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This study aimed to optimize velocity-selective magnetic resonance angiography (VS-MRA) protocols for whole-neck angiography and demonstrate its feasibility in healthy subjects with comparisons to clinical 3D time-of-flight (TOF) angiography. To help optimize VS-MRA protocols, 2D phase-contrast (PC) flow imaging and 3D B0 and B1 field mappings were performed on five healthy volunteers. Based on these measurements, a slab-selective (SS) inversion preparation was applied prior to a VS saturation preparation to further suppress venous blood, while the VS preparation pulse was designed with compensation for field offsets. VS-MRA and 3D TOF were performed on six healthy subjects, and relative contrast ratios (CRs) between artery and muscle signals were calculated for twenty arterial regions for comparisons. The pre-compensated VS pulse improved the visualization of the subclavian arteries and suppression of background tissues, which involved large B0 and B1 field errors. The combination of SS and VS preparations effectively suppressed venous blood. While the relative CR values were 0.78 ± 0.08 and 0.72 ± 0.10 for VS-MRA and 3D TOF, respectively, over the twenty segments, VS-MRA outperformed 3D TOF in visualizing arterial segments of a small size or with a horizontal orientation, such as subclavian, facial, and occipital arteries. The proposed neck VS-MRA with the field-error-compensated VS preparation combined with the SS preparation is feasible and superior to 3D TOF in visualizing small and/or horizontally oriented arterial segments. [ABSTRACT FROM AUTHOR]

The temporal resolution of dynamic magnetic resonance imaging (MRI) can be increased by sampling a fraction of k-space in an interleaved fashion, which introduces spatial and temporal aliasing. We describe algebraically and graphically the aliasing process caused by dynamic undersampled spiral imaging within 3-D xy f space (the Fourier transform of [k.sub.x][k.sub.y]t space) and formulate the unaliasing problem as a set of independent linear inversions. Since each linear system is numerically underdetermined, the use of prior knowledge in the form of bounded support regions is proposed. To overcome the excessive memory requirements for handling large matrices, a fast implementation of the conjugate gradient (CG) method is used. Numerical simulation and in vivo experiments using spiral twofold undersampling demonstrate reduced motion artifacts and the improved depiction of fine cardiac structures. The achieved reduction of motion artifacts and motion blur is comparable to simple filtering, which is computationally more efficient, while the proposed algebraic framework offers greater flexibility to incorporate additional algebraic acceleration techniques and to handle arbitrary sampling schemes. Index Terms--Algebraic reconstruction, dynamic imaging, fast imaging, spiral cardiac magnetic resonance imaging (MRI), temporal acceleration.

Objective. To develop a novel, free-induction-decay (FID)-calibrated single-shot simultaneous multi-slice fast spin echo (SMS-FSE) with very long hard pulse trains for high encoding efficiency and low energy deposition. Approach. The proposed single-shot SMS-FSE employs a mixed pulse configuration in which a long excitation pulse that is spatially multi-band (MB) selective is used in conjunction with short spatially nonselective refocusing pulses. To alleviate energy deposition to tissues while reducing signal modulation along the echo train, variable low flip angles with signal prescription are utilized in the refocusing pulse train. A time-efficient FID calibration and correction method is introduced before aliased voxels in the slice direction are resolved. Simulations and experiments are performed to demonstrate the feasibility of the proposed method as an alternative to conventional HASTE for generating T 2-weighted images. Main results. Compared with conventional HASTE, the proposed method enhances imaging speed effectively by an MB factor up to 5 without apparent loss of image contrast while successfully eliminating FID artifacts. Significance. We successfully demonstrated the feasibility of the proposed method as an encoding- and energy-efficient alternative to conventional HASTE for generation of T 2-weighted contrast. [ABSTRACT FROM AUTHOR]

Purpose: To evaluate both velocity and spatial responses of velocity‐selective arterial spin labeling (VS‐ASL), using velocity‐insensitive and velocity‐compensated waveforms for control modules, as well as a novel dynamic phase‐cycling approach, at different B0/B1+ field inhomogeneities. Methods: In the presence of imperfect refocusing, the mechanism of phase‐cycling the refocusing pulses through four dynamics was first theoretically analyzed with the conventional velocity‐selective saturation (VSS) pulse train. Numerical simulations were then deployed to compare the performance of the Fourier‐transform based velocity‐selective inversion (FT‐VSI) with these three different schemes in terms of both velocity and spatial responses under various B0/B1+ conditions. Phantom and human brain scans were performed to evaluate the three methods at B1+ scales of 0.8, 1.0, and 1.2. Results: The simulations of FT‐VSI showed that, under nonuniform B0/B1+ conditions, the scheme with velocity‐insensitive control was susceptible to DC bias of the static spins as systematic error, while the scheme with velocity‐compensated control had deteriorated velocity‐selective labeling profiles and, thus, reduced labeling efficiency. Through numerical simulation, phantom scans, and brain perfusion measurements, the dynamic phase‐cycling method demonstrated considerable improvements over these issues. Conclusion: The proposed dynamic phase‐cycling approach was demonstrated for the velocity‐selective label and control modules with both velocity and spatial responses robust to a wide range of B0 and B1+ field inhomogeneities. [ABSTRACT FROM AUTHOR]