Magnetic-Particle-Discrimination Method Using Difference of Relaxation Time for Magnetic Particle Imaging

Magnetic particle imaging (MPI) is an imaging modality that directly detects the nonlinear responses of magnetic nanoparticles (MNPs). Spatial encoding is achieved by saturating the magnetic moment of MNPs almost everywhere except in a special point called the field-free region in which a magnetic field vanishes. Recently, MPI sensitivity was improved using a field-free line (FFL) in which a field-free region was formed as a line. An MPI with an FFL device was developed using a neodymium magnet and an iron yoke to image objects with a small amount of MNPs, such as in biological systems. We have been developing MPI equipment for detecting amyloid-β, a causative agent of Alzheimer's disease. We attached amyloid-β probes to nanoparticles. In our development, we discriminated between magnetic particles that are bound to biological tissue from those that are suspended in the brain. We focused on the differences in relaxation times due to the change in the hydrodynamic diameter between the bound and unbound particles. Because the bound particles have a larger apparent particle size and do not rotate when an ac magnetic field is applied, the relaxation time is different from the unbound particles. Since the differences in the responses to the ac magnetic field appear as relaxation times, we investigated a particle-discrimination method using these differences and studied the magnetization response of MNPs using our developed MPI device.