Sharp set

An adaptation of conventional X-ray technology could offer detailed, sharp images of different tissues using low doses of radiation.

X-rays have been a standard part of the armoury of medical diagnosis for more than a century. However, the technique is limited — good for looking at bones but, without a contrast agent, less useful for differentiating between the various soft tissues of the body.

In conventional X-rays, the radiation is shone through the body, with a photographic film on the other side. The tissues absorb the X-rays to various degrees — bones absorb most of the radiation, while muscle, fat, tendons and so on let most pass straight through.

But rather than considering how the waves are absorbed as they pass through the body, researchers from the Paul Scherrer Institute in Villigen, Switzerland are looking at how much they are refracted. This phenomenon is caused by the waves slowing down as they pass through a material, and this slowing manifests itself as a phase shift — a difference in the relative positions of the peaks and troughs in the waves before and after they have passed through the body. This is relatively easy to detect.

The technique, known as phase-shift contrast, is a common method for improving detail in optical and transmission electron microscopy, but it has not been used for medical X-rays before. This, explained project leader Franz Pfeiffer, is because conventional phase-shift contrast requires high-intensity radiation sources that are not suitable for medical applications. To get around this, the team adapted conventional apparatus.

Phase-contrast imaging uses two diffraction gratings normally —one in front of the source, to ensure that the waves are delivered in coherent (in-phase) 'bundles', and the second after the object, to detect the phase shift.

Pfeiffer uses a standard X-ray machine with three gratings, made from silicon and gold. The first grating is, as usual, between the source and the patient, but the second and third are between the patient and the X-ray detector.

These convert the shift in phase into changes in the intensity of the radiation. The waves that have not been absorbed then form a standard X-ray image on the detector film, and this image is combined electronically with the phase contrast data to form the high- resolution image.

Pfeiffer's team tested the system on cardinal tetras, the tiny iridescent fish often seen in aquariums. A normal X-ray shows the skeleton of the fish and some blurred details of the eye and ear structures, but the phase-contrasted image showed clear images of the different areas of the eye, fine details of the structure of the fins, and the soft tissue inside the ear.

Pfeiffer believes this could be useful for medical imaging where tissues that have similar abilities to absorb X-rays need to be examined, especially in the detection of soft-tissue cancer and in imaging the blood vessels around the heart. And as the technique does not need high-intensity X-rays, it can reduce the potentially harmful radiation doses received by the patient.

Even better, the system can use standard X-ray sources and, as diffraction gratings are easy and relatively cheap to make, it should be easy to implement.