Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine
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Eddy-current (EC) and motion effects in diffusion-tensor imaging (DTI) bias the estimation of quantitative diffusion indices, such as the fractional anisotropy. Both effects can be retrospectively corrected by registering the strongly distorted diffusion-weighted images to less-distorted T2-weighted images acquired without diffusion weighting. Two different affine spatial transformations are usually employed for this correction: slicewise and whole-brain transformations. ⋯ Using this model, it is demonstrated that a more distinct evaluation of the whole-brain EC effects is possible if the through-plane distortion was considered in addition to the well-known in-plane distortions. Moreover, a comparison of different whole-brain registrations relative to a slicewise approach is performed, in terms of the relative tensor error. Our findings suggest that for appropriate intersubject comparison of DTI data, a whole-brain registration containing nine affine parameters provides comparable performance (between 0 and 3%) to slicewise methods and can be performed in a fraction of the time.
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Dynamic susceptibility contrast magnetic resonance perfusion imaging (DSC-MRI) is a useful method to characterize gliomas. Recently, support vector machines (SVMs) have been introduced as means to prospectively characterize new patients based on information from previous patients. Based on features derived from automatically segmented tumor volumes from 101 DSC-MR examinations, four different SVM models were compared. ⋯ A correct prediction of low-grade glioma was obtained at 83% (true positive rate) and for high-grade glioma at 91% (true negative rate) on the independent test data set. In conclusion, the combination of automated tumor segmentation followed by SVM classification is feasible. Thereby, a powerful tool is available to characterize glioma presurgically in patients.
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This work presents a new approach toward a fast, simultaneous amplitude of radiofrequency field (B(1)) and T(1) mapping technique. The new method is based on the "actual flip angle imaging" (AFI) sequence. However, the single pulse repetition time (TR) pair used in the standard AFI sequence is replaced by multiple pulse repetition time sets. ⋯ Error propagation theory was applied to identify optimal sequence parameters and to facilitate a systematic noise comparison to standard AFI. In terms of accuracy and signal-to-noise ratio, the presented method outperforms standard AFI B(1) mapping over a wide range of T(1). Finally, the capability of MTM to determine T(1) was analyzed qualitatively and quantitatively, yielding good agreement with reference measurements.
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The conventional stimulated-echo NMR sequence only measures the longitudinal component while discarding the transverse component, after tipping up the prepared magnetization. This transverse magnetization can be used to measure a spin echo, in addition to the stimulated echo. Two-dimensional single-shot spin- and stimulated-echo-planar imaging (ss-SESTEPI) is an echo-planar-imaging-based single-shot imaging technique that simultaneously acquires a spin-echo-planar image and a stimulated-echo-planar image after a single radiofrequency excitation. ⋯ We developed a method for amplitude of radiofrequency field inhomogeneity correction by acquiring an additional stimulated-echo-planar image with minimal mixing time, calculating the difference between the spin echo and the stimulated echo and multiplying the stimulated-echo-planar image by the inverse functional map. Diffusion-induced decay is corrected by measuring the average diffusivity during the prescanning. Rapid single-shot T(1) mapping may be useful for various applications, such as dynamic T(1) mapping for real-time estimation of the concentration of contrast agent in dynamic contrast enhancement MRI.
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In this study, we report the use of a novel ultrashort echo time T(1)rhoT(1) sequence that combines a spin-lock preparation pulse with a two-dimensional ultrashort echo time sequence of a nominal echo time 8 microsec. The ultrashort echo time-T(1)rho sequence was employed to quantify T(1)rho in short T(2) tissues including the Achilles tendon and the meniscus. T(1)rho dispersion was investigated by varying the spin-lock field strength. ⋯ A mean T(1)rho of 7.98 +/- 1.43 msec was observed in the meniscus of the healthy volunteers. There was significant T(1)rho dispersion in both the Achilles tendon and the meniscus. Mean T(1)rho increased from 2.06 +/- 0.23 to 7.85 +/- 0.74 msec in normal Achilles tendon and from 7.08 +/- 0.64 to 13.42 +/- 0.93 msec in normal meniscus when the spin-lock field was increased from 250 to 1,000 Hz.