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Readout Gradient (readout + gradient)
Selected AbstractsMinimization of Nyquist ghosting for echo-planar imaging at ultra-high fields based on a "negative readout gradient" strategyJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 5 2009Wietske van der Zwaag PhD Abstract Purpose: To improve the traditional Nyquist ghost correction approach in echo planar imaging (EPI) at high fields, via schemes based on the reversal of the EPI readout gradient polarity for every other volume throughout a functional magnetic resonance imaging (fMRI) acquisition train. Materials and Methods: An EPI sequence in which the readout gradient was inverted every other volume was implemented on two ultrahigh-field systems. Phantom images and fMRI data were acquired to evaluate ghost intensities and the presence of false-positive blood oxygenation level-dependent (BOLD) signal with and without ghost correction. Three different algorithms for ghost correction of alternating readout EPI were compared. Results: Irrespective of the chosen processing approach, ghosting was significantly reduced (up to 70% lower intensity) in both rat brain images acquired on a 9.4T animal scanner and human brain images acquired at 7T, resulting in a reduction of sources of false-positive activation in fMRI data. Conclusion: It is concluded that at high B0 fields, substantial gains in Nyquist ghost correction of echo planar time series are possible by alternating the readout gradient every other volume. J. Magn. Reson. Imaging 2009;30:1171,1178. © 2009 Wiley-Liss, Inc. [source] Improved slice selection for R2* mapping during cryoablation with eddy current compensation,JOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 1 2008Aiming Lu PhD Abstract Purpose To improve the slice profile and image quality of R2* mapping in the iceball during cryoablation with ultrashort echo time (UTE) imaging by compensating for eddy currents induced by the selective gradient when half-pulse radiofrequency (RF) excitation is employed to achieve UTEs. Materials and Methods A method to measure both B0 and linear eddy currents simultaneously is first presented. This is done with a least-square fitting process on calibration data collected on a phantom. Eddy currents during excitation are compensated by redesigning the RF pulse and the selective gradient accordingly, while that resultant from the readout gradient are compensated for during image reconstruction. In vivo data were obtained continuously during the cryoablation experiments to calculate the R2* values in the iceball and to correlate them with the freezing process. Results Image quality degradation due to eddy currents is significantly reduced with the proposed approaches. R2* maps of iceball throughout the cryoablation experiments were achieved with improved quality. Conclusion The proposed approaches are effective for compensating eddy currents during half-pulse RF excitation as well as readout. TEs as short as 100 ,sec were obtained, allowing R2* maps to be obtained from frozen tissues with improved quality. J. Magn. Reson. Imaging 2008;28:190,198. © 2008 Wiley-Liss, Inc. [source] Effect of slice angle on inhomogeneity artifact and its correction in slice-selective MR imagingCONCEPTS IN MAGNETIC RESONANCE, Issue 4 2009Kwan-Jin Jung Abstract The inhomogeneity of a local magnetic field causes an image artifact of geometric distortion and intensity abnormality because of the slice offset and readout shift in slice-selective MR imaging. It has been found that this artifact can be corrected by the projection of the slice offset onto the readout axis at a certain oblique slice angle. The slice angle for the artifact correction is determined by the amplitude of slice selection and readout gradients, and is independent of the magnetic field inhomogeneity and the main magnetic field direction. In addition, the existing view-angle tilting technique is found to be valid only for the slice orientation orthogonal to the object axis. The slice angle effect on the inhomogeneity artifact was confirmed experimentally through phantom and volunteer's head imaging for both regular and view-angle tilted spin echo sequences at 3 T. © 2009 Wiley Periodicals, Inc.Concepts Magn Reson Part A 34A: 238,248, 2009. [source] Multishot diffusion-weighted SPLICE PROPELLER MRI of the abdomenMAGNETIC RESONANCE IN MEDICINE, Issue 5 2008Jie Deng Abstract Multishot FSE (fast spin echo)-based diffusion-weighted (DW)-PROPELLER (periodically rotated overlapping parallel lines with enhanced reconstruction) MRI offers the potential to reduce susceptibility artifacts associated with single-shot DW-EPI (echo-planar imaging) approaches. However, DW-PROPELLER in the abdomen is challenging due to the large field-of-view and respiratory motion during DW preparation. Incoherent signal phase due to motion will violate the Carr-Purcell-Meiboom-Gill (CPMG) conditions, leading to destructive interference between spin echo and stimulated echo signals and consequent signal cancellation. The SPLICE (split-echo acquisition of FSE signals) technique can mitigate non-CPMG artifacts in FSE-based sequences. For SPLICE, spin echo and stimulated echo are separated by using imbalanced readout gradients and extended acquisition window. Two signal families each with coherent phase properties are acquired at different intervals within the readout window. Separate reconstruction of these two signal families can avoid destructive phase interference. Phantom studies were performed to validate signal phase properties with different initial magnetization phases. This study evaluated the feasibility of combining SPLICE and PROPELLER for DW imaging of the abdomen. It is demonstrated that DW-SPLICE-PROPELLER can effectively mitigate non-CPMG artifacts and improve DW image quality and apparent diffusion coefficient (ADC) map homogeneity. Magn Reson Med 59:947,953, 2008. © 2008 Wiley-Liss, Inc. [source] Centering the projection reconstruction trajectory: Reducing gradient delay errors,MAGNETIC RESONANCE IN MEDICINE, Issue 1 2003Dana C. Peters Abstract The projection reconstruction (PR) trajectory was investigated for the effect of gradient timing delays between the actual and requested start time of each physical gradient. Radial trajectories constructed with delayed gradients miss the center of k -space in an angularly dependent manner, causing effective echo times to vary with projection angle. The gradient timing delays were measured in phantoms, revealing delays on the x, y, and z gradients which differed by as much as 5 ,sec. Using this one-time calibration measurement, the trajectories were corrected for gradient delays by addition of compensatory gradient areas to the prephasers of the logical x and y readout gradients. Effective projection-to-projection echo time variability was reduced to less than 1 ,sec for all imaging orientations. Using corrected trajectories, artifacts were reduced in phantom images and in volunteer studies. This correction should potentiate greater clinical use of the PR trajectory. Magn Reson Med 50:1,6, 2003. Published 2003 Wiley-Liss, Inc. [source] | ||||||||||