US researchers have found a way to non-invasively track the movements of cells around the body, a discovery which could help scientists to better understand how some cancers spreadand how the immune system works. Assaf Gilad, from Johns Hopkins in Baltimore, developed a way to entice cells to produce a natural contrast that could be picked up by MRI (magnetic resonance imaging), enabling thelocation of the cells to be pinpointed. To achieve this the researchers created an artificial "reporter gene" which, when inserted into a cell causes it to produce a hydrogen-rich protein (containing multiple lysine amino acid building blocks). This makes the cell visible to an MRI scanner,which works by zapping cells in a magnetic field with a pulse of radiowaves. This temporarily knocks hydrogen atoms off kilter, which the scanner detects. Using the technique as proof of principle, the researcherswere able to detect tumour cells that had been transplanted into animals brains. In the future the team hope to come up with an array of different tracers which can be simultaneously and independently tracked around thebody, non-invasively, with MRI.