Monotone Signal Localization Using Magnetoresistive Sensor Array for Low-Field Magnetic Particle Imaging

Magnetic particle imaging (MPI) demands high sensitivity to be operable under low excitation fields as alternative countermeasure for specific absorption rate (SAR) and magnetostimulation effects. Owing to capability of magnetoresistive (MR) sensors to unidirectionally detect sub-pT signal, we built a prototype of brain MPI scanner by using MR sensor array. The arrays were 13 x 13 and 5 x 13 matrices with 15 mm sensor pitch and installed orthogonally relative to a 0.2 m drive coil. To preliminarily evaluate 10 kHz-signal localization, we used a current loop embedded with 20-turn coils. We then carefully measured pT field at 234 sensor coordinates for 1 mA ac current fed to the loop. The resulting field contour was numerically interpolated to identify the actual loop orientation. We found that the loop three-dimensionally rotated along xyz axes with (1.0 degrees, -1.5 degrees, 2.0 degrees) angles from the expected position. To further estimate spatial resolution of the array, we measured magnetic fields from two adjacent point sources (PSs) of small coils. The corresponding signal separation appeared dependent on the perpendicular distance to the nearest sensor. This fine signal localization of MR sensor array highlights its potential for tracing magnetic nanoparticles in low-field MPI system.