Enhanced Fluorine-19 MRI Sensitivity using a Cryogenic Radiofrequency Probe: Technical Developments and Ex Vivo Demonstration in a Mouse Model of Neuroinflammation

Abstract

Neuroinflammation can be monitored using fluorine-19 (19F)-containing nanoparticles and 19F MRI. Previously we studied neuroinflammation in experimental autoimmune encephalomyelitis (EAE) using room temperature (RT) 19F radiofrequency (RF) coils and low spatial resolution 19F MRI to overcome constraints in signal-to-noise ratio (SNR). This yielded an approximate localization of inflammatory lesions. Here we used a new 19F transceive cryogenic quadrature RF probe (19 F-CRP) that provides the SNR necessary to acquire superior spatially-resolved 19F MRI. First we characterized the signal-transmission profile of the 19 F-CRP. The 19 F-CRP was then benchmarked against a RT 19F/1H RF coil. For SNR comparison we used reference compounds including 19F-nanoparticles and ex vivo brains from EAE mice administered with 19F-nanoparticles. The transmit/receive profile of the 19 F-CRP diminished with increasing distance from the surface. This was counterbalanced by a substantial SNR gain compared to the RT coil. Intraparenchymal inflammation in the ex vivo EAE brains was more sharply defined when using 150 μm isotropic resolution with the 19 F-CRP, and reflected the known distribution of EAE histopathology. At this spatial resolution, most 19F signals were undetectable using the RT coil. The 19 F-CRP is a valuable tool that will allow us to study neuroinflammation with greater detail in future in vivo studies.

Performance of compressed sensing for fluorine‐19 magnetic resonance imaging at low signal‐to‐noise ratio conditions

“…26,38 NPs containing 5 µmol of PFCE were administered daily from day 5 after immunization until the end of the experiment. 6 Animals were sacrificed on day 10 using a lethal dose of anesthetic and transcardially perfused (20 mL of PBS and 20 mL of 4% paraformaldehyde [PFA] fixative). Tissue was prepared for ex vivo MRI as described previously 6 and secured within 15-mL tubes filled with 4% PFA, keeping the CNS as well as draining lymph nodes in situ.…”

“…6 Animals were sacrificed on day 10 using a lethal dose of anesthetic and transcardially perfused (20 mL of PBS and 20 mL of 4% paraformaldehyde [PFA] fixative). Tissue was prepared for ex vivo MRI as described previously 6 and secured within 15-mL tubes filled with 4% PFA, keeping the CNS as well as draining lymph nodes in situ. A cylindrical cap (diameter 3 mm, height 1 mm) containing NPs (2 mM of PFCE in 2% agarose) was fixed outside of the tube as an external standard.…”

“…Fluorine ( 19 F) MRI has been commonly used in cell tracking and molecular imaging. [1][2][3][4] Its widespread applicability, along with the challenge that 19 F is only sparsely available in vivo, has motivated the development of strategies that enhance signal-to-noise ratio (SNR) efficiency, for example, more sensitive hardware [5][6][7] and more time-efficient pulse sequences. [8][9][10] Digital signal processing, particularly compressed sensing (CS), [11][12][13] is a new avenue to boost detection performance.…”

“…EAE is a model of multiple sclerosis characterized by inflammatory lesions in the CNS, 29 which appear in arbitrary locations especially in the cerebrum, but can also follow fine anatomical structures, such as white matter tracts, in the cerebellum. 6 19 F nanoparticles (NPs) are applied intravenously in animal models to study the distribution of inflammatory cell lesions in vivo. [17][18][19][20][21] To study the performance of CS for 19 F MRI applications that include low-SNR conditions, we compared undersampled and CS reconstructed data with conventional Fourier and denoised reconstructions of fully sampled data.…”

Dielectric tetrahedrons as terahertz resonators switched from perfect absorber to reflector

“…Such properties can be engineered by tailoring materials, shapes and sizes of building blocks 11 , 12 . Both metallic and dielectric building blocks with different shapes 1 – 3 , such as cubes, disks and planar patterns from lithography, have been studied from microwave to visible wavelengths, while asymmetric three-dimensional (3D) geometries have shown different features in terahertz applications 13 , 14 . Therefore, the roles of 3D building blocks in the exotic EM properties in terahertz band (wavelength ) 15 raise wide interests accordingly 1 , 16 .…”