The particles are coated with antibodies to a particular virus, so they will form clumps that should be visible on conventional body scans if that virus is present.
The team working on the technology, from the Harvard Medical School's Center for Molecular Imaging Research in Charlestown, Massachusetts, have already managed to detect viruses in body fluids and tissue samples. They hope to be able to detect viruses in patients' bodies within a couple of years.
Much of the technology has already been tested in humans, so the scientists are confident that it will be safe.
Scans revealing where virus populations are - HIV, for example, tends to concentrate in the lymph nodes - could help doctors improve treatments. And a scan could reveal whether viruses used in gene therapy to ferry new DNA into patients have actually reached the parts of the body they are intended for- and in sufficient numbers to do any good (Journal of the American Chemical Society, DOI: 10.1021/ ja036409g).
Usually, viruses can only be detected indirectly, by capturing and amplifying viral DNA through the PCR technique, which takes about two hours. "It's cumbersome, takes time, gives you false positives and negatives, and only detects fragments of the virus," says Manuel Perez, head of the team developing the new technique - which gives an answer in half the time.
It relies on particles 50 nanometres wide. They have a core of iron oxide and a coating of dextran, a sugar to which antibodies stick easily. A top coat of antibodies to the virus under investigation is added, and the particles are ready for use.
In lab tests, the nanoparticles have been added to samples of body fluids; in patients, they would simply be injected.
If live viruses are present, they stick to the antibodies on the nanoparticles, forming a large cluster of particles which can then be detected through magnetic resonance imaging (MRI) or nuclear magnetic resonance (NMR) scans. "The antibodies recognise specific proteins on the surface of the virus, and it makes the nanoparticles clump with the virus," explains Perez.
But the scanners do not detect the nanoparticles directly. First, the particles' iron oxide cores are turned into magnets by a scanner's strong magnetic field, which also forces the nuclei of surrounding water molecules into different energy states.
A radio-frequency signal can then be used to make the water nuclei return to their original state. The time it takes to return to that state, the "relaxation time", is a measure of the density of molecules at that point. The magnetised particles lengthen the relaxation time of water molecules next to them, which makes the viral clusters visible in the MRI or NMR scans.
The Harvard team says its system has successfully detected the herpes simplex virus, which causes cold sores, and an adenovirus, which causes colds, in blood samples.
Perez says the magnetic nanoparticle technique could rapidly become a reality, as all the components it uses are already available commercially. Similar particles have already been shown to be safe in the body in earlier experiments on the detection of secondary prostate cancer tumours in lymph nodes.
Written by Andy Coghlan
New Scientist issue: 23rd August 2003
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