Eddy current-shielded x-space relaxometer for sensitive magnetic nanoparticle characterization

Abstract

The development of magnetic particle imaging (MPI) has created a need for optimized magnetic nanoparticles. Magnetic particle relaxometry is an excellent tool for characterizing potential tracers for MPI. In this paper, we describe the design and construction of a high-throughput tabletop relaxometer that is able to make sensitive measurements of MPI tracers without the need for a dedicated shield room.

FIG. 1.

High-level depiction of the transmit filter chain. A sinusoidal signal at the resonant frequency is generated by a National Instruments data acquisition system (DAQ) DAQ and fed to a power amplifier. The transmit filter chain consists of a matching inductor, common mode choke, and a differential low-pass filter. The final stage of the low-pass filter resonates with the transmit coil.

FIG. 2.

Optimized transmit filter layout. C_f are feedthrough capacitors, and C₁₋₃ are high-power Celem capacitors. L₁₋₂ were wound by hand using 10AWG insulated copper wire around PVC pipe elbows. R₁ is built in to the power amplifier.

FIG. 3.

Pictures of the completed relaxometer shield. ((a) and (b)) Picture of SOLIDWORKS model showing inserts that separate stages of the transmit and receive filter stages. ((c) and (d)) Complete shield including the cylindrical bore shield plus cap.

FIG. 4.

Results from COMSOL shielding simulation. The attenuation in both the receive chamber and the bore was predicted to be greater than 45 dB in the receive bandwidth. The difference in attenuation factors is due to the geometry of the bore.

FIG. 5.

Slices of the excitation field waveform and raw signal from the relaxometer.

FIG. 6.

(a) PSF from several dilutions of Chemicell 50 nm fluidMag nanoparticles. (b) PSFs normalized to each sample’s peak value show that PSF shape is fairly well preserved, though the detection limit is reached at 350 ng. (c) The MPI signal is linear with respect to iron concentration, as predicted by MPI theory. The noise floor is denoted by the dotted line.

FIG. 7.

Representative example of a PSF with error bars measured using a 250 μg sample of Chemicell 50 nm fluidMag nanoparticles. Four individual PSFs (red dashed lines) are plotted against their mean with error bars (black solid line). The error was ±0.002.

FIG. 8.

Comparison of PSFs of magnetite nanoparticles dispersed in toluene and embedded in a magnetic nanocomposite film.