Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine
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Intrasubject reproducibility of metabolite quantitation in three-dimensional proton magnetic resonance spectroscopic imaging (3D-MRSI) was investigated in 10 healthy volunteers over five separate sessions using two echo times (TEs): 144 and 30 ms. The use of a Gill-Thomas-Cosman (GTC) stereotactic head frame enabled precise subject repositioning and immobilization. Metabolite levels from each voxel in the volume of interest (VOI) were quantified using the Linear Combination of Model spectra (LCModel) analysis algorithm, and coefficients of variation (CVs) were calculated. ⋯ The 95% confidence interval (CI) was found for each metabolite, providing an indication of the normal fluctuation expected for 3D-MRSI. In vivo, median CVs at the %SD < or = 20 level were found to be (%CV for TE = 144 and 30 ms, respectively): N-acetyl-aspartate plus N-acetyl-aspartyl-glutamate (NAA): 10.2% and 13.5%; creatine plus phosphocreatine (Cr), 14.4% and 21.7%; and choline-containing compounds (Cho), 15.2% and 18.4%. Relaxing the statistical filtering criteria to %SD < or = 30 increased median CVs by less than 5% and permitted in vivo quantitation reproducibility to be evaluated for glutamine plus glutamate (Glx) and myoinositol (Ins) for TE = 30 ms, yielding CVs of 24.0% and 21.0%, respectively.
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In cerebral blood volume (CBV)-weighted functional MRI (fMRI) employing superparamagnetic contrast agent, iron dose and blood oxygenation level dependent (BOLD) contamination are two important issues for experimental design and CBV quantification. Both BOLD and CBV-weighted fMRI are based upon the susceptibility effect, to which spin-echo and gradient-echo sequences have different sensitivities. In the present study, CBV-weighted fMRI was conducted using spin-echo and gradient-echo sequences at 9.4T by systematically changing the doses of contrast agent. ⋯ A mathematical model was developed to quantify the extravascular (EV) BOLD effect. With a TE of 35 ms, the EV BOLD effect was estimated to account for 76+/-12% of the observed spin-echo fMRI signal at 9.4T. These data suggest that correcting BOLD effect may be necessary for accurately quantifying activation-induced CBV changes at high field.
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In this multicenter study, 2D spatial mapping of J-coupled resonances at 3T and 4T was performed using short-TE (15 ms) proton echo-planar spectroscopic imaging (PEPSI). Water-suppressed (WS) data were acquired in 8.5 min with 1-cm(3) spatial resolution from a supraventricular axial slice. Optimized outer volume suppression (OVS) enabled mapping in close proximity to peripheral scalp regions. ⋯ Metabolite concentrations were in the ranges reported in previous studies. Glu concentration was significantly higher in gray matter (GM) compared to white matter (WM), as anticipated. The short acquisition time makes this methodology suitable for clinical studies.
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Comparative Study
Noninvasive measurement of arterial cerebral blood volume using Look-Locker EPI and arterial spin labeling.
This paper describes a method of noninvasively measuring regional arterial cerebral blood volume fractions (CBV(a)) in vivo using the combination of Look-Locker echo-planar imaging (LL-EPI) with arterial spin labeling (ASL). Using this technique the arterial inflow curve is rapidly sampled and the regional CBV(a) is measured, while tissue perfusion signals are suppressed. Two methods of spin labeling (LL-EPI flow-sensitive alternating inversion recovery (LL-EPI-FAIR) and LL-EPI signal targeting using alternating radiofrequency (LL-EPI-STAR)) are assessed and their advantages discussed. ⋯ A 33 +/- 14% increase in CBV(a) on activation is found. CBV(a) is measured with a 6-s temporal resolution and the temporal response is compared with the BOLD signal change. CBV(a) is shown to increase more rapidly and return to baseline significantly faster than the BOLD signal change, which supports the suggestion that a change in CBV(a) is an input to the BOLD response.
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Previously published fast spin-echo (FSE) implementations of a Dixon method for water and fat separation all require multiple scans and thus a relatively long scan time. Further, the minimum echo spacing (esp), a time critical for FSE image quality and scan efficiency, often needs to be increased in order to bring about the required phase shift between the water and fat signals. This work proposes and implements a novel FSE triple-echo Dixon (fTED) technique that can address these limitations. ⋯ A fully automated postprocessing algorithm is then used to generate separate water-only and fat-only images for each slice. The technique was implemented with and without parallel imaging. We demonstrate that the new fTED technique enables both uniform water/fat separation and fast scanning with uncompromised scan parameters, including applications such as T(2)-weighted separate water and fat imaging of the abdomen during breath-holding.