Quantitative mapping of ventilation-perfusion ratios in lungs by 19F MR imaging of T1 of inert fluorinated gases

A new method is presented for quantitative mapping of ventilation-to-perfusion ratios (V-A/Q) in the lung: MRI of the F-19 longitudinal relaxation time (T-1) of an inert fluorinated gas at thermal polarization. The method takes advantage of the dependence of the F-19 T-1 on the local SF6 partial pressure, which depends on the local value of V-A/Q In contrast to hyperpolarized noble gases, with very long T(1)s, the T-1 of SF6 in mammal lungs is 0.8-1.3 ms. Thus, rapid signal averaging overcomes the low thermal equilibrium polarization. T-1 imaging of a phantom consisting of four different SF6/air mixtures with known T-1 values validates the modified Look-Locker T-1 imaging sequence. To demonstrate the method in vivo, partial obstruction of the left bronchus was attempted in three rats; 3D free induction decay (FID)-projection T-1 images (2 mm isotropic resolution) revealed obstructed ventilation in two of the animals. In those images, approximate to 1700 lung voxels contained sufficient SF6 for analysis and T-1 was determined in each voxel with a standard error of 8-10%. For comparison, independent V-A/Q images of the same animals were obtained using a previously described SF6 MRI technique, and good agreement between the two techniques was obtained. Relative to the previous technique the resolution achieved using the T-1 method is lower (for similar V-A/Q precision and imaging time); however, the T-1 method offers the potential advantages of eliminating the need for image coregistration and allowing patients with impaired lung function to breathe a 70% O-2 gas mixture during the entire imaging procedure.