Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine
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A radiofrequency (RF) excitation scheme is presented in which flip angle is encoded in the phase of the resulting excitation. This excitation is implemented with nonselective hard pulses, and is used to give flip angle maps over three-dimensional volumes. ⋯ The phase-sensitive method allows imaging over a much wider range of flip angles than double-angle methods. Phantom and in vivo results are presented comparing the phase-sensitive method with the conventional double-angle method, demonstrating the ability of the phase-sensitive method to measure a wider range of flip angles than double-angle methods.
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Although recent studies indicate that use of a single global transverse relaxation time, T(2), per metabolite is sufficient for better than +/-10% quantification precision at intermediate and short echo-time spectroscopy in young adults, the age-dependence of this finding is unknown. Consequently, the age effect on regional brain choline (Cho), creatine (Cr), and N-acetylaspartate (NAA) T(2)s was examined in four age groups using 3D (four slices, 80 voxels 1 cm(3) each) proton MR spectroscopy in an optimized two-point protocol. ⋯ The elderly NAA, Cr, and Cho T(2)s were 12%, 6%, and 10% shorter than the adolescents, a change of under 1 ms/year assuming a linear decline with age. Formulae for T(2) age-correction for higher quantification precision are provided.
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The advantages of predictive modeling in glioma grading from MR perfusion images have not yet been explored. The aim of the current study was to implement a predictive model based on support vector machines (SVM) for glioma grading using tumor blood volume histogram signatures derived from MR perfusion images and to assess the diagnostic accuracy of the model and the sensitivity to sample size. A total of 86 patients with histologically-confirmed gliomas were imaged using dynamic susceptibility contrast (DSC) MRI at 1.5T. ⋯ At optimal SVM parameters, the true positive rate (TPR) and true negative rate (TNR) of the SVM model on the 33 patients was 0.76 and 0.82, respectively. The interobserver agreement and the TPR increased significantly when the SVM model was based on an increasing sample size (P < 0.001). This result suggests that a predictive SVM model can aid in the diagnosis of glioma grade from MR perfusion images and that the model improves with increasing sample size.
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Single-shot echo-planar imaging (ss-EPI) has not been used widely for diffusion-weighted imaging (DWI) of the spinal cord, because of the magnetic field inhomogeneities around the spine, the small cross-sectional size of the spinal cord, and the increased motion in that area due to breathing, swallowing, and cerebrospinal fluid (CSF) pulsation. These result in artifacts with the usually long readout duration of the ss-EPI method. ⋯ In this work, a reduced FOV single-shot diffusion-weighted echo-planar imaging (ss-DWEPI) method is proposed, in which a 2D spatially selective echo-planar RF excitation pulse and a 180 degrees refocusing pulse reduce the FOV in the phase-encode (PE) direction, while suppressing the signal from fat simultaneously. With this method, multi slice images with higher in-plane resolutions (0.94 x 0.94 mm(2) for sagittal and 0.62 x 0.62 mm(2) for axial images) are achieved at 1.5 T, without the need for a longer readout.
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Comparative Study
Measurement of T1 and T2 in the cervical spinal cord at 3 tesla.
T(1) and T(2) were measured for white matter (WM) and gray matter (GM) in the human cervical spinal cord at 3T. T(1) values were calculated using an inversion-recovery (IR) and B(1)-corrected double flip angle gradient echo (GRE) and show significant differences (p = 0.002) between WM (IR = 876 +/- 27 ms, GRE = 838 +/- 54 ms) and GM (IR = 973 +/- 33 ms, GRE = 994 +/- 54 ms). IR showed significant difference between lateral and dorsal column WM (863 +/- 23 ms and 899 +/- 18 ms, respectively, p = 0.01) but GRE did not (p = 0.40). ⋯ WM relaxation times were similar to brain structures with very dense fiber packing (e.g., corpus callosum), while GM values resembled deep GM in brain. Optimized sequence parameters for maximal contrast between WM and GM, and between WM and cerebrospinal fluid (CSF) were derived. Since the spinal cord has rostral-caudal symmetry, we expect these findings to be applicable to the whole cord.