Material Decomposed Cargo Imaging with Dual Energy Megavoltage Radiography

1 Material Decomposed Cargo Imaging with Dual Energy Megavoltage Radiography a) Polad M. Shikhaliev Abstract Radiography imaging has well-known applications in medicine, industry, security, and scientific research. Megavoltage (MV) radiography has important applications in imaging large cargos for detecting illicit materials. A useful feature of MV radiography is the possibility of decomposing and quantifying materials with different atomic numbers. This can be achieved by imaging cargo at two different x-ray energies, or dual energy (DE) radiography. The performance of both single energy and DE radiography depends on beam energy, beam filtration, radiation dose, object size, and object content. The purpose of this work was to perform comprehensive qualitative and quantitative investigations of the image quality in MV radiography depending on the above parameters. A digital phantom was designed including Fe background with thicknesses of 2cm, 6cm, and 18cm, and materials samples of Polyethylene, Fe, Pb, and U. The single energy images were generated at x-ray beam energies 3.5MV, 6MV, and 9MV. The DE material decomposed images were generated using interlaced low and high energy beams 3.5/6MV and 6/9MV. The x-rays beams were filtered by low-Z (Polyethylene) and high-Z (Pb) filters with variable thicknesses. The radiation output of the accelerator was kept constant for all beam energies. The image quality metrics was signal-to-noise ratio (SNR) of the particular sample over a particular background. It was found that the SNR depends on the above parameters in a complex way, but can be optimized by selecting a particular set of parameters. For some imaging setups increased filter thicknesses, while strongly absorbed the beams, increased the SNR of material decomposed images. Beam hardening due to polyenergetic x-ray spectra resulted in material decomposition errors, but this could be addressed using region of interest decomposition. It was shown that it is not feasible to separate the materials with close atomic numbers using the DE method. Particularly, Pb and U were difficult to decompose, at least at the dose levels allowed by radiation source and safety requirements. Key words: megavoltage radiography, cargo imaging, dual energy, material decomposition a) Corresponding author: 4949 Stumberg Ln Baton Rouge, LA 70816 [email protected] 1. Introduction inspection in security imaging. DE MV (DEMV) radiography has been investigated for cargo imaging [1-8] and several Projection x-ray imaging (radiography) has well-known systems are now commercially available [9-13]. applications in medicine, industry, security, and scientific There are several conceptually different methods for research. In each of these areas certain requirements exist that acquiring DE radiography data. These include using (1) x-ray determine the data acquisition methods, detection systems, x-ray source and detector where the x-ray pulses are dynamically energies, radiation dose, and imaging time. One important switched (interlaced) between two energies, (2) x-ray source and application of projection radiography is imaging cargos with a two-layer detector where the detector layers can separate low large sizes for detecting illicit materials such as explosives, and high energy x-ray photons, and (3) x-ray source with an narcotics, nuclear materials, high density radiation shields, etc. energy-resolved photon counting detector that can split the x-ray These cargos may have sizes of up to a few meters in cross spectrum into several parts and acquire multi-energy x-ray sections and average densities of about thousand kilograms per images in a single acquisition. Each of these methods has its cubic meter. To image such a large object high energy x-ray advantages and limitations that determine its feasibility for beams with high penetration capabilities are required. X-rays cargo radiography. For DEMV cargo radiography, the method with mega-electronvolt energies are appropriate for this using a source with interlaced DE x-ray pulses has been shown purpose, and the corresponding imaging technique is called as feasible [3, 4, 6, 8], and corresponding x-ray sources are mega-voltage (MV) radiography. commercially available [12, 14]. Another useful feature of projection x-ray imaging is the There are several difficulties when DEMV cargo radiography possibility of separating and quantifying materials with different is acquired using x-ray beams with interlaced energies. One effective atomic numbers. In order to separate two materials limitation is that the low and high energy x-rays are filtered by from each other, two x-ray images of the object should be the same filter because both rays are emitted from the same acquired at two different energies in a single image acquisition. source. Such filtration may not be optimal for separation of the This imaging method is called dual energy (DE) radiography. energies of two beams. This problem is further complicated As in the case of single energy radiography, DE radiography because the x-ray attenuations of the high-Z (atomic number) also has various applications such as bone density and contrast materials do not change monotonically as opposed to low-Z measurements in medical imaging, and airport baggage materials due to higher pair production cross section. The image 2 quality of single energy radiography and material selective DE radiography depends on the types and thicknesses of the filter materials, background materials, and materials to be imaged. In decomposed images despite the fact that the thicker filter this work, we have performed comprehensive simulation studies absorbed more x-rays and decreased photon statistics. of single energy and DEMV radiography using (1)different Feasibility of decomposing high-Z materials such as Pb and U beam filter materials including low- and high-Z materials, by DE radiography was also tested. (2)different background thicknesses, (3)different material samples for imaging, (4)single energy radiography with 2. Methods and materials different energy settings, and (5) DE radiography with different energy settings. The signal to noise ratio (SNR) of the images 2.1. Imaging Phantom was used as image quality metrics while the radiation output of the source was kept constant for all image acquisitions. The As noticed above, the cargos may have various sizes, complex nature of the dependence of SNR on beam energies, material contents, area densities, and spatial distributions of the sample types, filter types, and background thicknesses was included materials. Therefore, it is difficult to identify a single demonstrated quantitatively and qualitatively. In general, high-Z phantom setup for performing comprehensive quantitative and and low-Z filters materials showed similar performances, while qualitative studies of DEMV cargo radiography. We designed a the higher beam energies were more preferable, particularly for digital phantom which meets many of the representative larger background thicknesses. Interestingly, for some imaging parameters of real objects including sizes, materials of interests, tasks, increasing filter thickness improved SNR in DE material and area densities, as shown in the figure 1. Figure 1. Digital Phantom used in the simulation study. It includes Fe background with thicknesses of 2cm, 6cm, and 18cm, respectively. The sample materials, including Pb, Fe, and Polyethylene with thicknesses of 1.2cm, 2.5cm, and 12cm, respectively, were placed at the top of Fe background with different thicknesses. The phantom includes a Fe background material with a the designed phantom covers probable background material type 50x70cm rectangular cross section and stepwise changing and thickness ranges as well as the range of the material samples thicknesses of 2cm, 6cm, and 18cm. The samples of the three to be imaged. materials, including Pb, Fe, and Polyethylene with thicknesses In one simulation, we substituted the Polyethylene sample of 1.2cm, 2.5cm, and 12cm, respectively, were placed over the with uranium (U) with 0.72cm thickness to evaluate possibility areas of the background material with different thicknesses. The of decomposing two high-Z materials Pb and U from each other. sample thicknesses were chosen such that they have similar area densities. Fe was chosen as the background material because it 2.2. X-ray beams is one of the most widely used industrial materials, including also the material of the cargo container walls, and can represent The x-ray energies used in cargo radiography should be an appropriate background material. Its thickness changes from sufficiently high to penetrate large cargos with sizes of minimal 2cm, which simulates cargo wall thickness, to 18cm, approximately 2.3x2.3m2 in cross-section [15]. The contents of which is similar to higher area densities of the cargos. The the cargos can be highly non-uniform in terms of material types sample materials Polyethylene, Fe, and Pb represent low-Z, and filled volume fractions. It is known that approximately 80% medium-Z, and high-Z materials, respectively, covering of cargos exhibit average area densities of up to 160g/cm2 which materials of interest to be imaged in cargo radiography. Thus, corresponds to approximately 20cm thickness of steel [3, 4, 6]. 3 the x-ray beam parameters to allow sufficient penetration of the beam filters fabricated typically from high-Z materials such as x-rays through the objects. In general, the system should be able W or Pb. to image the sample over the background when the area density In the current study, we used beam energies of 3.5MV, 6MV, of the sample is 20% of the area density of the background [16]. and 9MV for singe-energy radiography, and pairs of The radiation dose applied to the cargo should also be limited to 3.5MV/6MV and 6MV/9MV for DEMV radiography. The x-ray 500mRem effective dose (which is equivalent to In general, beams were filtered with two types of filters, low-Z imaging larger thicknesses of up to 40cm steel equivalent would (Polyethylene) and high-Z (Pb) filters because low-Z and high-Z be desirable.

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