator is substantially normal to the propagation axis <NUM>. The detector <NUM> is positioned such that its detection surface is substantially parallel to the output surface of the collimator <NUM>. The collimator <NUM> is used to constrain the incidence of electromagnetic radiation onto the detector <NUM>, which only collects data arising from a particular angle of scatter or diffraction. The collimator <NUM> is elongated and includes a plurality of channels extending between its input and its output. The channels may have a hexagonal cross section, or different shapes allowing the channels to be arranged in a tessellated fashion. The walls forming the channels may be formed from suitable material to block or substantially attenuate X-ray radiation, such as tungsten or lead antimony alloy.

The energy resolving detector <NUM> may be spatially resolving, such as a pixelated energy resolving X-ray detector. The energy resolving detector <NUM> and the absorption sensor <NUM> can be coupled to a data analyser to store and analyse the collected data.

It will be appreciated that various mechanical features (not shown) would be provided to hold the absorption sensor <NUM>, the collimator <NUM> and the energy resolving detector <NUM> in relative position with each other. A housing may also be provided to enclose or partially enclose the elements of the detector portion. Similarly mechanical features would be provided to hold the source <NUM> and the mask <NUM>. A housing may also be provided to enclose or partially enclose the elements of the emitter portion. A longitudinal member may also be provided to hold the emitter portion and the detector portion in a desired alignment with respect to each other.

In operation, the primary beam <NUM> produces a circular footprint on the plane of the absorption sensor <NUM>. When a sample material <NUM> is placed within the inspection volume, the detector <NUM> measures the energy of the photons scattered by the sample material at known angles of scatter, and the absorption sensor <NUM> measures an absorption signal of the ionising radiation. The data collection from the absorption sensor <NUM> and from the energy resolving detector <NUM> may occur sequentially or simultaneously.

<FIG> illustrates a cross section of the apparatus <NUM> of <FIG> along a line A-A', when a diffracting sample material <NUM> is provided within the inspection volume. For instance, the diffracting sample material may be an object or portion of an object present in some luggage. The conical shell beam <NUM> is formed of a plurality of rays of electromagnetic radiation, also referred to as primary rays. The sample material <NUM> intersects the primary beam <NUM> at point P3. This produces a ray <NUM> of diffracted photons at a fixed diffraction angle. The ray <NUM> passes through the collimator <NUM> and is collected by the pixel <NUM> of the energy-resolving detector <NUM>. The primary ray <NUM>, having passed through the sample material <NUM>, is measured at point P1 on the absorption sensor <NUM>. Similarly, the unattenuated primary ray <NUM> is detected at point P2 on the sensor <NUM>.

It will be appreciated that the system could also work with an amorphous sample producing a scattered ray at a fixed scattered angle.

The spatial distribution of the incident photons on the detector <NUM> provides the location of the diffracting sample <NUM>. Stated another way, the three dimensional coordinates (x,y,z) of the sample <NUM> can be retrieved based on the location of the pixels measuring the signal. For instance, the three-dimensional coordinates may be defined with respect to an origin provided at the centre of the surface of the detector <NUM> as shown in <FIG>. The nature of the diffracting sample can be retrieved by calculating the d-spacing of the sample from the energy of the photons.

The angular/energy distribution of the scattered intensity is unique to each different crystal structure and thus can be used to identify a material and determine characteristics such as lattice dimensions, crystallite size and percentage crystallinity. The relationship between the lattice spacing (d), and the angle (θ) subtended by the diffracted or scattered radiation from a plane of atoms inside a crystal is provided by the Bragg condition: nλ = 2d sinθ, in which λ is the wavelength of the incoming radiation and (n) is an integer. The angle subtended by the diffracted or scattered radiation and the interrogating or primary radiation is <NUM> (two theta). The two-theta angle is the angle between an incident X-ray beam and the diffracted X-ray. The two-theta angle at which scattered photons are collected by each collimator channel is determined by the angle subtended by the longitudinal axis of the collimator channel and the primary beam <NUM>. The collimator <NUM> collects scattered flux propagating normal to the detection surface and therefore the opening angle of the primary beam determines the two-theta angle in this case.

<FIG> illustrates example images created by raster-scanning the apparatus of <FIG> over a test-tray filled with differing sample materials. Raster-scanning can be achieved by sweeping the primary beam along a sawtooth trajectory. If the tray is on a conveyor belt the probe continuously traverses the belt whilst the belt is incrementally moved.

In this example the absorption sensor <NUM> was a dual-energy absorption ring sensor and the detector <NUM> was a pixelated energy-resolving detector. <FIG> illustrates a dual-energy absorption image <NUM> of the test tray obtained from absorption data collected by the dual-energy absorption ring sensor. <FIG> shows a colour coded scattering/diffraction image <NUM> of the test tray obtained from scattering/diffraction data collected by the pixelated energy-resolving detector. In this example, the information collected by the absorption sensor and the energy-resolving detector was captured simultaneously and the images <NUM> and <NUM> spatially aligned. Stated another way the images are co-registered.

X-rays interact with matter through different processes that include pair production, photoelectric effect absorption, elastic (Rayleigh or Thomson) and inelastic (Compton) scattering processes. Absorption based techniques, such as dual-energy X-ray absorptiometry (DXA) techniques can be used to establish a material atomic number Z and electron density by measuring the attenuation of an X-ray beam transmitted through a sample at broadly two different X-ray energies. X-ray photons that have undergone the photoelectric effect or Compton scattering are measured by their absence in the detected signal. However, dual-energy X-ray techniques do not provide structural information (d-spacing) of the sample. X-ray diffraction can be used to identify the nature of a material having a degree of structural order, for instance a repeating pattern of atoms, with a high degree of accuracy. X-ray diffraction techniques such as X-ray crystallography, use an elastic scattering process such as Rayleigh scattering in which an outgoing X-ray has the same wavelength as an incoming X-ray. A diffraction pattern produced by the Rayleigh scattered radiation is used to determine the lattice structure of the matter of the sample under inspection via Bragg's Law.

<FIG> illustrates a screening system. The screening system <NUM> includes a rotatable platform <NUM> to receive an item to be scanned, such as some luggage, and a detection apparatus <NUM> as described above in <FIG>. The detection apparatus <NUM> has an emitter portion to generate a primary beam of ionising radiation and a detector portion to detect an absorption signal and at least one of a diffraction signal and a scattering signal. A mechanical arrangement <NUM> and <NUM> is provided to translate the apparatus <NUM> along a translation axis <NUM> to scan the item with the primary beam. A controller <NUM> is provided to control the motion of the detection apparatus <NUM> and of the rotatable platform <NUM>. A data analyser <NUM> is provided to receive data from the detector portion of the apparatus <NUM> and perform data analysis.

As mentioned above in <FIG>, the detection apparatus <NUM> may be implemented in different ways. For instance, the absorption sensor <NUM> may be provided on the propagation axis of the primary beam <NUM> before the collimator <NUM>, or between the collimator <NUM> and the energy resolving detector <NUM> or after the energy resolving detector <NUM>. The system <NUM> is not limited to any particular implementation of the detection apparatus <NUM>.

The platform <NUM> should be made of a material that interacts weakly or not at all with the ionising radiation being used, such as X-ray radiations. For example, the platform <NUM> may be made of carbon fibre. The rotatable platform is rotatable around a rotational axis <NUM>, provided at the geometric centre, also referred to as centroid of the platform <NUM>. In this example the platform has a circular shape, however it will be appreciated that the platform may have a different geometrical shape. The translation axis <NUM> is substantially perpendicular to the rotational axis <NUM> and intersects with the rotational axis, illustrated as the virtual point A in <FIG>. As a result, the translation axis <NUM> is substantially parallel to the surface <NUM> of the rotatable platform on which the item <NUM> is provided. By providing an arrangement in which the rotational axis <NUM> is provided at the geometric centre of the platform <NUM>, and the translation axis <NUM> passing over the centre of the platform, a simple and reliable scanning system is obtained that limits the stress on the bearing of the system and facilitates the analysis of the data by using a relatively simple reference frame. However, it will be appreciated that the rotational axis <NUM> does not necessarily have to be at the geometric centre of the platform <NUM>, nor does the translation axis <NUM> necessarily needs to pass over the centre of the platform, and that off axis / off centre geometries could be envisaged.

The mechanical arrangement can be implemented in various ways such that the emitter portion (<NUM>/<NUM>) and the detector portion (<NUM>, <NUM>, <NUM>) of the apparatus <NUM> move as one unit. In this example the mechanical arrangement includes two parallel guide members <NUM> and <NUM> for translating the emitter portion and the detector portion, respectively. The first guide member <NUM> is provided above the platform <NUM> while the second guide member <NUM> is provided below the platform <NUM>. In an alternative embodiment a single guide member is used and the various parts of the apparatus <NUM> are held together by an arm coupled to the guide member.

In operation a user places an item <NUM> to be scanned on the platform <NUM>. The controller then starts the scanning process by activating the detection apparatus and controlling the motion of the mechanical arrangement and the motion of the platform. The emitter portion generates the primary beam of ionising radiation and the detector portion detect an absorption signal and a diffraction/scattering signal. The platform <NUM> rotates with respect to the rotational axis <NUM> and the detection apparatus <NUM> is translated along the translation axis <NUM>. Upon rotation of the rotatable platform and translation of the detector apparatus, the detector apparatus collects data along a curved trajectory in a reference frame of the rotational platform. For instance, the curved trajectory may be a two-dimensional spiral. The absorption sensor <NUM> and the energy resolving detector <NUM> may be operated to collect data simultaneously or sequentially. The controller <NUM> is configured to control the speed of rotation and the linear velocity of the translation motion of the detection apparatus. For instance, the controller <NUM> may be adapted to operate the screening system in a first mode in which the rotatable platform rotates with constant frequency and the detector apparatus is translated with a constant linear velocity, or in a second mode in which the angular velocity of the rotatable platform and the linear velocity of the detection apparatus are adjusted over time so that the velocity of the detection apparatus along a curved trajectory remains constant.

The data analyser <NUM> may include a storage medium, and a processor configured to execute instructions for carrying out processing of the data. The instructions may be downloaded or installed from a computer-readable medium which is provided for implementing data analysis according to the disclosure.

The processor may be configured to receive a first data set from the absorption sensor <NUM> to form an absorption image and a second data set from the energy resolving detector <NUM> to form a diffraction/ scattering image.

The processor is configured to execute a correction algorithm to correct the angular dependency of the data collected by the absorption sensor and the energy resolving detector, respectively. For instance, the processor may be configured to process the first data set to correct for the angular dependency of each absorption detector of the absorption sensor <NUM>. Similarly, the processor may be configured to process the second data set to correct for the angular dependency of each pixel of the energy resolving detector <NUM>.

The processor may also be configured to perform various steps to obtain an absorption image and a diffraction/scattering image that overlap spatially. The absorption and diffracted signals from any point in space are preferably measured simultaneously but could also be measured at different point in time. The method for reconstructing the absorption images and the diffraction/scattering images in each respective space is different.

The absorption signal is used to perform tomosynthesis. The processor is configured to execute a tomosynthesis reconstruction algorithm to obtain the absorption images. Images from different depth planes are obtained by, for instance, shifting and adding the same absorption signals. The shift amount determines which depth plane is in-focus and which alternate depth planes (above or below) are out of focus or blurred. There is no magnification in this approach.

A scattering/diffraction image may be produced based on linear discriminant analysis (LDA) in which the spectra for each pixel of the energy resolving detector <NUM> is multiplied by a weighting per energy before being summed. The weightings per energy can be calculated such that they accentuate the object of interest compared to its neighbours. The energy at each pixel could also be integrated to provide total scattering/diffraction. Scattering/diffraction images may also be obtained for a specific energy range. For instance, a 2D image may be obtained showing only photons having an energy between <NUM>-<NUM> keV. Each photon detected is isomorphic in that it can only come from one place in 3D space. There is also no magnification in this approach. Co-registration is achieved by selecting an absorption image from a depth plane and overlaying it with the diffracted/scattered image from the same depth plane.

The processor may execute a segmentation algorithm to perform image segmentation of the absorption image to identify one or more regions of interest. Then a classification algorithm may be executed to perform image classification of the regions of interest identified using the scattering/diffracted information. Image classification may also be performed on the absorption image. If two touching objects share the same absorption signal, then these cannot be segmented only using the absorption signal. By using both absorption and scattering/diffraction data segmentation and classification can be improved.

For example, the processor may be configured to execute an algorithm to calculate a parameter of the sample which may be used to identify the sample. For instance, the parameter may be a lattice spacing (d-spacing) of the sample. Classification may then be performed using a set of rules. Alternatively, specific materials may be classified as targets. For instance, a library of target materials may be used that lists d-values for each target of interest. Sample identification may then be communicated to a third party by a suitable display or other type of indicia such as an audible or visible alarm signal. The processor may be provided in a remote server in communication with the detection apparatus.

A benefit of the proposed system is the precision with which scattering/diffracting signals can be integrated. If the object of interest were for instance partially occluded by an interfering object or sandwiched within another object (for example a laptop); then the integrity of the scattered/diffracting photons collected would be diminished. The co-registered nature of the images means that scattered/diffracted signals can be chosen from only the object of interest (isolating and removing pixels that intersect the interfering object) or parts of the objects that have suffered the least interference (i.e., the parts not obscured by the metal parts of a laptop). By collecting both absorption and scattering/diffracting signals usinga common probe beam and detection system and by combining information derived from the absorption signal and the scattering /diffracting signal, the proposed system permits to reduce the probability of false alarms without unduly increasing the complexity of the system.

The screening system <NUM> may be used as a self-screening system. For instance, in an airport a passenger may load some luggage on the platform, wait for the scan to take place and receive a signal to go through a gate or alternatively an alarm may be raised if an item of interest has been identified.

The screening system <NUM> could also be used as a mail screening platform for identifying unauthorised objects, hazardous chemicals or other materials that may be included in letters or parcels.

<FIG> is a diagram illustrating the scanning motion or scanning path of the screening system of <FIG> in the reference frame of the platform. <FIG> is a top view of the diagram of <FIG>.

The linear motion of the emitter portion /detector portion assembly <NUM> combined with the rotational motion of the platform <NUM> produces a planar curved trajectory that forms a spiral.

With each revolution of the platform <NUM>, (θ - θ<NUM>) = n<NUM>π, the detection apparatus <NUM> will have travelled a distance r from its position at (θ - θ<NUM>) = (n - <NUM>)<NUM>π. This distance travelled with each full revolution is called the pitch (P). The velocity of the apparatus <NUM> referred to as linear probe velocity, <MAT>, is related to the pitch and angular velocity, ω by the following system of equations: <MAT> <MAT>.

The screening system <NUM> may operate in different modes. In a first mode, referred to as Constant Angular Velocity (CAV) mode, the rotating platform rotates with constant frequency and the apparatus <NUM> is translated with a linear velocity that also remains constant. In a second mode referred to as constant spiral velocity (constant Linear velocity in the spiral coordinate system), the angular velocity of the rotating platform <NUM> and the linear velocity of the apparatus <NUM> change over time, so that the velocity of the probe <NUM> along the spiral trajectory remains constant.

At time t, the position of the centre of the primary beam <NUM> (probe), in the reference frame of the rotating platform, is described by co-ordinates relative to the system Centre of Rotation (CoR), in the plane of rotation, (rp(t), θp(t)). The cartesian co-ordinates of the centre of the probe are given by: <MAT> <MAT>.

These co-ordinates describe the evolution of the spiral path as a function of time (See <FIG>). The reconstructed detector image is generated by combination of detector images at all time steps.

The number of rotations required for a full scan is defined by the distance between the starting position of the probe beam <NUM> at its extreme radial position and the centre of the platform <NUM> (plus an offset distance from the centre to take into account the fact that the probe does not have to go all the way to the centre to achieve full coverage) divided by the pitch distance P. For example, to scan a <NUM> radius disc with a <NUM> pitch, stopping at a radius of <NUM> from the centre of <NUM> would require <NUM> rotations.

<FIG> illustrates the position of a pixel of the energy detector with and without orientation correction. As described above, the energy resolving detector <NUM> may be a pixelated detector having an array of pixels. The position of each pixel at time t, can be orientation-corrected by a rotation about the centre-point of the detector. The correction angle is equal to the rotation angle θp of the rotating platform <NUM> at time t such that the two-dimensional cartesian co-ordinates of the ith pixel, in the stationary reconstruction reference frame, are: <MAT> <MAT> where Δxi and Δyi are the orthogonal cartesian vectors of the ith pixel in the detector array relative to the centre-point of the detector array in the rotating platform reference from θp = <NUM>.

This orientation correction method can also be used to correct data collected by absorption sensor <NUM>.

The scanning motion of a traditional raster scan is inefficient in that a significant amount of time is dedicated to accelerating and decelerating the probe. The proposed scanning motion of the disclosure is significantly more efficient than raster-scanning and permits the scanning of an entire security screening tray faster than a comparable traditional raster scan for a similarly sized object/tray.

Claim 1:
A screening system (<NUM>) for screening an item (<NUM>), the screening system comprising
a detection apparatus (<NUM>) comprising an emitter portion to generate a primary beam (<NUM>) of ionising radiation and a detector portion to detect an absorption signal and at least one of a diffraction signal and a scattering signal; the screening system further comprising
a rotatable platform (<NUM>) adapted to receive the item (<NUM>); and the screening system being characterized in
a mechanical arrangement (<NUM>, <NUM>) adapted to translate the detection apparatus (<NUM>) along a translation axis (<NUM>) to scan the item with the primary beam;
wherein the translation axis (<NUM>) is substantially parallel to a surface of the rotatable platform (<NUM>);
wherein the rotatable platform (<NUM>) is rotatable around a rotational axis (<NUM>);
wherein the primary beam (<NUM>) has a characteristic propagation axis (<NUM>) associated with it; and
wherein the rotational axis (<NUM>) is substantially parallel to the characteristic propagation axis (<NUM>); and in the primary beam (<NUM>) being a conical shell beam of ionising radiation.