Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TITLE: METHOD AND DEVICES FOR ASSESSING THE THREAT STATUS
OF AN ARTICLE AT A SECURITY CHECK POINT
CROSS-REFERENCE TO RELA TED APPLICATIONS
This application claims the benefit under 35 USC 120 of:
- U.S. provisional patent application serial number 61/094,743 filed on
September 5, 2008 by Michel Roux et al. and presently pending; and
- U.S. provisional patent application serial number 61/097,060 filed on
September 15, 2008 by Michel Roux et al. and presently pending.
This application also claims the benefit of priority under 35 USC 119 based
on
international PCT patent application no.: PCT/CA2007/001749 filed in the
Canadian
Receiving Office on October 1, 2007 by Aidan Doyle et al. and presently
pending.
The contents of the above-referenced patent documents are incorporated herein
by
reference.
FIELD OF THE INVENTION
The present invention relates to technologies for assessing the threat status
of materials by
means of penetrating radiation such as X-rays. The invention has numerous
applications,
in particular it can be used for scanning hand carried baggage at airport
security check
points.
BACKGROUND
Some liquid or combinations of liquid and other compounds may cause enough
damage to
bring down an aircraft. As no reliable technology-based solution currently
exists to
adequately address this threat, authorities have implemented a ban of most
liquid, gels and
aerosols in cabin baggage.
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As a result, there have been disruptions in operations (e.g., a longer
screening process;
additional line-ups), major inconveniences for passengers (as well as
potential health
hazards for some) and economic concerns (e.g., increased screening costs; lost
revenues
for airlines and duty free shops; large quantities of confiscated - including
hazardous -
merchandise to dispose of), and so on.
Clearly, there is a need to provide a technology-based solution to address the
threat of
fluids that are flammable, explosive or commonly used as ingredients in
explosive or
incendiary devices.
SUMMARY
As embodied and broadly described herein the invention provides a method for
performing security screening at a checkpoint. The method includes providing
an X-ray
imaging system having a scanning area and providing a supporting device for
supporting articles to be scanned in the scanning area, wherein the supporting
device
has at least two reference areas manifesting respective X-ray signatures when
exposed
to X-rays, the X-ray signatures being distinguishable from one another. The
method
further includes placing an article to be scanned on the supporting device,
introducing
the article to be scanned in the scanning area while the article is supported
by the
supporting device and using the X-ray imaging system for deriving the X-ray
signatures
of the reference areas and for obtaining an X-ray image of the article while
the
supporting device is in the scanning area. Yet, the method includes using the
X-ray
signatures to derive X-ray attenuation information from the X-ray image and
using the
X-ray attenuation information in determining if the article is a security
threat.
As embodied and broadly described herein the invention also includes a X-ray
inspection station for performing security screening on articles, the X-ray
inspection
station having an X-ray scanning area where one or more articles are exposed
to X-rays
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and a supporting device for supporting one or more articles while the articles
are
exposed to X-rays in the scanning area, wherein the supporting device has at
least two
reference areas manifesting respective X-ray signatures when exposed to X-
rays, the X-
ray signatures being distinguishable from one another. A computer based
processing
unit is provided for:
i) deriving the X-ray signatures of the reference areas and collecting X-ray
image data of the article while the supporting device is in the scanning
area;
ii) using the X-ray signatures to derive X-ray attenuation information from
the X-ray image data;
iii) using the X-ray attenuation information in determining if the article is
a
security threat.
As embodied and broadly described herein the invention also includes a tray
for
supporting an article while the article is subjected to an X-ray inspection in
an X-ray
imaging apparatus, the X-ray imaging apparatus including an array of X-ray
detectors,
the tray including at least two reference areas manifesting respective X-ray
signatures
that are distinguishable from one another, at least one of the reference areas
having an
extent such that X-rays passing through the reference area are received by a
majority of
the X-ray detectors of the array of X-ray detectors.
As embodied and broadly described herein the invention also includes a belt
for
carrying an article to be subjected to an X-ray inspection in and out of the
scanning area
of an X-ray imaging apparatus, the belt including at least two reference areas
manifesting respective X-ray signatures when exposed to X-rays, the X-ray
signatures
being distinguishable from one another.
Other aspects and features of the present invention will become apparent to
those
ordinarily skilled in the art upon review of the following description of
specific
embodiments of the invention in conjunction with the accompanying Figures.
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BRIEF DESCRIPTION OF THE DRA WINGS
A detailed description of examples of implementation of the present invention
is
provided herein below with reference to the following drawings, in which:
Figure 1 is a block diagram of an apparatus using X-rays to scan hand carried
baggage
at a security check point, according to a non-limiting example of
implementation of the
invention;
Figure 2 is a plan view of a tray for carrying materials as they undergo
security
screening, according to a non-limiting example of implementation of the
invention;
Figure 3 is a cross-sectional view taken along lines 3-3 in figure 2;
Figure 4 is a block diagram of the processing module of the apparatus shown in
Figure
l;
Figure 5 is a plan view of the tray according to a variant.
Figure 6 is a flow chart of a process according to a non-limiting example of
the
invention for performing threat assessment;
Figure 7 is a flow chart of a process according to a non-limiting example of
the
invention for performing self-calibration of the X-ray imaging apparatus of
Figure 1;
Figure 8 is a graph illustrating the relationship between grey scale values in
an X-ray
image and corresponding attenuation levels;
Figure 9 is another graph illustrating the relationship between grey scale
values in an X-
ray image and corresponding attenuation levels;
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Figure 10 is a top plan view of a belt of an X-ray imaging apparatus of the
type shown
in Figure 1, the X-ray imaging apparatus being omitted for clarity;
Figure 11 is a side elevation view of the belt shown in Figure 10;
Figure 12 is a front elevation view of the belt of Figures 10 and 11, also
showing the
detectors array of the X-ray imaging apparatus;
Figure 13 is an example of an X-ray image, showing various image segments and
how
they relate to respective detectors of the detectors array;
Figure 14 is a top plan view of the tray according to a second variant;
Figure 15 is a graph showing the relationship between the grey scale level in
X-ray
image data and attenuation values.
In the drawings, embodiments of the invention are illustrated by way of
example. It is
to be expressly understood that the description and drawings are only for
purposes of
illustration and as an aid to understanding, and are not intended to be a
definition of the
limits of the invention.
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DETAILED DESCRIPTION
With reference to Fig. 1, there is shown a specific non-limiting example of a
system 10
for use in screening containers with liquids, in accordance with a non-
limiting
embodiment of the present invention. The system 10 comprises an X-ray imaging
apparatus 100 that applies an X-ray screening process to a material such as a
liquid 104
contained in a container 102 that is located within a screening area of the X-
ray imaging
apparatus 100. In an airport setting, a passenger may place the container 102
in a tray
which is then placed onto a conveyor 114 that causes the container 102 to
enter the
screening area of the X-ray imaging apparatus 100. The X-ray imaging apparatus
100
outputs an image signal 116 to a processing module 500. The processing module
then
processes the X-ray image data conveyed by the signal 116.
The processing module 500 may be co-located with the X-ray imaging apparatus
100 or
it may be remote from the X-ray imaging apparatus 100 and connected thereto by
a
communication link, which may be wireless, wired, optical, etc. The processing
module 500 processes the image data and executes a method to produce a threat
assessment 118. The processing module 500 is computer based and its
functionality is
provided by suitable software executing on a computing platform.
The threat assessment 118 is provided to a console 128 and/or to a security
station 132,
where the threat assessment 118 can be conveyed to an operator 130 or other
security
personnel. The console 128 can be embodied as a piece of equipment that is in
proximity to the X-ray imaging apparatus 100, while the security station 132
can be
embodied as a piece of equipment that is remote from the X-ray imaging
apparatus 100.
The console 128 may be connected to the security station 132 via a
communication link
124 that may traverse a data network (not shown).
The console 128 and/or the security station 132 may comprise suitable software
and/or
hardware and/or control logic to implement a graphical user interface (GUI)
for
permitting interaction with the operator 130. Consequently, the console 128
and/or the
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security station 132 may provide a control link 122 to the X-ray imaging
apparatus 100,
thereby allowing the operator 130 to control motion (e.g., forward/backward
and speed)
of the conveyor 114 and, as a result, to control the position of the container
102 within
the screening area of the X-ray imaging apparatus 100.
In accordance with a specific non-limiting embodiment the X-ray imaging
apparatus
100 is a dual-energy X-ray imaging apparatus 100. However, persons skilled in
the art
will appreciate that the present invention is not limited to such an
embodiment. Such
dual-energy X-ray imaging apparatus 100 has a source which emits X-rays at two
distinct photon energy levels, either simultaneously or in sequence. Example
energy
levels include 50 keV (50 thousand electron-volts) and 150 keV, although
persons
skilled in the art will appreciate that other energy levels are possible.
The processing module 500 receives the image signal 116 and processes the
signal to
determine if the liquid 104 in the container 102 poses a security threat. The
determination can include an explicit assessment as to weather the liquid 104
is a threat
or not a threat. Alternatively, the determination can be an identification of
the liquid
104 or the class of materials to which the liquid 104 belongs, without
explicitly saying
whether the liquid 104 is threatening or not threatening. For example, the
processing
module can determine that the liquid 104 is "water" hence the operator 130
would
conclude that it is safe. In a different example, the processing module 500
determines
that the liquid 104 belongs to a class of flammable materials, in which case
the operator
130 would conclude that it would be a security threat. Also, the determination
can be
such as to provide an explicit threat assessment and at the same time also
provide an
identification of the liquid 104 in terms of general class of materials or in
terms of a
specific material. The results of the determination are conveyed in the threat
assessment signal 118 which is communicated to the console 128 and/or the
security
station 132 where it is conveyed to the operator 130.
Figure 4 is a high level block diagram of the processing module 500. The
processing
module 500 has a Central Processing Unit (CPU) 508 that communicates with a
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memory 502 over a data bus 504. The memory 502 stores the software that is
executed
by the CPU 508 and which defines the functionality of the processing module
500. The
CPU 120 exchanges data with external devices through an Input/Output (I/O)
interface
506. Specifically, the image signal 116 is received at the I/O interface 506
and the data
contained in the signal is processed by the CPU 508. The threat assessment
signal 118
that is generated by the CPU 508 is output to the console 128 and/or the
security station
132 via the I/O interface 506.
In a specific example of implementation, the system 10 is used in conjunction
with a
tray 200 shown in Figure 2 to perform security screening of liquid products.
The tray
200 is used as a receptacle in which objects to be screened, such as liquid
products or
other materials or articles, are placed and put on the conveyor belt of the X-
ray imaging
system 100. The tray 200 is provided with one or more distinct areas that have
X-ray
signatures which can be used as references against which the X-ray imaging
apparatus
100 can self-calibrate.
The tray 200 defines a surface 202 which is generally flat and on which the
liquid
product that is being screened rests. In the example shown in the drawings,
the surface
is shaped as a rectangle with rounded corners. Evidently, different shapes or
configurations can be used without departing from the spirit of the invention.
The surface 202 is provided with raised edges or rim 204 that extend in a
continuous
fashion around the periphery of the surface 202. The raised edges 204 prevent
articles
placed in the tray 200 to fall outside during the screening operation. The
height of the
raised edges 204 can vary without departing from the spirit of the invention.
The surface 202 defines five distinct areas. The first area 206 is the base
material from
which the tray 200 is made. That material may be any synthetic material that
has the
required strength and durability characteristics for the intended application.
The four
additional distinct areas 208, 210, 212 and 214 are in the form of inserts
that are placed
in respective receptacles in the base materia1206. The areas 208, 210, 212 and
214 are
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in the shape of rectangles placed near respective corners of the tray 200. It
is to be
expressly noted that the shape, placement in the tray 200 and the number of
the areas
208, 210, 212 and 214 can vary without departing from the spirit of the
invention.
The areas 206, 208, 210, 212 and 214 are distinct in that they have different
X-ray
signatures. Accordingly, when an X-ray image is taken of the tray 200 alone,
the areas
206, 208, 210, 212 and 214 will show up differently in the image. Preferably,
the area
206 is made of material that is selected to provide a weak X-ray signature
such as to
limit its effect in the image and thus make the other articles that are put on
the tray 200
more visible. In that sense, the area 206 attenuates the X-ray beam little or
not at all. In
contrast, the areas 208, 210, 212 and 214 are designed to provide different
levels of X-
ray attenuation, as it will be discussed later.
More specifically, each area 208, 210, 212 and 214 can be made from a material
providing the desired degree of X-ray attenuation. This solution can be
implemented by
providing an insert made from the selected material that is placed in the base
material
206 of the tray 200. This feature is best shown in Figure 3 which is a cross-
sectional
view of the tray 200 taken at the level of the area 214. Specifically, the
base material of
the tray is provided with a receptacle 300 in which is placed an insert 302
defining the
area 214. To ensure a snug fit the insert 302 is manufactured to be of about
the same
size as the receptacle 300. In this fashion, the insert 302 is held in the
receptacle 300 as
a result of friction fit. Evidently, other mounting methods can be provided
without
departing from the spirit of the invention. One possible variant is to use a
fastening
mechanism that would allow the insert 302 to be removed. In this fashion, the
insert
302 can be replaced with another insert, if the original insert is damaged or
if it is
deemed appropriate to change the X-ray signature of the area 214.
In a specific and non-limiting example of implementation the X-ray signature
of anyone
of the reference areas 206, 208, 210, 212 and 214 can be expressed as the gray
scale
level intensity of the pixels in the portion of the X-ray image that depicts
respective
reference area. Generally, the gray scale level intensity represents the
degree of
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attenuation of the X-rays as they pass through the object. The grey scale
level can be
relatively uniform across the reference area 206, 208, 210, 212 and 214. This
is the
case when the reference area 206, 208, 210, 212 and 214 is made of material
that is
homogenous and thus attenuates X-rays uniformly. Another example of an X-ray
signature is a situation when the area 206, 208, 210, 212 and 214 is not
homogeneous
and thus creates a certain gray scale level profile or pattern. The pattern
may be regular
or irregular.
Generally speaking, the X-ray signature of a reference area 206, 208, 210, 212
and 214
is the response produced by the reference area 206, 208, 210, 212 and 214 when
the
reference area 206, 208, 210, 212 and 214 interacts with X-rays. There are a
number of
interactions possible, such as:
= The Rayleigh scattering (coherent scattering)
= The photoelectric absorption
= The Compton scattering (incoherent scattering)
= The pair production
= Diffraction
The photoelectric absorption of X-rays occurs when an X-ray photon is
absorbed, resulting
in the ejection of electrons from the shells of the atom, and hence the
ionization of the
atom. Subsequently, the ionized atom returns to the neutral state with the
emission of
whether an Auger electron or an X-ray characteristic of the atom. This
subsequent X-ray
emission of lower energy photons is however generally absorbed and does not
contribute
to (or hinder) the X-ray image making process. This type of X-ray interaction
is dependent
on the effective atomic number of the material or atom and is dominant for
atoms of high
atomic numbers. Photoelectron absorption is the dominant process for X-ray
absorption up
to energies of about 25 keV. Nevertheless, in the energy range of interest for
security
applications (for today's state-of-the-art security screening systems, the
energy levels
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commonly utilized lie between 50 keV and 150 keV), the photoelectric effect
plays a
smaller role for low Zeff values with respect to the Compton scattering, which
becomes
dominant.
Compton scattering occurs when the incident X-ray photon is deflected from its
original
path by an interaction with an electron. The electron gains energy and is
ejected from its
orbital position. The X-ray photon looses energy due to the interaction but
continues to
travel through the material along an altered path. Since the scattered X-ray
photon has less
energy, consequently it has a longer wavelength than the incident photon. The
event is
also known as incoherent scattering because the photon energy change resulting
from an
interaction is not always orderly and consistent. The energy shift depends on
the angle of
scattering and not on the nature of the scattering medium. Compton scattering
is
proportional to material density and the probability of it occurring increases
as the incident
photon energy increases.
The diffraction phenomenon of the X-rays by a material with which they
interact is
related to the scattering effect described earlier. When the X-rays are
scattered by the
individual atoms of the material, the scattered X-rays may then interact and
produce
diffraction patterns that depend upon the internal structure of the material
that is being
examined.
As to the pair production interaction, it refers to the creation of an
elementary particle
and its antiparticle from an X-ray photon.
That response produced by a material as it interacts with X-rays can be
expressed in
terms of gray level value, gray level patterns seen in the X-ray image or
other physical
manifestation.
The selection of the proper material for making the inserts 302 for the
various reference
areas 208, 210, 212 and 214 can be made by in a number of ways. The insert may
or
may not be made from a homogenous material. An example of a non-homogeneous
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structure is an assembly of layers made from different materials that in
combination
would provide the desired X-ray signature. Another example is a mixture of
different
materials intended to create a pattern in the X-ray image. The person skilled
in the art
will recognize that an almost infinite number of different X-ray signatures
can be
developed by selecting the proper material or materials and by mixing or
assembling
them in the appropriate manner.
Examples of materials that can be used include plastics such as polyethylene,
polypropylene or others. Their density or composition can be varied to obtain
the
desired X-ray signature.
An advantage of performing a comparison between X-ray signatures extracted
from the
same image data is the elimination or at least the reduction of X-ray induced
variations
in the system response. In this fashion, the system is self-referencing.
The example of implementation shown in Figure 2 depicts the areas 208, 210,
212 and
214 placed in the respective corners of the tray 200. This is done in order to
reduce the
likelihood of obscuring anyone of those areas 208, 210, 212 and 214 by an
article that is
placed in the tray. For instance, if an article is put in the tray immediately
above
anyone of those areas 208, 210, 212 and 214, the X-ray signature of that area
may not
be correctly read since the X-ray image will be the result of a composite
response (the
area 208, 210, 212 and 214 and the article on top of it). In order to further
reduce the
possibility of obscuring the areas 208, 210, 212 and 214 it is possible to
place the areas
208, 210, 212 and 214 at a location that is outside the zone in the tray where
the articles
to be screened are located. An example of such embodiment is shown in Figure
5. The
tray 700 defines a central article receiving area 702 in which are placed the
articles to be
screened. The article receiving area 702 is surrounded by a rim portion 704
that extends
peripherally and fully encircles the article receiving area 702. The rim
portion 704 has
a top area 706 that is flat and that is sufficiently wide such as to accept
the reference
areas 208, 210, 212 and 214. In this fashion, articles to be screened are
unlikely obscure
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anyone of the areas 208, 210, 212 and 214 that remain outside the central
article
receiving area.
The tray 200 provides a material reference during the X-ray scanning process
which can
be used to limit or avoid altogether machine induced variations in the results
by
performing a self-calibration operation. Since in practice different X-ray
imaging
apparatuses are never identical and manifest some variations that can be
either at the
level of the X-ray detectors elsewhere in the machine, those variations can
impact the
detection results.
More specifically, the tray 200 can be used as a known reference for the X-ray
imaging
apparatus 10. Accordingly, when the X-ray scanning process is performed the X-
ray
imaging apparatus 100 can use the X-ray signature of the tray 200 to self-
calibrate.
Since in the course of an X-ray scanning operation the tray 200 will be used
repeatedly,
the self-calibration operation occurs with regularity, thus enhancing the
performance of
the X-ray imaging apparatus in terms accuracy in identifying security threats.
A general view of the threat assessment and self-calibration process is shown
at Figure
6. At step 800 the process starts. At step 802 a passenger at a security
checkpoint, such
as at an airport, removes articles from his/her hand carried luggage. Examples
of
articles include containers holding liquids or other articles such as
electronic equipment.
At step 804 the removed articles are placed in a tray that includes reference
areas, say
tray 700. In addition to the articles that are removed from the hand carried
luggage,
additional articles can also be included such as shoes (in the instance the
individual is
being requested by security personnel to have his/her shoes scanned), a belt
and a jacket
among others.
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At step 806 the tray 700 is placed on the belt 114 of the X-ray imaging
apparatus 100,
which carries the tray 700 with the articles therein inside the X-ray imaging
apparatus
100.
At step 808 an x-ray image of the tray 700 and of the articles therein is
taken. The X-
ray image data is then processed at step 810 to perform a self-calibration of
the X-ray
imaging apparatus 100. Once the X-ray imaging apparatus 100 is self-
calibrated, the x-
ray image data is processed to perform a threat assessment of the articles in
the tray
700.
In this example, the self-calibration of the X-ray imaging apparatus 100 and
the threat
status assessment are performed during the same X-ray scanning cycle. This
self-
calibration can be repeated at every scanning cycle, thus reducing as much as
possible
machine induced variations over time. If machine induced variations drift over
time,
such as the result of temperature, humidity of other environmental factors,
the repeated
self-calibration will track those drifts and thus enhance the detection
results.
Once the scanning operation is completed the person takes away his/her
belongings
from the tray. The empty tray is then brought back and placed near the entry
side of the
X-ray imaging apparatus such that it can be used by another person. If every
tray in the
set of trays provided with the X-ray imaging apparatus 100 use reference
areas, every
time a tray is used to perform a scan of articles, a self-calibration
operation occurs.
Note that it is not essential to perform self-calibration every time the X-ray
imaging
apparatus 100 scans articles to detect their threat status. One possibility is
to perform
the self-calibration operation at every other scanning cycle or at any other
frequency
deemed appropriate for the intended application. The selection of the
frequency at
which the self-calibration will occur can be done in a number of possible
ways, namely:
1. The set of trays that are used to scan articles in the X-ray imaging
apparatus
100 is provided with a sub-set that enable the self-calibration (trays with
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reference areas) and a sub-set that cannot be used to perform self-
calibration(trays without any reference areas). Since those trays are used
repeatedly, the self-calibration process will occur only when a tray with
reference areas is being scanned. The frequency at which the self-calibration
occurs can be set by determining the mix of trays that enable self-calibration
and those that do not, as desired.
2. The X-ray imaging apparatus 100 can be programmed such as to run the self-
calibration process for one tray in a sequence of trays that are being
scanned,
such as every second, third or fourth tray, for instance.
3. The X-ray imaging apparatus can be manually controlled to run the self-
calibration. The X-ray imaging apparatus has on its console 300 controls that
are actuated by the operator to run the self-calibration. The operator thus
takes
the decision at which frequency the self-calibration occurs.
The self-calibration operation, in terms of X-ray image data processing will
be
described in greater detail in connection with Figures 7, 8 and 9.
With reference to Figure 7, the process starts at step 900. At step 902 the X-
ray image
data is processed by the processing module 200 to locate the reference areas
in the tray.
At step 904 the X-ray signature of each reference area is acquired. In the
example
shown in the drawing, the X-ray signature is conveyed by the grey scale level
or value
of the reference area in the image. Accordingly, the grey scale value
associated with
each reference area is measured to acquire the X-ray signature.
As can be seen in the graph at Figure 8, each grey scale value can be mapped
to a
certain X-ray attenuation level. At step 906 the nominal X-ray signatures of
the
respective reference areas are obtained. In this specific example, the nominal
X-ray
signatures are associated with specific attenuation information. The nominal X-
ray
signatures can be obtained from different sources, as generally described
earlier. For
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instance, the nominal X-ray signatures are stored in a database that is
accessible to the
processing module 200. By performing the analysis of the X-ray image data, the
processing module 200 can extract the identities of the reference areas and on
the basis
of the identity information extract the signature information from the
database.
Alternatively, the nominal X-ray signatures can be encoded directly in the
tray such that
they can be read by the processing module in the X-ray image data. One example
is a
bar-code encoding that is machine readable.
At step 910 nominal X-ray signatures are used by the processing module 200 to
create a
relationship between the X-ray image data and corresponding attenuation
information.
This is best illustrated in the graph of Figure 8 which maps grey scale values
to
attenuation levels. The process is performed by software executed by the
processing
module 200.
Assume that the tray has three different reference areas, namely reference
area A, a
reference area C and an intermediate reference area B. Reference areas A, B
and C are
associated with progressively decreasing attenuation levels. For the sake of
this
example, consider that reference area A is associated with a 10% attenuation
level,
reference area B with 5% attenuation level and reference area C with 2%
attenuation
level.
The grey scale value associated with the reference area A is plotted against
an
attenuation values axis (at the known 10% attenuation level) to create a data
point 1000.
The grey scale value 1006 associated with the reference C is also plotted
against the
attenuation values axis at the known value of 5%, which creates a second data
point
1010.
The data points 1000 and 1010 can be used to establishing a linear
relationship between
the X-ray image data and corresponding attenuation levels, where the
relationship is
corrected with respect to known references (A and C). Note that in this
example, the X-
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ray image data is expressed in terms of grey scale values and the relationship
is
therefore established between the grey scale values read from the image and
the
attenuation levels. However, in situations where the X-ray image data is
conveyed in a
way other than grey scale values, the data conveying the image information can
equally
well be mapped to attenuation levels.
The two data points 1000 and 1010 allow creating a linear relationship between
the grey
scale values and the attenuation levels. In this fashion, any grey scale
values residing
between the grey scale values of data points 1000 and 1010 will be mapped to
attenuation levels according to a linear relation. In practice, this linear
relationship may
not accurately reflect the reality of the physics involved, in which case
additional data
points can be used to create a more accurate map. In the example shown in
Figure 10,
the intermediate reference B provides a data point 1012 that corresponds to 5%
attenuation. An algorithm can be used to create a best fit curve over the
three data
points which will define the relationship between the grey scale values and
the
attenuation levels.
While the above example illustrates a situation where the reference areas A, B
and C are
all located in the lower end of the attenuation level scale, in the range
between 2% and
10%, the references can be selected in a different area of the scale. More
particularly,
the references can be selected such as to cover a wider range of attenuations,
a range of
attenuations located near the upper end of the attenuation level scale (close
to 100%
attenuation), or anywhere else between the upper end and the lower end. For
instance,
in a variant, the references can be selected such as to span the entire scale.
~From the point of view of calibration results, the location of the reference
areas in the
lower end of the attenuation scale (low attenuation) allows calibrating the X-
ray
imaging apparatus 10 in the operating range where the apparatus usually
manifests the
most drift. Accordingly, performing a calibration in this area is likely to
improve in a
tangible way the accuracy of detection.
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In addition, the lower end of the attenuation scale (low attenuation) tends to
be non-
linear; accordingly the placement of reference areas in that region is likely
to produce a
more accurate map between the grey scale levels and the corresponding
attenuation
values. Figure 15 illustrates this point. Figure 15 shows a graph of the
relationship
between the grey scale levels and attenuation over the entire attenuation
range, namely
0% to 100%. The region 2200, in the lower end of the attenuation scale (low
attenuation) is non-linear. In a scenario where two reference points are used
for the
calibration and they are placed such as to locate one (point 2202) in the
region 2200 and
one (point 2204) outside the region 2200, the resulting map 2206 may not track
well the
curve. In a different scenario, where the reference points 2208 and 2210 are
all placed
in the region 2200, the resulting map 2212 better follows the curve.
Figure 9 provides an example of a different situation where the tray has 5
reference
areas spread over the entire attenuation range, namely reference areas D, E,
F, G and H
corresponding to 100%, 75%, 50% and 25% attenuation levels, respectively. By
using
a best fit algorithm a curve can be laid over the data points that establish
the relationship
between the grey scale values and the attenuation levels.
Referring back to Figure 7, the process terminates at step 912 by analyzing
the X-ray
image data to determine if the articles in the tray present a security threat.
To perform
the threat assessment the processing module 200 determines the levels of X-ray
attenuation associated to pixels or groups of pixels in the X-ray image. The
level of
attenuation is obtained on the basis of the relationship established earlier
where grey-
scale values are mapped to attenuation levels, and the resulting map is stored
in the
memory of the processing module 200. The processing module 200 uses the grey
scale
level as an input to the attenuation map and derives an attenuation values,
accordingly.
The attenuation values are then processed to determine the threat status of
the articles.
In a possible variant, the reference areas used to perform the self-
calibration operation
are associated with the belt 114 that is used to carry the tray and the
articles to be
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scanned through the X-ray imaging apparatus 100. This example is best shown in
Figures 10, 11 and 12.
Figure 10 is a plan view of the belt 1200. The belt is an endless sheet
mounted on two
rollers 1202 and 1204. One or both of the rollers can be used to drive the
belt such that
it advances articles through the scanning station of the X-ray imaging
apparatus 100.
The belt 1200 is provided with a plurality of reference areas. More
specifically, four
reference areas are used, namely reference areas 1206, 1208, 1210 and 1212.
Each
reference area is shaped as a strip of material that extends across the
direction of
movement of the belt 1200. In a specific example, the strips are oriented
generally
transversally with relation to the direction of movement of the belt 1200. The
length of
each reference area 1206, 1208, 1210 and 1212 is somewhat less than the
transverse
dimension of the belt 1200.
The materials that constitute the reference areas 1206, 1208, 1210 and 1212
can be
mounted on top of the belt 1200 surface and secured thereto in any suitable
fashion.
Alternatively, the materials can be mounted on the bottom of the surface or
imbedded in
the belt 1200 such that they are not visible to the eye.
In operation the articles to be scanned (with or without tray) should not be
placed
directly over the reference areas 1206, 1208, 1210 and 1212 to avoid obscuring
them. It
is better if the articles to be scanned are immediately adjacent the reference
areas 1206,
1208, 1210 and 1212 such that they do not overlap while at the same time the X-
ray
image data encompasses them both. To avoid an overlap an arrangement can be
provided to indicate to the user that articles should not be placed over the
reference
areas 1206, 1208, 1210 and 1212. The arrangement can include a physical
barrier, such
projections (not shown extending outwardly of the belt 1200 surface which
intuitively
indicate that no articles can be placed at location on the belt.
Alternatively, the
arrangement includes markings to indicate to a user that no articles should be
placed in
that area. The markings my include text, pictograms or a combination of both.
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Note that the markings can be used in conjunction with the physical barrier.
Yet the arrangement can also include markings to indicate where articles,
including a
tray, can be placed such as not to overlap with the reference areas. Those
markings may
be lines that delineate a boundary in which articles are to be placed. For
instance, the
marking may be in the shape of a rectangle 1214 indicating where articles to
be scanned
should be placed.
One advantage in using elongated strips of material to form the reference
areas which
are fed transversally to the direction of movement of the belt 1200 (the
direction of
movement is depicted by the arrow 1216) is to allow a majority of the X-ray
detectors
and preferably all of the X-ray detectors of the X-ray imaging apparatus 100
to sense
the reference areas.
With specific reference to Figure 12, which is a front view of the X-ray
imaging
apparatus 100, specifically showing the belt 1200 and a detector array 1400.
The
detector array 1400 has a plurality of detectors 1402 that are arranged to
form an L-
shape, including a horizontal arm and a vertical arm. The elongated strip of
material
can therefore be "seen" by each detector, which allows constructing for each
detector an
individual attenuation map of the type described earlier.
This is best shown in Figure 13 which is a representation of an X-ray image
obtained
from the detector array 1400. The image 1500 shows articles 1502 that are
being
scanned and also the reference areas. The X-ray image is assembled from
individual
image strips 1504, where each image strip is derived from the output of a
detector 1402.
Since the reference materials have geometry such that a portion of each of
them appears
in each image strip, that image strip can be processed independently to create
a detector
specific attenuation map.
Accordingly, the processing module 500 would therefore store in its memory a
series of
attenuation maps, one for each detector. In such case, when the X-ray image
data is
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processed, the portions of the image data that are derived from a given
detector are
processed against the attenuation map of that detector to determine the
attenuation
values. For instance, grey scale levels of pixels in any one of the strips
1504 are used as
inputs to the attenuation map of the detector associated with the image strip
to
determine the attenuation levels.
Note that similar reference material geometries can be used on trays as well.
Figure 14
is a top plan view of a tray 1600 having an area 1602 to receive articles to
be scanned
and a rim portion 1604 on which are placed reference materials to form
reference areas
1606 and 1608. In the example shown the tray 1600 is provided with two
reference
areas 1606 and 1608 but more can be provided if desired.
The tray has an imaginary longitudinal axis and an imaginary transverse axis.
In use, the
tray should be placed such that the reference areas 1606 and 1608 extend
across the
longitudinal axis, which coincides with the direction of movement of the tray
in the
scanning area of the X-ray imaging apparatus 10. In a specific example, the
reference
areas 1606 and 1608 are perpendicular to the direction of movement of the belt
(arrow
1608). Markings can be placed on the tray such that users can place the tray
in the
proper orientation. The markings can include pictograms of text. An arrow
placed on
the bottom of the area 1602 is an example of a marking.
Another example is to make the tray 1600 sufficiently long such that if it is
placed
transversely on the tray it will not fit in the entry of the X-ray machine. In
this fashion,
the tray can only be used in a single orientation.
Although various embodiments have been illustrated, this was for the purpose
of
describing, but not limiting, the invention. Various modifications will become
apparent
to those skilled in the art and are within the scope of this invention, which
is defined
more particularly by the attached claims.
