共查询到20条相似文献,搜索用时 15 毫秒
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Benedikt A. Poser Markus Barth Pål‐Erik Goa Weiran Deng V. Andrew Stenger 《Magnetic resonance in medicine》2013,69(1):37-47
Echo planar imaging (EPI) is most commonly used for blood oxygen level‐dependent fMRI, owing to its sensitivity and acquisition speed. A major problem with EPI is Nyquist (N/2) ghosting, most notably at high field. EPI data are acquired under an oscillating readout gradient and hence vulnerable to gradient imperfections such as eddy current delays and off‐resonance effects, as these cause inconsistencies between odd and even k‐space lines after time reversal. We propose a straightforward and pragmatic method herein termed “interleaved dual echo with acceleration (IDEA) EPI”: two k‐spaces (echoes) are acquired under the positive and negative readout lobes, respectively, by performing phase encoding blips only before alternate readout gradients. From these two k‐spaces, two almost entirely ghost free images per shot can be constructed, without need for phase correction. The doubled echo train length can be compensated by parallel imaging and/or partial Fourier acquisition. The two k‐spaces can either be complex averaged during reconstruction, which results in near‐perfect cancellation of residual phase errors, or reconstructed into separate images. We demonstrate the efficacy of IDEA EPI and show phantom and in vivo images at both 3 T and 7 T. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc. 相似文献
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Determination of neurological pathology in white matter disease can be made in a semiquantitative way from T(1)- or T(2)-weighted images. A higher level of quantification based on measured T(1) or T(2) values has been either limited to specific regions of interest or to low-resolution maps. Higher-resolution T(1) maps have proved difficult to obtain due to the excessively long scan times required using conventional techniques. In this study, clinically acceptable images are obtained by using single-shot echo planar imaging (EPI) with an acquisition scheme that maximizes signal-to-noise while minimizing the scan time. Magn Reson Med 45:630-634, 2001. 相似文献
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Combined acquisition of gradient‐echo and spin‐echo signals in MRI time series reveals additional information for perfusion‐weighted imaging and functional MRI because of differences in the sensitivity of gradient‐echo and spin‐echo measurements to the properties of the underlying vascular architecture. The acquisition of multiple echo trains within one time frame facilitates the simultaneous estimation of the transversal relaxation parameters R2 and R. However, the simultaneous estimation of these parameters tends to be incorrect in the presence of slice profile mismatches between signal excitation and subsequent refocusing pulses. It is shown here that improvements in pulse design reduced R2 and R estimation errors. Further improvements were achieved by augmented parameter estimation through the introduction of an additional parameter δ to correct for discordances in slice profiles to facilitate more quantitative measurements. Moreover, the analysis of time‐resolved acquisitions revealed that the temporal stability of R2 estimates could be increased with improved pulse design, counteracting low contrast‐to‐noise ratios in spin‐echo‐based perfusion and functional MRI. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc. 相似文献
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In this work a method for considerably improving the signal‐to‐noise ratio (SNR) in T1 maps based on the variable flip angle approach is proposed, employing spoiled fast low angle shot (FLASH) echo‐planar imaging (EPI) hybrid sequences with two echoes per excitation. In phantom measurements it could be verified that the SNR improvement in the underlying images translated into an SNR increase in the T1 maps exceeding theoretical predictions. Even a hybrid sequence with an 18% shorter measurement time than a standard FLASH readout with identical spatial coverage and resolution yielded an SNR gain of 23% in the resulting T1 maps. Hybrid sequences with either identical measurement time (9:05 min) or bandwidth (9:30 min) yielded gains of 60% and 67%, respectively. These results could be confirmed by measurements on four healthy volunteers. The image quality of T1 maps based on hybrid sequences was excellent and the SNR improvement was clearly visible. The measured SNR gains in T1 maps were between 20% (shortest sequence, white matter) and 66% (sequence with identical bandwidth, gray matter). The resulting T1 values were comparable, with a slight tendency toward higher values in the hybrid sequences. In summary, without prolonging experiment durations the method proposed yields SNR gains that are commonly achieved by acquiring two averages. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc. 相似文献
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Manojkumar Saranathan Mohammad Mehdi Khalighi Gary H. Glover Prachi Pandit Brian K. Rutt 《Magnetic resonance in medicine》2013,70(6):1669-1673
The Bloch‐Siegert (B‐S) B1+ mapping technique is a fast, phase‐based method that is highly SAR limited especially at 7T, necessitating the use of long repetition times. Spiral and echo‐planar readouts were incorporated in a gradient‐echo based B‐S sequence to reduce specific absoprtion rate (SAR) and improve its scan efficiency. A novel, numerically optimized 4 ms B‐S off‐resonant pulse at + 1960 Hz was used to increase sensitivity and further reduce SAR compared with the conventional 6 ms Fermi B‐S pulse. Using echo‐planar and spiral readouts, scan time reductions of 8–16 were achieved. By reducing the B‐S pulse width by a factor of 1.5, SAR was reduced by a factor of 1.5 and overall sensitivity was increased by a factor of 1.33 due to the nearly halved resonance offset of the new B‐S pulse. This was validated on phantoms and volunteers at 7 T. Magn Reson Med 70:1669–1673, 2013. © 2013 Wiley Periodicals, Inc. 相似文献
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Uwe Heinrichs Jane F. Utting Tobias Frauenrath Fabian Hezel Gabriele A. Krombach Michael A.J. Hodenius Sebastian Kozerke Thoralf Niendorf 《Magnetic resonance in medicine》2009,62(3):822-828
This study demonstrates the feasibility of applying free‐breathing, cardiac‐gated, susceptibility‐weighted fast spin‐echo imaging together with black blood preparation and navigator‐gated respiratory motion compensation for anatomically accurate T mapping of the heart. First, T maps are presented for oil phantoms without and with respiratory motion emulation (T = (22.1 ± 1.7) ms at 1.5 T and T = (22.65 ± 0.89) ms at 3.0 T). T relaxometry of a ferrofluid revealed relaxivities of R = (477.9 ± 17) mM?1s?1 and R = (449.6 ± 13) mM?1s?1 for UFLARE and multiecho gradient‐echo imaging at 1.5 T. For inferoseptal myocardial regions mean T values of 29.9 ± 6.6 ms (1.5 T) and 22.3 ± 4.8 ms (3.0 T) were estimated. For posterior myocardial areas close to the vena cava T‐values of 24.0 ± 6.4 ms (1.5 T) and 15.4 ± 1.8 ms (3.0 T) were observed. The merits and limitations of the proposed approach are discussed and its implications for cardiac and vascular T‐mapping are considered. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc. 相似文献
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A single‐shot T2 mapping protocol based on echo‐split gradient‐spin‐echo acquisition and parametric multiplexed sensitivity encoding based on projection onto convex sets reconstruction
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Mei‐Lan Chu Hing‐Chiu Chang Koichi Oshio Nan‐kuei Chen 《Magnetic resonance in medicine》2018,79(1):383-393
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A fast T1 measurement sequence using inversion recovery Look‐Locker echo‐planar imaging at steady state (IR LL‐EPI SS) is presented. Delay time for a full magnetization recovery is not required in the sequence, saving acquisition time significantly for high‐resolution T1 mapping. Imaging parameters of the IR LL‐EPI SS sequence were optimized to minimize the bias from the excitation pulses imperfection and to maximize the accuracy and reliability of T1 measurements, which are critical for its applications. Compared with the conventional inversion recovery Look‐Locker echo‐planar imaging (IR LL‐EPI) sequence, IR LL‐EPI SS method preserves similar accuracy and reliability, while saving 20% in acquisition time. Optimized IR LL‐EPI SS provided quantitative T1 mapping with 1 × 1 × 4 mm3 resolution and whole‐brain coverage (28 slices) in approximately 4 min. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc. 相似文献