Stem Cells Entering Heart Can Be Tracked With Nano-Hitchhikers

The promise of repairing damaged hearts through regenerative medicine -- infusing stem cells into the heart in the hope that these cells will replace worn out or damaged tissue -- has yet to meet with clinical success. But a highly sensitive visualization technique developed by Stanford University School of Medicine scientists may help speed that promise's realization.

Human and animal trials in which stem cells were injected into cardiac tissue to treat severe heart attacks or substantial heart failure have largely yielded poor results, said Sam Gambhir, PhD, MD, senior author of the study and professor and chair of radiology. "We're arguing that the failure is at least partly due to faulty initial placement," he said. "You can use ultrasound to visualize the needle through which you deliver stem cells to the heart. But once those cells leave the needle, you've lost track of them."

The new technique employs a trick that marks stem cells so they can be tracked by standard ultrasound as they're squeezed out of the needle, allowing their more precise guidance to the spot they're intended to go, and then monitored by magnetic-resonance imaging for weeks afterward.

To make this possible, the Gambhir lab designed and produced a specialized imaging agent in the form of nanoparticles whose diameters clustered in the vicinity of just below one-third of a micron -- less than one-three-thousandth the width of a human hair, or one-thirtieth the diameter of a red blood cell. The acoustical characteristics of the nanoparticles' chief constituent, silica, allowed them to be visualized by ultrasound; they were also doped with the rare-earth element gadolinium, an MRI contrast agent.