Imaging of Magnetic Nanoparticles Using Small TMR Sensor With an Excitation-Compensation Scheme

Magnetic nanoparticle imaging (MPI) is an emerging non-invasive technique with potential applications in bio-medical diagnostic and material science. In this paper, we propose a new method for MPI using tunnel magnetoresistance (TMR) sensor with an area of dimensions 0.45 mm*0.45 mm. The small size of the sensor promotes high level of integration and fine spatial resolution for magnetic field measurements. A solenoid excitation coil carrying alternating current is used to magnetize the magnetic nanoparticles (MNPs). We used commercial MNPs solution named synomag ®-D with iron concentration 10 mg/ml as sample. The response of the MNPs is measured by the TMR sensor during a 2D raster scan resulting in a magnetic image. In order to eliminate the negative effect of the strong background field from the excitation coil, a compensation coil which is located close to the TMR sensor is utilized to subtract the background signal in situ during measurements. Comparing the experimental results with and without the compensation scheme shows that the setup with the compensation coil cancels 95% of the background signal and increases the signal to noise ratio (SNR) from 7 dB to 10 dB. Mainly considering the tradeoff of 1/${f}$ noise and the MNPs response to the excitation frequency, the operating frequencies of 135 Hz, 235 Hz and 335 Hz were investigated in this study. Experimental results show that the system is capable of imaging $5~\mu \text{L}$ MNPs solution with a 5 mm lift-off from the sensor to the top surface of the MNPs sample. Furthermore, an `L' shape formed by 6 holes with $1~\mu \text{L}$ MNPs solution in each is imaged by the system. In comparison, it is very challenging for conventional MPI with coils as pickup sensors to image $10~\mu \text{g}$ irons in $1~\mu \text{L}$ MNPs solution at the low operating frequency. This is because TMR sensor has much better sensitivity than coils in the low frequency range. This work demonstrates the feasibility of using small TMR sensor with an excitation-compensation scheme for MPI. The advantages of high sensitivity, fine spatial resolution and low cost of the probe make it a promising alternative for future bio-medical research to measure the 3-D location and concentration of magnetic nanoparticles.