Purpose

Neuroinflammatory autoimmune diseases such as multiple sclerosis involve an early recruitment of inflammatory immune cells from the periphery into the central nervous system [1]. Previously we studied immune cell infiltration into brains of mice with experimental autoimmune encephalomyelitis (EAE), the animal model of MS, using fluorine-19 (¹⁹F)-loaded nanoparticles (NPs). Nanoparticles are administered intravenously (i.v.), and are taken up by inflammatory cells during their migration from the systemic circulation into the brain [2]. We detected the ¹⁹F-labeled cells in vivo using a room temperature (RT) dual ¹⁹F and ¹H radio frequency (RF) volume resonator employing 3D imaging with isotropic spatial resolutions above 600µm to overcome ¹⁹F signal-to-noise ratio (SNR) constraints [2]. This limited the information on the precise location of ¹⁹F-labeled inflammatory cells in the brain. To overcome this, we show here the first use of a new ¹⁹F transceive cryogenic quadrature RF surface probe (CRP) at ultrahigh field to substantially boost SNR beyond that of standard state-of-the-art RT coils, while facilitating the acquisition of better spatially-resolved images within shorter scan times.

Methods

Experiments were performed on a 9.4T animal MR system (Bruker BioSpin, Germany). The transceive quadrature CRP tuned to 376 MHz was compared with the volume resonator (ID=16mm) used previously [2]. SNR gain estimation: Using a 15ml-phantom filled with 30% 2,2,2-trifluoroethanol (TFE) in water, we acquired an axial 2D-RARE image (TR=3000ms, TE=42ms, ETL=8, FOV=20mm, matrix=96, NEX=8). B₁-mapping: On the same phantom we acquired 3D-SE images (TR=6000ms, TE=4.5ms, FOV=28x16x16mm³, matrix=91x52x52, NEX=1) with nominal excitation flip angles (FA) of 60 and 120 degrees and calculated the actual FA using the double-angle method. SNR vs number of ¹⁹F atoms: NMR tubes (ID=4mm) were filled with different concentrations of ¹⁹F-loaded NPs (Z-average diameter=164nm), which were prepared using perfluoro-15-crown-5-ether (PFCE, Fluorochem, UK) [3]. Each tube was placed below the CRP surface leaving a 1mm gap. SNR was calculated in an axial 2D-RARE image (TR=3000ms, TE=21ms, ETL=8, FOV=20x20mm², matrix=96x96, NEX=1). High-resolution brain image: Animal experiments were carried out in accordance with the local Animal Welfare Department. To induce EAE, SJL/J mice were immunized with proteolipid protein peptide139-151, and assessed daily for neurological symptoms [4]. PFCE NPs (10µmol) were administrated daily to EAE mice i.v. (D5-D9 following immunization). Mice were sacrificed on D10, fixed in paraformaldehyde and prepared in 15-ml tubes for ex-vivo MRI. 3D-RARE protocols were used for ¹⁹F MRI (TR=800ms, TE=4.9ms, FOV=28x16x16mm³, matrix=182x52x52, ETL=26, NEX=384) and a ¹H reference image (TR=1300ms, TE=39ms, FOV=30x20x20mm³, matrix=192x128x128, ETL=16, NEX=3).

Results

We first compared the performance of the CRP with the RT coil by estimating SNR of TFE phantom images acquired with both coils. The ¹⁹F CRP SNR gain was calculated by dividing the SNR for several ROIs (at increasing distances to the CRP surface) by the SNR of a large central ROI in the RT coil image (Fig. 1). Up to a depth of 8.5 mm the CRP had a superior SNR to the RT coil. Peak SNR gain exceeded a factor of 6. To investigate the SNR achieved as a function of the number of ¹⁹F atoms for a fast 2D-RARE protocol, we performed several scans with the 19F CRP using phantoms with varying concentrations of PFCE NPs (25mM-200mM) and with varying slice thickness (400µm-2000µm). The smallest number of 19F atoms per voxel measured (5.2×10¹⁵) resulted in an SNR of 18 (Fig. 2). B₁-Mapping (Fig. 3) demonstrated the typical inhomogeneity of a surface coil array, very similar to those reported for ¹H cryoprobes [5]. The significant flip angle variation must be compensated for by B₁-correction in order to allow quantitative ¹⁹F imaging (Fig. 3). Using a 3D-RARE protocol as previously described [2], we next acquired ¹⁹F MR images of a fixed EAE mouse brain using the CRP. In these measurements we increased the spatial resolution from 400x400x400µm to 154x308x308µm. Preliminary results with this newly designed ¹⁹F CRP show a superior delineation of the areas of immune cell infiltration, especially within the myelinated regions of the cerebellum (Fig. 4) when compared to previous images obtained with the RT ¹⁹F volume resonator [2].

Conclusion

We carried out first tests and high spatially-resolved MR measurements to determine the performance and feasibility of using a quadrature ¹⁹F CRP for application in future in vivo studies. An SNR gain exceeding 6 compared with a state-of-the-art room temperature ¹⁹F RF coil facilitated the acquisition of better spatially-resolved images. Preliminary results in ex-vivo mouse brain show the superiority of the quadrature CRP to delineate intricate areas of immune cell infiltration, thereby inspiring future in-vivo investigations in neuroinflammatory disease.