Spin‐echo magnetic resonance spectroscopic imaging at 7 T with frequency‐modulated refocusing pulses

Two approaches to high‐resolution SENSE‐encoded magnetic resonance spectroscopic imaging (MRSI) of the human brain at 7 Tesla (T) with whole‐slice coverage are described. Both sequences use high‐bandwidth radiofrequency pulses to reduce chemical shift displacement artifacts, SENSE‐encoding to reduce scan time, and dual‐band water and lipid suppression optimized for 7 T. Simultaneous B₀ and transmit B₁ mapping was also used for both sequences to optimize field homogeneity using high‐order shimming and determine optimum radiofrequency transmit level, respectively. One sequence (“Hahn‐MRSI”) used reduced flip angle (90°) refocusing pulses for lower radiofrequency power deposition, while the other sequence used adiabatic fast passage refocusing pulses for improved sensitivity and reduced signal dependence on the transmit‐B₁ level. In four normal subjects, adiabatic fast passage‐MRSI showed a signal‐to‐noise ratio improvement of 3.2 ± 0.5 compared to Hahn‐MRSI at the same spatial resolution, pulse repetition time, echo time, and SENSE‐acceleration factor. An interleaved two‐slice Hahn‐MRSI sequence is also demonstrated to be experimentally feasible. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Fully adiabatic 31P 2D‐CSI with reduced chemical shift displacement error at 7 T — GOIA‐1D‐ISIS/2D‐CSI

A fully adiabatic phosphorus (³¹P) two‐dimensional (2D) chemical shift spectroscopic imaging sequence with reduced chemical shift displacement error for 7 T, based on 1D‐image‐selected in vivo spectroscopy, combined with 2D‐chemical shift spectroscopic imaging selection, was developed. Slice‐selective excitation was achieved by a spatially selective broadband GOIA‐W(16,4) inversion pulse with an interleaved subtraction scheme before nonselective adiabatic excitation, and followed by 2D phase encoding. The use of GOIA‐W(16,4) pulses (bandwidth 4.3–21.6 kHz for 10–50 mm slices) reduced the chemical shift displacement error in the slice direction ～1.5–7.7 fold, compared to conventional 2D‐chemical shift spectroscopic imaging with Sinc3 selective pulses (2.8 kHz). This reduction was experimentally demonstrated with measurements of an MR spectroscopy localization phantom and with experimental evaluation of pulse profiles. In vivo experiments in clinically acceptable measurement times were demonstrated in the calf muscle (nominal voxel volume, 5.65 ml in 6 min 53 s), brain (10 ml, 6 min 32 s), and liver (8.33 ml, 8 min 14 s) of healthy volunteers at 7 T. High reproducibility was found in the calf muscle at 7 T. In combination with adiabatic excitation, this sequence is insensitive to the B₁ inhomogeneities associated with surface coils. This sequence, which is termed GOIA‐1D‐ISIS/2D‐CSI (goISICS), has the potential to be applied in both clinical research and in the clinical routine. Magn Reson Med, 2013. © 2012 Wiley Periodicals, 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 .     

A multislice gradient echo pulse sequence for CEST imaging

Chemical exchange–dependent saturation transfer and paramagnetic chemical exchange–dependent saturation transfer are agent‐mediated contrast mechanisms that depend on saturating spins at the resonant frequency of the exchangeable protons on the agent, thereby indirectly saturating the bulk water. In general, longer saturating pulses produce stronger chemical and paramagnetic exchange–dependent saturation transfer effects, with returns diminishing for pulses longer than T₁. This could make imaging slow, so one approach to chemical exchange–dependent saturation transfer imaging has been to follow a long, frequency‐selective saturation period by a fast imaging method. A new approach is to insert a short frequency‐selective saturation pulse before each spatially selective observation pulse in a standard, two‐dimensional, gradient‐echo pulse sequence. Being much less than T₁ apart, the saturation pulses have a cumulative effect. Interleaved, multislice imaging is straightforward. Observation pulses directed at one slice did not produce observable, unintended chemical exchange–dependent saturation transfer effects in another slice. Pulse repetition time and signal‐to noise ratio increase in the normal way as more slices are imaged simultaneously. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

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.     

MRI with zero echo time: Hard versus sweep pulse excitation

Zero echo time can be obtained in MRI by performing radiofrequency (RF) excitation as well as acquisition in the presence of a constant gradient applied for purely frequency‐encoded, radial centre‐out k‐space encoding. In this approach, the spatially nonselective excitation must uniformly cover the full frequency bandwidth spanned by the readout gradient. This can be accomplished either by short, hard RF pulses or by pulses with a frequency sweep as used in the SWIFT (Sweep imaging with Fourier transform) method for improved performance at limited RF amplitudes. In this work, the two options are compared with respect to T₂ sensitivity, signal‐to‐noise ratio (SNR), and SNR efficiency. In particular, the SNR implications of sweep excitation and of initial or periodical acquisition gaps required for transmit‐receive switching are investigated. It was found by simulations and experiments that, whereas equivalent in terms of T₂ sensitivity, the two techniques differ in SNR performance. With ideal, ungapped simultaneous excitation and acquisition, the sweep approach would yield higher SNR throughout due to larger feasible flip angles. However, acquisition gapping is found to take a significant SNR toll related to a reduced acquisition duty cycle, rendering hard pulse excitation superior for sufficient RF amplitude and also in the short‐T₂ limit. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Short echo spectroscopic imaging of the human brain at 7T using transceiver arrays

Recent advances in magnet technology have enabled the construction of ultrahigh‐field magnets (7T and higher) that can accommodate the human head and body. Despite the intrinsic advantages of performing spectroscopic imaging at 7T, increased signal‐to‐noise ratio (SNR), and spectral resolution, few studies have been reported to date. This limitation is largely due to increased power deposition and B₁ inhomogeneity. To overcome these limitations, we used an 8‐channel transceiver array with a short TE (15 ms) spectroscopic imaging sequence. Utilizing phase and amplitude mapping and optimization schemes, the 8‐element transceiver array provided both improved efficiency (17% less power for equivalent peak B₁) and homogeneity (SD(B₁) = ±10% versus ±22%) in comparison to a transverse electromagnetic (TEM) volume coil. To minimize the echo time to measure J‐modulating compounds such as glutamate, we developed a short TE sequence utilizing a single‐slice selective excitation pulse followed by a broadband semiselective refocusing pulse. Extracerebral lipid resonances were suppressed with an inversion recovery pulse and delay. The short TE sequence enabled visualization of a variety of resonances, including glutamate, in both a control subject and a patient with a Grade II oligodendroglioma. 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.     

Small‐tip fast recovery imaging using non‐slice‐selective tailored tip‐up pulses and radiofrequency‐spoiling

Small‐tip fast recovery (STFR) imaging is a new steady‐state imaging sequence that is a potential alternative to balanced steady‐state free precession. Under ideal imaging conditions, STFR may provide comparable signal‐to‐noise ratio and image contrast as balanced steady‐state free precession, but without signal variations due to resonance offset. STFR relies on a tailored “tip‐up,” or “fast recovery,” radiofrequency pulse to align the spins with the longitudinal axis after each data readout segment. The design of the tip‐up pulse is based on the acquisition of a separate off‐resonance (B0) map. Unfortunately, the design of fast (a few ms) slice‐ or slab‐selective radiofrequency pulses that accurately tailor the excitation pattern to the local B0 inhomogeneity over the entire imaging volume remains a challenging and unsolved problem. We introduce a novel implementation of STFR imaging based on “non‐slice‐selective” tip‐up pulses, which simplifies the radiofrequency pulse design problem significantly. Out‐of‐slice magnetization pathways are suppressed using radiofrequency‐spoiling. Brain images obtained with this technique show excellent gray/white matter contrast, and point to the possibility of rapid steady‐state T₂/T₁‐weighted imaging with intrinsic suppression of cerebrospinal fluid, through‐plane vessel signal, and off‐resonance artifacts. In the future, we expect STFR imaging to benefit significantly from parallel excitation hardware and high‐order gradient shim systems. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Phase‐sensitive,dual‐acquisition,single‐slab, 3D,turbo‐spin‐echo pulse sequence for simultaneous T2‐weighted and fluid‐attenuated whole‐brain imaging

Conventional T₂‐weighted turbo/fast spin echo imaging is clinically accepted as the most sensitive method to detect brain lesions but generates a high signal intensity of cerebrospinal fluid (CSF), yielding diagnostic ambiguity for lesions close to CSF. Fluid‐attenuated inversion recovery can be an alternative, selectively eliminating CSF signals. However, a long time of inversion, which is required for CSF suppression, increases imaging time substantially and thereby limits spatial resolution. The purpose of this work is to develop a phase‐sensitive, dual‐acquisition, single‐slab, three‐dimensional, turbo/fast spin echo imaging, simultaneously achieving both conventional T₂‐weighted and fluid‐attenuated inversion recovery–like high‐resolution whole‐brain images in a single pulse sequence, without an apparent increase of imaging time. Dual acquisition in each time of repetition is performed, wherein an in phase between CSF and brain tissues is achieved in the first acquisition, while an opposed phase, which is established by a sequence of a long refocusing pulse train with variable flip angles, a composite flip‐down restore pulse train, and a short time of delay, is attained in the second acquisition. A CSF‐suppressed image is then reconstructed by weighted averaging the in‐ and opposed‐phase images. Numerical simulations and in vivo experiments are performed, demonstrating that this single pulse sequence may replace both conventional T₂‐weighted imaging and fluid‐attenuated inversion recovery. Magn Reson Med 63:1422–1430, 2010. © 2010 Wiley‐Liss, 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.     

Transverse relaxometry with stimulated echo compensation

Presented is a fitting model for transverse relaxometry data acquired with the multiple‐refocused spin‐echo sequence. The proposed model, requiring no additional data input or pulse sequence modifications, compensates for imperfections in the transmit field and radiofrequency (RF) profiles. Exploiting oscillatory echo behavior to estimate alternate coherence pathways, the model compensates for prolonged signal decay from stimulated echo pathways yielding precise monoexponential T₂ quantification. Verified numerically and experimentally at 4.7 T in phantoms and the human brain, over 95% accuracy is readily attainable in realistic imaging situations without sacrificing multislice capabilities or requiring composite or adiabatic RF pulses. The proposed model allows T₂ quantitation in heterogeneous transmit fields and permits thin refocusing widths for efficient multislice imaging. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Optimized T1-weighted contrast for single-slab 3D turbo spin-echo imaging with long echo trains: application to whole-brain imaging.

T(1)-weighted contrast is conventionally obtained using multislice two-dimensional (2D) spin-echo (SE) imaging. Achieving isotropic, high spatial resolution is problematic with conventional methods due to a long acquisition time, imperfect slice profiles, or high-energy deposition. Single-slab 3D SE imaging was recently developed employing long echo trains with variable low flip angles to address these problems. However, long echo trains may yield suboptimal T(1)-weighted contrast, since T(2) weighting of the signals tends to develop along the echo train. Image blurring may also occur if high spatial frequency signals are acquired with low signal intensity. The purpose of this work was to develop an optimized T(1)-weighted version of single-slab 3D SE imaging with long echo trains. Refocusing flip angles were calculated based on a tissue-specific prescribed signal evolution. Spatially nonselective excitation was used, followed by half-Fourier acquisition in the in-plane phase encoding (PE) direction. Restore radio frequency (RF) pulses were applied at the end of the echo train to optimize T(1)-weighted contrast. Imaging parameters were optimized by using Bloch equation simulation, and imaging studies of healthy subjects were performed to investigate the feasibility of whole-brain imaging with isotropic, high spatial resolution. The proposed technique permitted highly-efficient T(1)-weighted 3D SE imaging of the brain.     

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.     

Design and use of tailored hard‐pulse trains for uniformed saturation of myocardium at 3 Tesla

Complete and uniform saturation of myocardium is essential for quantitative myocardial perfusion imaging using the first pass of a contrast agent. At 3 T, inhomogeneities of both the static (B₀) and radiofrequency (B₁) magnetic fields have led to the use of adiabatic B₁‐insensitive rotation type 4 (BIR‐4) pulses, which in practice are constrained by radiofrequency (RF) heating. In this study, we propose the use of trains of weighted hard pulses that are optimized for the measured variation of B₀ and B₁ fields in the myocardium. These pulses are simple to design, and require substantially lower RF power when compared with BIR‐4 pulses. In volunteers, at 3 T, we demonstrated that the proposed saturation pulse with three subpulses results in lower peak and lower average residual longitudinal magnetization over the heart, as compared with 8‐msec BIR‐4 pulses and conventional hard pulse trains (P < 0.05). Magn Reson Med 60:997–1002, 2008. © 2008 Wiley‐Liss, Inc.     

Spin‐echo micro‐MRI of trabecular bone using improved 3D fast large‐angle spin‐echo (FLASE)

Fast large‐angle spin echo (FLASE) is a common pulse sequence designed for quantitative imaging of trabecular bone (TB) microarchitecture. However, imperfections in the nonselective phase‐reversal pulse render it prone to stimulated echo artifacts. The problem is further exacerbated at isotropic resolution. Here, a substantially improved RF‐spoiled FLASE sequence (sp‐FLASE) is described and its performance is illustrated with data at 1.5T and 3T. Additional enhancements include navigator echoes for translational motion sensing applied in a slice parallel to the imaging slab. Whereas recent work suggests the use of fully‐balanced FLASE (b‐FLASE) to be advantageous from a signal‐to‐noise ratio (SNR) point of view, evidence is provided here that the greater robustness of sp‐FLASE may outweigh the benefits of the minor SNR gain of b‐FLASE for the target application of TB imaging in the distal extremities, sites of exclusively fatty marrow. Results are supported by a theoretical Bloch equation analysis and the pulse sequence dependence of the effective T₂ of triglyceride protons. Last, sp‐FLASE images are shown to provide detailed and reproducible visual depiction of trabecular networks in three dimensions at both anisotropic (137 × 137 × 410 μm³) and isotropic (160 × 160 × 160 μm³) resolutions in the human distal tibia in vivo. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Reduction of transmitter B1 inhomogeneity with transmit SENSE slice‐select pulses

Parallel transmitter techniques are a promising approach for reducing transmitter B₁ inhomogeneity due to the potential for adjusting the spatial excitation profile with independent RF pulses. These techniques may be further improved with transmit sensitivity encoding (SENSE) methods because the sensitivity information in pulse design provides an excitation that is inherently compensated for transmitter B₁ inhomogeneity. This paper presents a proof of this concept using transmit SENSE 3D tailored RF pulses designed for small flip angles. An eight‐channel receiver coil was used to mimic parallel transmission for brain imaging at 3T. The transmit SENSE pulses were based on the fast‐k_z design and produced 5‐mm‐thick slices at a flip angle of 30° with only a 4.3‐ms pulse length. It was found that the transmit SENSE pulses produced more homogeneous images than those obtained from the complex sum of images from all receivers excited with a standard RF pulse. Magn Reson Med 57:842–847, 2007. © 2007 Wiley‐Liss, Inc.     

Fast three‐dimensional 1H MR spectroscopic imaging at 7 Tesla using “spectroscopic missing pulse – SSFP”

The use of spectroscopic Missing Pulse ‐ SSFP (spMP‐SSFP) for fast three‐dimensional (3D) proton MR spectroscopic imaging (MRSI) at 7 Tesla (T) is demonstrated. Sequence modifications were required regarding the limits of the specific absorption rate as well as hardware limitations with respect to maximum B₁ field strength and B₀ gradient slew rate, as compared to previous studies performed at 3T. The combination of two spatially selective radiofrequency (RF) pulses (with orthogonal slice orientation) and a dual‐band chemical shift selective RF pulse for simultaneous water and lipid suppression proved to enable fast 3D MRSI measurements of the brain of healthy volunteers. Using a total measurement time of approximately 8.5 minutes and a nominal and real voxel size of 0.62 cm³ and 2.6 cm³, respectively, signals of N‐acetyl aspartate, total creatine, choline containing compounds, myo‐inositol, and glutamate+glutamine could be detected. Magn Reson Med, 2008. © 2008 Wiley‐Liss, Inc.     

Variable flip angle‐based fast three‐dimensional T1 mapping for delayed gadolinium‐enhanced MRI of cartilage of the knee: Need for B1 correction

Delayed gadolinium‐enhanced MRI of cartilage is a technique, which involves T₁ mapping to identify changes in the structural integrity of cartilage associated with osteoarthritis. Currently, the gold standard is 2D inversion recovery turbo spin echo, which suffers from long acquisition times and limited coverage. Three‐dimensional variable flip angle (VFA) is an alternate technique, which has been shown to be accurate when an estimate of T₁ is available a priori. This study validates the variable flip angle method for delayed gadolinium‐enhanced MRI of cartilage of the femoro‐tibial knee cartilage. When amplitude of (excitation) radiofrequency field inhomogeneities were minimized using nonselective pulses and amplitude of (excitation) radiofrequency field correction using an additional acquisition of a amplitude of (excitation) radiofrequency field map, the accuracy of T₁ measurements were improved, and slice‐to‐slice variations over the 3D volume were minimized. In conclusion, fast 3D T₁ mapping using the variable flip angle method with amplitude of (excitation) radiofrequency field correction appears to be an efficient and accurate method for delayed gadolinium‐enhanced MRI of cartilage of the knee. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.