| Summary: | A major problem currently in magnetic resonance imaging is the paucity of specificity and
accurate quantification of NMR parameters for clinical use. If the lack of specificity is eliminated
and accurate quantification is achieved, it would help physicians and radiologists to employ
current therapies more effectively.
Proton nuclear magnetic resonance ¹H NMR was used to investigate the entire signal from
excised lung tissue. The free induction decay signal contained a motionally restricted component
which decayed in a few 10's of μs and a mobile component which persisted about 10 ms or longer.
The motionally restricted component was characterized by the second moment of its lineshape
which had an average value of 3.42 ± (0.25) x 10[superscript 9]s[superscript]-2. This value was about 1/3 of the rigid
lattice M[subscript 2] value, indicating that long macromolecules undergo considerable anisotropic motion
on the NMR timescale.
The mobile component of the lung was characterized by its T[subscript 2] relaxation times which relate
to the microscopic tissue environment. Due to the inhomogeneous nature of the structure and
biochemical composition of lung, a smooth T[subscript 2] distribution was assumed. The mobile signal
consistently showed four resolvable components of T[subscript 2] range: 2-6, 10-40, 80-110, and 190-400
ms. The 2-6 ms component was present in a fully dehydrated preparation and was therefore
assigned to a non-aqueous lung constituent.
Collagen is a major protein present in lung tissue and has high tensile strength, rigidity, and
binding affinity for water. For this reason, the dependence of the second moment, T[subscript 2] relaxation
times, and T[subscript 2] relaxation amplitudes on collagen content were studied.
To determine the lung wet/dry ratio, the hydrogen content per unit mass for lung parenchyma
and water were estimated in two ways: 1) on the basis of chemical content and 2) on the basis
of comparison of restricted and mobile signals to the gravimetric (G) water content for a
lung sample studied at a wide range of water contents. Lung Wet/Dry weight ratios were estimated
from the free induction decays and compared with gravimetric measurements. The ratio
of (Wet/Dry)NMR/(Wet/Dry)G was 1.00 ± (0.08) and 1.00 ± (0.05) for the two methods of
estimation.
The water content measurements were validated and T[subscript 2] distributions were determined in
inflated, deflated, and perfused lungs on a clinical 1.5 T MRI scanner. The mean difference
between the gravimetric and MRI water contents was — 4.1g ± 7.6% and an excellent linear
correlation squared (R² = 0.98) was observed between the two independent measurements. A
voxel-by-voxel investigation of the T[subscript 2] distribution in inflated lung was particularly informative.
Plotting the global geometric mean T[subscript 2] versus lung-water density in inflated lung helped to
differentiate two distinct regions separated by the lung water density of about 0.4 g/ml.
A spherical shell model was tailored to characterize the susceptibility-induced magnetic field
gradients in inflated lung and a simulation was performed to assess the effect of diffusion alone
on the T[subscript 2] decay curve. This approach demonstrated that the multiexponential nature of the
T[subscript 2] distribution was largely due to diffusion of water molecules in the magnetic field gradients.
This study also enabled measurements of the inherent T[subscript 2] relaxation. In addition, it was found
that the inherent T[subscript 2] relaxation was dependent upon lung water density.
The estimation of the magnetic field gradients facilitated measurement of the apparent
diffusion coefficient by collecting images at a fixed imaging time using a multiecho pulse sequence
with different echo spacing. The apparent diffusion coefficient decreased from about 1.1 x
10[superscript -5]cm²/s to 1.7 x 10[superscript -6]cm²/s as diffusion time increased from 12 to 60 ms. [Scientific formulae used in this abstract could not be reproduced.] === Science, Faculty of === Physics and Astronomy, Department of === Graduate
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