Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine
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In echo-planar-based diffusion-weighted imaging (DWI) and diffusion tensor imaging (DTI), the evaluation of diffusion parameters such as apparent diffusion coefficients and anisotropy indices is affected by image distortions that arise from residual eddy currents produced by the diffusion-sensitizing gradients. Correction methods that coregister diffusion-weighted and non-diffusion-weighted images suffer from the different contrast properties inherent in these image types. Here, a postprocessing correction scheme is introduced that makes use of the inverse characteristics of distortions generated by gradients with reversed polarity. ⋯ Furthermore, the acquisition of an additional dataset with moderate diffusion-weighting as suggested by Haselgrove and Moore (Magn Reson Med 1996;36:960-964) is not required. With phantom data it is shown that the theoretically expected symmetry of distortions is preserved in the images to a very high degree, demonstrating the practicality of the new method. Results from human brain images are also presented.
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This work describes a new approach to multipoint Dixon fat-water separation that is amenable to pulse sequences that require short echo time (TE) increments, such as steady-state free precession (SSFP) and fast spin-echo (FSE) imaging. Using an iterative linear least-squares method that decomposes water and fat images from source images acquired at short TE increments, images with a high signal-to-noise ratio (SNR) and uniform separation of water and fat are obtained. This algorithm extends to multicoil reconstruction with minimal additional complexity. ⋯ Examples in the knee, ankle, pelvis, abdomen, and heart are shown, using FSE, SSFP, and spoiled gradient-echo (SPGR) pulse sequences. The algorithm was applied to systems with multiple chemical species, and an example of water-fat-silicone separation is shown. An analysis of the noise performance of this method is described, and methods to improve noise performance through multicoil acquisition and field map smoothing are discussed.
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Contrast-enhanced (CE) coronary magnetic resonance angiography (MRA) following intraarterial (IA) injection of contrast agent was compared using two sequences in swine: magnetization-prepared fast imaging with steady-state precession (True-FISP), and magnetization-prepared fast low-angle shot (FLASH). Thick-slice projection images were acquired with submillimeter in-plane spatial resolution (0.9 x 0.8 mm(2)). ⋯ The True-FISP acquisition resulted in an increase in signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) by approximately a factor of 2 over FLASH (P < 0.05). Magnetization-prepared True-FISP is a promising technique for catheter-directed CE thick-slice projection coronary MRA.
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Contrast reagents (CRs) may enter the tissue interstitium for a period after a vascular bolus injection. As the amount of interstitial CR increases, the longitudinal relaxographic NMR "shutter-speed" (T(-1)) for the equilibrium transcytolemmal water exchange process increases. The quantity T(-1) is given by |r(1o)[CR(o)] + R(1o0) - R(1i)| (where r(1o) and [CR(o)] represent the interstitial (extracellular) CR relaxivity and concentration, respectively, and R(1o0) and R(1i) are the extra- and intracellular (1)H(2)O relaxation rate constants, respectively, in the absence of exchange). ⋯ Thus, even relative pharmacokinetic quantities can be incorrectly represented in a parametric map that neglects this effect. The BOLERO analysis shows promise for in vivo vascular phenotyping in pathophysiology. It also includes a provision for approximating the separation of the perfusion and permeability contributions to CR extravasation.
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Comparative Study
Comparison of longitudinal metabolite relaxation times in different regions of the human brain at 1.5 and 3 Tesla.
In vivo longitudinal relaxation times of N-acetyl compounds (NA), choline-containing substances (Cho), creatine (Cr), myo-inositol (mI), and tissue water were measured at 1.5 and 3 T using a point-resolved spectroscopy (PRESS) sequence with short echo time (TE). T(1) values were determined in six different brain regions: the occipital gray matter (GM), occipital white matter (WM), motor cortex, frontoparietal WM, thalamus, and cerebellum. ⋯ The amounts of GM, WM, and cerebrospinal fluid (CSF) within the voxel were determined by segmentation of a 3D image data set. No influence of tissue composition on metabolite T(1) values was found, while the longitudinal relaxation times of water protons were strongly correlated with the relative GM content.