Rat lungs were imaged by 19F projection MRI of hexafluoroethane, mixed with 20% oxygen to form the inhaled gas. The 3D image had 700 microm resolution, and the data took 4.3 h to acquire. Free induction decays were collected in the presence of steady magnetic field gradients in 686 different directions. To take advantage of fast relaxation (T1 = 5.9 +/- 0.2 ms), the repetition time was 5 ms. To eliminate signal loss from magnetic field inhomogeneities, data were collected within 2 ms of spin excitation (from 80 micros to 2 ms after the 42-micros pi/2 pulses). The singular value decomposition of the transform from frequency to time domain was used to obtain projections despite the absence of data during and immediately after the RF pulses. Inert fluorinated gas imaging may be less expensive than polarized noble gas imaging and is appropriate for imaging steady-state rather than transient gas concentrations.

We partially obstructed the left bronchi of rats and imaged an inert insoluble gas, SF(6), in the lungs with NMR using a technique that clearly differentiates obstructed and normal ventilation. When the inhaled fraction of O(2) is high, SF(6) concentrates dramatically in regions of the lung with low ventilation-to-perfusion ratios (VA/Q); therefore, these regions are brighter in an image than where VA/Q values are normal or high. A second image, made when the inhaled fraction of O(2) is low, serves as a reference because the SF(6) fraction is nearly uniform, regardless of VA/Q. The quotient of the first and second images displays the low-VA/Q regions and is corrected for other causes of brightness variation. The technique may provide sufficient quantification of VA/Q to be a useful research tool. The noise in the quotient image is described by the probability density function for the quotient of two normal random variables. When the signal-to-noise ratio of the denominator image is >10, the signal-to-noise ratio of the quotient image is similar to that of the parent images and decreases with pixel value.

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, Ϸ1700 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. A crucial feature of a well-functioning lung is the locationby-location matching of ventilation (V A , gas replacement rate, L/min) and perfusion (Q, blood flow rate, L/min), which ensures that the arterial blood is fully oxygenated (1). The dimensionless ventilation-to-perfusion ratio (V A /Q) is therefore a key parameter for characterizing lung function. In healthy lungs the narrow distribution of V A /Q values is log normal, with a mean of Ϸ0.85, and a width (standard deviation [SD] of log V A /Q) of 0.15-0.30 (2,3). In lung disease the average V A /Q value may change, but of more importance is the increase in the width of the distribution of V A /Q values (i.e., V A and Q become mismatched in an increasing volume of the lung). In diseases such as chronic obstructive pulmonary disease (COPD) and asthma, obstructed ventilation results in regions of belownormal V A /Q. These regions are particularly important because there is significant blood flow, but the blood is not fully oxygenated before entering the arterial system, reducing arterial blood oxygen levels. Because of the critical importance of the heterogeneity of V A /Q in characterizing lung function, noninvasive methods capable of quantifying the spatial variation of V A and V A /Q are desired to enable a more detailed understanding of the progression and treatment of lung disease. In this article we present a new technique for quantitative mapping of v...

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MRI systems often use magnetic field gradient and shim pulse-shaping networks (pre-emphasis) to correct for magnetic field distortions caused by eddy currents. A pre-emphasis system that uses up to 16 fixed resistor-capacitor (RC) time constants per channel with programmable amplitude coefficients is described. The magnetic fields induced by the pre-emphasis RC time constants serve as a set of basis functions for compensating eddy-current fields induced by the gradient set. The resultant time-varying magnetic field gradient accurately reflects the gradient specified by the pulse programmer. Reductions in eddy-current fields are demonstrated for actively shielded and unshielded gradient sets.