From Wilhelm Conrad Röntgen's first snapshot of his wife's hand in 1895, to the security scanner that blows Arnold Schwarzenegger's cover in Total Recall, the use of X-rays to image dense objects has been part of common lore. Franz Pfeiffer and colleagues (Nature Phys. doi:10.1038/nphys265; 2006) now realign the popular view. They use X-rays to generate high-contrast images not only of bone, but also of the soft tissues that surround them. The approach could readily be used to improve the diagnostic power of existing medical-imaging equipment.
Conventional medical X-ray imaging uses the fact that the harder and denser the body tissues are, the more radiation they absorb, and the more contrast they produce on X-ray films. This makes it easy to distinguish bones and other dense bodies, such as tumours, from surrounding tissues. But discerning details of softer tissues from only the contrast in absorption is difficult.
When an X-ray passes through tissue, however, it is not just absorbed: its phase is changed too. And this phase shift is more sensitive to variations in the composition of soft-tissue structures than is absorption. But until now, extracting information about phase has required interferometric reconstruction techniques and bathing the target object in the ultra-high-intensity radiation of a synchrotron particle accelerator.
Pfeiffer et al. use a sequence of phase-contrast gratings to manipulate the relative phases of the X-rays that illuminate and subsequently emerge from an object. They can thus generate phase-contrast images — for example this 50-mm×50-mm picture of an angelfish — using commercial X-ray sources at much lower intensity, and cost, than has previously been possible. The authors note that, as well as improving the detail in X-ray images, their approach could be adapted for use with other low-intensity radiation sources, such as neutrons and ions.