READ MORE | AP NewsMagnetic tracking of immune cells could one day offer a better way to monitor organ transplants for rejection, researchers report.

A research team led by Chien Ho at Carnegie Mellon University found that they could tag immune cells with iron oxide and then track the cells using magnetic resonance imaging. Accumulation of immune cells in a transplanted organ can indicate rejection.

Ho and colleagues studied mice that had been given heart transplants. Their findings are reported in Tuesday's issue of the journal Proceedings of the National Academy of Sciences.

Currently, heart transplant patients are given drugs to suppress their immune systems to prevent rejecting the new organ.

In addition, frequent heart biopsies are done in which a catheter is threaded into the heart to remove a piece of tissue to be examined for signs of rejection. These procedures are uncomfortable and costly. In addition, Ho said, they may miss problems because they check only a sample of the heart rather than the whole organ.

Ho's procedure involved immune cells called macrophages, which ingest foreign particles in the body. They tagged these cells with minute iron oxide particles and then injected them into mice that had received a heart transplant three days earlier.

Using MRI they were able to track the immune cells and observe the rejection process, Ho reported, noting that rejection progressed from the outside the heart to the inside.

Ho's team is now working with researchers at the University of Pittsburgh School of Medicine on repeating the work in larger a