A softer option for x-ray imaging

17 Jun 2002

X-rays can be used to produce three-dimensional images of soft body tissues, say researchers in Japan. Until now, x-rays have mostly been used to image hard tissue such as bone, which absorbs the rays more strongly than soft tissue. The new technique opens up the possibility of viewing tumors and other soft tissue diseases easily and safely.

The research group, led by Atsuchi Momose of the Hitachi Advanced Research Laboratory in Saitama, published pictures of the internal structure of a cancerous rabbit liver in the latest issue of Nature Medicine. Instead of using x-ray absorption, they analyzed changes in the phase of the waves as they passed through soft tissue. These changes in phase (the timing of peaks and troughs) are caused by variations in the refractive index of the tissue through which the x-rays pass, which in turn depends on its density.

A beam of x-rays which has all its waves in phase is split into two. One beam passes through the tissue sample and the other is a reference. The two are brought together to form an interference pattern. If the X-ray waves are in opposite phase--peaks aligned with troughs--they cancel each other out, and where they are in phase they reinforce each other. This produces a characteristic pattern. The sample is then rotated in the x-ray beam to measure density as seen from different angles. These data are combined by computer into a series of 2D images, or "slices" through the sample. Several of these slices are then built into a 3D image.