New Phase-Contrast Microscopy
The Switzerland-based Paul Scherrer Institute (PSI), a leader in the field of imaging technology has introduced a new phase-contrast microscopy that enhances the sensitivity and contrast of classical X-ray images. Imaging techniques turn objects visually inside out, allowing ever greater precision - for instance in medical diagnosis. They also contribute to a better understanding of the mechanisms of certain diseases, like Alzheimer's or osteoporosis. Further applications occur in materials research, where imaging processes are decisive factors in achieving results that ultimately – as with medical progress – benefit society.
In conventional X-ray imaging, contrast is obtained through the differences in the absorption cross-section of the constituents of the object. The technique yields excellent results where highly absorbing structures such as bones are embedded in a matrix of relatively weakly absorbing material, for example the surrounding tissue of the human body. However, in cases where different forms of tissue with similar absorption cross-sections are under investigation (for example, mammography or angiography), the X-ray absorption contrast is relatively poor. Consequently, differentiating pathologic from non-pathologic tissue in an absorption radiograph obtained with a current hospital-based X-ray system remains practically impossible for certain tissue compositions.
Enhanced contrast enables the X-ray dose to be significantly reduced, which is particularly relevant to mammography techniques in screening for breast cancer. Phase-contrast microscopy is readily adaptable to existing X-ray equipment and could, therefore, trigger a major improvement in future X-ray diagnostic techniques.
As a majority of X-ray or neutron radiographic applications are based on the attenuation of the radiation inside the object, the imaging of the phase shift of the object can provide new and complementary information. In the case of X-rays, phase sensitive imaging yields an increased contrast for biological samples and thus is of interest for medical applications. Neutron phase measurements, on the other hand, have a long and distinguished history in the exploration of the fundamental properties of quantum mechanics. Phase-contrast imaging relies on refraction, or changes in the angular trajectory of X-rays or neutrons. Just as light rays bend when they enter water from air, X-rays or neutrons deflect as they travel through objects of varying densities.
Another aspect of medical technology currently at the top of PSI’s research programme is X-ray microtomography, a process that provides a detailed image of the interior of a sample. Three-dimensional images with a resolution of one thousandth of a millimeter (1 micrometer) can currently be produced within minutes.
A prerequisite for methods aiming for clinical applications is that they should work with standard and commercially available X-ray generators, and not only with highly brilliant and partially coherent X-rays from a syn-chrotron. In this particular aspect the grating interferometer is superior to the existing phase-sensitive techniques, because it can be used at low-brilliance sources.
The conventional X-ray transmission image is
shown in Fig. a, while Fig. b contains an image of the corresponding phase contrast signal. The skeleton of the fish and other highly absorbing structures, such as the calcified ear stones (otoliths) are clearly visible in the conventional radio-graph (Fig. a and e). However, small differences in the density of the soft tissue, e.g., the fine structure of the tail fin, eye, and the region around the otoliths are hardly visible in the absorption image (Fig. c, g). The corresponding phase contrast image, however, reveals these details (Fig. d, f, h).
Top quality 3D images
PSI’s equipment takes snapshots of aluminum alloys, ceramics, and prehistoric embryos, as well as bones affected by osteoporosis. PSI's two neutron radiography instruments, NEUTRA and ICON, are basically no more than large-scale cameras. But they have special powers – they can see through objects without destroying them. So can X-ray devices; but the difference is that neutrons can do this with heavy metals like lead or uranium. And they have other advantages, too. For examining finely structured organic substances (and water) the neutron beam is definitely the instrument of choice.
