Osteoporosis causes bones to become much weaker than normal, and more liable to fracture. According to worldwide figures from the World Health Organization, the condition causes some nine million fractures every year, more than 4.5 million of which are in the Americas and Europe.

Doctors normally diagnose osteoporosis with help from dual-energy X-ray absorptiometry (DEXA). In this technique, a patient is exposed to X-rays at two different energies. By comparing the resultant images, absorption from soft tissue can be subtracted out, leaving an image of the patient's bones that can be studied to measure bone density.

However, density is not necessarily the best parameter to judge a bone's health and strength. It cannot be used to predict where a fracture will occur, nor can it accurately show what effect new treatments are having. As a result, scientists have been exploring other ways to measure bone quality, using, for instance, the ratio of calcium to phosphorous (Ca/P).

Relative amounts of these elements are known to be crucial for correct bone metabolism. In the late 1990s, for instance, a two-dimensional gamma-ray study on animal bones found a decrease in the Ca/P ratio when osteoporosis had been induced. Then in 2005, researchers used synchrotrons to perform CT on osteoporotic animal bones. The results showed that a reduced Ca/P ratio was linked with bone loss.

The latest study – performed by medical physicists Robert Speller and Andria Hadjipanteli at University College London and colleagues – shows that such analyses are possible in three dimensions while using facilities that are available in hospitals. The researchers use a conventional CT scanner, but operate it similarly to normal DEXA so that the image subtraction reveals the different chemical elements present in the bone. "This has not been done before," says Speller. "To the best of my knowledge, these [measurements] are the first designed to show the spatial distribution [of Ca/P]."

Speller, Hadjipanteli and colleagues tested their CT technique on three healthy and three osteoporotic rabbit tibia (lower leg) bones. In the healthy bones, the average Ca/P ratios were within the range 1.67–1.68, whereas in the osteoporotic bones the values were within 1.64–1.65, confirming that osteoporosis reduces the ratio. Three-dimensional colour maps of the bones showed exactly where the bones were affected.

Speller thinks that the next step is to repeat the study with the bones placed under load. Already, he and his colleagues have designed a cell that can apply a force to bones while they are in a CT scanner, so that they can see how the regions with a lower Ca/P ratio are affected. But such measurements will still be of animal, not human, bones. "We are some way from using the technique in a hospital – maybe three or four years away," he said.

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