Single-Frequency Birdcage Coils for Deep Tissue Perfluorocarbon Magnetic Resonance Imaging in Mice.

Abstract

Fluorine-19 (¹⁹F) MRI has become an established tool for in vivo cell tracking following ex vivo or in vivo labelling of various cell types with ¹⁹F perfluorocarbons (PFCs). Here, we developed and evaluated novel mouse-specific radiofrequency (RF) hardware for improved dual ¹H anatomical imaging and deep tissue ¹⁹F MR detection of PFCs. Three linearly polarized birdcage RF coils were constructed-a dual-frequency ¹H/¹⁹F coil, and a pair of single-frequency ¹H and ¹⁹F coils, designed to be used sequentially. RF coil quality factors (Q values), signal homogeneity and sensitivity were benchmarked against a commercially constructed dual-frequency ¹H/¹⁹F surface coil. RF homogeneity was assessed using a phantom designed to mimic PFC localization at depth in a mouse. The single-frequency birdcage coils (¹H and ¹⁹F) displayed more uniform coverage and enhanced signal-to-noise ratios (SNRs) compared to both the birdcage and surface dual-frequency coils for ¹⁹F detection. Bilateral injection of a perfluoropolyether nanoemulsion into the footpads of female athymic nude mice, resulting in drainage to various lymph nodes and subsequent accumulation in lymph node macrophages, provided a platform to assess differences in SNRs and contrast-to-noise ratios (CNR) between both coil configurations as a function of depth and location. The single-frequency ¹H coil provided significantly increased CNR in anatomical images (p < 0.001) with increased anatomical coverage compared to the dual-frequency surface coil. The single-frequency ¹⁹F birdcage coil offered increased PFC detectability with significantly higher SNR in renal, lumbar, sciatic and popliteal lymph nodes (p < 0.01) compared to the dual-frequency surface coil. Interestingly, the percentage difference between SNR measurements in lymph nodes between the single-frequency ¹⁹F coil and the ¹H/¹⁹F surface coil had a linear relationship with increasing distance from the surface coil (R² = 0.6352; p < 0.0001), indicating a potential disagreement for imaging experiments that rely on ¹⁹F spin quantification at increasing depth within the mouse using surface RF coils.