Liquid crystals could slash cost of X-rays

A LOW-COST way of recording X-ray images electronically could make digital X-rays scanners affordable in the developing world. The method can produce high-resolution digital X-rays for one-tenth of the usual cost.

Digital X-ray machines are prized because the images they produce are simple to analyse, manipulate and store. Most of them work by using a layer of amorphous selenium to convert the X-rays into electric charge. This "charge image" is then recorded using an array of transistors and other electronic components, akin to those used in some digital cameras.

However, the machines are expensive because these arrays have to be large: X-rays cannot be easily focused, so X-ray machines work by recording the shadow of an object rather than a focused image. That means the recording medium, be it an electronic imager or conventional X-ray film, must be at least the same size as the object being scanned. Digitally imaging a human lung, for example, requires an array up to 40 centimetres square with 10 million pixels, which costs as much as $200,000. This puts them well out of reach of most hospitals in the developing world.

A new device developed by John Rowlands and colleagues at the University of Toronto in Ontario, Canada, could slash the cost of high-resolution digital X-rays. This low-cost alternative, which Rowlands calls the "X-ray light valve" consists of a layer of liquid crystal - which is opaque or transparent depending on whether an electric charge is present - covered with a layer of amorphous selenium. These layers are sandwiched between a pair of electrodes which generate an electric field across them.

When an X-ray is taken, the rays that hit the selenium layer generate a charge which is drawn towards the liquid crystal by the electric field. This makes the liquid crystal transparent at those locations. The overall pattern of transparency and opacity can be read off the liquid crystal layer using a light-based digital scanner (see diagram) and presented as a digital image. "We used an off-the-shelf light scanner and the X-ray images looked beautiful," says Rowlands.

After recording the image, the liquid crystal is reset by an electric field that restores its opacity.

Robert Street, an X-ray imaging expert at the Palo Alto Research Center in California, is intrigued. He reckons the X-ray light valve should be relatively cheap to fabricate.

Richard Lanza, an expert in X-ray imaging technology at the Massachusetts Institute of Technology, says the idea is significant because it separates the read-out system from the X-ray mechanism. "Two-thirds of the people in the world will never have a chest X-ray to diagnose a life-threatening illness such as tuberculosis," he says, so cutting the cost of digital X-ray machines in this way could make a big difference.