X-radiography

Two characteristics of the x-rays issued by synchrotrons markedly improve x-ray contrast.  First, the continuous energetic spectrum permits working with two parts of the absorption threshold of a marker atom and, via subtraction of the two images, the selective visualization of the marker (threshold x-ray).  Developed for coronary angiography, this technique has never been used as a substitute for a conventional examination, even though it renders the examination non-invasive.  There is currently a renewal of interest in the operational imagery of various organs, notably the brain and lungs.  X-ray may be used in a tomographic fashion, with resolution ranging from several hundred micrometers for the observation of entire organs to 0.5 micrometers (microtomography) for the examination of samples, leading, for example, to spectacular images of cerebral vascularization (figure 1) or trabecular bone structure.  

The partial coherence of a monochromatic synchrotron beam is the basis for the recent development of two techniques to strengthen contrast in interface between organs.  The first is D.E.I. (Diffraction Enhanced Imaging), a technique that consists of placing the detector far from the x-rayed object in order to permit the creation of interference between the beams, which are redirected at slightly different levels at the level of the interfaces.  A clinical study of D.E.I. mammography is currently being conducted at the Elettra synchrotron facility in Trieste.  The second technique, R.E.I. (Refraction Enhanced Imaging), consists of using the same phenomenon, but this time with an acute analysis of the beams redirected at the interfaces with a crystal analyzer.  This technique may make up for the lack of sensitivity of M.R.I. for the early detection of arthrosis in its reversible stage.  The advantages of these two techniques in comparison with conventional x-rays are shown in figure 1.

 
Figure 1 : Volume renderings of a rat brain cortical sample injected with barium (600mg/ml), obtained at 20 keV in absorption mode with a voxel size equal to 1.4 micrometre. Maximum intensity projections have been performed for volumes with a size equal to    x = 1.5 mm, y = 1.5 mm, z = 1 mm. (a) projection in the x-y plane of a sample extracted from the fronto-central cortex, (b) x-z projection for a sample from the frontal region.

References
[1] A.S. Popel, A.R. Pries, and D.W. Laaf, J. Neuro. Meth, 111, 911-913, (1998); R. Weissleder and U. Mahmood, Radiology, 219, 316-333, (2001).
[2] H. Elleaume, A.M.. Charvet, S. Corde, F. Estève and J.F. Le Bas, Physics in Medecine & Biology, 47, 3369-3385, (2002).
Principal publication and Authors
F. Plouraboué (a), P. Cloetens (b), C. Fonta (c), A. Steyer (d), F. Lauwers (e) and J-P Marc-Vergnes (e), submitted to J. Microsc. (2003). (a) IMFT, UMR 5502, Toulouse (France) (b) ESRF, Grenoble, (France) (c) CERCO, UMR 5549, Toulouse, (France) (d) Université de Paris I, (France) (e) INSERM U455, Toulouse, (France)
 

Figure 2 : Radiographies de doigt humain  (a) par technique conventionnelle (b) par DEI, (c) par REI. Crédit : Rob Lewis