Introduction

Cystic fibrosis (CF) is the most common life-shortening genetic disease in Caucasians. CF leads to the production of abnormally viscous mucus that blocks airways and results in local infections and inflammation. In CF patients, MRI with hyperpolarized (HP) ¹²⁹Xe gas has been shown to provide regional information about lung ventilation that is complimentary to standard spirometry¹. Fluorinated gases have been proposed as a lower cost alternative to HP ¹²⁹Xe gas and a comparison between the two gases for lung ventilation was recently performed in healthy volunteers². The scope of this work is to perform a comparison between ¹⁹F and HP ¹²⁹Xe gas for the detection of lung ventilation defects in subjects with CF at 3T.

Methods

Three subjects with CF, lung function >40% and clinical stability, underwent a HP ¹²⁹Xe and ¹⁹F lung MRI scan during the same imaging session in a randomized order as part of a study aiming at comparing ¹⁹F and HP ¹²⁹Xe gas imaging in subjects with CF. All scans were performed on a 3T clinical system (PRISMA, Siemens Medical Solutions, USA). Xenon images were acquired using a flexible ¹²⁹Xe chest coil (Clinical MR Solutions, WI) during a single breath hold of 750 ml of isotopically enriched ¹²⁹Xe polarized up to ~14% and mixed with 250ml of N₂. 3D images were acquired using a 2D GRE sequence with a resolution of 2.73mm x 4.37mm x 10.5mm. ¹⁹F images were acquired using an 8-channel ¹⁹F-tuned chest coil (ScanMed, NE). For the acquisition of ¹⁹F images, subjects inhaled of a pre-mixed, medical grade gas mixture of 79% perfluoropropane (PFP) to 21% oxygen delivered using a custom gas delivery device³. A series of wash in and wash out ¹⁹F ventilation images were acquired from each subject to extract wash in and wash out rates. Each image in the series was acquired during a 12-second breath hold that followed two tidal volume breaths and a full breath of the contrast gas. Images were acquired using a 3D GRE sequence with a resolution of 6.25mm x 6.25mm x 15mm. 3D ¹H scans were run before and after each ¹²⁹Xe and ¹⁹F scan to obtained anatomical information and to facilitate co-registration of ¹²⁹Xe images with ¹⁹F images. Images were first processed in MIM Software (Cleveland OH) and then in MATLAB (Mathworks, MA) for B1 correction⁴, and for ventilation defect percentage (VDP) and low ventilation defect percentage (LVP) maps calculation.

Results

Figure 1 shows ¹²⁹Xe and ¹⁹F lung ventilation images acquired in two CF subjects that show different lung ventilation phenotypes. The ¹⁹F ventilation images display a lower SNR despite their lower resolution. While subject 1 exhibits large wedge shaped ventilation defects that were observed in both imaging procedures, subject 2 exhibits smaller patchy ventilation defects that were not visible on ¹⁹F images. Figure 2 shows ¹⁹F and ¹²⁹Xe VDP maps of the bias corrected images shown in Figure 1. While in subject 2 the ¹⁹F VDP maps mirror the defects in the ¹²⁹Xe VDP map, this is not the case in subject 1. This is reflected in the VDP and LVP (ventilation < 10% of maximum ventilation) numbers, being much lower in subject 1 and slightly lower in subject 2 for the ¹⁹F images.

Conclusion

¹⁹F images demonstrated, in general, lower SNR and lower VDP values than ¹²⁹Xe images, despite the use of a low spatial resolution for the ¹⁹F images. From this initial study, VDP values were more similar in subjects with wedge shaped ventilation defects than in subjects with smaller patchy ventilation defects. The lower VDP value in ¹⁹F images is most likely due to PFP diffusion during the breath holds preceding the imaging breath. PFP diffuses into regions of the lungs that are not completely obstructed and are characterized by a low wash in time. Comparison between ¹²⁹Xe and ¹⁹F images using the same inhalation protocol is currently underway.

Acknowledgements

This work was supported by the Cystic Fibrosis Foundation Mucociliary Clearance Consortium Grant DONALD14XX0, by the NIDDK RTCC CF core grant P30 DK065988, and by the NIDDK grant R01 DK108231.