Search

Purpose: To initiate the archive of relaxation-weighted images that may help discriminate between pulmonary pathologies relevant to acute respiratory distress syndrome. MRI has the ability to distinguish pathologies by providing a variety of different contrast mechanisms. Lungs have historically been difficult to image with MRI but image quality is sufficient to begin cataloging the appearance of pathologies in T 1 - and T 2 -weighted images. This study documents T 1 and the use of T 1 contrast with four experimental rat lung pathologies., Methods: Inversion-recovery and spoiled steady state images were made at 1.89 T to measure T 1 and document contrast in rats with atelectasis, lipopolysaccharide-induced inflammation, ventilator-induced lung injury (VILI), and injury from saline lavage. Higher-resolution Ernst-angle images were made to see patterns of lung infiltrations., Results: T 1 -weighted images showed minimal contrast between pathologies, similar to T 1 -weighted images of other soft tissues. Images taken shortly after magnetization inversion and displayed with inverted contrast highlight lung pathologies. Ernst-angle images distinguish the effects of T 1 relaxation and spin density and display distinctive patterns. T 1 for pathologies were: atelectasis, 1.25 ± 0.046 s; inflammation from instillation of lipopolysaccharide, 1.24 ± 0.015 s; VILI, 1.55 ± 0.064 s (p = 0.0022 vs. normal lung); and injury from saline lavage, 1.90±0.080 s (p = 0.0022 vs. normal lung; p = 0.0079 vs. VILI). T 1 of normal lung and erector spinae muscle were 1.25 ± 0.028 s and 1.02 ± 0.027 s, respectively (p = 0.0022)., Conclusions: Traditional T 1 -weighting is subtle. However, images made with inverted magnetization and inverted contrast highlight the pathologies and Ernst-angle images aid in distinguishing pathologies., (© 2018 International Society for Magnetic Resonance in Medicine.)

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.

MR images of laboratory rat lungs that resolve the thin membranes that separate lung lobes are presented. It appears that the capabilities of in vivo small-animal pulmonary MRI may rival those of in vivo small-animal X-ray CT. Free induction decay (FID)-projection imaging was employed with particular attention to the choice of acquisition time. For a given nominal resolution, one obtains optimal point discrimination when the acquisition time T(acq) normalized by the signal decay time constant T(2)(*) is approximately 0.8-0.9, although a better signal-to-noise ratio (SNR) is obtained when this quotient is 1.6. Currently available equipment should be able to even exceed the results presented herein.

The lung volumes of mechanically ventilated rats were measured over the course of the respiratory cycle using the NMR signal strength from inhaled sulfurhexafluoride. Rats with elastase-induced emphysema showed larger lung volumes and slower exhalation than control rats. For humans the technique should be able to provide lung volume measurements at least 20 times a second., (Copyright 2002 Wiley-Liss, Inc.)

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.

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.

Because clinical studies have indicated that shear in fluids may be one of the major causes of magnetic resonance image artifacts, we imaged an apparatus that would separate the effects of shear from the plethora of other motion artifacts. The apparatus contained fluid between two concentric cylinders. To produce shear, we turned the outer cylinder. To produce motion without shear, we turned both cylinders as one. Shear rates near 200 s-1 (similar to those in large arteries) can cause pixel intensities either to increase 40% or decrease to background levels. Increases can occur when the velocities are nearly parallel to the phase-encoding gradient. Decreases can occur when velocities are in the direction of the frequency gradient. The decreases are similar to those predicted from phase dispersion within a "voxel" so long as the voxel's phase warp does not exceed 2 pi.