Abstract

The proton NMR line shape of rabbit lens was investigated to explain the extremely short value of the <i>T₂</i> relaxation time that determines the decay time of the lens free induction decay (FID) signal. The proton lens spectra were measured at 300 MHz, and a characteristic, antisymmetric profile was found. To determine whether the line shape is caused by unaveraged residual dipolar interaction from immobile protein protons, which would yield a homogeneously broadened line, we performed a spectral hole-burning experiment on the lens. In these experiments we could show that the line is clearly inhomogenously broadened. The inhomogeneity of the external field (Δ<i>B</i>₀) was excluded by comparing at room temperature (295 K) the normalized proton NMR line shape of the whole rabbit lens measured at 300 MHz with the NMR line of a reference sample of pure water of similar size and shape. The measurements of the NMR spectra of the lens cortex and nucleus alone, as well as the spin-lattice relaxation time data obtained for the lens at different frequencies, indicate that the distribution of the chemical shift values is not responsible for the lens profile. Therefore, to understand our data we have to assume that the magnetic susceptibility effect and the shape of the lens are responsible for the observed the NMR line shape and also for the short value of the <i>T₂</i> relaxation time. We calculated the magnetic field inside the lens, using the model of two concentric spheres with different susceptibilities. The results of these calculations are in very good agreement with the experimental data of the lens. The consistency of the assumed model was checked by measurements of the NMR line shapes for the lens phantom and for the lens at higher fields (500 MHz).

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