MRI Pulse Sequences (mri + pulse_sequence)

Distribution by Scientific Domains


Selected Abstracts


General algorithm for automated off-center MRI

MAGNETIC RESONANCE IN MEDICINE, Issue 1 2006
J. Magland
Abstract A general formula was derived that automatically modifies any MRI pulse sequence to realize arbitrary field-of-view (FOV) shifts. Unlike conventional techniques for implementing off-center MRI, the new method is completely automatic and can therefore be incorporated into the scanner hardware or software, thereby simplifying the development of MRI pulse sequences. The algorithm was incorporated into a visual pulse sequence programming environment, and several pulse sequences were programmed and tested at various off-center locations using the new technique. Unless there is significant background field inhomogeneity or gradient nonlinearity, research sequences employing the automatic technique need only be programmed and tested at the gradient isocenter, whereas with conventional methods, artifacts can sometimes depend on the position of the FOV. Magn Reson Med, 2006. 2006 Wiley-Liss, Inc. [source]


Pulsed Z-spectroscopic imaging of cross-relaxation parameters in tissues for human MRI: Theory and clinical applications

MAGNETIC RESONANCE IN MEDICINE, Issue 5 2002
Vasily L. Yarnykh
Abstract A new method of pulsed Z-spectroscopic imaging is proposed for in vivo visualization and quantification of the parameters describing cross-relaxation between protons with liquid-like and solid-like relaxation properties in tissues. The method is based on analysis of the magnetization transfer (MT) effect as a function of the offset frequency and amplitude of a pulsed off- resonance saturation incorporated in a spoiled gradient-echo MRI pulse sequence. The theoretical concept of the method relies on an approximated analytical model of pulsed MT that provides a simple three-parameter equation for a pulsed steady-state Z-spectrum taken far from resonance. Using this model, the parametric images of cross-relaxation rate constant, content, and T2 of the semisolid proton fraction can be reconstructed from a series of MT-weighted images and a coregistered T1 map. The method was implemented on a 0.5 T clinical MRI scanner, and it provided high-quality 3D parametric maps within an acceptable scanning time. The estimates of cross-relaxation parameters in brain tissues were shown to be quantitatively consistent with the literature data. Clinical examples of the parametric images of human brain pathologies (multiple sclerosis and glioma) demonstrated high tissue contrast and clear visualization of the lesions. Magn Reson Med 47:929,939, 2002. 2002 Wiley-Liss, Inc. [source]


Handbook of MRI pulse sequences

JOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 1 2006
Michael Jacobs Ph.D.
[source]


Routine clinical brain MRI sequences for use at 3.0 Tesla

JOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 1 2005
Hanzhang Lu PhD
Abstract Purpose To establish image parameters for some routine clinical brain MRI pulse sequences at 3.0 T with the goal of maintaining, as much as possible, the well-characterized 1.5-T image contrast characteristics for daily clinical diagnosis, while benefiting from the increased signal to noise at higher field. Materials and Methods A total of 10 healthy subjects were scanned on 1.5-T and 3.0-T systems for T1 and T2 relaxation time measurements of major gray and white matter structures. The relaxation times were subsequently used to determine 3.0-T acquisition parameters for spin-echo (SE), T1 -weighted, fast spin echo (FSE) or turbo spin echo (TSE), T2 -weighted, and fluid-attenuated inversion recovery (FLAIR) pulse sequences that give image characteristics comparable to 1.5 T, to facilitate routine clinical diagnostics. Application of the routine clinical sequences was performed in 10 subjects, five normal subjects and five patients with various pathologies. Results T1 and T2 relaxation times were, respectively, 14% to 30% longer and 12% to 19% shorter at 3.0 T when compared to the values at 1.5 T, depending on the region evaluated. When using appropriate parameters, routine clinical images acquired at 3.0 T showed similar image characteristics to those obtained at 1.5 T, but with higher signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR), which can be used to reduce the number of averages and scan times. Recommended imaging parameters for these sequences are provided. Conclusion When parameters are adjusted for changes in relaxation rates, routine clinical scans at 3.0 T can provide similar image appearance as 1.5 T, but with superior image quality and/or increased speed. J. Magn. Reson. Imaging 2005;22:13,22. 2005 Wiley-Liss, Inc. [source]


Experimental determination of human peripheral nerve stimulation thresholds in a 3-axis planar gradient system

MAGNETIC RESONANCE IN MEDICINE, Issue 3 2009
Rebecca E. Feldman
Abstract In MRI, strong, rapidly switched gradient fields are desirable because they can be used to reduce imaging time, obtain images with better resolution, or improve image signal-to-noise ratios. Improvements in gradient strength can be made by either increasing the gradient amplifier strength or by enhancing gradient efficiency. Unfortunately, many MRI pulse sequences, in combination with high-performance amplifiers and existing gradient hardware, can cause peripheral nerve stimulation (PNS). This makes improvements in gradient amplifiers ineffective at increasing safely usable gradient strength. Customized gradient coils are one way to achieve significant improvements in gradient performance. One specific gradient configuration, a planar gradient system, promises improved gradient strength and switching time for cardiac imaging. The PNS thresholds for planar gradients were characterized through human stimulation experiments on all three gradient axes. The specialized gradient was shown to have significantly higher stimulation thresholds than traditional cylindrical designs (y-axis SRmin = 210 18 mT/m/ms and ,Gmin = 133 13 mT/m; x-axis SRmin = 222 24 mT/m/ms and ,Gmin = 147 17 mT/m; z-axis SRmin = 252 26 mT/m/ms and ,Gmin = 218 26 mT/m). This system could be operated at gradient strengths 2 to 3 times higher than cylindrical configurations without causing stimulation. Magn Reson Med, 2009. 2009 Wiley-Liss, Inc. [source]


XTC MRI: Sensitivity improvement through parameter optimization

MAGNETIC RESONANCE IN MEDICINE, Issue 6 2007
Kai Ruppert
Abstract Xenon polarization Transfer Contrast (XTC) MRI pulse sequences permit the gas exchange of hyperpolarized xenon-129 in the lung to be measured quantitatively. However, the pulse sequence parameter values employed in previously published work were determined empirically without considering the now-known gas exchange rates and the underlying lung physiology. By using a theoretical model for the consumption of magnetization during data acquisition, the noise intensity in the computed gas-phase depolarization maps was minimized as a function of the gas-phase depolarization rate. With such optimization the theoretical model predicted an up to threefold improvement in precision. Experiments in rabbits demonstrated that for typical imaging parameter values the optimized XTC pulse sequence yielded a median noise intensity of only about 3% in the depolarization maps. Consequently, the reliable detection of variations in the average alveolar wall thickness of as little as 300 nm can be expected. This improvement in the precision of the XTC MRI technique should lead to a substantial increase in its sensitivity for detecting pathological changes in lung function. Magn Reson Med 57:1099,1109, 2007. 2007 Wiley-Liss, Inc. [source]


General algorithm for automated off-center MRI

MAGNETIC RESONANCE IN MEDICINE, Issue 1 2006
J. Magland
Abstract A general formula was derived that automatically modifies any MRI pulse sequence to realize arbitrary field-of-view (FOV) shifts. Unlike conventional techniques for implementing off-center MRI, the new method is completely automatic and can therefore be incorporated into the scanner hardware or software, thereby simplifying the development of MRI pulse sequences. The algorithm was incorporated into a visual pulse sequence programming environment, and several pulse sequences were programmed and tested at various off-center locations using the new technique. Unless there is significant background field inhomogeneity or gradient nonlinearity, research sequences employing the automatic technique need only be programmed and tested at the gradient isocenter, whereas with conventional methods, artifacts can sometimes depend on the position of the FOV. Magn Reson Med, 2006. 2006 Wiley-Liss, Inc. [source]