Improved quantitative ¹⁹F MR molecular imaging with flip angle calibration and B₁-mapping compensation

Purpose To improve ¹⁹F flip angle calibration and compensate for B₁ inhomogeneities in quantitative ¹⁹F MRI of sparse molecular epitopes with perfluorocarbon (PFC) nanoparticle (NP) emulsion contrast agents. Materials and Methods Flip angle sweep experiments on PFC-NP point source phantoms with three custom-designed ¹⁹F/¹H dual-tuned coils revealed a difference in required power settings for ¹⁹F and ¹H nuclei, which was used to calculate a calibration ratio specific for each coil. An image-based correction technique was developed using B₁-field mapping on ¹H to correct for ¹⁹F and ¹H images in two phantom experiments. Results Optimized ¹⁹F peak power differed significantly from that of ¹H power for each coil (P-<-0.05). A ratio of ¹⁹F/¹H power settings yielded a coil-specific and spatially independent calibration value (surface: 1.48-±-0.06; semicylindrical: 1.71-±-0.02, single-turn-solenoid: 1.92-±-0.03). ¹H-image-based B₁ correction equalized the signal intensity of ¹⁹F images for two identical ¹⁹F PFC-NP samples placed in different parts of the field, which were offset significantly by ∼66% (P-<-0.001), before correction. Conclusion ¹⁹F flip angle calibration and B₁-mapping compensations to the ¹⁹F images employing the more abundant ¹H signal as a basis for correction resulted in a significant change in the quantification of sparse ¹⁹F MR signals from targeted PFC NP emulsions.