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

Abstract

A new method is presented for quantitative mapping of ventilation-to- perfusion ratios (VA/Q) in the lung: MRI of the 19F longitudinal relaxation time (T1) of an inert fluorinated gas at thermal polarization. The method takes advantage of the dependence of the 19F T1 on the local SF6 partial pressure, which depends on the local value of VA/Q. In contrast to hyperpolarized noble gases, with very long T1s, the T1 of SF6 in mammal lungs is 0.8-1.3 ms. Thus, rapid signal averaging overcomes the low thermal equilibrium polarization. T1 imaging of a phantom consisting of four different SF6/air mixtures with known T1 values validates the modified Look-Locker T1 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 T1 images (2 mm isotropic resolution) revealed obstructed ventilation in two of the animals. In those images, ≈1700 lung voxels contained sufficient SF 6 for analysis and T1 was determined in each voxel with a standard error of 8-10%. For comparison, independent VA/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 T1 method is lower (for similar VA/Q precision and imaging time); however, the T1 method offers the potential advantages of eliminating the need for image coregistration and allowing patients with impaired lung function to breathe a 70% O2 gas mixture during the entire imaging procedure. © 2008 Wiley-Liss, Inc.