GB2517654A - Inspection Apparatus - Google Patents

Abstract

Inspection apparatus for indicating the presence of a body of noxious material, such as explosive, in a container 10 comprises nuclear magnetic resonance apparatus including a magnet 3 which applies a magnetic field to the material. An RF field is applied to the material to cause nuclear magnetic resonance of the material. The resulting RF signals are detected 17,19,23. The detected signals are processed by an imager 25 to construct an image of the body. The image is displayed on a visual display unit 27.

Description

Inspection Apparatus This invention relates to inspection apparatus, and particularly to apparatus for indicating the presence of certain explosives, narcotics, drugs or other dangerous substances hidden in, for example, baggage, parcels, packages or letters. A particularly inportant application of the invention lies in the checking of baggage prior to its being loaded on to an aircraft.

The checking of baggage at airports conventionally involves the use of an x-ray scanner through which each item of baggage, such as a suitcase, is carried by a conveyor belt. Such a scanner will produce an image of the item on a display unit, which Is watched by security personnel. The system will, however, detect only those items within the suitcase which are at least partially opaque to X-ra,ys. Currently-used explosives such as semtex, PE4, gelignite, TNT and Cordtex (Trade Mark), narcotics such as cocaine and heroin, and drugs such as canabis resin are not opaque to X-rays, and would not be detected by such a system.

More recently, checking of baggage by use of nuclear magnetic resonance (NMR) has been proposed. In the known NMR checking systems, a static magnetic field is applied to the item of baggage under investigation to define an equilibrium axis of magnetic alignment in the baggage. An r.f. field is then applied to the baggage in a direction orthogonal to the static magnetic field direction to excite magnetic resonance in the materials in the baggage, and resulting r.f. signals are detected and processed. The detected r.f. signals indicate the presence of compounds which contain nuclei which e4iibit particular nuclear magnetic resonance characteristics. Since the above-mentioned materials contain such nuclei, the system gives a warning that such materials may be present in the baggage item. It is then necessary for the security personnel to open the item to determine whether it does, in fact, contain dangerous material or whether the material responsible for the NMR signal is harmless. Examples of such known systens are disclosed in, for exanple, GB-2,200,462 and W084/04173.

However, such systeas merely provide a signal which acts as a warning of the possible presence of dangerous materials.

It is an object of the present invention to provide an improved NMR inspection apparatus.

According to the invention there is provided inspection apparatus operative to indicate the presence in a container of a body of a material which is diaracterised by a relatively long spin-lattice relaxation time T and a relatively very short spin-spin relaxation time T2, the apparatus comprising means to apply a magnetic field to a region of the container; means to apply to said region a radio-frequency (RE) field perpendicular to the magnetic field at a predetermined frequency to cause nuclear magnetic resonance in said material present in the region; means to detect RI signals resulting from the nuclear magnetic resonance; and neans to process the detected RI signals to construct an image of said material in the region.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing, which is a schematic blockdiagramof inspection apparatus in accordance with the invention.

The nuclei of some elements (most notably 1H, 191, 31P) have a magnetic moment. In quantum mechanical terms, this is described by a nuclear spin angular momentum quantum nunter, I, the value of which is an odd nultiple of for nuclei with odd mass number and is integral or zero for nuclei with even mass numbers. Nuclei with zero spin have no nuclear magnetic moment and therefore give no NP dipole signal. All the nuclei of interest to the present application have spin 4.

In a static magnetic field, quantum mechanical considerations dictate that the nuclear magnetic moments of a spin + nucleus may orient either parallel' and antiparallel' to the field. It Is more favourable from an energy viewpoint for the nuclei to align parallel to the field so that, in equilibrium, a small difference in the population densities of the two states (only one part in 2 million at 0.15 Tesla) occurs and a net magnetisation is produced in a macroscopic volume. The value of the net magnetisation is proportional to the nuclear magnetic moment and to the abundance of that particular nucleus in the material to be studied. If, for any reason, a non-equilibrium population distribution is produced (e.g. by the application of an RE field) then the subsequent return of the magnetisatlon to its equilibrium value is described by an exponential rise with a characteristic long time constant T, the so-called Uspin_latticell relaxation time.

The nuclear magnetisation is extremely small (very auth smaller than the total magnetic susceptibility of water) so that its presence is detected by its interaction with AC magnetic fields usually at RI frequencies. In NF'I imaging, it is usual to consider only the net magnetic moment of an assembly of nuclei and to treat this as a classical variable. The quantum mechanical properties of individual nuclei may be overlooked. In equilibrium, the net magnetic moment vector is aligned with the applied field. If, however, this moment is made to rotate to an angle, 8, to the applied field, it will precess around the direction of the applied field at the Larmor precession frequency, which is proportional to the field and to a constant (y) which is characteristic of that nucleus (Y/21r 42.6 MHz/Tesla for hydrogen). In N1IR imaging, this rotation is produced by the application of a small RI magnetic field perpendicular to the main field, the angle 8 being determined by the anqlitude and duration of the RE pulse. Normally, rotations of 9O (ff12 pulse) and (-wpulse) are used. Following one or more RI pulses to rotate the net magnetisation vector, it is the precession of the magnetisation around the static field vthicth is detected by inductive coupling to a tuned pick-up coil.

Inunediately following the RF pulse, the nuclear moments from all contributing nuclei are perpendicular to the applied field and start to precess in perfect synchronisation. However, two effects contributed to dephase' the rotation of the nuclear moments, so that an exponential decay in the effective anlitude of the signal occurs with a time constant nudi shorter than the T relaxation tine to equilibrium. The first of these is described by a relaxation time T2 (the so-called "spin-spin" relaxation time) and is a characteristic of the material. This relaxation time describes the way In which neighbouring nuclei interact due to their relative motion at the molecular level and is therefore well correlated to the viscosity of the material. In biological material, T2 is generally in the range to 500 milliseconds, but for explosives is typically less than 20 mill iseconds.

Examples of T2 values for various explosives are given in

Table 1 below.

Material T2 (milliseconds) PE4 15 Gelignite 10 Cordtex 2 TNT C0.1

Table 1

The second process, described by a relaxation time T2*, is due to the variation in the magnetic field, and hence precession frequency, across the macroscopic volume of the object, or part of an object, from which the NMR signal is derived. T2* is approximately given by 11145o where Y is the gyromagnetic ratio for the nuclei of interest and Is the variation of field across the volume. It is therefore a reflection of the homogeneity of the magnetic field and the size of the object, and is in no way characteristic of the material. In general, the effective relaxation time, 12 eff, is given by 1 = 1 + 1 T2eff 12 12* To produce an image from an object, small magnetic fields are produced by additional coils, which fields are in the sane direction as the main magnetising field but the value of which depends linearly on the position within the object. For a one dinEnsional object1 the signal from each part of the object is therefore simply encoded in terms of the frequency of the NMR signal. The detected NW signal is the sum of NMR signals from all parts of the object so that the signal amplitude from any part of the object may be derived from a Fourier transform of the signal. In two dimensions, the principle of imaging is similar, but imaging gradients rrust be produced in many directions, and reconstruction performed from the complete set of NW signals.

In r-theta' imaging a uniform field gradient amplitude is used, the direction of which is rotated under computer control between successive data lines' to build up the data set.

An example of inspection apparatus In accordance with the invention which produces an image of any piece of a material having a long I and a very short 12 which is secreted in a container is illustrated schematically in the drawing. The apparatus comprises an electromagnet 1 which produces a strong uniform static main magnetic field across a gap between two pole pieces 3 of the electromagnet 1, the pole pieces 3 being joined by a yoke 5 carrying an energising coil (not shown). Alternatively, the main magnetic field may be provided by a tubular electromagnet.

A conveyor belt 7, meved by a roller 8 whidt is driven by a motor 9, conveys baggage, such as a suitcase 10, through the magnetic

field produced by the magnet 1.

The strength of the magnetic field in the gap btween the pole pieces 3, and hence in the suitcase 10, is controlled by a computer 11 via a main magnet control 11 wliidi controls the supply of energising current to the energising coil of the electromagnet.

The apparatus further includes a gradient coil system 13 thereby a gradient may be imposed on the static magnetic field in the gap between the pole pieces 3 in any one or more of three orthogonal directions x, y and z. The coil system 13 is energised by a gradient field control 15 isliith is in turn controlled by the computer 11. An r.f. antenna system 11 is selectively connectable by way of a transmit-receive switd, 19 to either an r.f. transmitter 21 or a receiver 23. The transmitter 21 is operated under control by the computer 11 to apply r.f. field pulses to the suitcase 10 for excitation of magnetic resonance in any relevant material contained in the suitcase. R.f. signals resulting from magnetic resonance excited in the material are sensed by the antenna system 17 and are passed via the receiver 23 to an imager 25 shich, under control of the computer 11, processes the signals to produce signals representing an image of the material. These signals are, in turn, passed to a display device 27 to provide a visual display of the image.

In operation of the apparatus, the field provided by the electromagnet 1 defines an equilibrium axis of magnetic alignment in the material. To obtain an Image of a selected region, e.g. a cross-sectional slice of the suitcase and its contents, an r.f. field pulse is first applied to the suitcase by means of the r.f.

transmitter 21 and antenna system 11 to excite magnetic resonance in the selected region. To this end the antenna system 17 produces a field in a direction orthogonal to the static field diection so as to tip the spins of nuclei of material in the selected region from the direction of the static field into a plane orthogonal to the static field direction. To restrict excitation to the selected region, the r.f. field pulse is applied in conjunction with magnetic field gradients imposed by the coil system 13, the frequency of the r.f.

field being thosen in conjunction with the magnitudes and directions of the imposed gradients so that the Larmor frequency of diosen protons in the body, e.g. hydrogen or nitrogen protons, is equal to the

r.f. field frequency only in the selected region.

The r.f. signals resulting from excitation are then spatially encoded by application of one or more further gradient magnetic fields, detected by the antenna system 17 and processed to produce an Image.

Normally a number of excitation and signal detection sequences are required to produce sufficient data to produce a satisfactory Image.

As conpared with medical magnetic resonance imaging (MRI), the values of 12 encountered in the imaging of explosives are very nuch shorter. In medical MRI a variety of imaging sequences are enployed to discriminate between materials with differing proton densities or differing 11, 12 values and to counteract the effects of iithomogeneous fields. These sequences rely on the fact that in human tissue 12 values do not fall below 30 ms, so that even after a sequence which might involve the application of three or more RE pulses, the nuclear magnetisation is still large enough to be detected with a good signal to noise ratio.

In the imaging of explosives, where 12 values less than 15 ms are of primary interest, only the two sinplest and fastest sequences have been considered. Structural detail between materials of differing T and 12 values is not required, and only a proton density imaging sequence is appropriate. The fastest, and therefore the one best suited to materials with short 12, is the repeated free induction decay (REID) sequence, in which a singleW/2 RF pulse is iirnnediately followed by data collection in the presence of the imaging field gradients. The spin-echo sequence is designed to minimise the effects of inhomogeneous magnetic fields and is therefore best suited to imaging in the presence of magnetic artefacts. This sequence consists of a 1T/2 pulse followed by a 71 pulse a tine V later.

After the first pulse, the nuclear moments dephase as described by the tine constant. However, the following 71pulse flips the magnetic moments In such a way that they now start to rephase. A time t later the moments return to conplete synchronisation and an echo' signal is produced. In this sequence the peak signal during the echo occurs a tine t after the RE pulse so that data colbction during this period is not affected by the pulse itself.

However, since the peak signal occurs a time 2t after the first RI pulse; the nuclear magnetisatlon has already decayed at a rate determined by the 12 of the material so that this sequence Is not well suited to materials with very short 12.

In both cases, after a waiting period In shidi the nuclear magnetisatlon relaxes towards its equilibrium value, the sequence is repeated with a different imaging field gradient. To speed imaging, the period between sequences is made less than T, so that the nuclear magnetisation never readies its equilibrium value. A compromise Is therefore made between speed of imaging and maximisation of the signal amplitude. A repeat period of 500 mIlliseconds provides an acceptable compromise for materials with Ii values approaching one second.

Because the 12 values of explosive materials are so short, short RI pulses, large field gradients and very fast data collection are essential, and these requirements are far nore onerous than for medical MRI.

There aretwo conventional imaging tediniques, namely radial Cr-theta) mapping and parallel (x-y) mapping. For r-theta mapping, the data collection commences iñrediately after the end of the RE pulse, sinultaneously with the application of the radial imaging field gradient. It is therefore the faster imaging sequence and is therefore preferred for the imaging of materials with very short 12.

In x-y mapping, a gradient In the x direction is applied for a short period prior to data collection during the application of the y gradient. It is therefore slightly slower but is less susceptible to field inhomogeneities which would create iosting and other image distortions in r-theta mapping. The r-theta reconstruction is performed by filtered back projection and x-y reconstruction is performed by two-dimensional Fourier transform. The former relies on the equal spacing of data points in the spatial frequency domain (k-space) and is therefore very sensitive to inhomogeneities in the main field and to any non-linearities in the imaging field gradients.

These appear as artefacts in the image. In x-y imaging, on the other hand, exact image reconstruction is performed Wuidi translates non-linearities in the field into non-linearities of the image. The quality of the image is therefore higher. For these reasons r-theta imaging has been entirely superceded by x-y imaging In medical systems.

However, it will be seen that for explosive imaging applications, Miere fast data collection Is required in the presence of inhomogeneous fields, the preferred technique is the use of r-theta data collection but with an x-y two-dimensional Fourier transform reconstruction algorithm. This can be achieved by interpolating the circularly symmetric r-theta data on to a rectangular grid of data points in the spatial frequency domain followed by a 2-dimensional Fourier frequency transform reconstruction. Two interpolation algorithms have been implemented. The first takes each point on the rectangular grid and assigns to it the value of the nearest data point on the radial grid. The second assigns to each point on the rectangular grid the mean value of the four nearest points on the radial grid. More sophisticated interpolation methods can be envisaged but, in practice, it has been found that the results from the two interpolation algorithms mentioned above are very similar, suggesting that there is little to be gained from such extension to more sophisticated methods.

As regards RF pulse production, the duration of the RF magnetic field pulse at the NMR resonance frequency must be short compared with 12 and its amplitude mist be sufficient to produce a 1t12 rotation of the nuclear magnetisation.

The imaging field gradients are produced by the

low-inductance untuned copper coils 13 within the magnet 1. These gradients mist be applied instantaneously', and therefore the inductance of the gradient coils imist be small If the drive voltage is not to be excessive. This necessitates the use of large currents to

produce useful fields.

In a realistic baggage handling situation, the presence of ferromagnetic objects, such as hinges and locks of the baggage, and radios, hairdriers etc. within the baggage, adjacent to any explosive material must be anticipated. Such objects produce large local field gradients, the most dramatic effect of which is to reduce the effective 12 value of any voxel to a value below the inherent T2 of ye the material, to the local 12* value. The more irthonogeneous the field, the more difficult becomes the operation of obtaining a good image. A method of dealing with such a problem would be to ban the use of ferromagnetic components in the baggage manufacture and to ban the carriage of electrical and electronic devices within the bagga9e. Failing that, it is necessary to try to

compensate for the field inhomogeneities.

The tolerance to sudi inhornogeneities can be improved, contrary to expectations, by increasing the resolution of the imaging sequence from a typical medical image of 256 x 256 pixels to a high-resolution, say, 1012 x 1012 pixel image.

In order to reduce field Inhomogeneities, it is proposed to use a technique of field mapping, in vkiith the field is measured at various points around the outside of the baggage and the form of a reverse field needed to compensate for any irthomogeneities is calculated. This field is then generated by nutually orthogonal field compensating coils or sets of coils. The mapping may be effected by encircling the baggage with a water jacket of known shape, for example a toroid, and producing an image of the water jacket. Any distortion of the image from the true shape of the water jacket will be due to field irtomogeneities introduced by the baggage. The compensating field can then be adjusted to bring the image back to the true shape, and it follows that the irthomogeneities have then been substantially overcome. Imaging of the baggage can then be performed. As many field compensating coils may be provided as are necessary to achieve adequate compensation.

In general, it is not necessary to produce a complete 3-dimensional reconstruction of the baggage, because any 2-dimensional slice may be sufficient to identify the presence of a suspicious material. In an NMR Imaging system, image planes may be constructed in any of three orthogonal directions by suitable energisation of the field coils. It is therefore proposed to map the field at various points around the baggage, such as at the eight corners of a suitcase, using, for example, a flux-gate probe, and to choose the imaging direction which is subject to the least distortion. /

In practice, the baggage will preferably be moved by the conveyor to a central position within the magnetic field, and the selection of slices will be effected electronically, rather than moving the baggage In a series of steps for imaging respective slices at a predetermined imaging position.

Data collection nust start as soon as possible following the RF pulse, and must be coripleted as rapidly as possible before the signal has decayed to the system noise level. To flEet the first requirement, an RF preairplifier with a rapid recovery from the saturation effects of the RF pulse rust be used. The recovery time of the preamplifier developed by us is less than 5/As and this limits the time delay between the RF pulse and data collection to a mininvm of 3Ops. In practice, the minimum delay permitted will be limited by the Q of the tuned RF detector coil, this being typically 400 at 6MHz, leading to a decay time constant of 6Ops and a minimum delay time of 500pis. In some circumstances, it may be beneficial to reduce the Q of the coil in order to speed recovery at the expense of signal amplitude. To meet the second requirement, the fastest available analog to digital converter rust be used. To obtain adequate dynamic range, 12-bit conversion is required, at v4iidi resolution conversion rates of 3ps per point are possible. For a low-resolution 128 x 128 image format, data collection therefore takes a minimum of lms which, in practice, allows a lower limit of 0.5 ins for T2. -It has been found that the presence of non-magnetic electrically-conductive materials, such as aluminium, aluminium foil, copper, brass or stainless steel, in the construction of the baggage or contained within the baggage, will distort the RF and gradient magentic fields used in the imaging process, but will not appreciably distort the static magnetising field. The intensity of the image will be reduced,but the image is found to suffer little, if any, distortion.

As mentioned above, it has previously been proposed to detect the presence of explosives, narcotics and other such dangerous substances by the use of NMR techniques, but the imaging of such materials, to provide an indication of the shape of the body of material and of its location within the baggage, has not previously been proposed. It would be possible to use NIIR detection in accordance with the prior art to provide a rapid indication as to whether the baggage contains a suspect material and, if so, to change the system to an imaging mode in accordance with the present invention, to provide an Image of the material.

Imaging, by itself, presents a problem in that it takes an appreciable time to gather enough data to produce an Image, and this might cause passenger delays. Furthermore, the operator must remain alert while viewing the image of every item of baggage as it passes along the conveyor. NMR is the only available technique which provides the possibility of both detection and imaging, so that it is possible to provide a system in which a fast-moving stream of baggage is monitored by an NMR apparatus which, on detection of the presence of a suspect material, operates an alarm and diverts the suspect baggage to a more slowly moving stream in which NMR imaging takes place. The operator is then aware that there is good reason to scan each image.especially carefully.

In either the detection case or the imaging case it would be possible to deliberately incorporate in an explosive material a material, such as fluorine or phosphorus which has a distinctive nucleus which Is easily detected by NMR and which will provide a distinct "signature" to the explosive material.

It would also be possible to use a uphase mapping technique wherein the RE field would cause simultaneous resonance in two nuclei which have slightly different frequencies, for example the same nuclei occurring in different materials. Inunediately after the 90° pulse, the phase vectors from the two nuclei will be in phase but, because one nucleus has a higher frequency than the other, after a certain time they develop a phase difference of 1800. At an equal time later, the signals come back Into phase again, and this process repeats itself. As the signals add vectorlally, when they are in phase opposition there is a net reduction in the resulting RE signal, and if the signals are equal there may, in fact, be net cancellation of the RF signal. This alternate presence and absence of signal is readily detectable in the NMR image.

Although the invention is described above In relation to the imaging of explosives or other dangerous materials in baggage, it wou1d also be applicable to such imaging in respect of smaller packages, parcels and letters.

Claims (11)

1. CLAIMS1. Inspection apparatus operative to indicate the presence in a container of a body of a material which Is characterised by a relatively long spin-lattice relaxation time 1, and a relatively very short spin-spin relaxation time T2, the apparatus comprising means to apply a magnetic field to a region of the container; means to apply to said region a radio-frequency (RF) field perpendicular to the magnetic field at a predetermined frequency to cause nuclear magnetic resonance in said material present in the region; means to detect RF signals resulting from the nuclear magnetic resonance; and means to process the detected RF signals to construct an image of said material In the region.
2. 2. Apparatus as claimed in Claim 1, wherein the processing means is operative to collect data from the RF signals In an r-theta radial mode and to interpolate the data into an x-y rectangular format for constructing the image.
3. 3. Apparatus as claimed in Claim 1 or Claim 2, whereIn high resolution of the imaging is achieved to improve the tolerance of the apparatus to inhomogeneities in the magnetic field caused by the container and/or its contents.
4. 4. Apparatus as claimed in Claim 3, wherein the resolution is 1012 x 1012 pixels.
5. 5. Apparatus as claimed in any preceding claim, wherein inhomogeneities in the magnetic field caused by the container and/or its contents are determined by field mapping; and wherein the magnetic field applying means is operative to apply a compensating magnetic field to substantially compensate for said inhomogeneities.
6. 6. Apparatus as claimed in Claim 5, including a water jacket to encircle a said container; wherein the processing means is operative to image the water jacket when said container is positioned therein and to determine the magnitude of any compensating magnetic field required to reduce distortion of the water jacket image caused by said container and/or its contents.
7. 7. Apparatus as claimed in any preceding claim, including means to monitor distortion in the magnetic field In a plurality of directions around the container; wherein the processing means is operative to select as the imaging direction that direction exhibiting the least distortion in the magnetic field.
8. 8. Apparatus as claimed in any preceding claim, wherein the 1W field applying means is operative to cause nuclear magnetic resonance simultaneously in two nuclei having slightly different resonance frequencies, thereby causing cyclic amplitude changes in the detected 1W signal.
9. 9. Apparatus as claimed In any preceding claim, including means to convey the container into position for application of themagnetic field thereto.
10. 10. Inspection apparatus substantially as herelnbefore described with reference to the accompanying drawing.
11. 11. Apparatus as claimed in any preceding claim, operative to provide an initial indication of the presence of a said material in the container; and operable subsequently to construct said image.