Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine
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Comparative Study
New insights into the mechanisms of signal formation in RF-spoiled gradient echo sequences.
RF spoiling is a well established method to produce T(1)-weighted images with short repetition-time gradient-echo sequences, by eliminating coherent transverse magnetization with appropriate RF phase modulation. This paper presents 2 novel approaches to describe signal formation in such sequences. Both methods rely on the formulation of RF spoiling as a linear increase of the precession angle between RF pulses, which is an alternative to the commonly used quadratic pulse phase scheme. ⋯ This provides a physical interpretation of the deviations from ideal spoiling behavior in FLASH and echo-shifted sequences. The results of the partition method in the small flip angle regime are compared with numerical simulations based on a Fourier decomposition of magnetization states. Measurements performed with in vitro solutions were in good agreement with numerical simulations at short relaxation times (T(1)/TR = 32 and T(2)/TR = 4); larger deviations occurred at long relaxation times (T(1)/TR = 114 and T(2)/TR = 82).
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In 3D MRI, sampling k-space with traditional trajectories can be excessively time-consuming. Fast imaging trajectories are used in an attempt to efficiently cover the k-space and reduce the scan time without significantly affecting the image quality. In many applications, further reductions in scan time can be achieved via undersampling of the k-space; however, no clearly optimal method exists. ⋯ This can be particularly efficient because in these types of trajectories the contribution of new information by later shots is less significant. In this work the performance of progressive trajectories for different degrees of undersampling is assessed with trajectories based on missile guidance (MG) ideas. The results show that the approach can be efficient in terms of reducing the scan time, and performs better than the stack of spirals (SOS) technique, particularly under nonideal conditions.
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A method for simultaneous multislice imaging is presented that uses a multislice RF pulse that imparts a different linear phase profile to each slice. During readout, slices are unaliased by using extra slice-select gradient lobes, which rephase and dephase individual slices one at a time. Compared to other simultaneous slice methods, this method avoids distortion by slice-select gradients, and does not require extra views or additional hardware. ⋯ This can cause non-ideal rephasing of the individual slices due to susceptibility gradients, which manifests itself as crosstalk between slices. There is also a concomitant increase in the minimum TR of the sequence. The method is demonstrated with phantom and in vivo images using gradient-echo and spin-echo versions.
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In balanced steady-state free precession (b-SSFP) sequences, uncompensated first-order moments of encoding gradients induce a nonconstant phase evolution for moving spins within the excitation train, resulting in signal loss and image artifacts. To restore these flow-related phase perturbations, "pairing" of consecutive phase-encoding (PE) steps is compared with a fully flow-compensated sequence using compensating gradient waveforms along all three encoding directions. ⋯ Nevertheless, the results of phantom experiments indicate that the pairing technique becomes ineffective at flow velocities exceeding roughly 0.5-1 m/s. Consequently, the additional scan time required to null the first gradient moments in a flow-compensated b-SSFP sequence makes the "pairing" technique preferable for applications in which slow to moderate flow velocities can be expected.
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The noise analysis for three-point decomposition of water and fat was extended to account for the uncertainty in the field map. This generalization leads to a nonlinear estimation problem. The Crámer-Rao bound (CRB) was used to study the variance of the estimates of the magnitude, phase, and field map by computing the maximum effective number of signals averaged (NSA) for any choice of echo time shifts. ⋯ With this choice of echo time shifts it is possible to achieve the maximum NSA uniformly across all fat:water ratios. The optimization is also carried out for the estimation of phase and field map. These theoretical results were verified using Monte Carlo simulations with a newly developed nonlinear least-squares reconstruction algorithm that achieves the CRB.