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Impact case study

X-ray Imaging for Security Screening Applications

Unit(s) of assessment: General Engineering

Research theme: Safety and Security of Citizens and Society

School: School of Science and Technology


(Header image courtesy of 3DX-Ray)

A major problem for aviation security is the total lack of depth and material composition information in simple X-ray scans. The screener searches routinely for threats such as improvised explosive devices and weapons amongst everyday objects. The superposition of object features along the direction of the X-ray beam (or depth) is exploited to hide or camouflage threats. Serious misinterpretations and mischaracterizations of objects have caused increased false alarms and reduced throughput.

Pioneering research at NTU on 3D X-ray imaging has produced a breakthrough in multiple-view imaging, which has had significant and worldwide impact on the security industry and public safety. It is a prime tool in detecting weapons and explosives concealed in luggage. The worldwide total number of the travelling public that have been screened and kept safe by NTU-enabled X-ray systems is estimated at 9.8 billion. Transportation Security Administration (TSA) officers have used NTU-enabled X-ray systems to find and confiscate a total of 20,884 guns at US checkpoints.

NTU research is central to Advanced Technology Systems (through the end of 2019 it is estimated that 6,446 units containing NTU-enabled X-ray systems are deployed for aviation security worldwide). The total revenue generated from NTU-enabled X-ray based system sales is around $1 billion. The AIM listed NTU spinoff, 3DX-RAY Ltd, manufactures a portfolio of advanced X-ray screening systems for the security, industrial inspection; nuclear, pharmaceutical and medical device markets.

NTU’s research excellence and exceptional impact in the field of X-ray security imaging was recognised by a Queen’s Anniversary Award for Higher Education 2015 (Safety and Security of Citizens). Evans won the Times Higher Education Award in 2016 (outstanding contribution to innovation and technology), and The Institute of Physics Dennis Gabor Medal and Prize 2017 (industrial application of physics), and appointment as a Royal Society Wolfson Fellow in 2018-2023.

Research background

To address fundamental problems in aviation security, NTU researchers have developed three distinct, compact and cost-effective advanced X-ray technologies.

Divergent Beam Technology

This technology was pioneered by Professor Paul Evans and his team at NTU in collaboration with the Home Office and funded by EPSRC, to employ a single stationary X-ray source producing divergent fan beams incident upon a folded array of dual-energy X-ray detectors. This method enables well-balanced colour stereoscopic image pairs to be collected simultaneously to provide information on depth. Prior to this innovation, two separate X-ray sources would have been required to provide physically intersecting (convergent) beams. In comparison, NTU’s technology provides a highly compact and cost-effective solution. It reduces significantly the physical footprint/length, weight/radiological shielding and hardware complexity of an X-ray scanner requiring more than one view, which is the fundamental requirement for volumetric analysis. Critically, NTU’s technology also offered the same throughput as standard single view ‘workhorse’ machines.

Multi-View Technology

This was invented and developed by Evans and his team at NTU to leverage the advantages of divergent beam technology but with far greater depth fidelity and angular coverage, without the requirement for stereo viewing mechanisms. A series of highly correlated perspective views are collected during a single linear scan using multiple divergent fan-beams and folded dual-energy detector arrays. The resultant images realise a number of different 2D and 3D imaging modes. A pilot version was demonstrated to the Home Office. The technique exploited kinetic depth effect (KDE), which enables the observer to work out the shapes of objects with remarkable accuracy during a rotation. This was the first demonstration of KDE using 1D detectors.

Building on this initial success a unique full-scale multi-view dual-energy (colour) scanner was built to NTU’s design in collaboration with Professor Richard Lacey’s (former Chief Scientist CBRNE chemical, biological, radiological, nuclear and explosives) team at the Home Office and EPSRC in collaboration with the US Dept of Homeland Security (DHS), UK Home Office, and 3DX-RAY Ltd. The successful results from the NTU/Home Office collaboration led to the award of a six-year rolling research programme by US Dept of Homeland Security to NTU to evaluate the performance of kinetic depth X-ray imaging for luggage screening. To support this programme the experimental scanner was extensively reengineered by the NTU team. Comparative studies were undertaken of various full-scale (luggage) imaging modes, which included single view, stereo, and multi-view/KDEX incorporating dynamic 3-view, 7-view or more (divergent beam) views. The team’s work culminated in a Broad Agency Announcement (BAA) by the US Dept of Homeland Security to US industry to build prototype scanners. Evans liaised with nine different US security manufacturers under the BAA, which provided funding for prototypes and field trials e.g. Astrophysics Inc (USA) incorporated the NTU Multi-View technology into a new range of scanners.

Castellated (dual-energy) Detector Technology

This was invented and developed by Evans and his team at NTU to reduce the cost of dual-energy detectors providing colour-coded materials discrimination for Multi-View (or single view) scanners including computed tomography (CT) scanners. Assisted by Chan he demonstrated a 50% reduction in the total number of detector elements without affecting image resolution. This was achieved by creating a patchwork (1D or 2D) of detector elements to replace the industry standard configuration of a double layer, with low-energy elements positioned in front of the high-energy elements. The work formed the basis for a range of products now commercially available through 3D X-RAY Ltd. Funded by the EPSRC £50k and in collaboration with the Home Office this work was featured in a “Crime and security special report” Journal of EPSRC.


Commercial adoption of new innovative imaging technologies that substantially enhance the safety and security of the travelling public

NTU-enabled systems have an estimated installed base of over 6,400 machines and include the Advanced Multi-View Technology (AT) scanner class, used in cabin baggage screening systems, hold baggage screening systems, and staff screening X-ray systems. In the US alone (where 100% of passengers are screened using NTU-enabled technologies), 5.56 billion passengers have been protected by NTU enabled technologies from 2014-2019. From 2014-19, a total of 9.8 billion passengers worldwide are estimated to have been screened and protected by NTU-enabled X-rays. In the US alone, the 100% deployment of NTU-enabled X-ray systems resulted in the discovery and confiscation of 20,884 guns.

Through end of 2019 a total of 11,761 total units, of which an estimated 55% - 6446 units - contain NTU-enabled X-ray systems are deployed for aviation security worldwide.The estimated total sales revenue of these systems is roughly $4.9 billion. From August 2013-2019, the total revenue generated from NTU-enabled X-ray based system sales is at minimum $1.1 billion (based on available company press releases). In addition to annual sales, service and support revenue (at an industry average of 7% of system costs) is roughly $33 million.

3DX-RAY has applied NTU’s technology to additional commercial security sectors and industrial inspection markets. The company is the main trading subsidiary of Image Scan Holdings plc, a technology group, which was listed on the London Stock Exchange Market AIM in 2002. The group was formed to exploit novel tools and technologies invented and developed by the University.


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  • Hon H.W.; *Evans, J.P.O.; Multiple-view line-scan imaging; (2002); IEE PROCEEDINGS - OPTOELECTRONICS; Vol. 149, (2), pp. 45-50; DOI: 10.1049/ip-opt:20020231
  • Evans, J.P.O., Liu, Y., Chan, J.W.; Downes, D.; View synthesis for depth from motion 3D X-ray imaging; (2006); PATTERN RECOGNITION LETTERS; Vol. 27 (15), pp. 1863-1873.
  • Chan, J.W.; *Evans, J.P.O.; Yen, S.Y.; Monteith, A. (Home Office); Wire transfer function analysis for castellated dual-energy X-ray detectors; (2004); OSA - APPLIED OPTICS; Vol. 43 p6413;