Calculated Néel relaxation time plotted against parameter of MNPs of 20 nm diameter. The inset shows the Néel relaxation time vs magnetic core diameter in zero field . Only MNPs of sizes 19–21 nm produce observable signal decay (assumptions: and ).
(a) Schematic illustration of the MRXI setup. MNPs are located at different positions (A, B, and C) inside the ROI. (b) MRX curves of MNPs located at different positions inside the ROI. Only MNPs in the ZFV (position A) produce measurable signal decay in the time window as shown in the inset. The magnetic field distribution vanishes in the center of the imaging volume and increases toward the edges.
On-axis distribution profile of the magnetic field near ZFV for three different selection field gradients . The intersection between the threshold line and the magnetic field gradients determine the size of ZFV or the spatial resolution (, , and , indicated by the vertical arrows). The inset shows the effect of magnetic field gradient on the spatial resolution .
Room temperature Néel relaxation curves of the magnetic particles cluster of size (iron content ) measured after applying magnetizing pulse ( and ). The Néel relaxation curve in a finite magnetic field is faster than that of zero field.
Magnetic images of the MNP cluster measured at 20 mm above the cluster by a 12 mm SQUID gradiometer. The cluster size is 4 mm and it is located at ; ; 0. (a) shows the stray field magnetic image and (b) shows the MRXI of the cluster.
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