The International Commission of Radiation Protection (ICRP) was established in 1928 and evaluates the risk to humans when exposed to electromagnetic radiation. The ICRP also sets limits on maximum permissible levels of exposure for the public and for radiation workers. Exposure levels were initially set in 1928 and revised in 1977 and 1991. Table 1 provides some information on the 1977 and 1991 levels oR exposure to radiation as set by the ICRP.

Table 1 : ICRP dose limits for members of the public and occupationally exposed radiation workers. Existing limits for 1977 and current values updated in 1991 are given Permissible dose limits in rem and sievert Public Occupationally exposed Exposed part of body radiation workers Permissible Annual Quarterly dose/year exposure dose exposure dose rem mSv rem mSv rem mSv Gonads, Bone marrow, Total body 0,5 5** 5 50** 3 30* Bone, Thyroid and Skin of whole body 3,0 30# 30 300# 15 150 Hands, Feet, Forearms, Ankles 7,5 75@ 75 750@ 40 400 Any single organ (excluding those 1, 5 15*** 15 150*** 8 80 mentioned above) Data from J E Coggle-Biological effects of radiation, Wykeham Publications (London), 1977 p 135 and 136 * 1,3 rem (13 mSv) to abdomen of women of reproductive capacity ** Respectively reduced to 1 mSv/year and 20 mSv/year in 1991 &num For the skin respectively increased to 50 mSv/year and 500 mSv/year in 1991 @ Reduced to 500 mSv for radiation workers in 1991 *** For the lens of the eye remained at 15 mSv/year and 150 mSv/year in 1991.

Units used in earlier radiation biology are the Roentgen (R) and the rad.

Roentgen is the unit of exposure, i. e. the amount of energy directed at a material and not that fraction actually absorbed by the material. One roentgen is the quantity of X-or gamma-radiation produced in one cm3 of air, at standard temperature and pressure, ions carrying one electrostatic unit of electricity of either sign. This translates to 58x10-4 coulomb of electricity per kilogram of air.

It is relatively easy to measure the ionisations produced in air by X-and gamma-radiations below 3 MeV, but not at higher energies. Therefore, the International Commission of Radiation Protection (ICRP) adopted the rad for use in 1956. One rad is the absorption of 10-2 J of radiation energy per kilogram of material. The Sl-unit of absorbed dose is the gray (Gy), that can be converted to roentgen.

1 Gy = 1 J/kg = 100 rad (= 87 roentgen for only X-and gamma rays E<3 MeV) Although all ionising radiation interacts with living matter in a similar way, different types of radiation differ in their effectiveness (efficiency) in damaging a biological system. Differences in the distribution of the ionisations and excitations are responsible for this. The relative biological effectiveness is always expressed in relatior to a dose of a standard type of radiation. The rem (roentgen equivalent man) is a unit ol dose equivalent (or effective dose) that reflects the relative biological effectiveness ol different radiations. A dose in rem is numerically equal to the dose in rad multiplied by s weight factor, WR, of the radiation. Table 2 summarizes accepted values of these weight factors.

Table 2: Weight factors, WR, used in defining maximum permissible doses * Data from J E Coggle-Biological effects of radiation, Wykeham Publications (London), # 1977 p 130.

** Fourth annual report ICRP-1991. Type of radiation Weight factor since Weight factor in 1977* 1991** X and. rays, electrons,, rays 1, 0 1,0 and muons (all energies) Neutrons : < 10 keV 5 10 10 keV-100 keV 10 10 > 100 keV to 2 MeV 20 10 >2MeVto20MeV 10 10 > 20 MeV 5 10 Protons other than recoil 5 10: includes naturally protons, energy >2 MeV occurring particles Alpha particles, fission 20 20: excludes naturally fragments, heavy nuclei occurring particles

The Sl-unit for dose equivalent is the sievert. Note that dose in sievert is given by dose in Gy multiplied by WR.

1 Sv = 100 rem (= 87 R for X-and gamma rays) To calculate the effective dose for a particular organ or part of the body the absorbed dose in Gy must be multiplied by the weight factor WR and by a tissue weight factor WT. The total effective dose is then defined as the sum of the weighted equivaleni doses. This"effective"dose is not really the dose, but rather a risk, expressed in dose units. The adopted tissue weighing factors, WT, are summarized in the Fourth Annua Report of the ICRP and include the factors shown in Table 3.

Table 3: Tissue weighting factors, WT Fourth annual report ICRP-1991 Bone surface and Skin 0,1 Bladder, Breast, Liver, Oesophagus, 0,05 Thyroid, Remainder Colon, Lung, Red bone marrow, 0,12 Stomach Gonads 0,20

The concept of Background Equivalent Radiation Time (BERT) wa : introduced to make radiation dose more understandable to lay people, such as patient :

undergoing X-ray examination. BERT is the time required to obtain the same dose from background radiation as is being obtained from the exposure to a particular radiation procedure, e. g. an X-ray examination. The advantages of BERT are that it provides a comparative figure that does not imply risk, it emphasises that radiation is a naturally occurring phenomenon and it is a concept understandable to lay people. An obvious potential problem with the BERT concept is the possibility of different background radiation levels being used when calculating BERT values. In this specification, BERT values are given based on a background radiation of 1 mSv/year (100 mrem/year).

There exists a need at airports and other places to perform security checks on people to ensure that dangerous objects or materials or other unauthorised or stolen objects or materials are not concealed in or underneath clothing. Frequent exposure to conventional X-ray equipment providing high radiation dosages may however expose passengers or the like to radiation which is higher than permissible BERT values, especially to high-risk body areas such as the reproductive organs, where the ICRP dose limit is 1 mSv/year (a BERT value of 1 year).

According to one aspect of the invention, there is provided a method of checking for concealed items or materials carried on the body or in the clothing on the body of a person, the method including locating a clothed person between radiographic image capturing means and a source of penetrating electromagnetic radiation; exposing at least part of the person to penetrating electromagnetic radiation from the radiation source at a dose which does not exceed a BERT value of three weeks; and capturing a radiographic image of an outline of the irradiated part of the persor thereby to determine if an item or material is concealed on that part of the person or ir the clothing worn by the person.

For the purposes of this specification, it is to be understood that the capturinc of a radiographic image includes not only the capturing of an image for visua observation of the image, but also the capturing of an image in other forms, e. g electronic forms, allowing non-visual inspection methods to be used to determine if ar item or material is concealed. Indeed, from an ethical and public acceptability point o

view, it is likely that non-visual inspection methods, e. g. the use of shape recognition computer software, will be prevalent.

Preferably, the dose does not exceed a BERT value of one week. More preferably, the dose does not exceed a BERT value of 1/3 day. Most preferably, the dose does not exceed a BERT value of one hour. As will be appreciated, in view of the dose limits set by the ICRP for members of the public, the lower the dose, the more often a person can be checked or screened for concealed items or materials. This will be particularly advantageous for the checking or screening of people at airports, as many people fly often and sometimes more than one flight per day. At a dose of less than a BERT value of one day, the method of the invention thus provides a 5-fold dose reduction compared to a conventional diagnostic X-ray of an ankle, and a dose reduction which can be as high as 1500-fold compared to a conventional diagnostic X- ray of a large body part.

The source of penetrating electromagnetic radiation may have a line energy spectrum in contrast to a broad polychromatic energy spectrum. Preferably, the source of penetrating radiation has a line energy spectrum with no characteristic peak X or gamma ray emissions above 90 keV. More preferably, the source of penetrating electromagnetic radiation has a line energy spectrum with no characteristic peak X or gamma ray emissions above 60 keV.

Typically, the source of penetrating electromagnetic radiation includes a radioisotope material, although the source may be an X-ray source which employs an accelerating voltage of no more than about 200 kV. The source of penetrating electromagnetic radiation may include as a major radioisotope constituent, radioisotope having a half-life of at least one year. It is however to be appreciated thai the method of the invention may employ a radiation source which provides Bremsstrahlung, e. g. those from an X-ray tube making use of a tungsten target or e molybdenum target, although a tungsten target is preferred. However, when using ar X-ray tube, as will be appreciated by those skilled in the art, the radiation dose pe check may then be increased by a factor of between about four and about ten, wher compared to the use of an appropriate radioisotope material.

The source of penetrating radiation may include as a major radioisotope constituent, a radioisotope selected from the group consisting of americium-241, thulium-170, gadalinium-153, iodine-125, cadmium-109 and tin-119m.

In a preferred embodiment of the invention, the source of penetrating electromagnetic radiation is an americium-241 radioisotope gamma ray source. The source may have a diameter between 3 mm and 40 mm, typically between about 18 mm and about 25 mm. The radioisotope gamma ray source may have a gamma ray emission surface area which is larger than the projected area of the gamma ray emission surface, as disclosed in WO 00/55866.

Preferably, when the source of penetrating radiation includes as a major radioisotope constituent, americium-241, the americium-241 is in the form of a hydroxide, but an oxide may also suffice.

The method may include placing the clothed person in a shielded environment to inhibit accidental exposure of other people to irradiation from the source of penetrating radiation. However, it is to be appreciated that the method of the invention can be employed without using a radiation shielded environment. The method can thus be used in areas with poor infrastructure, with security personnel wearing protective aprons.

Typically, the method includes exposing various parts of a person tc penetrating electromagnetic radiation from a plurality of radiation sources and capturinc a plurality of radiographic images of the outline of the various irradiated parts of the person.

As mentioned hereinbefore, the radiographic images are not necessary images allowing visual inspection, but may be images in, e. g. electronic form, which allows a software algorithm to check for anomalies such as would be presented bz concealed objects or materials. If one or more anomalies are detected by whateve non-human checking means is used to check the images, an alarm, e. g. a visual o audible alarm may be raised by the checking means. Said checking means may als (

then provide an indication of the location on the body of the person where the anomaly was determined, e. g. a number indicating a body area.

The method may include, for a person for whom the radiographic image indicates that it is likely that an item or material is concealed, e. g. when the checking means raises an alarm, again exposing the person to penetrating electromagnetic radiation at a higher dose than initially used to provide a radiographic image of improved quality that can be stored and displayed visually. In this case, the method usually includes retaining the radiographic image as forensic image.

However, when the radiographic image or images does not show anything suspicious, the method typically includes erasing or destroying the radiographic image.

If preferred, a signal, e. g. visual or audible, may indicate conclusion of the check and that no anomalies were detected.

According to another aspect of the invention, there is provided a security arrangement for checking for concealed items or materials carried on the body or in the clothing on the body of a person, the arrangement including a radiation shielded environment within which a clothed person can be located; at least one source of penetrating electromagnetic radiation configured to irradiate a person located within the shielded environment with a dose of penetrating radiatior which does not exceed a BERT value of three weeks; and radiographic image capturing means for capturing a radiographic image of ar outline of at least a portion of an irradiated person located in the shielded environment.

Preferably, the source of penetrating electromagnetic radiation is configurec to irradiate a person located within the shielded environment with a dose of penetratinc electromagnetic radiation which does not exceed a BERT value of one week. More preferably, the source of penetrating electromagnetic radiation is configured to irradiate a person located within the shielded environment with a dose of penetrating electromagnetic radiation which does not exceed a BERT value of 1/3 day. Mos preferably, the source of penetrating electromagnetic radiation is configured to irradiate a person located within the shielded environment with a dose of penetrating radiatior which does not exceed a BERT value of one hour.

The source of penetrating electromagnetic radiation may be as hereinbefore described. Typically, the security arrangement includes a plurality of sources of penetrating electromagnetic radiation and the radiographic image capturing means is capable of capturing a plurality of radiographic images of an outline of various parts of the body of an irradiated person located in the shielded environment.

The radiographic image capturing means may include an electromagnetic radiation converter to convert incident electromagnetic radiation into electrical signals.

An example of a suitable electromagnetic radiation converter is the Direct Ray (trade name) detector array supplied by Hologic, that comprises an amorphous selenium coating over a thin-film-transistor matrix with associated readout electronics. Instead, the electromagnetic radiation converter may comprise a scintillator for converting X-rays or gamma rays into visible light, e. g. a caesium iodide or CdZnTe scintillator, that may include dopants, such as thulium, europium, zirconium, or the like, in combination with a converter for converting the visible light into electrical charge, e. g. a silicon photodiode. Suitable flat panel scintillators are sold by, for example Agfa, GE Medical, Canon and Trixell.

Instead of employing flat panels, the image capturing means may include a plurality of said converters in strip form, i. e. strip detectors, such as supplied by eV- products, a division of Il-VI Incorporated of Saxonburg, PA, USA. Such strip detectors will typically be used with a source of penetrating electromagnetic radiation which produces a fan beam of radiation.

Instead, or in addition, the radiographic image capturing means may include an electromagnetic radiation converter to convert incident electromagnetic radiatior directly into electrical signals, or first into light and then into electrical signals, via c Charge Coupled Device (CCD) camera or a Back Entrance Charge Coupled Device (BCCD) camera to display the captured image on a monitor or alternatively, subjecting i to electronic processing, using suitable anomaly detection algorithms. The radiographic image capturing means may be configured to employ frame addition, or, preferably frame integration to enhance the quality of the captured image.

In one embodiment of the invention, the radiographic image capturing means includes one or more computerised radiography (CR) flat or curved panels comprising phosphors to receive incident electromagnetic radiation and to capture a latent radiographic image. The CR panel may be displaceable between a first position in which it captures the latent radiographic image and a second position in which the latent radiographic image is read and the panel reset.

The security arrangement may include a collimator to provide a collimated cone or fan beam of penetrating electromagnetic radiation from the source. When using a cone beam of radiation, the source detector and person to be checked remain stationary relative to each other during exposure of the person to the radiation. However, when using a fan beam, depending on the detector configuration, at least one of these three system components must be displaceable relative to the other two, so that scanning along a line can be effected to obtain an image of an outline of the irradiated part of the person.

The shielded environment may include an entrance door and an exit door, with an interlock arrangement which inhibits release of electromagnetic radiation from the source unless both doors are closed.

The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings.

In the drawings, Figure 1 shows a schematic plan view of one embodiment of a security arrangement in accordance with the invention, with a person to be checked located within a radiation-shielded environment; Figure 2 shows a side view into the security arrangement of Figure 1; Figure 3 shows an enlarged plan view of the security arrangement of Figure 1 with more detail ; Figure 4 shows a schematic side view of one embodiment of a security arrangement in accordance with the invention, which allows checking of the feet of c person located within a radiation-shielded environment; and

Figure 5 shows a schematic side view of another embodiment of a security arrangement in accordance with the invention which allows checking of the feet of a person located within a radiation-shielded environment.

Referring to Figure 1 of the drawings, reference numeral 10 generally indicates a security arrangement in accordance with the invention for checking for concealed items or materials carried on the body or in the clothing of a person. The security arrangement 10 includes a radiation-shielded cubicle 12 comprising hollow side walls 14,16, a hollow entrance door 18 and a hollow exit door 20. A person 22 is shown standing inside the cubicle 12. Also shown in Figure 1, are four radioactive sources of penetrating electromagnetic radiation 24.1, 24.2, 24.3 and 24.4. The cubicle 12 is typically rectangular or square, although other configurations may also be used.

The radioactive sources 24.1 to 24.4 are radioisotope sources and are located on different sides of the cubicle 12, in the side walls 14,16 and doors 18,20 respectively. The sources 24.1 to 24.4 are located at different elevations inside the cubicle 12.

The entrance door 18 has a hinge 19 (see Figure 3) and opens outwardly as shown by arrow 26 and the exit door 20 has a hinge 21 and opens outwardly as showr by the arrow 28.

Each radioactive source 24.1 to 24.4 includes as a major radioisotope constituent, americium-241. The sources 24.1 to 24.4 are configured to irradiate the person 22 located within the cubicle 12 with a dose of penetrating radiation (X-anc gamma rays) which does not exceed a BERT value of three weeks, typically a BER1 value of less than one day, e. g. about one hour within a brief period of no more thar about a few seconds. The radioactive sources 24.1 to 24.4 each have a line energy spectrum which has no significant characteristic peak X or gamma ray emissions abov 60 keV. Although not shown in the drawings, the sources 24.1 to 24.4 are shutterec and are interlocked with the doors 18,20 by an interlock arrangement which prevent : the shutters of the sources 24.1 to 24.4 from opening unless the doors 18,20 ar closed, as shown in Figure 1 of the drawings. This feature prevents unintentions escape of electromagnetic radiation from the cubicle 12.

The security arrangement 10 further includes radiographic image capturing means for capturing a radiographic image of an outline of at least a portion of the person 22 located in the cubicle 12. In one embodiment of the invention, as shown in Figure 2 of the drawings, the radiographic image capturing means comprises a plurality of flat panels, two of which are shown in Figure 2 of the drawings and indicated by reference numerals 30 and 32, located against an inside surface of the side wall 14, as shown in Figure 3 of the drawings. Figure 3 also shows that two similar flat panels 34, 36 are located inside the opposite side wall 16.

In the embodiment of the invention shown in Figures 1 to 3 of the drawings, the flat panels 30 to 36 are so-called CR (computerised radiography) panels employing phosphors to receive incident electromagnetic radiation and to capture a latent radiographic image. These kinds of panels are marketed by, e. g. Agfa, Canon and Fudji. The latent image stored by the panel can be read by a laser of suitable frequency which irradiates the phosphor pixel by pixel, thereby prompting light emission for detection. Thus, as shown in Figure 2 of the drawings, after having been exposed to penetrating electromagnetic radiation, the panel 32 is displaced towards a position indicated by reference numeral 38 for reading of the latent image and resetting of the panel 32. This is effected with appropriate laser and associated equipment not shown in the drawings. The panel 30 simultaneously moves into the position previously occupied by the panel 32 for irradiation. In the opposite side wall 16, the panels 34 and 36 are operated in the same fashion.

As will be appreciated, instead of using CR panels, direct ray (DR) panels, such as those available from Hologic, Agfa and others may be used. The DR panels will not be displaceable and will be connected to monitors for direct display of images or, more likely, to computerised checking means which can evaluate electronic images for anomalies without visually displaying the images. Although a DR panel is more costly than a CR panel, an advantage of the use of DR panels is that the cubicle 12 car be reduced in size and even be circular cylindrical.

The radioactive sources 24.1 to 24.4 each include a suitable collimator o lead, or preferably tungsten, to provide a desired fan or cone beam of electromagnetic radiation. The radioisotope source 24.1 is located at an elevation which correspond :

more or less with the elevation of the centres of the flat panels 30,32 and is used to check the upper part of the body of the person 22 with a cone beam. The radioisotope source 24.3 (not shown in Figure 3) is located at a lower elevation and is used to check the lower legs with a cone beam. As will be appreciated, the flat panels 34 and 36 are thus also at a lower elevation than the flat panels 30,32. The radioisotope source 24.2 is used to check in the vicinity of the groin of the person 22 using a fan beam whereas the radioisotope source 24.4 is used to check the lower inside legs of the person 22 using a fan beam. These sources are not shown in Figure 3. Although not shown in the drawings, appropriate flat panels similar to the flat panels 30 to 36, are thus housed in the doors 18,22 for use with the sources 24.2 and 24.4.

If desired, the door 20 may also house further flat panels for checking under the armpits of the person 22 and the door 18 may house a radioactive source providing a fan beam for checking under the armpits.

The side walls 14,16 and the doors 18,20 are lined with a 3 mm thick or thicker lead lining or coating 40 on inside surfaces thereof. A floor and a roof of the cubicle 12 are similarly protected. Although not shown in the drawings, the cubicle 12 may include one or more windows, which will be of lead glass thick enough to adequately shield bystanders outside the cubicle 12 from radiation.

In use, a person to be checked, e. g. a passenger at an airport terminal, walks through the entrance door 18 into the cubicle 12. The door 18 closes automatically after the person and, with both the doors 18 and 20 closed, the person is irradiated with penetrating electromagnetic radiation from the radioisotope sources 24.1 to 24.4. Only one flat panel 30 et al associated with each radioisotope source 24.1 to 24.4 is in a position to be irradiated and captures a latent radiographic image of an outline of thai part of the body of the person 22 covered by the particular radioactive source 24.1 tc 24.4. After having been irradiated for a brief period, the person 22 exits through the exi door 20, which opens automatically, the irradiated flat panels are displaced from theii operative positions into positions where they can be read by the laser and reset Simultaneously, another flat panel moves into the operative position for the next persor to be irradiated, with this process repeating itself over and over again. When DR panel,, or strip detectors are used, the images are read directly from the panels or detectors.

In the embodiment of the invention shown in the drawings, the radiographic images captured on the flat panels are shown on monitors, with the monitors constantly being screened by security personnel. If nothing of a suspicious nature is observed, the images are erased. If a suspect item or material is being noticed, a person can be stopped by the security personnel and subjected to a further radiographic check at a higher dose to provide a better quality image. Instead, a person can be subjected to frisking and questioning.

As however mentioned hereinbefore, preferably no images are displayed at all for ethical and public acceptability reasons, with a machine checking the electronic images for anomalies and providing a signal declaring the images free of anomalies or providing an alarm if an anomaly has been determined. Such an arrangement can be used to check male and female subjects.

For ethical reasons, when a monitor is used, males and females should be checked separately, with a male security officer manning the monitor for males being checked and a female security officer being used for the checking of females. Thus, typically, when a monitor is used, two cubicles 12 will be employed simultaneously, one for males and one for females.

Figures 4 and 5 show schematically some features of two further embodiments of a security arrangement in accordance with the invention and respectively indicated by reference numerals 50 and 60. The security arrangements 50, 60 include parts or features similar to the security arrangement 10, and unless otherwise indicated, the same reference numerals are used to indicate the same o similar parts or features as were used in relation to the security arrangement 10.

In Figure 4 of the drawings, the security arrangement 50 includes E radioactive source 24.5 providing a fan beam for checking under the armpits of the person 22, and a flat panel or a strip detector 52 associated with the radioactive source 24.5. The security arrangement 50 further includes a radioactive source 24.6 and a fla panel or strip detector 54 provided in a floor 56 of the security arrangement 50. The radioactive source 24.6 and the flat panel 54 are used to check the feet of the person 2 for concealed items or materials. However, it is to be appreciated that the possibility

exists that the radioactive source 24.2, using a suitable collimator providing a cone beam, may suffice for the checking of the feet, thus obviating the need for the additional source 24.6. To reduce radiation exposure risk further, the source 24.2 may be located rather behind the person 22, as shown in Figure 1 of the drawings.

In Figure 5, there are shown two radioactive sources 24.7 and 24. 8, each providing a fan beam for checking a single foot of the person 22. Each radioactive source 24.7, 24.8 is associated with a flat panel or strip detector 62,64 respectively for capturing a radiographic image of one of the feet of the person 22.

It is an advantage of the method and security arrangement of the invention, as illustrated and described, that they employ radiation dosages that are orders of magnitude lower than conventional diagnostic or security X-ray equipment. Assuming that a person has to be exposed to radiation four times to properly check the body of the person for concealed items or material (at a dose each time of less than a BERT value of one hour), the method and security arrangement of the invention allows screening of airport passengers or the like about seven times daily, with a built-in safety factor of at least two, before the person has been exposed to a dose equivalent to a BERT value of one year. It is thus expected that the method and security arrangement of the invention will find particular application at airports or the like. Application in other areas are also possible, e. g. to check workers for smuggled valuable metal salts impregnated into their clothes.

It will also be appreciated that should the isotope sources be replaced by a conventional X-ray tube, the exposure dose of the person screened for anomaly, is four to ten fold higher than that from an americium-241 isotope source, but still low compared to diagnostic radiographic images. When using the isotope iodine-125, the dose approximately quadruples but even then is still low enough for practica application.

It will also be appreciated that the use of conventional X-rays allows using c cylindrically-shaped security arrangement permitting one or more X-ray tube image capturing means assemblies confined to a rigid gantry, that moves to desired position : for body checks.

It will further be appreciated that the use of fan beams instead of cone beams may require additional repetitive scanning motions suitably matched to the type of detector used. For example, a passenger may scan or move across a fixed fan beam and a fixed line detector. Should the line detector be replaced by a flat screen, only the fan beam may remain stationary during image capture, with both the person and the flat screen moving past the stationary fan beam.