Intuitive design guidelines for fast spin echo imaging with variable flip angle echo trains.

We present a simple and intuitive means for determining the flip angles (FAs) required for smooth transitions between static pseudo steady states (SPSSs) in fast spin echo (FSE) imaging with variable FA (VFA) echo trains. We demonstrate the effectiveness of single and multiple transition pulses to successfully vary refocusing FAs while retaining high signal levels. The graphical interpretation presented here is consistent with previous analytical techniques and permits accurate signal-intensity predictions along the echo train.     

Optimized three‐dimensional fast‐spin‐echo MRI

Spin‐echo‐based acquisitions are the workhorse of clinical MRI because they provide a variety of useful image contrasts and are resistant to image artifacts from radio‐frequency or static field inhomogeneity. Three‐dimensional (3D) acquisitions provide datasets that can be retrospectively reformatted for viewing in freely selectable orientations, and are thus advantageous for evaluating the complex anatomy associated with many clinical applications of MRI. Historically, however, 3D spin‐echo‐based acquisitions have not played a significant role in clinical MRI due to unacceptably long acquisition times or image artifacts associated with details of the acquisition method. Recently, optimized forms of 3D fast/turbo spin‐echo imaging have become available from several MR‐equipment manufacturers (for example, CUBE [GE], SPACE [Siemens], and VISTA [Philips]). Through specific design strategies and optimization, including short non–spatially selective radio‐frequency pulses to significantly shorten the echo spacing and variable flip angles for the refocusing radio‐frequency pulses to suppress blurring or considerably lengthen the useable duration of the spin‐echo train, these techniques permit single‐slab 3D imaging of sizeable volumes in clinically acceptable acquisition times. These optimized fast/turbo spin‐echo pulse sequences provide a robust and flexible approach for 3D spin‐echo‐based imaging with a broad range of clinical applications. J. Magn. Reson. Imaging 2014;39:745–767. © 2014 Wiley Periodicals, Inc .     

Time‐efficient fast spin echo imaging at 4.7 T with low refocusing angles

An implementation of fast spin echo at 4.7 T designed for versatile and time‐efficient T₂‐weighted imaging of the human brain is presented. Reduced refocusing angles (α < 180°) were employed to overcome specific absorption rate (SAR) constraints and their effects on image quality assessed. Image intensity and tissue contrast variations from heterogeneous RF transmit fields and incidental magnetization transfer effects were investigated at reduced refocusing angles. We found that intraslice signal variations are minimized with refocusing angles near 180°, but apparent gray/white matter contrast is independent of refocusing angle. Incidental magnetization transfer effects from multislice acquisitions were shown to attenuate white matter intensity by 25% and gray matter intensity by 15% at 180°; less than 5% attenuation was seen in all tissues at flip angles below 60°. We present multislice images acquired without excess delay time for SAR mitigation using a variety of protocols. Subsecond half Fourier acquisition single‐shot turbo spin echo (HASTE) images were obtained with a novel variable refocusing angle echo train (20° < α < 58°) and high‐resolution scans with a voxel volume of 0.18 mm³ were acquired in 6.5 min with refocusing angles of 100°. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

3D fast spin echo with out‐of‐slab cancellation: A technique for high‐resolution structural imaging of trabecular bone at 7 tesla

Spin‐echo‐based pulse sequences are desirable for the application of high‐resolution imaging of trabecular bone but tend to involve high‐power deposition. Increased availability of ultrahigh field scanners has opened new possibilities for imaging with increased signal‐to‐noise ratio (SNR) efficiency, but many pulse sequences that are standard at 1.5 and 3 T exceed specific absorption rate limits at 7 T. A modified, reduced specific absorption rate, three‐dimensional, fast spin‐echo pulse sequence optimized specifically for in vivo trabecular bone imaging at 7 T is introduced. The sequence involves a slab‐selective excitation pulse, low‐power nonselective refocusing pulses, and phase cycling to cancel undesired out‐of‐slab signal. In vivo images of the distal tibia were acquired using the technique at 1.5, 3, and 7 T field strengths, and SNR was found to increase at least linearly using receive coils of identical geometry. Signal dependence on the choice of refocusing flip angles in the echo train was analyzed experimentally and theoretically by combining the signal from hundreds of coherence pathways, and it is shown that a significant specific absorption rate reduction can be achieved with negligible SNR loss. Magn Reson Med 63:719–727, 2010. © 2010 Wiley‐Liss, Inc.     

Gradient moment nulling in fast spin echo

The fast spin echo sequence combines data from many echo signals in a Carr-Purcell-Meiboom-Gill echo train to form a single image. Much of the signal in the second and later echoes results from the coherent addition of stimulated echo signal components back to the spin echo signal. Because stimulated echoes experience no dephasing effects during the time that they are stored as M, magnetization, they experience a different gradient first moment than does the spin echo. This leads to flow-related phase differences between different echo components and results in flow voids and ghosting, even when the first moment is nulled for the spin echo signal. A method of gradient moment nulling that correctly compensates both spin echo and stimulated echo components has been developed. The simplest solution involves nulling the first gradient moment at least at the RF pulses and preferably at both the RF pulses and the echoes. Phantom and volunteer studies demonstrate good suppression of flowrelated artifacts.     

Multiecho sequences with variable refocusing flip angles: optimization of signal behavior using smooth transitions between pseudo steady states (TRAPS).

A variation of the rapid acquisition with relaxation enhancement (RARE) sequence (also called turbo spin-echo (TSE) or fast spin-echo (FSE)) is presented. This technique uses variable flip angles along the echo train such that magnetization is initially prepared into the static pseudo steady state (PSS) for a low refocusing flip angle (alpha < 180 degrees ). It is shown that after such a preparation, magnetization will always stay very close to the static PSS even after significant variation of the subsequent refocusing flip angles. This allows the design of TSE sequences in which high refocusing flip angles yielding 100% of the attainable signal are applied only for the important echoes encoding for the center of k-space. It is demonstrated that a reduction of the RF power (RFP) by a factor of 2.5-6 can be achieved without any loss in signal intensity. The contribution of stimulated-echo pathways leads to a reduction of the effective TE by a factor f(t), which for typical implementations is on the order of 0.5-0.8. This allows the use of longer echo readout times, and thus longer echo trains, for acquiring images with a given T(2) contrast.     

Intracranial black-blood MR angiography with high-resolution 3D fast spin echo

Three-dimensional fast spin-echo (3DFSE) techniques are promising for black-blood imaging of cerebral vessels. In this study, flow-related signal dephasing was demonstrated as the primary mechanism for blood signal attenuation. Parameter optimization of TR (1500 to 3000 ms), receiver bandwidth (25 to 31.25 kHz), effective TE (25.7 to 30.1 ms), and ETL (7 to 8) was accomplished by making measurements of vessel-totissue contrast-to-noise ratios on vessels. A comparison of high-resolution 3DFSE and 3DTOF magnetic resonance angiography demonstrated that 3DFSE can generate images with equivalent or better small vessel detail than conventional techniques. 3DFSE black-blood techniques may provide improved sensitivity of small arteries and veins with slow or in-plane flow and immunity to flow-related distortions. Future studies with optimized parameters will determine the clinical efficacy of this technique.     

Reduced RF power without blurring: correcting for modulation of refocusing flip angle in FSE sequences.

In order to reduce the RF power deposition of fast spin echo sequences operated at high field strength, the flip angles of the refocusing pulse train are varied from pulse to pulse using a modulated angle refocusing train method. The technique employs high flip angle pulses prior to sampling the center of k-space in order to preserve T(2) contrast, low flip angles after sampling the center of k-space to reduce power and prolong relaxation, and a smooth transition between the high and low flip angle regimes in order to maintain the pseudosteady-state, maximizing signal and avoiding artifact-inducing oscillations. An analytical expression is used to predict and correct for the flip angle dependence of the signal, thus eliminating any deleterious effects of flip angle modulation on the point spread function. Analysis of resolution and SNR were performed in simulation and phantom studies. In human imaging studies, it is shown that RF energy deposition per slice in a single-shot fast spin echo application can be reduced by up to 75%, making the sequence as practical at 3 T as it is has been at 1.5 T.     

Investigation and modeling of magnetization transfer effects in two‐dimensional multislice turbo spin echo sequences with low constant or variable flip angles at 3 T

Magnetization transfer effects represent a major source of contrast in multislice turbo spin echo sequences (TSE)/fast spin echo sequences. Generally, low refocusing flip angles have become common in such MRI sequences, especially to mitigate specific absorption rate problems. Since the strength of magnetization transfer effects is related to the radiofrequency power and therefore specific absorption rate applied, magnetization transfer induced signal attenuations are investigated for a variety of TSE sequences with low constant and variable flip angles. Noticeable differences between the sequences have been observed. In particular, fewer signal attenuations are observed for TSE with low flip angles such as hyperecho‐TSE and smooth transitions between pseudo steady states–TSE, leading to contrast that is less dependent on the number of slices. It is shown that the strength of the magnetization transfer‐induced signal attenuations can be understood and described by a physical framework, which is based on the mean square flip angle of a given TSE sequence. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Reduction of B1 sensitivity in selective single‐slab 3d turbo spin echo imaging with very long echo trains

Single‐slab 3D turbo/fast spin echo (SE) imaging with very long echo trains was recently introduced with slab selection using a highly selective excitation pulse and short, nonselective refocusing pulses with variable flip angles for high imaging efficiency. This technique, however, is vulnerable to image degradation in the presence of spatially varying B₁ amplitudes. In this work we develop a B₁ inhomogeneity‐reduced version of single‐slab 3D turbo/fast SE imaging based on the hypothesis that it is critical to achieve spatially uniform excitation. Slab selection was performed using composite adiabatic selective excitation wherein magnetization is tipped into the transverse plane by a nonselective adiabatic‐half‐passage pulse and then slab is selected by a pair of selective adiabatic‐full‐passage pulses. Simulations and experiments were performed to evaluate the proposed technique and demonstrated that this approach is a simple and efficient way to reduce B₁ sensitivity in single‐slab 3D turbo/fast SE imaging with very long echo trains. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.