Audrius Brazdeikis and colleagues used a SQUID (superconducting quantum interference device) to accurately measure how iron oxide magnetic nanoparticles relax in a finite magnetic field of a few hundred µT after the particles have been aligned by magnetic pulses of 2 mT that last 1 s. The technique exploits the fact that magnetic nanoparticles relax faster in a finite magnetic field than in no field at all. The researchers directly obtained an image of the magnetic nanoparticle tracers by mapping how their magnetism decays.

Although such a technique has been proposed before, it was limited to just 2D and had poor spatial resolution. This is not the case for Brazdeikis and colleagues' method, which does not depend on pick-up coil size or the distance between sample and sensor.

Lower dose of nanoparticles
Images obtained using the magnetic relaxation method are not affected by bioelectric activity in the heart, which makes it potentially useful for detecting and characterizing inflammatory atherosclerotic plaques. And beacuse no radiation is used, the dose of magnetic nanoparticles employed can be orders of magnitude below human tolerance levels.

"The technique is not only useful for generating direct images of magnetic nanoparticles by measuring the magnetic flux at fixed magnetic gradient after saturation, it is also capable of distinguishing between bound and unbound magnetic labels," lead author Subhasis Sarangi told "This means that it could be used for high-resolution direct imaging of molecular interactions in vivo by employing functionalized magnetic nanoparticles."

The team now hopes to improve its technique by parallel image acquisition using a pick-up coil array on the SQUID and advanced sensitivity encoding imaging techniques (SENSE). Here, imaging time can be reduced without significantly sacrificing the signal-to-noise ratio, explains Sarangi.

The work was reported in J. Appl. Phys.