Introduction: 

Fluorine-19 (¹⁹F) magnetic resonance imaging (MRI) serves as a dynamic technique for in vivo detection of cellular and molecular events, offering quantifiable, background-free signals from ¹⁹F spins. To maximize ¹⁹F detectability and signal quantification, several important considerations must be made. For instance, it is necessary to have full coverage of the imaging subject to facilitate ¹⁹F detection without prior knowledge of the signal location. Additionally, since cellular events occur at depth within tissue, it is important to maintain accurate signal quantification within a range of imaging depths. The choice of radiofrequency (RF) coil for an imaging experiment can have a significant impact on these factors. For example, birdcage coils provide full animal coverage with uniform B₁ field generation. In this work, a new suite of small-bore birdcage coils were explored for their improved animal coverage and homogenous B₁ field generation and compared their ability to detect ¹⁹F in deep tissue to a commercially available dual-tuned ¹H/¹⁹F surface coil. 

Methods: 

Three whole-mouse birdcage coils were constructed (¹H single-frequency, ¹⁹F single-frequency, and ¹H/¹⁹F switch tuned) with inner diameters of 31 mm and lengths of 85 mm. For practical use of single frequency birdcage coils, a 3D-printed animal bed was constructed that could be fixed to the scanner bed to help prevent animal re-positioning during coil switching (< 60 second coil switching times). To assess coil homogeneity, a dual-frequency phantom was constructed using gadolinium-doped water, with an internal concentric tube of the perfluorocarbon tracer Celsense (CelSense Inc.) [1] to mimic the presence of ¹⁹F in deep tissue. T₁-weighted spoiled gradient echo images were acquired for ¹H and 3D balanced steady state free precession for ¹⁹F imaging. To assess coil homogeneity, SNR as a function of depth was calculated for each of the slices within our phantom for both frequencies. Finally, to evaluate the detectability of ¹⁹F in deep tissue in vivo, 50 mL of the perfluorocarbon V-Sense (CelSense Inc.) was injected bilaterally into the foot pads of female athymic nude mice (n = 3) to promote uptake into deep lymph nodes. Images were taken at one, eight, and 24-days post injection with both our surface and single-frequency birdcage coils to compare the detectability of V-Sense within ¹⁹F-containing organs. 

Results: 

In vivo imaging revealed a significant increase in contrast-to-noise ratios (p < 0.0001) for the anatomical proton images using the single frequency ¹H birdcage coil compared to the surface coil. SNR within the subiliac, popliteal, accessory axillary, lumbar, renal, and sciatic lymph nodes were measured and compared between the single frequency birdcage coils and the dual-tuned surface coil. The single frequency ¹⁹F birdcage coil provided significantly higher SNR within renal, sciatic, and lumbar lymph nodes (p < 0.001) (Figure 1A). The percentage difference between SNR measurements as a function of depth into the mouse revealed an R² fitting factor of 0.51, suggesting superior signal homogeneity of the birdcage coils at depth in vivo. When the popliteal lymph nodes which are prone to streaking artifacts from the residual PFC tracer found in the nearby footpads, the R² fit improved to 0.64 (Figure 1B). 

Conclusions: 

In this work, single frequency coils were implemented with the use of a customized 3D animal bed to allow for the collection of whole mouse ¹H and ¹⁹F images. Our single frequency coils demonstrated increased sensitivity for ¹⁹F in deep tissue, as well as improved homogeneity. These improvements in homogeneity should translate to into improved quantification of cellular/molecular events, which is the focus of our current work. 

Image/Figure:

Click to view full size

Image/Figure Caption:

Figure 1: Sagittal ¹H/¹⁹F MR overlays demonstrating the PFC signal reception at depth within the single frequency birdcage and dual-tuned surface coils (A). Quantitative measure of percentage differences in inter-lymph node SNR measurements between each coil (above) (B).

Author