Quantitative magnetization transfer imaging using balanced SSFP.

Department of Radiology, Division of Radiological Physics, University Hospital Basel, Basel, Switzerland.
Magnetic Resonance in Medicine (Impact Factor: 3.27). 10/2008; 60(3):691-700. DOI: 10.1002/mrm.21705
Source: PubMed

ABSTRACT It is generally accepted that signal formation in balanced steady-state free precession (bSSFP) is a simple function of relaxation times and flip angle only. This can be confirmed for fluids, but for more complex substances, magnetization transfer (MT) can lead to a considerable loss of steady-state signal. Thus, especially in tissues, the analytical description of bSSFP requires a revision to fully take observed effects into account. In the first part of this work, an extended bSSFP signal equation is derived based on a binary spin-bath model. Based on this new model of bSSFP signal formation, quantitative MT parameters such as the fractional pool size, corresponding magnetization exchange rates, and relaxation times can be explored. In the second part of this work, model parameters are derived in normal appearing human brain. Factors that may influence the quality of the model, such as B(1) field inhomogeneities or off-resonances, are discussed. Overall, good correspondence between parameters derived from two-pool bSSFP and common quantitative MT models is observed. Short repetition times in combination with high signal-to-noise ratios make bSSFP an ideal candidate for the acquisition of high resolution isotropic quantitative MT maps, as for the human brain, within clinically feasible acquisition times.

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First, simulations were used to evaluate the theoretical sensitivity of each technique to the characteristics of a model of WM that incorporates four pools of magnetization, based on published in vitro measurements. Next, the experimental reproducibility of each technique was investigated, and the impact of certain basic variations in the data acquisition and analysis procedures was evaluated. In the final stage, both methods were applied longitudinally in vivo to assess the dynamic changes that occur in acute, contrast-enhancing lesions of MS. The theoretical results illustrate the sensitivity and limitations of QMTI and QT2 to specific pathology-inspired modifications of WM, and shed new light on the potential specificity of often-neglected QMTI parameters. The reproducibility of both techniques is acceptable for use in repeated clinical measurements, and QMTI has lower variability overall. The importance of corrections for magnetic field inhomogeneity in QMTI is demonstrated, and a simple optimization of the QMTI data acquisition is introduced. Both techniques were sensitive to active disease pathology in the longitudinal study of MS patients. Overall, this thesis demonstrates the complementary nature and usefulness of QMTI and QT2 in the characterization of the natural disease course of a degenerative disease of the human central nervous system. With further refinement, these techniques could play an important role in the study of other diseases, and have the potential to serve as outcome measures in clinical trials. L'objectif principal de cette thèse est la réconciliation de deux techniques quantitatives d'imagerie par résonance magnétique, en apparence difféerentes, utilisées pour la caractérisation de la susbtance blanche du cerveau humain en santé ou affectée par la maladie. 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