Patent Application
20070286329
Hereafter, an embodiment of the present invention is described in detail with reference to the drawings.
As shown in
In the present embodiment, the RF linac 1 alternately generates X-rays having two different energy levels. The X-rays each interact with and penetrate through the same inspected object 7, and then detected by the detector 8. The detection results of the detector 8 are analyzed by the computer 13 to obtain the radiation images of the inspected object and to further distinguish the material attributes of the inspected object.
As shown in
Therefore, based on the trigger signals, the RF linac 1 can alternately generate X-rays of two different energy spectra in which different energy levels predominate respectively. Since the spectrum of the X-rays generated by the accelerator is wide, energy spectrum modulation is needed to further increase the proportion of X-rays with desired energy levels in the spectrum. Considering the energy levels of the X-rays generated by the RF linac 1, various materials can be utilized to perform energy spectrum modulation, thereby obtaining the energy spectra most suitable for material discrimination.
In addition, since the energy distribution domains vary in the energy spectra of the X-rays, the materials suitable for energy spectrum modulation differ. For example, when the lower limit of the principal domain of an X-ray beam's energy distribution is higher than a threshold value (e.g., ˜3 MeV) of higher energy level, a low Z material, such as B, C, polyethylene and any other hydrogen-rich organic material, should be chosen for energy spectrum modulation of this X-ray beam.
Meanwhile, in order to absorb the scatter component of low energy level in the rays, it is preferred to additionally utilize a thin high Z material for energy spectrum modulation after using a thick low Z material for energy spectrum modulation. When the lower limit of the principal domain of an X-ray beam's energy distribution is higher than a threshold value (e.g., ˜300 keV) of lower energy level, a high Z material, such as Pb, W, U, etc., should be chosen for energy spectrum modulation of this X-ray beam; a medium Z material such as Cu can also be chosen.
Here, the first energy spectrum modulation part 202, which is made of high Z material and coupled to the rotation axis 201, is used for energy spectrum modulation of the low-energy rays. As shown in
The second energy spectrum modulation part 203, which is made of low Z material such as compound material, for example polyethylene plus Pb, and formed into one or several vanes, is used for energy spectrum modulation of the high-energy ray. As shown in
To implement energy spectrum modulation, the vanes rotate around the axis at a preset frequency, and the position detector generates a trigger signal as a synchronization signal when detecting that the vanes rotate to a fixed position. The signal is sent to the synchronization control part 4 and the control part 9 via line 3 and 4 respectively, and the RF linac 1 and the detector 8 are made synchronized with the energy spectrum modulation apparatus 2 under the control of the synchronization control part 4 and the control part 9 respectively.
In this way, it can be ensured that the rays having a high-energy spectrum all interact with the material of the vanes, i.e., they all undergo-the modulation by the second energy spectrum modulation part 203, while all the rays of a low-energy spectrum are subjected to the absorption by the material on the axis, i.e., they all undergo the modulation by the first energy spectrum modulation part 202.
As above described, the material of the first energy spectrum modulation part 202 can be a high Z material such as Pb, W, U, etc., which is selected as the material for energy spectrum modulation of X-rays; a medium Z material such as Cu can also be selected. On the contrary, the material of the second energy spectrum modulation part 203 can be a low Z material such as B, C, polyethylene and any other hydrogen-rich organic material, which is selected as the material for energy spectrum modulation of X-rays. As a result of the modulation, the energy spectra 2P of high- and low-energy rays are obtained, where the energy spectra of two different energy levels are taken sufficiently apart from each other.
Further,
The optimized rays having both high and low energy levels, which are obtained after the modulation by the energy spectrum modulation apparatus 2, pass through the first and second collimators 6A and 6B and then interact with the inspected object 7. As shown in
The data signals outputted from the detector 8 is sent to the material discrimination and image processing part 13 via line 12. As described above, the detection values by the detector 8 are a detection value HEL for high energy level and a detection value LEL for low energy level. The obtained detection values HEL and LEL can be substituted into classification functions to determine the effective atomic number range of the material in the inspected object, thereby determining the material attributes.
Here, the classification functions are acquired as follows: using the rays with two energy levels from the dual-energy system to scan an atomic-number-known material, such as polyethylene standing for organic matter, Al for light metal, Fe for inorganic matter and Pb for heavy metal, etc., with the material's mass thickness varying, and thus obtaining a series of collected values; calculating two function values from the signals for high and low energy levels collected each time, for example, calculating In(HEL/HEL0) from the signals for high or low energy level, and calculating a*{In(LEL/LEL0)−In(HEL/HEL0)} from the signals for high energy level, where a is a coefficient and HEL0 and LEL0 each are predetermined reference detection values; then obtaining the fitting functions of the material based on the statistical values of the above two function values, as shown in
Then, the classification curves are obtained from the fitting functions by use of statistical methods such as K-means or leader clustering, vector machine, etc. For example, computing the statistical variance of the fitting function value, and then displacing the fitting, curve by the corresponding variance according to the optimum classification criterion as required. In discriminating an unknown material, the classification function values for the detection values are computed from the two function values of the detection values. Then the computed values are compared with the predetermined classification function values to obtain the effective atomic number range of the material and to further determine the material attributes of the object.
As shown in
Then, in Step S120, the above-mentioned energy spectrum modulation apparatus 2 is utilized to modulate the X-rays having different energy spectra. For example, both controlled by the synchronization signal, the first energy spectrum modulation part 202 modulates the first X-ray while the second energy spectrum modulation part 203 modulates the second X-ray.
Next, in Step S130, after passing through the first and second collimators 6A and 6B, the modulated X-rays radiate and interact with the inspected object 7.
In Step S140, the detector 8 collects data for high and low energy levels based on the synchronization signal from the control system 9. Here, the detector 8 can change the multiple of its amplifying gain to change its dynamic range, thereby obtaining with higher accuracy the signal values after the interaction between the dual-energy rays and the object.
In Step S150, the imaging signals for high and low energy levels are sent to the material discrimination and image processing part 13, in which it is judged that whether the sent signal is imaging signal for high energy level or for low energy level.
The imaging signals for high and low energy levels are processed in Steps S160 and S170, respectively.
In Step S180, the classification function values for the detection values are computed from the two function values for high and low energy levels. Then the computed values are compared with the predetermined classification function values to obtain the effective atomic number range of the material and to further determine the material attributes of the object.
In Step S190, in order to obtain a clear image of the inspected object, a number of images obtained after the X-rays having different energy levels scan the inspected object can be combined to acquire a scan image of better quality.
It is well known that the penetration factor of high-energy rays is strong, and the detection data can be obtained with a high accuracy after the rays penetrate through an object of large mass thickness, therefore, a clear gray-scale image can be acquired for the object of large mass thickness. However, when high-energy rays penetrate through an object of small mass thickness, the obtained gray-scale image is blurred and the detail information tends to be lost. Fortunately, the above disadvantage can be compensated by the gray-scale image obtained after low-energy rays penetrate through the object.
In Steps S191 and S192, the material attributes of the inspected object are determined, for example, whether the mass thickness of the inspected object 7 is thick or thin. Here, the approximate range of the mass thickness of the object is judged from the attenuation of the rays, that is, when the attenuation is great, for example, the detection value is less than a predetermined threshold value, the mass thickness of the material is referred as large; when the attenuation is little, for example, the detection value is more than a predetermined threshold value, the mass thickness of the material is referred as small.
In Step S193, for the material of a small mass thickness, a smaller weighting factor, such as 30%, is given to the data for the high energy level, and a bigger weighting factor, such as 70%, is given to the data for the low energy level.
In Step S194, for the material of a large mass thickness, a bigger weighting factor, such as 70%, is given to the data for the high energy level, and a smaller weighting factor, such as 30%, is given to the data for the low energy level.
Then, in Step S195, the images for the high and low energy levels are synthesized using the above weighting factors to acquire the final clear image.
Therefore, the present invention proposes that the detection values obtained after the X-rays of different energy levels interact with the object are compared with the corresponding predetermined threshold values, and different weighting factors are given to the data for the high and low energy levels, thereby obtaining the gray-scale information of the finally synthesized image.
Although the images, which are detected after the rays interact with objects of various mass thickness, have different image characteristics, with the processing of the above method, even if the mass thickness of objects varies greatly, a clear gray image of the material can be acquired in the object scanning.
The above-mentioned is only the specific embodiments of the present invention, while the scope of the present invention is not limited to it. Any modification or substitution, which is obvious to the skilled in the art within the technical range disclosed in the present invention, should be included in the scope of the present invention, which is thus defined by the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 200610011945.9 | May 2006 | CN | national |
