The frequency-selective nature of quadrupolar relaxation enhancement offers a high potential for designing smart molecular probes for the usage as novel MRI contrast agents by cycling the main magnetic field. Their validation and application requires a fast field-cycling MRI system. In this work, we present the first implementation and validation of such a system at the clinical field strength of 3T. The complete FFC-MRI setup was successfully validated by R1 dispersion imaging with dispersive iron oxide magnetic nanoparticles, thus providing a ready-to-use hardware setup for the future investigation of new compounds.
The FFC-MRI was implemented on a clinical 3T MRI scanner (Skyra, Siemens Healthineers, Germany) using a shielded B0 insert coil custom-built by Resonance Research Inc. (RRI, USA). The offset coil is driven by a gradient power amplifier (GPA) from International Electric Company (IECO, Finland) with a field efficiency of 0.668 mT/A. This allows for an offset field ∆B0 of up to ± 100mT with possible ramp times of 1ms.
For system validation, previously published iron oxide magnetic nanoparticles (IOMNPs) dispersed in hexane and with different shapes (spherical and cubic) were selected as FFC-MRI contrast agent7 because of a suspected R1 dispersion at 3T. Samples containing three different concentrations (1mM, 0.5mM and 0.25 mM) for each shape and a non-dispersive reference sample (hexane only) were prepared (see figure 2a). Nuclear magnetic relaxation dispersion (NMRD) profiles were measured at room temperature (295K) to verify the R1 magnetic field dependence around the nominal B0 (2.89T) of our MRI system. The measurements were performed using a Spinmaster FFC 1T relaxometer (Stelar, Italy) and an external HTC-110 3T superconducting magnet equipped with a standalone PC-NMR console (Stelar, Italy) for 1H Larmor frequency range 10kHz-30MHz and 40Mhz-128MHz, respectively.
FFC-MRI images were acquired at three different field strengths (2.79T, 2.89T, 2.99T) using a modified saturation recovery spin echo sequence including B0 eddy current compensation8. After saturation preparation the B0 field was cycled to B0 ±100 mT within a rise time of 1ms for various evolution times Tevol (60, 100, 150, 300, 500, 800, 1500ms) to allow for relaxation at different field strengths prior image acquisition at the nominal field (TR/TE = 10000/15ms, matrix = 64x64, field of view = 40x40mm2, slice thickness = 5mm, bandwidth = 130Hz/Px). For each field strength, R1 was determined using a three parameter non-linear saturation recovery model in a region of interest (ROI) for each sample. Additionally, images at 2.79T and 2.99T with an isotropic in-plane resolution of 0.2x0.2mm2 were acquired with an evolution time of 150ms. The images were normalized to account for different equilibrium magnetizations and effective field-shifts6. The dreMR image was than obtained by magnitude subtraction of the normalized images.
This project receives financial support by the
European Commission in the frame of the H2020 Programme (FET-open) under grant
agreement 665172.
This article is partially based upon work from COST Action CA15209, supported
by COST (European
Cooperation in Science and Technology).
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