Magnetic resonance behavior of normal and diseased lungs: spherical shell model simulations

Microstructural Analysis of Peripheral Lung Tissue through CPMG Inter-Echo Time R2 Dispersion

Since changes in lung microstructure are important indicators for (early stage) lung pathology, there is a need for quantifiable information of diagnostically challenging cases in a clinical setting, e.g. to evaluate early emphysematous changes in peripheral lung tissue. Considering alveoli as spherical air-spaces surrounded by a thin film of lung tissue allows deriving an expression for Carr-Purcell-Meiboom-Gill transverse relaxation rates R₂ with a dependence on inter-echo time, local air-tissue volume fraction, diffusion coefficient and alveolar diameter, within a weak field approximation. The model relaxation rate exhibits the same hyperbolic tangent dependency as seen in the Luz-Meiboom model and limiting cases agree with Brooks et al. and Jensen et al. In addition, the model is tested against experimental data for passively deflated rat lungs: the resulting mean alveolar radius of R_A = 31.46 ± 13.15 μm is very close to the literature value (∼34 μm). Also, modeled radii obtained from relaxometer measurements of ageing hydrogel foam (that mimics peripheral lung tissue) are in good agreement with those obtained from μCT images of the same foam (mean relative error: 0.06 ± 0.01). The model’s ability to determine the alveolar radius and/or air volume fraction will be useful in quantifying peripheral lung microstructure.

Microstructural analysis of foam by use of NMR R2 dispersion

The spin-spin relaxation rate R2 (=1/T2) in hydrogel foams measured by use of a multiple spin echo sequence is found to be dependent on the echo time spacing. This property, referred to as R2-dispersion, originates to a large extent from molecular self-diffusion of water within internal field gradients that result from magnetic susceptibility differences between the gel and air phase. Another contribution to the R2 relaxation rate is surface relaxation. Numerical simulations are performed to investigate the relation between the foam microstructure (the mean air bubble radius and standard deviation of the air bubble radius) and foam composition properties (such as magnetic susceptibilities, diffusion coefficient and surface relaxivity) at one hand and the R2-dispersion at the other hand. The simulated R2-dispersions of gel foam are in agreement with the measured R2-dispersions. By correlating the R2-dispersion parameters and simulated microstructure properties a semi-empirical relationship is obtained that enables the mean air bubble size to be derived from measured R2-dispersion curves. The R2-derived mean air bubble size of a hydrogel foam is in agreement with the bubble size measured with X-ray micro-CT. This illustrates the feasibility of using 1H R2-dispersion measurements to determine the size of air bubbles in hydrogel foams and of alveoli in lung tissue.

Quantification of lung water by transpulmonary thermodilution in normal and edematous lung

PURPOSE

To analyze the accuracy of the transpulmonary thermodilution method in the determination of extravascular lung water (EVLW).

MATERIAL AND METHODS

Acute lung injury was produced in eight adolescent pigs weighing 28 to 35 kg by bronchoalveolar lung lavage. EVLW was measured by transpulmonary thermodilution method before and after the intratracheal introduction of 250 or 500 mL of saline solution in different lung injury conditions. No corrections for anatomic dead space were made.

RESULTS

When 250 mL was introduced, 195 +/- 17 mL was detected in normal (uninjured) lungs versus 74 +/- 57 mL in edematous (injured) lungs (P <.05). When 500 mL was introduced, 343 +/- 67 mL was detected in normal lungs versus 160 +/- 51 mL in edematous lungs (P <.001). Considering all determinations together, there was a very high negative correlation between the baseline EVLW and the percentage of EVLW detected (r = -0.92, P <.001).

CONCLUSION

The transpulmonary thermodilution method is very accurate to detect changes in EVLW in normal lungs. In edematous lung, this method may underestimate the EVLW.