Introduction

Fluorinated gas lung imaging has the potential to revolutionise how chronic respiratory diseases are both diagnosed and their progress is monitored. After being breathed in it is possible to detect a signal in vivo from SF₆¹, C₂F₆² and C₃F₈³. Unlike hyperpolarised ¹²⁹Xe/ ³He lung imaging, fluorinated gasses aren’t first hyperpolarised. Instead the short T1 of ¹⁹F in the compounds is exploited, allowing for multiple repeats within a short duration.Fluorinated gasses have the advantage over ¹²⁹Xe that they don’t possess anesthetic properties and consequently be free breathed allowing for data to be obtained on wash in/ wash out kinetics⁴. Fluorinated compounds are also significantly cheaper and simpler to use than hyperpolarised ¹²⁹Xe or ³He.Fluorinated gas MRI has previously always been performed with the volunteer horizontal in a conventional scanner. Lung function and volume, as well as arterial oxygen levels are affected by body position⁵. By lying supine lung function is reduced and for patients at the latter stages of chronic conditions, such as COPD, many simply can’t lay in a supine position for an extended period of time. At Nottingham the 0.5T Paramed MROpen Upright MRI scanner has two parallel TechMag Redstone spectrometers; making it the first of its kind to allow for both proton and multinuclear imaging. The subject can be imaged in a variety of positions, including supine, seated or standing. The open design is also excellent in terms of patient experience; claustrophobic reactions are virtually eliminated and the easy access and different orientations can be exploited to make the scanner better suited to disabled or paediatric cohorts.Before buying a dedicated coil tuned to ¹⁹F. The feasibility of imaging ¹⁹F was assessed for the upright scanner, through imaging the ¹H of a fluorinated gas at standard temperature and pressure.

Methods

An Electrolube GDP 400 air duster provided the fluorinated gas. This is cheap and contains 60-100% 1,1,1,2-Tetrafuroethane (HFC). A 10L Tedlar Bag was half filled with HFC.Neal Et. Al.’s ⁶ work on ¹⁹F imaging was taken as a template when working out the feasibility of imaging ¹⁹F at 0.5T. Neal Et. Al.⁶ imaged PFP (C₃F₈) at 3T with a TR of 7.5ms. The T1 of PFP in vivo is ~12.4ms. The T1 of 1H on HFC was first measured. The Clinical 3D spoiled GRE sequence from the Paramed with the minimum TE=8ms and minimum resolution (256*128*24) was modified to have a TE=2.6ms and resolution 128*32*1 (Infinite slice). The TR was then set so the ratio of T1 to TR was the same as Neal Et. Al⁶ and the flip angle to the Ernst Angle (57°).

Results

Using two 90° RF pulses at varying TRs the drop in NMR signal was measured. The data was fitted with an R²=0.998. From this fit the T1 was calculated to be 1600ms +/-62ms with a 95% confidence interval. Using the same ratio of T1:TR as Neal Et. Al⁶ (TR=968ms), the half filled 10L Tedlar bag. First, with a Field of View (FOV) of 1m*1m (figure 1) then at an FOV of 0.5m*0.5m (figure 2) . A 1L bag of HFC gas was then imaged with an FOV 0.5m*0.5m (figure 3). SNRs can be seen in figure captions.

Discussion

HFC can be imaged within a single scan at 0.5T and the signal is expected to improve when moving to ¹⁹F. The Bright spot in the bottom left quadrant of the images is suspected to be the hard plastics that form the imaging coil casing and is still visible when imaging an empty tedlar bag. These plastics only cause problems at short TEs due to the short T1 of ¹H in the plastic. At 3T there would be expected to see 2.5 fold improvement in signal when moving to the ¹⁹F frequency with PFP from the proton frequency with HFC. ¹⁹F nuclei have a relative sensitivity of 0.84 compared to ¹H. For every HFC molecule there are 2 protons and for every PFP molecule there are 8 ¹⁹F atoms. However, PFP has the structure CF₃-CF₂-CF₃ and there is chemical shift between the CF₃ and CF₂ resonances of 48ppm³. At 3T only the CF₃ resonances are excited. However, at 0.5T the chemical shift will be 6 times smaller. This shift may be small enough to also excite the CF₂ part of each molecule, making the improvement in signal even greater then 2.5 fold.

Conclusions

From this preliminary work a grant submission to MRC CiC has been submitted to support the implementation of a ¹⁹F chest coil and gas delivery system. At Nottingham there is also the capability of imaging using hyperpolarised ¹²⁹Xe on this scanner and there is the intention to compare both techniques in the future.

Acknowledgements

Big thank you to Pete Thelwall for his expertise in fluorinated gas imaging. Also to the Haydn Green Foundation for funding my PhD.