SuperParamagnetic Relaxometry (SPMR) has been extensively studied in physics and is based on a simple and measurable attribute of magnetic nanoparticles – i.e. superparamagnetic nanoparticles do not behave magnetically until they are exposed to a magnetic field for a brief time and then the particle “relaxes” after the field is turned off.
By measuring the magnitude, or size, of the magnetic field emitted by the nanoparticles and the timing, i.e. how long after exposure to the magnetic field it takes for the particle to relax, the nanoparticles can be located when bound to cells.
Free particles release a low magnetic field very quickly – less than a millisecond after they have been magnetized (known as Brownian relaxation).
Bound particles release a larger magnetic field over a longer period, e.g. seconds (known as Néel relaxation).