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New tracer improves the tracking of cells during MRI

Scientists have developed a new, highly sensitive chemical probe that tags cells for detection by magnetic resonance imaging. The discovery, which could contribute to both research and the development of future therapies, is described in Nature Materials.Contrast materials, or contrast media, are commonly used to provide clearer results in magnetic resonance imaging (MRI).

Advanced techniques are now enabling observers not only to distinguish normal from abnormal conditions, but also to visualize cell populations in the body, including the in-vivo trafficking and behavior of immune or stem cells.

Now, scientists from the University of California-San Diego School of Medicine have produced a new, non-invasive technique to “see” and track cells in living persons that is stronger than previously used.

The work is expected to enhance the progress of treatments involving stem cells and immune cells, as it will give researchers a clear picture of how cells behave after being introduced to the body.

The research team was led by Eric T. Ahrens, PhD, professor of radiology, and Roger Tsien, PhD, professor of pharmacology, chemistry and biochemistry, who was part of a team that won the Nobel Prize in chemistry in 2008 for their work with fluorescent proteins.

In previous work, Tsien and colleagues have mentioned that effective fluorescent proteins should feature brightness and photostability, but they emphasized that they must also be non-toxic and not likely to disrupt the pH balance. Fluorine-19 and iron enhance MRI images

The team synthesized a new probe for labeling cells, using fluorine-19 (19F), the stable isotope of the element fluorine. The probe uses agents that are formulated as a “nanoemulsion.” These agents contain microscopic droplets of an inert fluorine-based agent.

The agent is taken up by cells of interest, and it can be detected by MRI. Using this technology, researchers can observe how the cells move.

Ahrens explains that 19F tracer agents produce positive-signal hot-spot images. They are easy to see, since tissues contain virtually no fluorine, which means that no background signal is produced.

Now, Ahrens’ team has made “a major leap in sensitivity,” by discovering how to dissolve and encapsulate metals inside the fluorine-based droplets. They have combined highly fluorinated nanoemulsions with the magnetic properties of metals to create a new imaging medium with a much stronger MRI signal than was previously possible.

In what they believe is the first attempt to use iron to enhance 19F MRI signals, the researchers have found that it is more effective than any other metal ion at enhancing the signal.

Iron is also cheaper and biologically more friendly than Gadolinium, which is also used to improve the clarity of MRI scans.

The authors believe that 19F MRI aided by iron will be a significant advance in tracking cells, benefiting a range of emerging therapeutic areas, including immunotherapy, stem cells and treatment of inflammation.