Quantitative mapping of ventilation-perfusion ratios in lungs by 19F MR imaging of T1 of inert fluorinated gases.
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abstract
A new method is presented for quantitative mapping of ventilation-to-perfusion ratios (V(A)/Q) in the lung: MRI of the (19)F longitudinal relaxation time (T(1)) of an inert fluorinated gas at thermal polarization. The method takes advantage of the dependence of the (19)F T(1) on the local SF(6) 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 SF(6) 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 SF(6)/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, approximately 1700 lung voxels contained sufficient SF(6) 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 SF(6) 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.
