In this work, we investigate (3)He magnetic resonance imaging as a noninvasive morphometric tool to assess emphysematous disease state on a local level. Emphysema was induced intratracheally in rats with 25 U/100 g body wt of porcine pancreatic elastase dissolved in 200 microl saline. Rats were then paired with saline-dosed controls. Nine three-dimensional (3D) (3)He diffusion-weighted images were acquired at 1, 2, or 3 wk postdose, after which the lungs were harvested and prepared for histological analysis. Recently introduced indexes sensitive to the heterogeneity of the air space size distribution were calculated. These indexes, D(1) and D(2), were derived from the moments of the mean equivalent airway diameters. Averaged over the entire lung, it is shown that the average (3)He diffusivity (D(ave)) correlates well with histology (R = 0.85, P < 0.0001). By matching small (0.046 cm(2)) regions in (3)He images with corresponding regions in histological slices, D(ave) correlates significantly with both D(1) and D(2) (R = 0.88 and R = 0.90, respectively, with P < 0.0001). It is concluded that (3)He MRI is a viable noninvasive morphometric tool for localized in vivo emphysema assessment.

We have measured longitudinal nuclear relaxation rates of Xe in Xe-N2 mixtures at densities below 0.5 amagats in a magnetic field of 8.0 T. We find that intrinsic spin relaxation in this regime is principally due to fluctuations in the intramolecular spin-rotation SR and chemical-shift-anisotropy CSA interactions, mediated by the formation of Xe-Xe persistent dimers. Our results are consistent with previous work done in one case at much lower applied fields where the CSA interaction is negligible and in another case at much higher gas densities where transient xenon dimers mediate the interactions. We have verified that a large applied field suppresses the persistent-dimer mechanism, consistent with standard relaxation theory, allowing us to measure room-temperature gas-phase relaxation times T1 for Xe greater than 25 h at 8.0 T. These data also yield a maximum possible low-field T1 for pure xenon gas at room temperature of 5.45±0.2 h. The coupling strengths for the SR and CSA interactions that we extract are in fair agreement with estimates based both on previous experimental work and on ab initio calculations. Our results have potential implications for the production and storage of large quantities of hyperpolarized Xe for use in various applications.

The response of the NMR relaxation times ( T 1 , CPMG T 2 , and Hahn T 2 ) to bleomycin‐induced lung injury was studied in excised, unperfused rat lungs. NMR, histologic, and biochemical (collagen content measurement) analyses were performed 1, 2, 4, and 8 weeks after intratracheal instillation of saline (control lungs) or 10 U/kg bleomycin sulfate. The control lungs showed no important NMR, water content, histologic, or collagen content changes. The spin‐spin relaxation times for the fast and intermediate components of the CPMG decay ( T 2f and T 2i , respectively) increased 1 week after bleomycin injury (acute inflammatory stage) and then progressively decreased during the following 2–8 weeks (i.e., with the development of the chronic, fibrotic stage of the injury). The slow component ( T 2s ) showed no significant changes. The response of T 1 and the slow component of the Hahn T 2 was, on the whole, similar to that of CPMG T 2f and T 2i . T 1 changes were very small. Lung water content increased 1 week after injury. Histologic and biochemical assessment of collagen showed that collagen content was close to control at 1 week, but markedly increased at 2, 4, and 8 weeks. T 1 and T 2 data were directly correlated with lung water content and inversely correlated with collagen content. Our results indicate that NMR relaxation time measurements (particularly T 2 ) reflect the structural changes associated with bleomycin injury. The prolonged T 2 relaxation times observed in the acute stage are related to the presence of edema, whereas the subsequent decrease in these values marks the stage of the collagen deposition (fibrotic stage). CPMG‐ T 2 and Hahn‐ T 2 measurements can be valuable as a potentially noninvasive method for characterizing bleomycin‐induced lung injury and pathologically related lung disorders. Magn Reson Med 47:246–256, 2002. © 2002 Wiley‐Liss, Inc.

The effects of endotoxin injury on lung NMR relaxation times (T1, CPMG T2, and Hahn decay constant (Hahn T2)) were studied in excised unperfused rat lungs. Blinded histologic examination showed no clear-cut separation between endotoxin and control lungs. Morphometric lung tissue volume density and gravimetric lung water content did not differ significantly between the two groups. In contrast, the values of the fast, intermediate, and slow T2 components, obtained by multiexponential analysis of the CPMG decay curve, increased markedly after endotoxin administration, with minimal overlap between endotoxin and control values. The response of Hahn T2 was, in general, in the same direction as that of CPMG T2; however, Hahn T2 may be more affected by measurement errors and may be less sensitive to the presence of lung injury. T1 showed minimal changes after injury. The present data suggest that CPMG T2 measurements can consistently detect the presence of lung injury even when conventional histologic, morphometric, and gravimetric studies provide negative or equivocal results, and that the CMPG T2 method is superior, in this respect, to the Hahn decay method. T1 does not appear to be sensitive to lung injury in the absence of significant lung water accumulation.

The spin-lattice relaxation time T1 was measured in excised degassed (airless) rat lungs over the frequency range 6.7 to 80.5 MHz. The observed frequency dependence was fitted successfully to the water-biopolymer cross-relaxation theory proposed by H. E. Rorschach and C. F. Hazlewood (RH) [J. Magn. Reson. 70, 79 (1986)]. The rotating frame spin-lattice relaxation time T1(rho) was also measured in rat lung fragments over the frequency range 0.56 to 5.6 kHz, and the observed frequency dependence was explained with an extension of the RH model. The agreement between the theory and the experimental data in both cases is good.

The water-biopolymer cross-relaxation model, proposed by H. E. Rorschach and C. F. Hazlewood (RH) [J. Magn. Reson. 70, 79 (1986)], explains the Larmor frequency dependence of T1 in many biological systems. However, the RH theory fails at low Larmor frequencies. In this paper, a more general version of the RH theory has been developed. This theory is valid at all frequencies. Use of the new expression for the spin-lattice relaxation rate (1/T1), earlier published experimental data in H2O/D2O bovine serum albumin, which had been measured over a wide frequency range (10 kHz to 100 MHz), were fitted over the entire frequency range. The agreement between theory and the experimental data is excellent. Theoretical expressions for the rotating-frame spin-lattice relaxation rate (1/T1(rho)) were also obtained.

Water self diffusion in excised rat lungs has been measured using pulsed-field-gradient (PFG) techniques. The apparent diffusion coefficient,Dapp, was measured from a plot of the magnetizationMvsg2ato be 4.0 × 10−6cm2/s in the limit of small γδga, where γ is the gyromagnetic ratio, δ is the duration of the applied gradient pulses, andgais the applied gradient strength.Dappis independent of the diffusion time,t, for values oftbetween 18 and 106 ms. For larger values of γδga, an additional smaller value of the slope ofMvsg2awas observed, indicating the existence of other, more slowly dephasing spins. Variation oftrevealed that the relative magnetization associated with the more slowly dephasing spins decreases astis increased. In addition, the relative magnetization of the slowly dephasing spins decreases as the temperature,T, of the excised rat lung is increased. Slow exchange from the compartment of the more rapidly to that of the more slowly dephasing spins may explain some of the observed dependence of the relative magnetizations ontandT. Measurements of water self diffusion in rat lung at various levels of water content indicate a correlation betweenT2components and diffusion components. A new technique that combines the PFG with the Carr–Purcell–Meiboom–Gill technique is presented. The application of this technique to excised rat lung confirms the correlation betweenT2and diffusion components.

We performed P-31 NMR measurements of the temperature-dependent spin-lattice relaxation time in several mixed single crystals of Rb1-x(ND4)xD2PO4 [DRADP] having different ammonium concentration x (x=0.22, 0.44, 0.78) as well as in three pure single crystals (ND4)D2PO4 [DADP], (NH4)H2PO4 [ADP], and RbH2PO4 [RDP]. The P-31 NMR spin-lattice relaxation-time measurements in mixed crystals show that the phosphorus nuclei are strongly influenced by the gradual slowing down of the motions of protons in the nearby O-D...O acid bonds in the structural spin-glass states. In addition to a common T1 minimum observed in all ammonium-containing systems, a second T1 minimum is observed in the temperature region of the glass transition in mixed spin-glass systems but not in pure ADP, DADP, or RDP. We attribute this lower temperature T1 minimum to the extreme slowing down of the O-D...O intrabond motion, which is unique to the glass system. In addition, the correlation times and activation energies for the T1 minima in all samples were determined. A comparison between the pure and mixed systems reveals that the mixed system T1 minimum must be fit to a distribution of correlation functions but that each pure system requires only a single correlation function. Furthermore, measurements on systems having x=0.78 and 0.22 show that P-31 NMR can be used to determine the threshold concentrations that characterize the glass phase.