Narrowband magnetic nanoparticle imaging using cooled pickup coil and gradient field

Abstract

Magnetic particle imaging (MPI) has been extensively studied for in-vivo biomedical diagnosis. We developed a narrowband MPI system utilizing third harmonic detection. The third harmonic signal from the magnetic nanoparticles (MNPs) was detected with a pickup coil cooled to 77 K, and its output was read out with a resonant circuit. The noise of the detection system was S 1=2 B ¼ 12 fT/Hz 1/2 at a signal frequency of 8.79 kHz. We also introduced the so-called gradient field with a field gradient of 0.3 T/m in order to improve the MPI spatial resolution. We first clarified the properties of MNPs, which provided the basis for MPI using the gradient field. Next, we measured the signal-field map generated from the MNPs when an excitation field with a root mean square value of 1.6 mT and frequency of 2.93 kHz was applied. Using a mathematical technique called singular value decomposition (SVD), we reconstructed an image of the MNP distribution from the measured map. We demonstrated the detection of MNP samples as small as 1 µg at a distance of 50 mm. The spatial resolution of the reconstructed MNP distribution was approximately 10 mm. These results will indicate the feasibility of the system for the application to breast cancer detection.

Relationship between harmonic spectra and coercive field of immobilized magnetic nanoparticles

“…The lines were calculated using Eqs. ( 5)- (8). The value of β = 1.35 was used for μ 0 H 0 = 14 and 21 mT, while β = 1.60 for μ 0 H 0 = 7 mT.…”

“…As shown above, the harmonic spectra when the m values are distributed in the sample can be evaluated using the empirical expression given by Eqs. ( 5)- (8). However, there is no theoretical basis for this empirical expression to date.…”

“…1) The MNPs are detected by measuring the magnetic fields from the MNPs on the surface of the patient's body. Harmonic-space MPI, [1][2][3] x-space MPI, 4,5) and narrowband MPI [6][7][8] measurement techniques have been developed. In harmonic-space MPI, the measurements are performed using the harmonic signals produced by the nonlinear magnetization curves of the MNPs.…”