X-ray or gamma ray systems or devices – Specific application – Diffraction – reflection – or scattering analysis
Reexamination Certificate
2001-05-21
2003-04-29
Dunn, Drew A. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Diffraction, reflection, or scattering analysis
C378S057000, C378S086000, C378S087000, C378S088000, C378S089000
Reexamination Certificate
active
06556653
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to X-ray imaging systems, and more particularly the present invention relates to composition, density and geometry imaging of an object by measuring and analyzing incident and scattered radiation passing through that object.
BACKGROUND OF THE INVENTION
The following publications relate to the subject of x-ray imaging systems and methods. Various teachings from these publications are cited herein to facilitate the description of the present invention.
Glasstone S. and Sesonske, A., Nuclear Reactor Engineering, Chapter 2, Chapman & Hall, New York, 1994.
Zieglier, C. A., Bird, L. L. and Chel{dot over (e)}k, “X-Ray Raleigh Scattering Method for Analysis of Heavy Atoms in Low Z Media”, Analytical Chemistry, Vol. 13, pp. 1794-1798, 1956.
Bray, D. E. and Stanley, R. K., Nondestructive Evaluation, Chapter 20, McGraw-Hill, New York, 1989.
Battista, J. J. and Bronskill, M. J., “Compton scatter imaging of transverse sections: an overall appraisal and evaluation for radiotherapy planning”, Physics in Medicine and Biology, Vol. 26, pp. 81-99, 1981.
Lale, P. G., “The examination of internal tissues, using gamma-ray scatter with a possible extension to megavoltage radiography”, Physics in Medicine and Biology., Vol. 4, pp. 159-167, 1959.
Hussein, E. M. A., “Compton Scatter Imaging Systems”, in Bioinstrumentation: Research, Development and Applications, Butterworths Publ., Stoneham, M A, D. L. Wise, Ed., Chapter 35, pp. 1053-1086, 1990.
Harding, G. and Kosanetzky, J, “Scattered X-ray Beam Nondestructive Testing”, Nuclear Instruments and Methods, Vol. A280, pp. 517-528,1989.
Prettyman, T. H., Gardner, R. P., Russ, J. C. and Verghese, K., “A Combined Transmission and Scattering Tomographic Approach to Composition and Density Imaging”, Applied Radiation and Isotopes, Vol. 44, pp. 1327-1341, 1993.
Arendtsz, N., V. and Hussein E. M. A., “Energy-spectral Scatter Imaging. Part I: Theory and Mathematics”, IEEE Transactions on Nuclear Science, Vol. 42, pp. 2155-2165, 1995.
Arendtsz, N., V. and Hussein E. M. A., “Energy-spectral Scatter Imaging. Part II: Experiments”, IEEE Transactions on Nuclear Science, Vol. 42, pp. 2166-2172, 1995.
MCNP 4C, Monte Carlo N-Particle Transport Code System, RSICC Code Package CCC-700, Oak Ridge National Laboratory.
Conventional x-ray radiographic systems, commonly used in airports to detect weapons, sharp objects and the likes, are not suited for the detection of plastic explosives. This is due to the fact that such systems typically utilize low-energy photons, where the photoelectric effect (photon absorption) dominates. The probability of photoelectric absorption per atom can be roughly expressed as follows as taught by Glasstone et al.:
&tgr;≅constant
Z
n
/E
3
(1)
where Z is the atomic number of the medium, E is the photon energy, and the exponent n varies between 3 for low-energy photons to 5 for high-energy rays. Therefore, the low Z-number of nitrogen-based explosives makes it difficult to distinguish them from other common materials, with the photoelectric effect on which conventional radiography relies. Alternative techniques were therefore developed.
If the Compton scattering modality of photons is allowed to come into play, then additional information can be brought in to assist in detecting explosives. The probability of Compton scattering per atom, &sgr; depends on the number of electrons available as scattering targets and therefore increases linearly with Z, and can be expressed as follows, as taught by Glasstone et al.
&sgr;=constant
Z/E
(2)
Therefore, Compton scattering provides density-related information. The electron density is directly proportional to Z, and the mass density is proportional to the electron density (given that the ratio of the atomic-number to the mass-number is equal to about one-half for most elements, except hydrogen) as taught by Zieglier et al.
Compton scattering can provide such mass-density information, which if used in conjunction with the Z-number information given by the photoelectric effect can help in identifying nitrogen-based explosives; that are characterized by having higher mass density than most common organic materials. A dual-energy (high and low) radiographic system can be used for this purpose; with the higher energy providing electron-density information and the lower energy strongly reflecting the effect of the Z-number, according to equations (1) and (2). This is the concept of the E-scan system. Alternatively, scattering can be monitored, typically back scattering, to obtain density information.
Another useful photon-interaction modality is the coherent Rayleigh scattering process, where photons are deflected by a small angle without losing energy. The probability of this reaction is however small and is proportional to Z
3
making the reaction more sensitive to metals, as taught by Zieglier et al.
X-ray fluorescence depends on the production of x-rays characteristic of the target atom. However the technique is best suited for high Z atoms, and even then the measured flux is low, as taught by Zieglier et al.
The remaining photon interaction of significance is pair production, where a high energy photon disintegrates into an electron-positron pair in the presence of the electromagnetic field of the atom. Once again, this is an interaction that dominates at high Z number and high photon energy, as taught by Glasstone et al. This leaves the photoelectric effect and Compton scattering as the most suitable photon-interaction modalities for use in imaging.
Radiography techniques are disadvantaged by the fact that they provide integrated information, along the chord of radiation transmission, thus mixing the attributes of overlying objects. This can lead to masking and smearing out of information. Computed tomography (CT) solves this problem by unfolding the radiation-attenuation measurements into pixel-specific information at individual slice of the object. While solving the masking problem of radiography, the fact still remains that CT determines the attenuation coefficient of the material present in the pixel. Therefore, at the commonly used X-ray operating range of 80-200 kV, keeping in mind that the average X-ray energy in keV is equal to about one-third the peak energy which corresponds to the operating voltage in kV as taught by Bray et al., the photoelectric effect dominates in CT, as can be seen by comparing the equations (1) and (2). Therefore, in essence one obtains physical information that is identical in nature to that obtained by basic radiography, although de-convoluted into individual pixels. This comes at considerable cost due to the involvement of a complex mechanical scanning mechanism and a sophisticated numerical image reconstruction process.
The question now is whether a material can be uniquely identified from the value of its attenuation coefficient, or CT number. This question has bewildered medical physicists who plan for radiotherapy (at high photon-energy where Compton-scattering is dominant) from CT images (produced by low-energy X-rays where the photoelectric effect prevails). With the known nature of the body, some empirical formulations are devised, relating CT numbers to the electron density of tissue, muscles and bones, as taught by Battista et al. Given however the wide variety of materials that may be present in a passenger luggage, CT numbers may not necessarily be uniquely related to density, thus resulting in ambiguous and perhaps false indications. Like the case with conventional radiography, more information is needed to uniquely identify an explosive material from CT images. Such information can come from a suspicious object geometry, or other non-technical supplementary information. Alternatively, one can expect CT to progress in the same fashion as conventional radiography to provide additional physical information.
Progress of CT as an explosive detection system (EDS) requires that it provides both density and Z-number information. This can be achieved, similar to E-scan, by using a dua
Dunn Drew A.
Ho Allen C.
Theriault Mario
University of New Brunswick
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