Radiation modulator

Abstract

In an example, it is disclosed an array of a plurality of detectors, the plurality of detectors being arranged along a longitudinal direction and being configured to detect pulsed ionizing radiation having a spectrum of energies, the spectrum comprising at least a higher portion of energies and a lower portion of energies, each detector corresponding to a pixel of an inspection image of cargo after the cargo has been scanned in a scanning direction by the array, the scanning direction being substantially perpendicular to the longitudinal direction, wherein the array comprises at least one radiation modulator located over at least one detector of the array, and wherein the radiation modulator is configured to filter the ionizing radiation through at least two zones in a direction substantially perpendicular to the longitudinal direction, such that: transmission of the lower portion of the spectrum is inhibited through a first zone of the modulator, and transmission of the higher portion through the first zone of the modulator is enabled, and transmission of both the lower and higher portions of the spectrum through a second zone of the modulator is enabled.

Description

[0001] RADIATION MODULATOR

[0002] Field of Invention

[0003] The invention relates but is not limited to an array of a plurality of detectors of pulsed ionizing radiation. The invention also relates to a matrix comprising at least two arrays.

[0004] Background of Invention

[0005] The performances of x-ray cargo inspection systems comprising arrays of detectors are a trade-off between capabilities whose requirements are contradictory.

[0006] Contrast and penetration both require larger detectors, in order to collect more x-ray signal to provide a better signal-to-noise ratio.

[0007] On the contrary, resolution requires detectors with a smaller pitch, in order to separate objects close to each other.

[0008] In other words, detectors with a smaller pitch generate a lower contrast and a lower penetration, whereas larger detectors suffer from lower resolution.

[0009] Summary of Invention

[0010] Aspects and embodiments of the invention are set out in the appended claims. These and other aspects, and aspects and embodiments which are useful in understanding the invention set out in the claims, are also described in the present disclosure.

[0011] Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination.

[0012] Brief Description of Drawings

[0013] Embodiments of the present disclosure will now be described, by way of example, with reference to the accompanying drawings, in which:

[0014] Figures 1A, 1 B and 1C schematically represent a first example array according to the disclosure;

[0015] Figure 2 schematically represents a spectrum of energies of ionizing radiation;

[0016] Figures 3A and 3B schematically represent a second example array according to the disclosure;

[0017] Figure 4A schematically shows respective typical point spread functions as a result of using a modulator according to Figures 3A and 3B, for different energies, including a 6MeV x-ray accelerator spectrum point spread function;

[0018] Figure 4B schematically shows a modulation transfer function of the 6MeV x-ray accelerator spectrum point spread function curve of Figure 4A, compared to a modulation transfer function of a point spread function curve obtained without using a modulator placed on the detectors;

[0019] Figures 5A and 5B schematically represent a third example array according to the disclosure;

[0020] Figures 6A and 6B schematically represent a fourth example array according to the disclosure;

[0021] Figure 7 schematically shows a point spread function as a result of using a modulator according Figures 6A and 6B, compared to a point spread function as a result of using a modulator according to Figures 3A and 3B;

[0022] Figure 8 schematically represents a fifth example array according to the disclosure;

[0023] Figure 9 schematically shows a 2D modulation transfer function of a 6MeV x-ray accelerator spectrum point spread function obtained using a modulator according to Figure 8, compared to a 2D modulation transfer function of a point spread function obtained without using a modulator placed on the detectors;

[0024] Figure 10A schematically shows a matrix where a plurality of modulators are arranged in bands which are parallel to each other;

[0025] Figure 10B schematically shows a matrix where a plurality of modulators are arranged in bands which are in a zigzag configuration to each other;

[0026] Figure 10C schematically shows a matrix comprising a plurality of arrays with detectors divided into four zones.

[0027] In the figures, similar elements bear identical numerical references. Embodiments

[0028] Overview

[0029] The disclosure discloses an array of detectors making use of a polychromatism of a flux of ionizing radiation (such as an x-ray radiation having a spectrum). The array comprises a radiation modulator on at least some of the detectors, or close to some of the detectors with respect to the ionizing radiation. The radiation modulator is configured to filter differently the different energies associated with the polychromatism. The radiation modulator is configured to filter lower energies on some spatial parts of the detectors only, and to transmit most of higher energies.

[0030] Energies of the spectrum which contribute to resolution of the detectors are not the same energies of the spectrum which contribute to the penetration of the radiation.

[0031] Resolution of the detectors is dependent on lower energies, because small objects are usually also thin: only lower energies are absorbed by thin objects.

[0032] Penetration on the contrary is dependent on higher energies, because only higher energies are transmitted by thick objects and thus detected by the detectors. Furthermore, penetration is measured with large objects which do not usually require a high resolution to be detected by the detectors. Contrast measured on large objects hidden behind a certain amount of matter benefits from lower and higher energies, but the hidden large objects do not require a high resolution to be detected.

[0033] As high energies are transmitted by the radiation modulator, the radiation penetration is not degraded by the modulator. The filtering by the modulator has a resolution which depends strongly on energy, with higher resolution for lower energies.

[0034] Detailed Description of Example Embodiments

[0035] Figures 1A, 1 B and 1C schematically show an array 1 of a plurality of detectors 2. In Figures 1A, 1 B and 1 C, the plurality of detectors 2 is arranged along a longitudinal direction (OZ).

[0036] Each detector 2 corresponds to a pixel of an inspection image of cargo after the cargo has been scanned in a scanning direction (OX) by the array 1.

[0037] In Figure 1A, the scanning direction (OX) is substantially perpendicular to the longitudinal direction (OZ).

[0038] As shown in Figures 1 B and 1C, the plurality of detectors 2 is configured to detect pulsed ionizing radiation 5 (such as x-ray radiation) having a spectrum of energies. As shown in Figures 1 B and 1C, the pulsed ionizing radiation 5 has a main direction (OY) of propagation, the main direction of propagation (OY) being perpendicular to both the scanning direction (OX) and the longitudinal direction (OZ).

[0039] Figure 2 schematically represents a spectrum 10 of energies of the ionizing radiation.

[0040] In Figure 2, the spectrum 10 of energies comprises at least a higher portion 12 of energies and a lower portion 11 of energies.

[0041] In Figures 1A, 1 B and 1C, the array 1 comprises at least one radiation modulator 3 located over at least one detector 2 of the array 1. In other words, the modulator 3 is configured to be located between a source of the ionizing radiation 5 (the source is not shown in the Figures) and the array 1. In Figures 1A, 1 B and 1C, the radiation modulator 3 is located over all of the detectors 2 of the array 1 in the longitudinal direction (OZ), separated from the array 1 by a distance counted in a direction substantially parallel to the main propagation direction (OY). In Figures 1A, 1 B and 1C, the modulator 3 hovers over the array 1 without touching the array 1 . In Figures 1A, 1 B and 1C, the modulator 3 is located quite close to the array 1 , that is between the cargo under inspection (not shown in the Figures) and the detectors. In some examples, the modulator may also be placed between the source and the cargo under inspection. Embodiments where the modulator is located on the array 1 are also envisaged, as shown in Figure 3B discussed in more detail below.

[0042] As shown in Figures 1A and 1 C, the radiation modulator s is configured to filter the ionizing radiation through at least two zones 31 and 32 in a direction substantially perpendicular to the longitudinal direction (OZ). In Figures 1A and 1C, the radiation modulator 3 is configured to filter the ionizing radiation through three successive zones 31-1 , 32 and 31- 2 corresponding to the first zone 31 and the second zone 32, in the direction (OX), substantially perpendicular to the longitudinal direction (OZ). In other words, in Figures 1A and 1C, the first zone 31 is divided into two subzones 31-1 and 31-2 separated by the second zone 32 in the direction (OX) substantially perpendicular to the longitudinal direction (OZ).

[0043] As shown in Figure 10, the radiation modulators is configured to filter the ionizing radiation 5 through the three successive zones 31-1 , 32 and 31-2, such that transmission of the lower portion 11 of the spectrum 10 is inhibited through the first zone 31 of the modulator 3 (i.e., the subzones 31-1 and 31-2 of the first zone 31 of Figure 1C) and transmission of the higher portion 12 through the first zone (i.e., the subzones 31-1 and 31-2 of the first zone 31 of Figure 1C) of the modulator 3 is enabled, and such that transmission of both the lower portion 11 and the higher portion 12 of the spectrum 10 through the second zone 32 of the modulator 3 (i.e., the second zone 32 of Figure 1C) is enabled.

[0044] As shown in Figures 1A and 1 C, the radiation modulator s is configured to filter the ionizing radiation 5 through the at least two zones 31 and 32 in the direction (OX) substantially perpendicular to the longitudinal direction (OZ). As explained in greater detail below, in some examples, the radiation modulator may be configured to further filter the ionizing radiation through at least two zones in a direction substantially parallel to the longitudinal direction (OZ).

[0045] As shown in Figure 1C, the modulator 3 is also continuous in the direction substantially perpendicular to the longitudinal direction (OZ). In Figure 1C, the first zone 31 comprises a block of material being of a first homogeneous thickness Hi in the direction (OY) of transmission of the radiation 5, and the second zone 32 comprises a block of material being of homogeneous thickness H2corresponding to an equal thickness of material in the modulator 3 compared to the first thickness Hi , in the direction (OY) of transmission of the radiation 5. In Figures 1A, 1 B and 1 C, the second zone 32 comprises a block of material being different from the material of the first zone 31 , but transmitting both the lower and higher portions 11 and 12 of the spectrum.

[0046] Embodiments of the disclosure make use of oversampling in the scanning direction (OX). As can be seen from Figures 1 A, 1 B and 1 C, the detectors 2 have a width W and are wider in the direction (OX) of scanning than in the longitudinal direction (OZ). The relatively wide dimension W of the detectors 2 in the (OX) direction allows to keep a larger area of interaction with the incident ionizing radiation 5, while limiting any spatial resolution degradation, because a displacement 6 of the array 1 between two pulses of the ionizing radiation 5 in the scanning direction (OX) is much lower than the detector size W in the scanning direction (OX). Furthermore, as shown in Figure 1A, a ratio rof a width w2of the second zone 32 in the scanning direction (OX) substantially perpendicular to the longitudinal direction (OZ) over a width wi of the first zone 31 , is such that: r < 1.

[0047] In Figure 1A, wr= + w±-2, in which WM is the width of the subzone 31-1 and wi-2is the width of the subzone 31-2.

[0048] Preferably r is such that:

[0049] 1 / 4 < r < 1.

[0050] As also shown in Figure 1A, a width w2of the second zone 32 in the direction (OX) substantially perpendicular to the longitudinal direction (OZ) is greater than a displacement 6 of the array in the scanning direction (OX) between two pulses of ionizing radiation during the scan of the cargo by the array, such that: w2> 8.

[0051] In some examples, the width w2of the second zone 32 in the direction (OX) is such that, preferably: w2> 28.

[0052] As already stated above and as shown in Figure 1 B, the first zone 31 of the radiation modulator 3 is configured to cover each of the detector 2 homogeneously in the longitudinal direction (OZ). As described in more detail below, the disclosure applies to embodiments wherein the at least one radiation modulator is located on at least one detector of the array, and embodiments where not all of the detectors are at least partly covered by the modulator are also envisaged.

[0053] As shown in Figure 3, the subzone 31-1 and the subzone 31-2 may have the same width such that w1-1= wr-2-

[0054] In Figures 3A and 3B, the first zone 31 comprises a block of material being of homogeneous thickness Hi in the direction (OY) of transmission of the radiation 5, and the second zone 32 is configured to correspond to a void (i.e. , an aperture) in the modulator 3, in the direction of transmission (OY) of the radiation 5.

[0055] Figure 4A schematically shows three typical point spread functions as a result of using a modulator 3 according to Figures 3A and 3B, i.e., a modulator having a first zone 31 having a thickness Hi of 3cm of Iron and having a second zone 32 corresponding to a void (i.e., an aperture as shown in Figures 3A and 3B) having a width w2equal to ! of the total width W of the detectors 2, for two different specific energies (i.e. a first energy of 6MeV, see lower curve in Figure 4A, and a second energy of 300keV, see upper curve in Figure 4A) and as well as a total point spread function for a full 6MeV x-ray accelerator spectrum (see middle curve of Figure 4A).

[0056] Figure 4B schematically shows a modulation transfer function (MTF, i.e. the Fourier transform) of the full 6MeV x-ray accelerator spectrum point spread function of Figure 4A (i.e. the middle curve of Figure 4A): see upper curve of Figure 4B. The upper curve of Figure 4B is compared to a MTF of a point spread function curve obtained without using a modulator placed on the detectors: see lower curve of Figure 4B. The lower curve of Figure 4B, being a MTF of the curve obtained without using a modulator placed on the detectors, corresponds to a Nyquist frequency span. As can be seen from Figure 4B, the upper curve of Figure 4B has a frequency span larger than the Nyquist frequency span of the lower curve of Figure 4B. In other words, more frequencies are transmitted to the detectors when a modulator is placed over the array of detectors, which provides a better resolution of the cargo under inspection.

[0057] In Figures 5A and 5B, the first zone 31 is continuous in the direction (OX) substantially perpendicular to the longitudinal direction (OZ), in other words the first zone 31 is not divided into subzones. In Figures 6A and 6B, the first zone 31 comprises a block of material being of a first homogeneous thickness Hi in the direction (OY) of transmission of the radiation 5, and the second zone 32 comprises a block of material being the same as the material of the first zone but being of a second varying thickness H in a direction of transmission of the radiation, the second thickness H corresponding to a lower thickness of material in the modulator compared to the first thickness, in the direction of transmission of the radiation. In Figure 6B, H first decreases from Hi to substantially zero and then increases from substantially zero to Hi. The block of the second zone 32 thus has a triangular indent.

[0058] Figure 7 schematically shows a point spread function obtained as a result of using a modulator having a triangular indent according to Figures 6A and 6B, compared to a point spread function obtained as a result of using a modulator according to Figures 3A and 3B. As it is apparent from Figure 7, the peaked shape of the modulator according to Figures 6A and 6B allows a better transmission of the higher frequencies.

[0059] Other shapes of indents in the block of the second zone are envisaged.

[0060] In the developments above, the modulator improves resolution in the direction (OX) of scanning, while preserving the penetration.

[0061] In the development below, resolution is improved in both directions (OX) and (OZ) by having a filtering part oriented along the two directions (OX) and (OZ) simultaneously, as explained in greater detail below. In the development below, the displacement 6 of the array 1 between two pulses of the ionizing radiation is lower than half the width of the detector 2.

[0062] In some examples and as shown in Figure 8, the radiation modulator 3 is configured to further filter the ionizing radiation through at least two zones 31 and 32 in the longitudinal direction (OZ). In Figure 8, for each detector 2, the first zone 31 of the modulator 3 is configured to cover an upper part or a lower part of the detector 2, the upper part and the lower part being defined by at least a diagonal of the detector 2. In Figure 8, for each detector 2, the second zone 32 of the modulator 3 is configured to cover a lower part or an upper part of the detector 2. In some non-limiting examples, the thickness Hi of the first part 31 of the modulator 3 in the main direction (OY) of propagation of the radiation is such that the first part 31 of the modulator 3 is configured to absorb half of the full-scale flux (e.g., an x-ray flux without any objects).

[0063] Figure 9 schematically shows a 2D modulation transfer function of the full 6MeV x-ray accelerator spectrum point spread function obtained using a modulator 3 according to Figure 8: see upper curve of Figure 9. The upper curve of Figure 9 is compared to a 2D MTF of a point spread function curve obtained without using a modulator placed on the detectors: see lower curve of Figure 9. The resolution improvement is demonstrated in Figure 9 by the comparison of the modulation transfer functions.

[0064] As shown in Figures 10A, 10B and 10C, the disclosure applies to a matrix 4 comprising at least two arrays 1 according to any aspect of the disclosure. In Figures 10A, 10B and 10C, the matrices 4 comprise five arrays 1 , but other numbers of arrays are envisaged.

[0065] As shown in Figures 10A and 10B, the disclosure applies in cases where an array 1 according to any aspect of the disclosure comprises a plurality of radiation modulators 3 located over a plurality of detectors 2 of the array 1 .

[0066] As shown in Figures 10A and 10B, the plurality of modulators 3 are arranged in bands which are parallel to each other (Figure 10A) and / or in a zigzag configuration to each other (Figure 10B).

[0067] In the case of Figure 10A, when the displacement of the array 1 between two pulses of the ionizing radiation is lower than half the width of the matrix 4, the modulators are configured to act as splitting the detectors 2 into two, along their diagonal, hence increasing the resolution in both the (OX) and (OZ) directions.

[0068] In the case of Figure 10B, when the displacement of the array 1 between two pulses of the ionizing radiation is lower than half the width of the pitch along the scanning direction of an array of the matrix 4, the modulators are configured to act as splitting the detectors 2 into four as shown in Figure 10C, along their two diagonals, hence further increasing the resolution in both the (OX) and (OZ) directions.

[0069] In Figure 10B, four arrays 1 of the matrix 4 are covered by two types of filter along the two diagonals. If the displacement of the matrix 4 between two pulses of the ionizing radiation is lower than the width of each array 1 , the modulator 3 is configured to act as splitting the pixel in four.

[0070] In the present disclosure the material of the modulator may comprise at least one of Iron, Copper and Lead.

[0071] The present disclosure, the thickness Hi of the filtering material of the first zone 31 may be defined such that the contribution of the second zone 32 (which can be a non-filtered zone, such as an aperture) to the signal collected by the detector 2 is at least equal to the contribution of first, filtered zone 31. In some examples according to Figures 3A, 3B, 5A and 5B, for a detector 2 with a width W of 20mm, a modulator 3 with second zone w2(e.g., an aperture of 5 mm), and for a 6 MeV spectrum of 6 MeV, the thickness Hi is 3 cm of Iron. In such examples, a thinner second zone is paired with a thicker (i.e., greater Hi) filtering part of the first zone, while a broader second zone is paired with a thinner (i.e., smaller Hi) filtering part of the first zone.

[0072] In the developments above, the modulator is aligned with the array in the scanning direction, but it is envisaged that the modulator can have a wider dimension in the scanning direction than the detector, to make alignment easier.

[0073] In some examples, the modulator may also be moved with respect to the array, in time. In such a case (not shown in the Figures), the modulator may be placed next to the source of ionizing radiation (to minimise the size of the modulator), and be moved by an actuator in the scanning direction, based on a scanning speed of the cargo under inspection (as detected e.g., by a radar as a non-limiting example). Other embodiments are envisaged.

[0074] The disclosure applies to x-ray radiation, but other types of pulsed ionizing radiation are also envisaged. The disclosure applies to any type of polychromatic ionizing radiation used for transmission imaging, provided that attenuation of the radiation is energy (or wavelength) dependant.

[0075] Other embodiments than those shown in the Figures are also envisaged. The disclosure also applies to a modulator configured such that its thickness varies, e.g., continuously or discontinuously, along the direction substantially perpendicular to the longitudinal direction (i.e., the thickness varies in the scanning direction). For example, the thickness of the modulator may vary sinusoidally, with a spatial period smaller than the width of the detector in the direction substantially perpendicular to the longitudinal direction (i.e., the pitch of the detector in the scanning direction) and with an amplitude that filters the spectrum of the radiation substantially in the same way as the above-described modulators. Similarly to the above-described modulators, the radiation modulator, whose thickness varies along the direction substantially perpendicular to the longitudinal direction, is configured to filter the ionizing radiation through at least two zones in the direction substantially perpendicular to the longitudinal direction, such that: transmission of the higher portion through the first zone of the modulator is enabled, the first zone of the modulator being defined as a zone of the modulator where transmission of the lower portion of the spectrum is inhibited, and transmission of both the lower and higher portions of the spectrum through a second zone of the modulator is enabled.

Claims

CLAIMS1. An array of a plurality of detectors, the plurality of detectors being arranged along a longitudinal direction and being configured to detect pulsed ionizing radiation having a spectrum of energies, the spectrum comprising at least a higher portion of energies and a lower portion of energies, each detector corresponding to a pixel of an inspection image of cargo after the cargo has been scanned in a scanning direction by the array, the scanning direction being substantially perpendicular to the longitudinal direction, wherein the array comprises at least one radiation modulator located over at least one detector of the array, and wherein the radiation modulator is configured to filter the ionizing radiation through at least two zones at least in a direction substantially perpendicular to the longitudinal direction, such that: transmission of the lower portion of the spectrum is inhibited through a first zone of the modulator, and transmission of the higher portion through the first zone of the modulator is enabled, and transmission of both the lower and higher portions of the spectrum through a second zone of the modulator is enabled.

2. The array of claim 1 , wherein a width w2of the second zone in the direction substantially perpendicular to the longitudinal direction is greater than a displacement <5 of the array in the scanning direction between two pulses of ionizing radiation during the scan of the cargo by the array, such that: w2> 6, optionally wherein w2> 28.

3. The array of claim 1 or 2, wherein a ratio r of a width w2of the second zone in the direction substantially perpendicular to the longitudinal direction over a width wi of the first zone, is such that: r < 1, preferably such that:

4. The array of any of claims 1 to 3, wherein the first zone is continuous in the direction substantially perpendicular to the longitudinal direction.

5. The array of any of claims 1 to 3, wherein the first zone is divided into two subzones separated by the second zone in the direction substantially perpendicular to the longitudinal direction.

6. The array of any of claims 1 to 5, wherein the first zone of the radiation modulator is configured to cover the detector homogeneously in the longitudinal direction.

7. The array of any of claims 1 to 5, wherein the radiation modulator is configured to filter the ionizing radiation through at least two zones in the longitudinal direction.

8. The array of claim 7, wherein the first zone of the modulator is configured to cover an upper part or a lower part of the detector, the upper part and the lower part being defined by at least a diagonal of the detector, and wherein the second zone of the modulator is configured to cover a lower part or an upper part of the detector.

9. The array of any of claims 4 to 8, wherein the modulator is configured such that its thickness varies, continuously or discontinuously, along the direction substantially perpendicular to the longitudinal direction, the modulator being configured to filter the ionizing radiation such that: the transmission of the higher portion is enabled through a zone corresponding to the first zone of the modulator, the first zone of the modulator being defined as a zone of the modulator where transmission of the lower portion of the spectrum is inhibited, and the transmission of both the lower and higher portions of the spectrum is enabled through a zone corresponding to the second zone of the modulator, optionally wherein the thickness of the modulator varies sinusoidally in the direction substantially perpendicular to the longitudinal direction, with a spatial period smaller than the width of the detector.

10. The array of any of claims 4 to 8, wherein the first zone comprises a block of material being of a first homogeneous thickness in a direction of transmission of the radiation, and wherein the second zone is configured to correspond to a void or comprises a block of material : being the same as the material of the first zone but being of a second, homogeneous or varying, thickness in a direction of transmission of the radiation, the second thickness corresponding to a lower thickness of material in the modulator compared to the first thickness, in the direction of transmission of the radiation, and / or being different from the material of the first zone, but transmitting both the lower and higher portions of the spectrum, and being of a second homogeneous thickness in a direction of transmission of the radiation, the second thickness corresponding to an equal thickness of material in the modulator compared to the first thickness, in the direction of transmission of the radiation.11 . The array of any of claims 1 to 10, wherein the modulator is located between a source of the ionizing radiation and the array, and wherein the modulator is configured to be located between a cargo under inspection and the array of detector, either quite close to the array but not touching the array or the at least one radiation modulator being located on the at least one detector of the array, or wherein the modulator is configured to be located between the source and the cargo under inspection.

12. The array of any of claims 1 to 11 , wherein the array comprises a radiation modulator respectively located over each detector of the array.

13. The array of any of claims 1 to 12, wherein the modulator is configured to be moved with respect to the array, in the scanning direction.

14. The array of any of claims 1 to 13, wherein the array comprises a plurality of radiation modulators located over a plurality of detectors of the array, andwherein the plurality of modulators are arranged in bands which are parallel and / or in a zigzag configuration to each other.

15. A matrix comprising at least two arrays of any of the preceding claims.

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