Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine
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The magnetization transfer ratio of the lumbar discs was spatially quantified from age-matched subjects and the nucleus pulposus magnetization transfer ratio was correlated with T2-weighted Pfirrmann grades. A moderate and significant linear correlation between magnetization transfer ratio and Pfirrmann grades was observed, suggesting that nucleus pulposus collagen relative density increases with degeneration. ⋯ This observation may suggest a possible increase in absolute collagen content, in addition to increased collagen relative density. In summary, magnetization transfer MRI of the disc may serve as a noninvasive diagnostic tool for disc degeneration, in addition to other MRI techniques specific to proteoglycan content.
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Proton resonance frequency shift-based MR thermometry (MRT) is hampered by temporal magnetic field changes. Temporal changes in the magnetic susceptibility distribution lead to nonlocal field changes and are, therefore, a possible source of errors. The magnetic volume susceptibility of tissue is temperature dependent. ⋯ To study the implications for a clinical case, simulations were performed in a 3D breast model. Temperature errors were quantified by computation of magnetic field changes in the glandular tissue, resulting from susceptibility changes in a thermally heated region. The results of the experiments and simulations showed that the temperature-induced susceptibility changes of water and fat lead to significant errors in proton resonance frequency shift-based MR thermometry.
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Measuring T(2) at 7.0 T is not trivial due to RF inhomogeneity effects, however, gradient echo sampling of a spin echo is insensitive to RF pulse errors, does not suffer from significant distortions, and allows T'(2) and T(2) to be estimated simultaneously. Gradient echo sampling of a spin echo results are relatively sensitive to noise and therefore fitting methods and timing parameters were optimized: a weighted linear fit reduced the errors in T(2) compared to a nonlinear fit, the optimum spin echo time was approximately equal to the expected T(2) and decreasing the number of gradient echoes minimized the error of the estimated T(2). T(2), T'(2), and T*(2) decreased with field strength in frontal gray matter, occipital gray matter, and white matter, with T(2) having a linear dependence (frontal gray matter: 87, 76, 47 ms, occipital gray matter: 80, 68, 46 ms and white matter: 80, 71, 47 ms at 1.5, 3.0 and 7.0 T, respectively).