Low-Concentration Magnetic Particle Spectroscopy Using Gradiometric Receive Coil-Coupled Magnetoresistive Sensor

To realize clinical magnetic particle imaging (MPI), scanner sensitivity must be designed to recognize magnetic nanotracers within safe dosage range. As zero-dimensional MPI scanner, magnetic particle spectroscopy (MPS) is often used to initially characterize complex dynamic of the tracers by applying a sinusoidal magnetic field with high amplitude. However, the resulting magnetization signal can be very small to dominate electromagnetism of cellular matrices with tracer concentration below
$2 \mu_{\mathrm{Fe}} \mathrm{mL}^{-1}$
. Here, we demonstrated an enhanced MPS sensitivity by coupling typical gradiometric receive coil with a magnetoresistive (MR) sensor under
$25 \mathrm{mT} / \mu_0$
at 10 kHz. This magnetometric system could distinguish the particle signal of Resovist(Registered trademark.) sample from its 0.1 mL liquid medium while diluting iron concentration up to 70 ngFe.For this case, we confirmed that the field-induced background signal from the water-based medium became significant. Depending on the ion concentration, such polar solvent exhibited both diamagnetism and eddy current effect. Even though its harmonic components might disappear technically, magnetization signal of low-concentration sample had phase and magnitude differences relative to those of the medium at fundamental frequency. Owing to its high sensitivity, MPS system using MR sensor can be potentially implemented for liquid-phase biosensing, in addition to MPI technology.