Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine
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Diffusion tensor MRI (DTI) using conventional single-shot (SS) 2D diffusion-weighted (DW)-EPI is subject to severe susceptibility artifacts. Multishot DW imaging (DWI) techniques can reduce these distortions, but they generally suffer from artifacts caused by motion-induced phase errors. Parallel imaging can also reduce the distortions if the sensitivity profiles of the receiver coils allow a sufficiently high reduction factor for the desired field of view (FOV). ⋯ Because the DW magnetization is stored in the longitudinal direction until readout, it undergoes T(1) rather than T(2) decay. Inner volume imaging (IVI) is used to limit the imaging volume. This reduces the time required for EPI readout of each complete k(x)-k(y) plane, and hence reduces T(2)(*) decay during the readout and T(1) decay between the readout of each k(z). 3D ss-STEPI images appear to be free of severe susceptibility and motion artifacts. 3D ss-STEPI allows high-resolution DTI of limited volumes of interest, such as localized brain regions, cervical spinal cord, optic nerve, and other extracranial organs.
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An inhomogeneous radiofrequency (RF) magnetic field is an essential source of error for the quantification of MRI and MRS parameters. To correct for effects of RF inhomogeneities in 3D data sets, it is necessary to have knowledge of the 3D RF distribution in the sample. In this paper a method for fast 3D RF mapping is presented. ⋯ The acquisition of the 3D RF map using 64 partitions and TR = 500 ms requires 1.5 min. The use of the sequence in vivo is demonstrated by the calculation of the RF maps in the human brain at 3T. The comparison of calculated flip angles with the flip angles obtained by fitting signal behavior in the 3D stimulated-echo acquisition mode (STEAM)-EPI sequence and the analysis of errors due to spatially dependent T(1) values in the brain show that the accuracy of the calculated flip angles in the human brain is about 2 degrees.
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The mechanism behind previously observed changes in the apparent diffusion coefficient (ADC) during brain activation is not well understood. Therefore, we investigated the signal source and spatial specificity of functional magnetic resonance imaging (fMRI) ADC changes systematically in the visual cortex of cats using diffusion-weighted (DW) spin-echo (SE) fMRI with b-values of 2, 200, and 800 s/mm(2), and echo times (TE) of 16, 28, and 60 ms at 9.4 T. For b > or = 200 s/mm(2), no ADC changes were detected in brain parenchyma, suggesting a minimal tissue contribution to the ADC change. ⋯ At TE = 16 ms, the highest ADC changes occurred at the cortical surface with its large draining veins, which can mainly be explained by an additional contribution from the venous blood oxygenation changes. Our TE-dependent ADC results agree with computer simulations based on a three-compartment model. The contribution of arterial blood volume changes in ADC fMRI offers an improvement in spatial localization for SE-BOLD fMRI studies.
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The proton NMR transverse relaxation time T(2) of glutamate (Glu) in human brain was measured by means of spectrally selective refocusing at 3.0 T in vivo. An 81.4-ms-long dual-band Gaussian 180 degrees RF pulse, designed for refocusing at 2.35 and 3.03 ppm, was employed within point-resolved spectroscopy (PRESS) to generate the Glu C4-proton target multiplet and the total creatine (tCr) singlet. ⋯ Apparent T(2) values of Glu and tCr were estimated as 201 +/- 18 and 164 +/- 12 ms for the medial prefrontal (PF) cortex, and 198 +/- 22 and 169 +/- 15 ms (mean +/- SD, N = 5) for the left frontal (LF) cortex, respectively. With water segmentation data, the magnetization values of Glu and tCr of the two adjacent voxels, calculated from the T(2) values and spectra following the thermal equilibrium magnetization, were combined to give the Glu and tCr concentrations as 10.37 +/- 1.06 and 8.87 +/- 0.56 mM for gray matter (GM), and 5.06 +/- 0.57 and 5.16 +/- 0.45 mM (mean +/- SD, N = 5) for white matter (WM), respectively.
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Cardiac pulsatility causes a nonrigid motion of the brain. In multi-shot diffusion imaging this leads to spatially varying phase changes that must be corrected. A conjugate gradient based reconstruction is presented that includes phase changes measured using two-dimensional navigator echoes, coil sensitivity information, navigator-determined weightings, and data from multiple coils and averages. ⋯ In a higher slice, fiber directions derived from single-shot data show distortions from anatomical scans by as much as 7 mm compared to less than 2 mm for our multi-shot reconstructions. The reduced distortions imply that phase encoding can be applied in the shorter left-right direction, enabling time savings through the use of a rectangular field of view. Higher resolution diffusion imaging in the spine permits visualization of a nerve root.