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.     

Single‐shot T1 mapping using simultaneous acquisitions of spin‐ and stimulated‐echo‐planar imaging (2D ss‐SESTEPI)

The conventional stimulated‐echo NMR sequence only measures the longitudinal component while discarding the transverse component, after tipping up the prepared magnetization. This transverse magnetization can be used to measure a spin echo, in addition to the stimulated echo. Two‐dimensional single‐shot spin‐ and stimulated‐echo‐planar imaging (ss‐SESTEPI) is an echo‐planar‐imaging‐based single‐shot imaging technique that simultaneously acquires a spin‐echo‐planar image and a stimulated‐echo‐planar image after a single radiofrequency excitation. The magnitudes of the spin‐echo‐planar image and stimulated‐echo‐planar image differ by T₁ decay and diffusion weighting for perfect 90° radiofrequency and thus can be used to rapidly measure T₁. However, the spatial variation of amplitude of radiofrequency field induces uneven splitting of the transverse magnetization for the spin‐echo‐planar image and stimulated‐echo‐planar image within the imaging field of view. Correction for amplitude of radiofrequency field inhomogeneity is therefore critical for two‐dimensional ss‐SESTEPI to be used for T₁ measurement. We developed a method for amplitude of radiofrequency field inhomogeneity correction by acquiring an additional stimulated‐echo‐planar image with minimal mixing time, calculating the difference between the spin echo and the stimulated echo and multiplying the stimulated‐echo‐planar image by the inverse functional map. Diffusion‐induced decay is corrected by measuring the average diffusivity during the prescanning. Rapid single‐shot T₁ mapping may be useful for various applications, such as dynamic T₁ mapping for real‐time estimation of the concentration of contrast agent in dynamic contrast enhancement MRI. Magn Reson Med, 2010. © 2010 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.     

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.     

Simultaneous B0‐ and B1+‐Map acquisition for fast localized shim,frequency, and RF power determination in the heart at 3 T

High‐field (≥3 T) cardiac MRI is challenged by inhomogeneities of both the static magnetic field (B₀) and the transmit radiofrequency field (B₁+). The inhomogeneous B fields not only demand improved shimming methods but also impede the correct determination of the zero‐order terms, i.e., the local resonance frequency f₀ and the radiofrequency power to generate the intended local B₁+ field. In this work, dual echo time B₀‐map and dual flip angle B₁+‐map acquisition methods are combined to acquire multislice B₀‐ and B₁+‐maps simultaneously covering the entire heart in a single breath hold of 18 heartbeats. A previously proposed excitation pulse shape dependent slice profile correction is tested and applied to reduce systematic errors of the multislice B₁+‐map. Localized higher‐order shim correction values including the zero‐order terms for frequency f₀ and radiofrequency power can be determined based on the acquired B₀‐ and B₁+‐maps. This method has been tested in 7 healthy adult human subjects at 3 T and improved the B₀ field homogeneity (standard deviation) from 60 Hz to 35 Hz and the average B₁+ field from 77% to 100% of the desired B₁+ field when compared to more commonly used preparation methods. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Rapid radiofrequency field mapping in vivo using single‐shot STEAM MRI

Higher field strengths entail less homogeneous RF fields. This may influence quantitative MRI and MRS. A method for rapidly mapping the RF field in the human head with minimal distortion was developed on the basis of a single‐shot stimulated echo acquisition mode (STEAM) sequence. The flip angle of the second RF pulse in the STEAM preparation was set to 60° and 100° instead of 90°, inducing a flip angle‐dependent signal change. A quadratic approximation of this trigonometric signal dependence together with a calibration accounting for slice excitation‐related bias allowed for directly determining the RF field from the two measurements only. RF maps down to the level of the medulla could be obtained in less than 1 min and registered to anatomical volumes by means of the T₂‐weighted STEAM images. Flip angles between 75% and 125% of the nominal value were measured in line with other methods. Magn Reson Med 60:739–743, 2008. © 2008 Wiley‐Liss, Inc.     

Short‐echo,single‐shot,full‐intensity proton magnetic resonance spectroscopy for neurochemical profiling at 4 T: Validation in the cerebellum and brainstem

A short echo time (TE = 24 ms) semiadiabatic localization by adiabatic selective refocusing (LASER) sequence was designed and optimized for full‐intensity proton magnetic resonance spectroscopy (¹H MRS) at 4 T. The sequence was combined with VAPOR water suppression and three‐dimensional outer volume suppression for improved localization and suppression of unwanted coherences. Artifact‐free, single‐shot spectra were obtained from the human brain with a spectral pattern almost identical to that obtained with an ultra‐short TE (TE = 5 ms) stimulated‐echo acquisition mode (STEAM) sequence as a result of the train of adiabatic refocusing pulses in semi‐LASER that reduce the apparent TE. Approximately 2‐fold higher signal intensity relative to STEAM was demonstrated in phantoms and the human brain. To test the performance of the sequence in clinically relevant brain regions with a volume coil, semi‐LASER spectra were acquired from three cerebellar and brainstem volumes of interest (VOIs) in 23 healthy subjects. Ultra‐short echo STEAM spectra were acquired from the same VOIs to compare neurochemical profiles obtained with semi‐LASER with those obtained with STEAM. Neurochemical profiles of the cerebellum and brainstem acquired by these two techniques were nearly identical, validating the accuracy of the metabolite concentrations obtained with semi‐LASER at the longer TE relative to STEAM. A high correlation between metabolite concentrations obtained by these two proton ¹H MRS techniques indicated the sensitivity to detect intersubject variation in metabolite levels. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

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.     

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.     

In vivo free induction decay based 3D multivoxel longitudinal hadamard spectroscopic imaging in the human brain at 3 T

We propose and demonstrate a full 3D longitudinal Hadamard spectroscopic imaging scheme for obtaining chemical shift maps, using adiabatic inversion pulses to encode the spins' positions. The approach offers several advantages over conventional Fourier‐based encoding methods, including a localized point spread function; no aliasing, allowing for volumes of interest smaller than the object being imaged; an option for acquiring noncontiguous voxels; and inherent outer volume rejection. The latter allows for doing away with conventional outer volume suppression schemes, such as point resolved spectroscopy (PRESS) and stimulated echo acquisition mode (STEAM), and acquiring non‐spin‐echo spectra with short acquisition delay times, limited only by the excitation pulse's duration. This, in turn, minimizes T₂ decay, maximizes the signal‐to‐noise ratio, and reduces J‐coupling induced signal decay. Results are presented for both a phantom and an in vivo healthy volunteer at 3 T. Magn Reson Med 69:903–911, 2013. © 2012 Wiley Periodicals, Inc.     

Consequences of T2 relaxation during half‐pulse slice selection for ultrashort TE imaging

A “half‐pulse” slice selection approach is used in the ultrashort echo time pulse sequence and is required to give minimal transverse relaxation in a two‐dimensional acquisition. This method splits the normal excitation radiofrequency pulse in half and acquires a pair of images, each using one of these half‐pulses. These half‐pulses are used without a refocusing gradient since summing the pair of images yields images with accurate slice selection. When the radiofrequency pulse duration is similar to the sample T₂, characteristics such as the effective echo time and choice of radiofrequency pulse require careful evaluation as some of the approximations in conventional slice selection do not apply. We derive a theory that includes relaxation during excitation using Pauly's excitation k‐space formalism. Further, this theory is tested on phantoms with a range of values of T₂ demonstrating the effect on the slice profile. We conclude that relaxation during excitation is significant and should be included in our estimate of the T₂ weighting of the sequence. In general, the T₂ weighting should be measured from the time of the centroid of the excitation pulse. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

TROMBONE: T1‐relaxation‐oblivious mapping of transmit radio‐frequency field (B1) for MRI at high magnetic fields

Fast, 3D radio‐frequency transmit field (B₁) mapping is important for parallel transmission, spatially selective pulse design and quantitative MRI applications. It has been shown that actual flip angle imaging—two interleaved spoiled gradient recalled echo images acquired in steady state with two very short time delays (TR₁, TR₂)—is an attractive method of B₁ mapping. Herein, we describe the TROMBONE method that efficiently integrates actual flip angle imaging with EPI imaging, alleviates very short TR requirement of actual flip angle imaging and through their synergy yields up to 16 times higher precision in B₁ estimation in the same experimental time. High precision of TROMBONE can be traded for faster scans. The map of B₁ reconstructed from the ratio of intensities of two images is insensitive to longitudinal relaxation time (T₁) in the physiologically relevant range. A table of the optimal acquisition protocol parameters for various target experimental conditions is provided. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Improved T2 mapping accuracy with dual‐echo turbo spin echo: Effect of phase encoding profile orders

Turbo spin echo (TSE) pulse sequences have been applied to estimate T₂ relaxation times in clinically feasible scan times. However, T₂ estimations using TSE pulse sequences has been shown to differ considerable from reference standard sequences due to several sources of error. The purpose of this work was to apply voxel‐sensitivity formalism to correct for one such source of error introduced by differing phase encoding profile orders with dual‐echo TSE pulse sequences. The American College of Radiology phantom and the brains of two healthy volunteers were imaged using dual‐echo TSE as well as 32‐echo spin‐echo acquisitions and T₂ estimations from uncorrected and voxel‐sensitivity formalism‐corrected dual‐echo TSE and 32‐echo acquisitions were compared. In all regions of the brain and the majority of the analyses of the American College of Radiology phantom, voxel‐sensitivity formalism correction resulted in considerable improvements in dual‐echo TSE T₂ estimation compared with the 32‐echo acquisition, with improvements in T₂ value accuracy ranging from 5.2% to 18.6%. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Rapid B1 mapping using orthogonal,equal‐amplitude radio‐frequency pulses

We present a new phase‐based method for mapping the amplitude of the radio‐frequency field (B₁) of a transmitter coil in three‐dimension. This method exploits the noncommutation relation between rotations about orthogonal axes. Our implementation of this principle in the current work results in a simple relation between the phase of the final magnetization and the flip angle (FA). In this study, we focus on FAs less than 90°. Our method is rapid and easy to implement compared with the existing B₁ mapping schemes. The mapping sequence can be simply obtained by adding to a regular three‐dimensional gradient‐echo sequence a magnetization preparation radio‐frequency pulse of the same FA but orthogonal in phase to the excitation radio‐frequency pulse. This method is demonstrated capable of generating reliable maps of the B₁ field within 1 min using FAs no larger than 60°. We show that it is robust against T₁, small chemical shift, and mild background inhomogeneity. This method may especially be suitable for B₁ mapping in situations (e.g., long‐T₁ and hyperpolarized‐gas imaging) where magnitude‐based methods are not readily applicable. A noise calculation of the FA map using this method is also presented. Magn Reson Med, 2012. © 2011 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.     

Fast radiofrequency flip angle calibration by Bloch–Siegert shift

In a recent work, we presented a novel method for B field mapping based on the Bloch–Siegert shift. Here, we apply this method to automated fast radiofrequency transmit gain calibration. Two off‐resonance radiofrequency pulses were added to a slice‐selective spin echo sequence. The off‐resonance pulses induce a Bloch–Siegert phase shift in the acquired signal that is proportional to the square of the radiofrequency field magnitude B₁². The signal is further spatially localized by a readout gradient, and the signal‐weighted average B₁ field is calculated. This calibration from starting system transmit gain to average flip angle is used to calculate the transmit gain setting needed to produce a desired imaging sequence flip angle. A robust implementation is demonstrated with a scan time of 3 s. The Bloch–Siegert‐based calibration was used to predict the transmit gain for a 90° radiofrequency pulse and gave a flip angle of 88.6 ± 3.42° when tested in vivo in 32 volunteers. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

Phase‐sensitive sodium B1 mapping

Quantitative sodium MRI requires accurate knowledge of factors affecting the sodium signal. One important determinant of sodium signal level is the transmit B₁ field strength. However, the low signal‐to‐noise ratio typical of sodium MRI makes accurate B₁ mapping in reasonable scan times challenging. A new phase‐sensitive B₁ mapping technique has recently been shown to work better than the widely used dual‐angle method in low‐signal‐to‐noise ratio situations and over a broader range of flip angles. In this work, the phase‐sensitive B₁ mapping technique is applied to sodium, and its performance compared to the dual‐angle method through both simulation and phantom studies. The phase‐sensitive method is shown to yield higher quality B₁ maps at low signal‐to‐noise ratio and greater consistency of measurement than the dual‐angle method. An in vivo sodium B₁ map of the human breast is also shown, demonstrating the phase‐sensitive method's feasibility for human studies. Magn Reson Med, 2010. © 2010 Wiley‐Liss, 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.     

Eigenmode analysis of transmit coil array for tailored B1 mapping

In vivo radiofrequency (RF) field B₁ mapping represents an essential prerequisite for parallel transmit applications. However, the large dynamic range of the transmit fields of the individual coil elements challenges the accuracy of MR‐based B₁ mapping techniques. In the present work, the B₁ mapping error and its impact on the RF performance are studied based on a coil eigenmode analysis. Furthermore, the linear properties of the transmit chain are exploited to virtually adjust the weighting of the different coil eigenmodes in the B₁ mapping procedure, resulting in considerably reduced mapping errors. In addition, the weighting of the eigenmodes is tailored to potential target applications, e.g., specific absorption rate (SAR) reduced RF shimming or multidimensional RF pulses, resulting in improved RF performance. The basic theoretic principles of the concept are elaborated and validated by corresponding simulations. Furthermore, results on B₁ mapping and RF shimming experiments, performed on phantoms and in vivo using a 3‐T scanner equipped with an eight‐channel transmit/receive body coil, are presented to prove the feasibility of the approach. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Time‐interleaved acquisition of modes: An analysis of SAR and image contrast implications

As the magnetic field strength and therefore the operational frequency in MRI are increased, the radiofrequency wavelength approaches the size of the human head/body, resulting in wave effects which cause signal decreases and dropouts. Especially, whole‐body imaging at 7 T and higher is therefore challenging. Recently, an acquisition scheme called time‐interleaved acquisition of modes has been proposed to tackle the inhomogeneity problems in high‐field MRI. The basic premise is to excite two (or more) different B modes using static radiofrequency shimming in an interleaved acquisition, where the complementary radiofrequency patterns of the two modes can be exploited to improve overall signal homogeneity. In this work, the impact of time‐interleaved acquisition of mode on image contrast as well as on time‐averaged specific absorption rate is addressed in detail. Time‐interleaved acquisition of mode is superior in B homogeneity compared with conventional radiofrequency shimming while being highly specific absorption rate efficient. Time‐interleaved acquisition of modes can enable almost homogeneous high‐field imaging throughout the entire field of view in PD, T₂, and T₂*‐weighted imaging and, if a specified homogeneity criterion is met, in T₁‐weighted imaging as well. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.