Transverse relaxometry with reduced echo train lengths via stimulated echo compensation

Transverse relaxation (T₂) mapping has many applications, including imaging of iron accumulation in grey matter. Using the typical multiecho spin‐echo sequence with long echo trains, stimulated echo compensation can enable T₂ fitting under conditions of variable radio frequency homogeneity arising from slice profile and in‐plane radio frequency variation. Substantial reduction in the number of refocusing pulses could enable use at high magnetic fields where specific absorption rate is a major limitation, and enable multislice use with reduced incidental magnetization transfer at all field strengths. We examine the effect of reduced echo train lengths and multislice imaging on T₂ fitting using stimulated echo compensation applied to iron‐rich subcortical grey matter in human brain at 4.7 T. Our findings indicate that reducing from 20 echoes to as few as four echoes can maintain consistent T₂ values when using stimulated echo compensation in grey and white matter, but not for cerebrospinal fluid. All territories produce marginal results when using standard exponential fitting. Savings from reduced echoes can be used to substantially increase slice coverage. In multislice mode, the resulting incidental magnetization transfer decreased brain signal but had minimal effect on measured T₂ values. Magn Reson Med 70:1340–1346, 2013. © 2013 Wiley Periodicals, Inc.     

Stabilization of echo amplitudes in FSE sequences

The classical CPMG sequence and its extension as an imaging sequence, fast spin echo (FSE, based on RARE), suffer from signal magnitude variations in the early echoes when the re-focusing pulses are not set exactly to 180°. It has been suggested that by varying the value of the nutation angle of each refocusing pulse the signal magnitude could be made constant. This article describes an algorithm permitting the generation of sequences of nutation angles yielding series of echsoes with constant signal magnitudes. This result is then usesd to design selective pulses for the FSE imaging technique.     

Spin‐echo MRI using π/2 and π hyperbolic secant pulses

Frequency‐modulated (FM) pulses have practical advantages for spin‐echo experiments, such as the ability to produce a broadband π rotation, with an inhomogeneous radiofrequency (RF) field. However, such use leads to a nonlinear phase of the transverse magnetization, which is why FM pulses like the hyperbolic secant (HS) pulse are not commonly used for multislice spin‐echo magnetic resonance imaging (MRI). Here, a general theory and methods are described for conventional spin‐echo imaging using a π HS pulse for refocusing. Phase profiles produced by the HS pulse are analytically described. The analysis is extended to yield the specific relationships between pulse parameters and gradients, which must be satisfied to compensate the nonlinear phase variation produced with a spin‐echo sequence composed of π/2 and π HS pulses (the π/2 HS ? π HS sequence). The latter offers advantages for multislice spin‐echo MRI, including excellent slice‐selection and partial compensation for RF inhomogeneity. Furthermore, it can be implemented with a shorter echo time and lower power deposition than a previously described method using a pair of π HS pulses. Magn Reson Med 61:175–187, 2009. © 2008 Wiley‐Liss, Inc.     

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.     

SNR‐optimized phase‐sensitive dual‐acquisition turbo spin echo imaging: A fast alternative to FLAIR

Phase‐sensitive dual‐acquisition single‐slab three‐dimensional turbo spin echo imaging was recently introduced, producing high‐resolution isotropic cerebrospinal fluid attenuated brain images without long inversion recovery preparation. Despite the advantages, the weighted‐averaging‐based technique suffers from noise amplification resulting from different levels of cerebrospinal fluid signal modulations over the two acquisitions. The purpose of this work is to develop a signal‐to‐noise ratio‐optimized version of the phase‐sensitive dual‐acquisition single‐slab three‐dimensional turbo spin echo. Variable refocusing flip angles in the first acquisition are calculated using a three‐step prescribed signal evolution while those in the second acquisition are calculated using a two‐step pseudo‐steady state signal transition with a high flip‐angle pseudo‐steady state at a later portion of the echo train, balancing the levels of cerebrospinal fluid signals in both the acquisitions. Low spatial frequency signals are sampled during the high flip‐angle pseudo‐steady state to further suppress noise. Numerical simulations of the Bloch equations were performed to evaluate signal evolutions of brain tissues along the echo train and optimize imaging parameters. In vivo studies demonstrate that compared with conventional phase‐sensitive dual‐acquisition single‐slab three‐dimensional turbo spin echo, the proposed optimization yields 74% increase in apparent signal‐to‐noise ratio for gray matter and 32% decrease in imaging time. The proposed method can be a potential alternative to conventional fluid‐attenuated imaging. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

A simple low‐SAR technique for chemical‐shift selection with high‐field spin‐echo imaging

We have discovered a simple and highly robust method for removal of chemical shift artifact in spin‐echo MR images, which simultaneously decreases the radiofrequency power deposition (specific absorption rate). The method is demonstrated in spin‐echo echo‐planar imaging brain images acquired at 7 T, with complete suppression of scalp fat signal. When excitation and refocusing pulses are sufficiently different in duration, and thus also different in the amplitude of their slice‐select gradients, a spatial mismatch is produced between the fat slices excited and refocused, with no overlap. Because no additional radiofrequency pulse is used to suppress fat, the specific absorption rate is significantly reduced compared with conventional approaches. This enables greater volume coverage per unit time, well suited for functional and diffusion studies using spin‐echo echo‐planar imaging. Moreover, the method can be generally applied to any sequence involving slice‐selective excitation and at least one slice‐selective refocusing pulse at high magnetic field strengths. The method is more efficient than gradient reversal methods and more robust against inhomogeneities of the static (polarizing) field (B₀). Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Contrast‐enhanced,three‐dimensional,whole‐brain,black‐blood imaging: Application to small brain metastases

Contrast‐enhanced three‐dimensional T₁‐weighted imaging based on magnetization‐prepared rapid‐gradient recalled echo is widely used for detecting small brain metastases. However, since contrast materials remain in both blood and the tumor parenchyma and thus increase the signal intensity of both regions, it is often challenging to distinguish brain tumors from blood. In this work, we develop a T₁‐weighted, black‐blood version of single‐slab three‐dimensional turbo/fast spin echo whole‐brain imaging, in which the signal intensity of the brain tumor is selectively enhanced while that of blood is suppressed. For blood suppression, variable refocusing flip angles with flow‐sensitizing gradients are employed. To avoid a signal loss resulting from the flow‐sensitizing scheme, the first refocusing flip angle is forced to 180°. Composite restore pulses at the end of refocusing pulse train are applied to achieve partial inversion recovery for the T₁‐weighted contrast. Simulations and in vivo volunteer and patient experiments are performed, demonstrating that this approach is highly efficient in detecting small brain metastases. Magn Reson Med 63:553–561, 2010. © 2010 Wiley‐Liss, Inc.     

Vessel size imaging using dual contrast agent injections

Purpose:

To investigate the feasibility of a vessel size imaging (VSI) technique with separate contrast agent injections for evaluation of the vessel caliber in normal tissues and in brain tumors.

Materials and Methods:

Computer simulation was first performed to assess the potential errors in the estimation of vessel caliber that could result from time shifts between the dual contrast agent injections. Eight patients (four female, four male, 37–77 years old) with brain tumors (three high‐grade gliomas, two low‐grade gliomas, and three meningiomas) were recruited for clinical study. Dynamic susceptibility contrast magnetic resonance imaging (MRI) using gradient echo (GE) and spin echo (SE) echo‐planar imaging sequences were performed separately with a 10‐minute interval on a 3.0T scanner. Vessel caliber maps were calculated and analyzed in regions of interest at cortical gray matter (GM), thalamus, white matter (WM), and tumors.

Results:

From the computer simulation, the error of vessel caliber measurement was less than 8% when the difference between the time‐to‐peak of the GE and the SE studies was 1.5 seconds, and reduced to within 5% when the difference was 1 second. From the patient datasets of a 64 × 64 matrix, the estimated vessel calibers were 37.4 ± 12.9 μm for cortical gray matter, 20.7 ± 8.8 μm for thalamus, and 15.0 ± 5.1 μm for white matter, comparable to results in the literature. Two patients had a VSI with 128 × 128 matrix and showed similar results in vessel calibers of normal tissues. All the tumors had larger mean vessel diameter than normal‐appearing tissues. The difference in vascular size between normal tissue and tumor was demonstrated clearly in both the VSIs of regular and high spatial resolution.

Conclusion:

This study suggests that VSI with a dual injection method is a feasible technique for estimating microvascular calibers of normal tissues and brain tumors in clinical scanners. J. Magn. Reson. Imaging 2009;30:1078–1084. © 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.     

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.     

Hyperechoes.

A novel spin-echo-based refocusing strategy called a hyperecho mechanism is introduced by which the full coherence of magnetization submitted to a sequence of arbitrary RF pulses can be reinstalled. First implementations illustrate the potential of hyperecho formation-especially for Rapid Acquisition with Relaxation Enhancement (RARE) imaging, in which the full image intensity can be retrieved using a fraction of the RF power of a fully refocused sequence. The contribution of stimulated echo pathways to the hyperecho signal leads to an increased signal intensity at a given refocusing time for tissues with T(1) > T(2). For identical T(2) contrast, longer echo times have to be used. Further possibilities for using hyperechoes in gradient-echo sequences and for spin selection are discussed. Magn Reson Med 46:6-12, 2001.     

SPECIAL semi‐LASER with lipid artifact compensation for 1H MRS at 7 T

The measurement of full metabolic profiles at ultrahigh fields including low concentrated or fast‐relaxing metabolites is usually achieved by applying short echo time sequences. One sequence beside stimulated echo acquisition mode that was proposed in this regard is spin echo full intensity‐acquired localized spectroscopy. Typical problems that are still persistent for spin echo full intensity‐acquired localized spectroscopy are B₁ inhomogeneities especially for signal acquisition with surface coils and chemical shift displacement artifacts due to limited B₁ amplitudes when using volume coils. In addition, strong lipid contaminations in the final spectrum can occur when only a limited number of outer volume suppression pulses is used. Therefore, an adiabatic short echo time (= 19 ms) spin echo full intensity‐acquired localized spectroscopy semilocalization by adiabatic selective refocusing sequence is presented that is less sensitive to strong B₁ variations and that offers increased excitation and refocusing pulse bandwidths than regular spin echo full intensity acquired localized spectroscopy. Furthermore, the existence of the systematic lipid artifact is identified and linked to unfavorable effects due to the preinversion localization pulse. A method to control this artifact is presented and validated in both phantom and in vivo measurements. The viability of the proposed sequence was further assessed for in vivo measurements by scanning 17 volunteers using a surface coil and moreover acquiring additional volume coil measurements. The results show well‐suppressed lipid artifacts, good signal‐to‐noise ratio, and reproducible fitting results in accordance with other published studies. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Effects of refocusing flip angle modulation and view ordering in 3D fast spin echo

Recent advances have reduced scan time in three‐dimensional fast spin echo (3D‐FSE) imaging, including very long echo trains through refocusing flip angle (FA) modulation and 2D‐accelerated parallel imaging. This work describes a method to modulate refocusing FAs that produces sharp point spread functions (PSFs) from very long echo trains while exercising direct control over minimum, center‐k‐space, and maximum FAs in order to accommodate the presence of flow and motion, SNR requirements, and RF power limits. Additionally, a new method for ordering views to map signal modulation from the echo train into k_y‐k_z space that enables nonrectangular k‐space grids and autocalibrating 2D‐accelerated parallel imaging is presented. With long echo trains and fewer echoes required to encode large matrices, large volumes with high in‐ and through‐plane resolution matrices may be acquired with scan times of 3–6 min, as demonstrated for volumetric brain, knee, and kidney imaging. Magn Reson Med 60:640–649, 2008. © 2008 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.     

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 .     

Shielded dual‐loop resonator for arterial spin labeling at the neck

Purpose

To construct a dual‐loop coil for continuous arterial spin labeling (CASL) at the human neck and characterize it using computer simulations and magnetic resonance experiments.

Materials and Methods

The labeling coil was designed as a perpendicular pair of shielded‐loop resonators made from coaxial cable to obtain balanced circular loops with minimal electrical interaction with the lossy tissue. Three different excitation modes depending on the phase shift, Δψ, of the currents driving the two circular loops were investigated including a “Maxwell mode” (Δψ = 0°; ie, opposite current directions in both loops), a “quadrature mode” (Δψ = 90°), and a “Helmholtz mode” (Δψ = 180°; ie, identical current directions in both loops).

Results

Simulations of the radiofrequency field distribution indicated a high inversion efficiency at the locations of the carotid and vertebral arteries. With a 7‐mm‐thick polypropylene insulation, a sufficient distance from tissue was achieved to guarantee robust performance at a local specific absorption rate (SAR) well below legal safety limits. Application in healthy volunteers at 3 T yielded quantitative maps of gray matter perfusion with low intersubject variability.

Conclusion

The coil permits robust labeling with low SAR and minimal sensitivity to different loading conditions. J. Magn. Reson. Imaging 2009;29:1414–1424. © 2009 Wiley‐Liss, Inc.     

Brain MR perfusion‐weighted imaging with alternate ascending/descending directional navigation

In this study, a new arterial spin labeling technique that requires no separate spin preparation pulse was developed. Sequential two‐dimensional slices were acquired in ascending and descending orders by turns using balanced steady state free precession for pair‐wise subtraction. Simulation studies showed this new technique, alternate ascending/descending directional navigation (ALADDIN), has high sensitivity to both slow‐ (1–10 cm/sec) and fast‐moving (>10 cm/sec) blood because of the presence of multiple labeling planes proximal to imaging planes and sensitivity of balanced steady state free precession to initial magnetization differences. ALADDIN provided high‐resolution multislice perfusion‐weighted images in ～3 min. About 80–90% of signals in a slice were ascribed to spins saturated in the four prior slices. Three to five edge slices on each side of imaging group were affected by transient magnetization transfer effects and incomplete T₁ recovery between successive acquisitions. ALADDIN signals were dependent on many imaging parameters, implying room for improvement. Sagittal and coronal ALADDIN images demonstrated perfusion direction in gray matter regions was mostly from center to lateral, anterior, or posterior, whereas that in some white matter regions was reversed. ALADDIN is likely useful for many studies requiring perfusion‐weighted imaging with short scan time, insensitiveness to arterial transit time, directional information, high resolution, and/or wide coverage. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

Sensitive J‐coupled metabolite mapping using Sel‐MQC with selective multi‐spin‐echo readout

The selective multiple quantum coherence technique is combined with a read gradient to accelerate the measurement of a specific scalar‐coupled metabolite. The sensitivities of the localization using pure phase encoding and localization with the read gradient are compared in experiments at high magnetic field strength (17.6 T). Multiple spin‐echoes of the selective multiple quantum coherence edited metabolite are acquired using frequency‐selective refocusing of the specified molecule group. The frequency‐selective refocusing does not affect the J‐modulation of a coupled spin system, and the echo time is not limited to a multiple of 1/J to acquire pure in‐phase or antiphase signal. The multiple echoes can be used to accelerate the metabolite imaging experiment or to measure the apparent transverse relaxation T₂. A simple phase‐shifting scheme is presented, which enables the suppression of editing artifacts resulting from the multiple spin‐echoes of the water resonance. The experiments are carried out on phantoms, in which lactate and polyunsaturated fatty acids are edited, and in vivo on tumors, in which lactate content and T₂ are imaged. The method is of particular interest when a fast and sensitive selective multiple quantum coherence editing is necessary, e.g., for spatial three dimensional experiments. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Efficient fat suppression by slice‐selection gradient reversal in twice‐refocused diffusion encoding

Most diffusion imaging sequences rely on single‐shot echo‐planar imaging (EPI) for spatial encoding since it is the fastest acquisition available. However, it is sensitive to chemical‐shift artifacts due to the low bandwidth in the phase‐encoding direction, making fat suppression necessary. Often, spectral‐selective RF pulses followed by gradient spoiling are used to selectively saturate the fat signal. This lengthens the acquisition time and increases the specific absorption rate (SAR). However, in pulse sequences that contain two slice‐selective 180° refocusing pulses, the slice‐selection gradient reversal (SSGR) method of fat suppression can be implemented; i.e., using slice‐selection gradients of opposing polarity for the two refocusing pulses. We combined this method with the twice‐refocused spin‐echo sequence for diffusion encoding and tested its performance in both phantoms and in vivo. Unwanted fat signal was entirely suppressed with this method without affecting the water signal intensity or the slice profile. Magn Reson Med 60:1256–1260, 2008. © 2008 Wiley‐Liss, Inc.