Affiliations: [a] Department of Computer Science, University College London, London, UK
| [b] Department of Security and Crime Science, University College London, London, UK
| [c]
Rapiscan Systems Ltd., Stoke-On-Trent, Staffordshire, UK
| [d] Rapiscan Systems, Torrance, California, USA
Correspondence:
[*]
Corresponding author: Lewis D. Griffin, Department of Computer Science, University College London, 66-72 Gower Street, London WC1E 6EA, UK. Tel.: +44 0 20 3108 7107; E-mail: l.griffin@cs.ucl.ac.uk.
Abstract: BACKGROUND: Large-scale transmission radiography scanners are used to image vehicles and cargo containers. Acquired images are inspected for threats by a human operator or a computer algorithm. To make accurate detections, it is important that image values are precise. However, due to the scale (∼5 m tall) of such systems, they can be mechanically unstable, causing the imaging array to wobble during a scan. This leads to an effective loss of precision in the captured image. OBJECTIVE: We consider the measurement of wobble and amelioration of the consequent loss of image precision. METHODS: Following our previous work, we use Beam Position Detectors (BPDs) to measure the cross-sectional profile of the X-ray beam, allowing for estimation, and thus correction, of wobble. We propose: (i) a model of image formation with a wobbling detector array; (ii) a method of wobble correction derived from this model; (iii) methods for calibrating sensor sensitivities and relative offsets; (iv) a Random Regression Forest based method for instantaneous estimation of detector wobble; and (v) using these estimates to apply corrections to captured images of difficult scenes. RESULTS: We show that these methods are able to correct for 87% of image error due wobble, and when applied to difficult images, a significant visible improvement in the intensity-windowed image quality is observed. CONCLUSIONS: The method improves the precision of wobble affected images, which should help improve detection of threats and the identification of different materials in the image.
