Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine
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Comparative Study
In vivo method for correcting transmit/receive nonuniformities with phased array coils.
Phased array coils are finding widespread applications in both the research and the clinical setting. However, intensity nonuniformities with such coils can reduce the potential benefits of these coils, particularly for applications such as tissue segmentation. In this work, a method is described for correcting the nonuniform signal response based on in vivo measures of both the transmission field of body coil and the reception sensitivity of phased array coils, separately. ⋯ This approach reduces the ratio between signal intensity SD of an image and its mean intensity from approximately 21% before correction to 13% after correction. Results are also shown demonstrating the utility of this approach in vivo with human brain images. The method is general and can be applied with most pulse sequences, any coil combination for transmission and reception, and in any anatomic region.
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The goal of this study was to measure the interindividual reproducibility of glutamate quantification in 1.5-T (1)H MRS of human brains. To determine the effective echo time (TE) for glutamate quantification, spectra from a phantom and 12 participants were obtained with TE = 30, 35, 40, and 144 ms (repetition time (TR) = 2000 ms and volume of interest = 4 cm(3)). The average Cramer-Rao lower bounds for glutamate quantification using LCModel was lowest in two experiments when TE = 40 ms. ⋯ Spectra were acquired with TR = 6000 ms and TE = 40 ms. Results showed that the coefficients of variance were 11.0 and 13.1% in the anterior cingulate cortex and insula, respectively. This suggests that glutamate can be reproducibly measured from 1.5-T (1)H MRS with long TR, effective TE, and the LCModel.
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The standard pharmacokinetic model applied to contrast reagent (CR) bolus-tracking (B-T) MRI (dynamic-contrast-enhanced) data makes the intrinsic assumption that equilibrium transcytolemmal water molecule exchange is effectively infinitely fast. Theory and simulation have suggested that this assumption can lead to significant errors. Recent analyses of animal model experimental data have confirmed two predicted signature inadequacies: a specific temporal mismatch with the B-T time-course and a CR dose-dependent underestimation of model parameters. ⋯ It is effectively eliminated by use of the shutter-speed model. The size of the mismatch is considerably greater for the IDC lesions than for the FA lesions, causing the shutter-speed model to exhibit improved discrimination of malignant IDC tumors from the benign FA lesions compared with the standard model. Furthermore, the shutter-speed model clearly reveals focal "hot spots" of elevated CR perfusion/permeation present in only the malignant tumors.
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Attenuating the static signal in arterial spin tagging (ASSIST) was initially developed for 3D imaging of cerebral blood flow. To enable the simultaneous collection of cerebral blood flow and BOLD data, a multi-slice version of ASSIST is proposed. ⋯ The utility of the sequence was demonstrated by simultaneously acquiring ASSIST and BOLD data during a functional task and by collecting resting-state ASSIST data over a large number of slices. In addition, the temporal stability of the perfusion signal was found to be 60% greater at 3 T compared to 1.5 T, which was attributed to the insensitivity of ASSIST to physiologic noise.