B 1 inhomogeneity correction of RARE MRI at low SNR: Quantitative in vivo 19 F MRI of mouse neuroinflammation with a cryogenically-cooled transceive surface radiofrequency probe

Low SNR in fluorine-19 ( F) MRI benefits from cryogenically-cooled transceive surface RF probes (CRPs), but strong B inhomogeneities hinder quantification. Rapid acquisition with refocused echoes (RARE) is an SNR-efficient method for MRI of neuroinflammation with perfluorinated compounds but lacks an analytical signal intensity equation to retrospectively correct B inhomogeneity. Here, a workflow was proposed and validated to correct and quantify F-MR signals from the inflamed mouse brain using a F-CRP. In vivo F-MR images were acquired in a neuroinflammation mouse model with a quadrature F-CRP using an imaging setup including 3D-printed components to acquire co-localized anatomical and F images. Model-based corrections were validated on a uniform F phantom and in the neuroinflammatory model. Corrected F-MR images were benchmarked against reference images and overlaid on in vivo H-MR images. Computed concentration uncertainty maps using Monte Carlo simulations served as a measure of performance of the B corrections. Our study reports on the first quantitative in vivo F-MR images of an inflamed mouse brain using a F-CRP, including in vivo T calculations for F-nanoparticles during pathology and B corrections for F-signal quantification. Model-based corrections markedly improved F-signal quantification from errors > 50% to < 10% in a uniform phantom (p < 0.001). Concentration uncertainty maps ex vivo and in vivo yielded uncertainties that were generally < 25%. Monte Carlo simulations prescribed SNR ≥ 10.1 to reduce uncertainties < 10%, and SNR ≥ 4.25 to achieve uncertainties < 25%. Our model-based correction method facilitated F signal quantification in the inflamed mouse brain when using the SNR-boosting F-CRP technology, paving the way for future low-SNR F-MRI applications in vivo.