An advance computed tomography (CT) system was recently built for the U.S. Army Armament Research, Development and Engineering Center, Picatinny Arsenal, NJ, for the inspection of munitions. The system is a charged coupled device (CCD) camera based CT system designated with the name “experimental Imaging Media” (XIM). The design incorporated shielding for use up to 4MeV x-ray photons and integrated two separate cameras into one single field of view (FOV). Other major distinguishing characteristics include its processing functions to digitally piece the two cameras together, use of advanced artifact reduction principles, performing reconstruction simultaneously during acquisition, and its development in accurate beam hardening corrections through digital means.
The overall setup of the system, as shown, depicts the internal layout of the cameras, shielding, scintillation screen, and rotational fixture. The general layout is comparative to a common 16-bit CCD camera radiographic imaging system. The x-ray photons pass through the inspection piece and impinge onto a scintillation/phosphor screen where the energy is converted into visible light. From there, the light is redirected off a series of mirrors that allow the cameras to be out of the direct line of sight of the main radiation beam. The light is then focused through the camera lens and into a cooled CCD chip.
At this point, the information is converted and digitized into a radiographic image. This process is repeated for multiple planes/projections as the part is rotated, acquired, and rotated again until enough information is obtained to achieve a volumetric file with a specified resolution. Figure 1a is the internal drawing of the XIM, Figure 1b is a photograph of the layout of the XIM CT system, and Figure 1c is a photograph of the shielded camera housing inside the XIM CT system.
The radiation source matched with the XIM is a Varian M3A dual energy linear accelerator (LINAC) with a 1.2MeV and 3MeV setting. For the purposes of this research, all of the imaging was performed at 3MeV. This was mainly a result of a lower dose rate produced by the accelerator, a loss in conversion efficiency, lower phosphorescence of the conversion screen, and an overall reduction in the signal received by the cameras at 1.2MeV. This issue could have been adjusted for by reducing the source to detector distance (SDD), but to sustain consistent spatial relationships, a fixed SDD was used.
The overall techniques used within this discussion were performed using 360-degree rotation, a SDD of 162.4 in. (4.1 m), a fixed spot size of roughly 2 mm in diameter, and a source to object distance of 152.7 in. (3.9 m). The useable FOV, collimation, exposure/pulse settings, and the positioning of the inspection piece in relation to the floor were optimized as needed in relation to the part under investigation. The achieved spatial resolution ranged from 0.033 in. (0.883 mm) to 0.011 in. (0.279 mm). The number of pixels for each camera was more than 4008 length by 2672 width. The limitation on the resolution was primarily a factor of the final file size of the volume reconstruction and not a general limitation of the imaging portion of the system. The images were acquired with 540 to 720 projections and varied with the use-able FOV to sustain a consistent volume size to prevent processing issues related to the use of a central processing unit based computer.
This work was done by Stephan C. Zuber of Army ARDEC, ESIC Quality Engineering & System Assurance Directorate (QESA). ARL-0202
This Brief includes a Technical Support Package (TSP).
HIGH ENERGY COMPUTED TOMOGRAPHIC INSPECTION OF MUNITIONS
(reference ARL-0202) is currently available for download from the TSP library.
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