B1 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

Abstract

Purpose: Low SNR in fluorine-19 (¹⁹ F) MRI benefits from cryogenically-cooled transceive surface RF probes (CRPs), but strong B₁ 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 B₁ inhomogeneity. Here, a workflow was proposed and validated to correct and quantify ¹⁹ F-MR signals from the inflamed mouse brain using a ¹⁹ F-CRP.

Methods: In vivo ¹⁹ F-MR images were acquired in a neuroinflammation mouse model with a quadrature ¹⁹ F-CRP using an imaging setup including 3D-printed components to acquire co-localized anatomical and ¹⁹ F images. Model-based corrections were validated on a uniform ¹⁹ F phantom and in the neuroinflammatory model. Corrected ¹⁹ F-MR images were benchmarked against reference images and overlaid on in vivo ¹ H-MR images. Computed concentration uncertainty maps using Monte Carlo simulations served as a measure of performance of the B₁ corrections.

Results: Our study reports on the first quantitative in vivo ¹⁹ F-MR images of an inflamed mouse brain using a ¹⁹ F-CRP, including in vivo T₁ calculations for ¹⁹ F-nanoparticles during pathology and B₁ corrections for ¹⁹ F-signal quantification. Model-based corrections markedly improved ¹⁹ 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%.

Conclusion: Our model-based correction method facilitated ¹⁹ F signal quantification in the inflamed mouse brain when using the SNR-boosting ¹⁹ F-CRP technology, paving the way for future low-SNR ¹⁹ F-MRI applications in vivo.

Keywords: 19F-MRI; B1 correction; RARE; inflammation; transceive surface RF probe.