Sub-pT oscillatory magnetometric system using magnetoresistive sensor array for a low-field magnetic particle imaging

Implementing low ac excitation fields toward clinical magnetic particle imaging (MPI) system is critical to avoid magnetostimulation effects. However, low-field MPI scenario demands high sensitivity to probe the change in monotone magnetization response of magnetic nanoparticles. Here, we use an array of ultrasensitive MR sensors to detect sub-pT magnetic signal and obtain its spatial distribution. While each sensor is operated at 5 V, signal processing circuit rises its sensitivity to 20 mV pT−1 at 10 kHz with 0.25 pT noise level. In evaluating sensor performance, we initially measured a 10-kHz magnetic field from a 40-turns coil with 1 mm in diameter. MR sensor recognizes magnetic signal of mini coil to be linear with the coil input current. We then recorded the signals simultaneously from a 6 × 6 sensor channels to map the fields at 0.2 kHz. While placing mini coil 50 mm apart from the array, we obtained high-contrast field image showing coil position and its change in field polarity. For practical nanoparticle detection, we later used a 3 × 3 array configuration for the 10 kHz oscillatory magnetometry while compensating the MR sensor magnetically from the ac excitation field. Despite a 20-pT noise floor associated with the field compensation circuit, we were able to map stray fields of a 37-mgFe iron oxide nanoparticle sample distanced 100 mm from the array, under 12.4 µT/μ0 field amplitude. This result highlights usability of MR sensor array for low-field MPI system.