Research by Professor Alessandro Olivo, Department of Medical Physics and Biomedical Engineering, UCL
Image formation in x-ray imaging relies on materials with different attenuation properties showing up differently subject to the amount of radiation passing through them. This approach is suboptimal when there are small differences in attenuation (because of similar characteristics in the “target” and its background or because the detail is very thin). This problem is well-known in medical imaging (since soft tissues have similar attenuation); but is encountered in all fields, including security inspections.
One way to overcome this limitation is to exploit the fact that x-rays are electromagnetic waves which change speed when moving through different materials. This leads to a wave front distortion, as parts of the wave front are “phase-shifted” with respect to others.
The physical quantity causing changes in x-ray speed is much larger than that driving attenuation effects. If properly exploited, this can lead to much stronger image contrast. This is known as X-ray Phase Contrast Imaging.
Efforts to get a detector to see these changes in speed have traditionally been extremely complex and extremely expensive, involving very specialized technology such as synchrotron radiation. There are only about 50 synchrotron facilities in the world; each costs >£100M, with the footprint of a football stadium. This does not suit the technique to everyday use in clinics or at airports.
Through a project funded in 2007 by the Innovative Research Call (IRC) in Explosives and Weapons Detection (a cross-government programme sponsored by a number of Departments and Agencies under the UK Government’s CONTEST strategy), and through an EPSRC-funded “Career Acceleration” fellowship, which was also part of the Global Uncertainties Programme, Professor Olivo’s team at UCL devised an approach that can overcome this problem and lead to enhanced threat detection.
The team observed that wave front distortions translate into changes in x-ray direction. This is effectively x-ray refraction just as, in visible light, a straw appears bent in a glass of water; but with x-rays, the deviations are measured in the micro-radian range: this is the equivalent to seeing a 1mm deflection from 1km away.
Systems to detect these miniscule shifts of angle are within reach. By placing a pair of apertured masks either side of the imaged object, with the two masks are slightly mismatched, the transmission of x-rays deviating in one direction in reduced, while increasing it for those going the other way. Small angular deviations are turned into significant intensity differences – even when a standard x-ray source is used.
Recently, they have observed a more powerful effect. If a beamlet hits a detail that is larger than itself, the entire beamlet is deviated; but if it strikes smaller details, multiple refraction occurs inside the beamlet, sending x-rays in different directions. These smaller events occur below the resolution limit of the imaging system, but a “global” effect (a broadening of the beamlet) can be observed and measured.
This is effectively a measure of the degree of inhomogeneity of the sample on the microscopic scale; for example, a perfectly homogenous sample would not produce any broadening. Preliminary evidence shows that this could help to distinguish between harmless and harmful materials (eg explosives), since it is unlikely for two materials to have the same microscopic structure even if they have similar attenuation characteristics.
A new project funded by the IRC in Explosives and Weapons Detection 2013 initiative (a Cross-Government programme sponsored by a number of Departments and Agencies under the UK Government’s CONTEST strategy, in partnership with the US Department of Homeland Security, Science and Technology Directorate) is developing a demonstrator system, with the UCL team working in collaboration with Nikon Metrology. It is envisaged that, following a successful demonstration, prototype systems will be developed and subsequently commercialised and marketed by Nikon.