An injection of microscopic iron particles could help scans reveal the hiding places of viruses in the body.
MRI scans could reveal viruses
The particles could allow doctors to see if gene therapy is working - or perhaps detect "reservoirs" of HIV in patients.
Researchers at Harvard Medical School in Massachusetts, created the particles using a tiny core of iron oxide, coated with antibodies which lock onto specific viruses.
If live viruses are present, they will stick onto the particles - which can be detected on MRI scans.
So far, the nanoparticles have been added to samples of human body fluids and proved successful in locating viruses.
However, the Harvard team does not believe there is any great obstacle to using them in humans - iron is an element found in plentiful quantities in humans.
Dr Manuel Perez, who led the project, told New Scientist magazine that the technique would work far better than current hit-and-miss methods of finding viruses which look for traces of viral DNA.
He said: "It's cumbersome, takes time, gives you false positives and negatives and only detects fragments of the virus."
He said that the technique had managed to find the herpes simplex virus and a cold virus.
One of the prime reasons for developing it is to help gene therapy scientists improve the chances of success.
Gene therapy boost
Gene therapy aims to alter the genetic makeup of cells to correct gene-related medical conditions.
Most researchers believe that the best way to get genes into the cell is using modified viruses, which hijack the cell's genetic machinery when they infect it in order to replicate.
However, there are considerable problems in getting the viruses to infect enough of the right type of cells in the right locations in the body to make a significant difference.
The Harvard technique would allow scientists to check exactly where their modified viruses are heading.
Dr Jimmy Bell, a researcher at the Medical Research Council Clinical Sciences Centre at Imperial College, London, is developing similar imaging techniques designed to help doctors track individual cells.
He said that the field offered "exciting potential".
"This is all quite feasible - perhaps not tomorrow, but certainly human trials should start before too long.
"It will offer doctors the chance to measure objectively the effect of the therapy they are giving."