XPCi
New technique enables transfer of X-ray phase contrast imaging to clinical practice
By Professor Robert D Speller, Head of the Radiation Physics Group, University College London, and Dr Alessandro Olivo, of the Medical Physics & Bioengineering Dept. University College London
The impressive image improvements provided by XPCi (b) compared to the standard image quality of conventional X-ray absorption imaging (a). This sort of improvement, up to now accessible only to synchrotron radiation scientists, could soon be available in clinical environments
- lung imaging, in which the technique showed the potential to spot small lesions with conventional planar imaging without having to rely on expensive (both in terms of cost and patient dose) CT scans
- vascular imaging/coronary angiography, in which the potential to image blood vessels without contrast agents was demonstrated
- bone imaging, where minimal details on the bone trabecular structure are easily and effectively depicted due to the substantially increased sensitivity
- and many others, to include improved resolution and lesion detectability in liver imaging, kidney imaging, etc. Moreover, refraction/interference effects are less subject to decreasing with increasing X-ray energy that absorption effects: as a consequence, images could be acquired at higher X-ray energies, which could translate into dose reductions also of one order of magnitude.
Despite being probably the ideal X-ray imaging technique, the problem so far with XPCi was that its use seemed to be restricted to synchrotron radiation environments. All early implementations of the technique seemed in fact to require levels of spatial coherence (i.e. small focal spot plus large source-to-sample distance) and monochromaticity not available with state-of-the-art clinical sources. Although pilot experiments with synchrotron radiation like the one on mammography currently underway in Trieste have a high scientific significance, a real world-scale impact would be achieved only by developing a relatively small-sized, cost effective prototype. This clearly requires taking XPCi out of synchrotron environments and into laboratory practice.
Researchers at University College London, after having demonstrated that polychromatic radiation can provide the same level of phase contrast image quality of its monochromatic counterpart, have developed a new technique based on the use of coded apertures, which makes all advantages of XPCi achievable with conventional sources.
The new technique was fully modelled through a computer simulation, a small imaging prototype was realised, and the pilot experiments carried out provided results perfectly matching the simulated ones. Such experimentation confirmed beyond doubt that the advantages of XPCi demonstrated by synchrotron radiation studies can now be achieved by means of conventional X-ray sources.
A system based on the new technique would therefore be based around conventional, state-of-the-art sources and detectors. As a consequence, its cost would not differ substantially from radiography units currently in use.
The main difference would lay in the introduction of the coded-aperture arrays, which consist of extremely thin (20-30mm) gold layers deposited on graphite substrates. The cost of such a device is currently in the range of a few thousand Euros, but could be reduced by at least one order of magnitude if the devices were mass-produced. Alignment stages to achieve the correct positioning of such devices would be the only further addition to the system, meaning that its practical realisation would thus be very cost-effective.
Contact: aolivo@medphys.ucl.ac.uk
This article was published on 03/01/2008