Designing adiabatic radio frequency pulses using the Shinnar-Le Roux algorithm

B 1 + inhomogeneity mitigation for diffusion weighted MRI at 7T using TR-FOCI pulses

PURPOSE

The purpose of this study is to improve the image quality of diffusion-weighted images obtained with a single RF transmit channel 7 T MRI setup using time-resampled frequency-offset corrected inversion (TR-FOCI) pulses to refocus the spins in a twice-refocused spin-echo readout scheme.

METHODS

We replaced the conventional Shinnar-Le Roux-pulses in the twice refocused diffusion sequence with TR-FOCI pulses. The slice profiles were evaluated in simulation and experimentally in phantoms. The image quality was evaluated in vivo comparing the Shinnar-Le Roux and TR-FOCI implementation using a b value of 0 and of 1000 s/mm2.

RESULTS

The b0 and diffusion-weighted images acquired using the modified sequence improved the image quality across the whole brain. A region of interest-based analysis showed an SNR increase of 113% and 66% for the nondiffusion-weighted (b0) and the diffusion-weighted (b = 1000 s/mm2) images in the temporal lobes, respectively. Investigation of all slices showed that the adiabatic pulses mitigatedB 1 + $$ {B}_1^{+} $$ inhomogeneity globally using a conventional single-channel transmission setup.

CONCLUSION

The TR-FOCI pulse can be used in a twice-refocused spin-echo diffusion pulse sequence to mitigate the impact ofB 1 + $$ {B}_1^{+} $$ inhomogeneity on the signal intensity across the brain at 7 T. However, further work is needed to address SAR limitations.

Application of 7T MRS to High-Grade Gliomas

MRS, including single-voxel spectroscopy and MR spectroscopic imaging, captures metabolites in high-grade gliomas. Emerging evidence indicates that 7T MRS may be more sensitive to aberrant metabolic activity than lower-field strength MRS. However, the literature on the use of 7T MRS to visualize high-grade gliomas has not been summarized. We aimed to identify metabolic information provided by 7T MRS, optimal spectroscopic sequences, and areas for improvement in and new applications for 7T MRS. Literature was found on PubMed using "high-grade glioma," "malignant glioma," "glioblastoma," "anaplastic astrocytoma," "7T," "MR spectroscopy," and "MR spectroscopic imaging." 7T MRS offers higher SNR, modestly improved spatial resolution, and better resolution of overlapping resonances. 7T MRS also yields reduced Cramér-Rao lower bound values. These features help to quantify D-2-hydroxyglutarate in isocitrate dehydrogenase 1 and 2 gliomas and to isolate variable glutamate, increased glutamine, and increased glycine with higher sensitivity and specificity. 7T MRS may better characterize tumor infiltration and treatment effect in high-grade gliomas, though further study is necessary. 7T MRS will benefit from increased sample size; reductions in field inhomogeneity, specific absorption rate, and acquisition time; and advanced editing techniques. These findings suggest that 7T MRS may advance understanding of high-grade glioma metabolism, with reduced Cramér-Rao lower bound values and better measurement of smaller metabolite signals. Nevertheless, 7T is not widely used clinically, and technical improvements are necessary. 7T MRS isolates metabolites that may be valuable therapeutic targets in high-grade gliomas, potentially resulting in wider ranging neuro-oncologic applications.

Ultrahigh field brain magnetic resonance imaging using semiadiabatic radiofrequency pulses

Great attention is being paid to solving, or mitigating, the technical problems associated with MRI at ultrahigh field strengths of 7 T and higher. This paper explores the use of the semiadiabatic spin-echo (SA-SE) pulse sequence, which uses semiadiabatic radiofrequency (RF) pulses to remove and/or mitigate the effects of the nonuniform B₁ excitation field and B₀ inhomogeneity associated with the electromagnetic properties of the human brain. A semiadiabatic RF pulse version of the recently published serial transmit excitation pulse (STEP) RF pulse sequence is also presented that now incorporates semiadiabatic pulses, henceforth is called SA-STEP. As demonstrated by computer simulation, and confirmed using head imaging, both techniques can produce multislice SE MR imaging at 7 T. These new methods use relatively low RF power and achieve good coverage of the human brain in a single scan.

Echo-planar imaging is superior to fast spin-echo diffusion-weighted imaging for cranioplasty using titanium mesh in brain magnetic resonance imaging: a phantom study

This study aimed to compare the radiofrequency (RF) shielding effects of titanium mesh of echo-planar imaging (EPI) versus fast spin-echo (FSE) diffusion-weighted imaging (DWI) to establish a suitable sequence for patients who undergo cranioplasty and for whom titanium mesh was used in brain magnetic resonance imaging (MRI). A 1.5-T MRI scanner with clinical setting sequences was used. A phantom for the examination constructed using a sucrose solution in a plastic container was used to compare the signal attenuation (SA) ratio, area of RF shielding effect (Area), normalized absolute average deviation (NAAD), and apparent diffusion coefficient (ADC) between EPI and FSE-DWI. EPI provided significantly better SA ratio, Area, and NAAD (P < 0.01). When the number of slices increased, the RF shielding became more negative. There was no significant difference in the ADC. Regardless of the k-trajectory, EPI-DWI had a lower RF shielding effect than FSE-DWI in patients undergoing cranioplasty.

Parallel transmit optimized 3D composite adiabatic spectral-spatial pulse for spectroscopy

PURPOSE

To develop a 3D composite adiabatic spectral-spatial pulse for refocusing in spin-echo spectroscopy acquisitions and to compare its performance against standard acquisition methods.

METHODS

A 3D composite adiabatic pulse was designed by modulating a train of parallel transmit-optimized 2D subpulses with an adiabatic envelope. The spatial and spectral profiles were simulated and validated by experiments to demonstrate the feasibility of the design in both single and double spin-echo spectroscopy acquisitions. Phantom and in vivo studies were performed to evaluate the pulse performance and compared with semi-LASER with respect to localization performance, sequence timing, signal suppression, and specific absorption rate.

RESULTS

Simultaneous 2D spatial localization with water and lipid suppression was achieved with the designed refocusing pulse, allowing high-quality spectra to be acquired with shorter minimum TE/TR, reduced SAR, as well as adaptation to spatially varying B0 and B 1 + field inhomogeneities in both prostate and brain studies.

CONCLUSION

The proposed composite pulse can serve as a more SAR efficient alternative to conventional localization methods such as semi-LASER at ultrahigh field for spin echo-based spectroscopy studies. Subpulse parallel-transmit optimization provides the flexibility to manage the tradeoff among multiple design criteria to accommodate different field strengths and applications.

Rapid, B 1 -insensitive, dual-band quasi-adiabatic saturation transfer with optimal control for complete quantification of myocardial ATP flux

PURPOSE

Phosphorus saturation-transfer experiments can quantify metabolic fluxes noninvasively. Typically, the forward flux through the creatine kinase reaction is investigated by observing the decrease in phosphocreatine (PCr) after saturation of γ-ATP. The quantification of total ATP utilization is currently underexplored, as it requires simultaneous saturation of inorganic phosphate ( P i ) and PCr. This is challenging, as currently available saturation pulses reduce the already-low γ-ATP signal present.

METHODS

Using a hybrid optimal-control and Shinnar-Le Roux method, a quasi-adiabatic RF pulse was designed for the dual saturation of PCr and P i to enable determination of total ATP utilization. The pulses were evaluated in Bloch equation simulations, compared with a conventional hard-cosine DANTE saturation sequence, before being applied to perfused rat hearts at 11.7 T.

RESULTS

The quasi-adiabatic pulse was insensitive to a >2.5-fold variation in B 1 , producing equivalent saturation with a 53% reduction in delivered pulse power and a 33-fold reduction in spillover at the minimum effective B 1 . This enabled the complete quantification of the synthesis and degradation fluxes for ATP in 30-45 minutes in the perfused rat heart. While the net synthesis flux (4.24 ± 0.8 mM/s, SEM) was not significantly different from degradation flux (6.88 ± 2 mM/s, P = .06) and both measures are consistent with prior work, nonlinear error analysis highlights uncertainties in the P_i -to-ATP measurement that may explain a trend suggesting a possible imbalance.

CONCLUSIONS

This work demonstrates a novel quasi-adiabatic dual-saturation RF pulse with significantly improved performance that can be used to measure ATP turnover in the heart in vivo.

UltraHigh Field MR Imaging in Epilepsy

More than one million people in the United States suffer from seizures that are not controlled with antiseizure medications. Targeted interventions such as surgery and deep brain stimulation can confer seizure reduction or even freedom in many of these patients with drug-resistant epilepsy, but success critically depends on identification of epileptogenic zones through MR imaging. Ultrahigh field imaging facilitates improved sensitivity and resolution across many imaging modalities and may facilitate better detection of epileptic markers than is achieved at lower field strengths. The increasing availability and clinical adoption of ultrahigh field scanners play an important role in characterizing drug-resistant epilepsy and planning for its treatment.

A multi spin echo pulse sequence with optimized excitation pulses and a 3D cone readout for hyperpolarized 13 C imaging

PURPOSE

Imaging tumor metabolism in vivo using hyperpolarized [1-13 C]pyruvate is a promising technique for detecting disease, monitoring disease progression, and assessing treatment response. However, the transient nature of the hyperpolarization and its depletion following excitation limits the available time for imaging. We describe here a single-shot multi spin echo sequence, which improves on previously reported sequences, with a shorter readout time, isotropic point spread function (PSF), and better signal-to-noise ratio.

METHODS

The sequence uses numerically optimized spectrally selective excitation pulses set to the resonant frequencies of pyruvate and lactate and a hyperbolic secant adiabatic refocusing pulse, all applied in the absence of slice selection gradients. The excitation pulses were designed to be resistant to the effects of B0 and B1 field inhomogeneity. The gradient readout uses a 3D cone trajectory composed of 13 cones, all fully refocused and distributed among 7 spin echoes. The maximal gradient amplitude and slew rate were set to 4 G/cm and 20 G/cm/ms, respectively, to demonstrate the feasibility of clinical translation.

RESULTS

The pulse sequence gave an isotropic PSF of 2.8 mm. The excitation profiles of the optimized pulses closely matched simulations and a 46.10 ± 0.04% gain in image SNR was observed compared to a conventional Shinnar-Le Roux excitation pulse. The sequence was demonstrated with dynamic imaging of hyperpolarized [1-13 C]pyruvate and [1-13 C]lactate in vivo.

CONCLUSION

The pulse sequence was capable of dynamic imaging of hyperpolarized 13 C labeled metabolites in vivo with relatively high spatial and temporal resolution and immunity to system imperfections.

B1 inhomogeneity correction of RARE MRI with transceive surface radiofrequency probes

PURPOSE

The use of surface radiofrequency (RF) coils is common practice to boost sensitivity in (pre)clinical MRI. The number of transceive surface RF coils is rapidly growing due to the surge in cryogenically cooled RF technology and ultrahigh-field MRI. Consequently, there is an increasing need for effective correction of the excitation field ( B 1 + ) inhomogeneity inherent in these coils. Retrospective B₁ correction permits quantitative MRI, but this usually requires a pulse sequence-specific analytical signal intensity (SI) equation. Such an equation is not available for fast spin-echo (Rapid Acquisition with Relaxation Enhancement, RARE) MRI. Here we present, test, and validate retrospective B₁ correction methods for RARE.

METHODS

We implemented the commonly used sensitivity correction and developed an empirical model-based method and a hybrid combination of both. Tests and validations were performed with a cryogenically cooled RF probe and a single-loop RF coil. Accuracy of SI quantification and T₁ contrast were evaluated after correction.

RESULTS

The three described correction methods achieved dramatic improvements in B₁ homogeneity and significantly improved SI quantification and T₁ contrast, with mean SI errors reduced from >40% to >10% following correction in all cases. Upon correction, images of phantoms and mouse heads demonstrated homogeneity comparable to that of images acquired with a volume resonator. This was quantified by SI profile, SI ratio (error < 10%), and percentage of integral uniformity (PIU > 80% in vivo and ex vivo compared to PIU > 87% with the reference RF coil).

CONCLUSION

This work demonstrates the efficacy of three B₁ correction methods tailored for transceive surface RF probes and RARE MRI. The corrected images are suitable for quantification and show comparable results between the three methods, opening the way for T₁ measurements and X-nuclei quantification using surface transceiver RF coils. This approach is applicable to other MR techniques for which no analytical SI exists.

Spin Lock Adiabatic Correction (SLAC) for B1-insensitive pulse design at 7T

A new framework for B₁ insensitive adiabatic pulse design is proposed, denoted Spin Lock Adiabatic Correction (SLAC), which counteracts deviations from ideal behaviour through inclusion of an additional correction component during pulse design. SLAC pulses are theoretically derived, then applied to the design of enhanced BIR-4 and hyperbolic secant pulses to demonstrate practical utility of the new pulses. At 7T, SLAC pulses are shown to improve the flip angle homogeneity compared to a standard adiabatic pulse with validation in both simulations and phantom experiments, under SAR equivalent experimental conditions. The SLAC framework can be applied to any arbitrary adiabatic pulse to deliver excitation with increased B₁ insensitivity.

Characterization of Brain Metabolism by Nuclear Magnetic Resonance

The noninvasive, quantitative ability of nuclear magnetic resonance (NMR) spectroscopy to characterize small molecule metabolites has long been recognized as a major strength of its application in biology. Numerous techniques exist for characterizing metabolism in living, excised, or extracted tissue, with a particular focus on 1 H-based methods due to the high sensitivity and natural abundance of protons. With the increasing use of high magnetic fields, the utility of in vivo 1 H magnetic resonance spectroscopy (MRS) has markedly improved for measuring specific metabolite concentrations in biological tissues. Higher fields, coupled with recent developments in hyperpolarization, also enable techniques for complimenting 1 H measurements with spectroscopy of other nuclei, such as 31 P and 13 C, and for combining measurements of metabolite pools with metabolic flux measurements. We compare ex vivo and in vivo methods for studying metabolism in the brain using NMR and highlight insights gained through using higher magnetic fields, the advent of dissolution dynamic nuclear polarization, and combining in vivo MRS and ex vivo NMR approaches.

RF pulse methods for use with surface coils: Frequency-modulated pulses and parallel transmission

The first use of a surface coil to obtain a 31P NMR spectrum from an intact rat by Ackerman and colleagues initiated a revolution in magnetic resonance imaging (MRI) and spectroscopy (MRS). Today, we take it for granted that one can detect signals in regions external to an RF coil; at the time, however, this concept was most unusual. In the approximately four decade long period since its introduction, this simple idea gave birth to an increasing number of innovations that has led to transformative changes in the way we collect data in an in vivo magnetic resonance experiment, particularly with MRI of humans. These innovations include spatial localization and/or encoding based on the non-uniform B1 field generated by the surface coil, leading to new spectroscopic localization methods, image acceleration, and unique RF pulses that deal with B1 inhomogeneities and even reduce power deposition. Without the surface coil, many of the major technological advances that define the extraordinary success of MRI in clinical diagnosis and in biomedical research, as exemplified by projects like the Human Connectome Project, would not have been possible.

A specific absorption rate reduction method for simultaneous multislice magnetic resonance imaging

This study proposes a modified Shinnar-Le Roux method to synthesize the excitation radio frequency (RF) pulse for a 2D gradient echo (GRE) based simultaneous multi-slice (SMS) magnetic resonance imaging (MRI) with features of low specific absorption rate (SAR) and small out-of-slice ripple. This synthesis method for SMS RF pulses employs thinner slice bandwidth and lower multislice offset frequencies to reduce SAR values and adopts a weighted Parks-McClellan algorithm to reduce sidelobes. Formulas for estimating relative SAR values of the SMS pulses are also introduced. Relative SAR values and out-of-slice ripples of the proposed and typical RF pulses with different parameters are presented. In simultaneous 5-slice phantom and 3-slice human brain imaging, SMS pulses synthesized with the proposed method achieve 32% and 28% SAR values of standard pulses while providing similar image qualities. Typical RF pulses such as sinc x cos can also take advantage of the proposed method and offer lower SAR values for SMS imaging. The RF pulse synthesized using the proposed method features low SAR, small sidelobes, and consistent image quality for 2D GRE-based SMS MRI. This method is applicable to the synthesis of typical SMS RF pulses for significant SAR reduction.

Sculpting 3D spatial selectivity with pairs of 2D pulses: A comparison of methods

Enhancing the specificity of the spins' excitation can improve the capabilities of magnetic resonance. Exciting voxels with tailored 3D shapes reduces partial volume effects and enhances contrast, particularly in cases where cubic voxels or other simple geometries do not provide an optimal localization. Spatial excitation profiles of arbitrary shapes can be implemented using so-called multidimensional RF pulses, which are often limited in practice to 2D implementations owing to their sensitivity to field inhomogeneities. Recent work has shown the potential of spatio-temporally encoded (SPEN) pulses towards alleviating these constraints. In particular, 2D pulses operating in a so-called hybrid scheme where the "low-bandwidth" spatial dimension is sculpted by a SPEN strategy while an orthogonal axis is shaped by regular k-space encoding, have been shown resilient to chemical shift and B₀ field inhomogeneities. In this work we explore the use of pairs of 2D pulses, with one of these addressing geometries in the x-y plane and the other in the x-z dimension, to sculpt complex 3D volumes in phantoms and in vivo. To overcome limitations caused by the RF discretization demanded by these 2D pulses, a number of "unfolding" techniques yielding images from the centerband RF excitation while deleting sideband contributions - even in cases where center- and side-bands severely overlap - were developed. Thus it was possible to increase the gradient strengths applied along the low bandwidth dimensions, significantly improving the robustness of this kind of 3D sculpting pulses. Comparisons against conventional pulses designed on the basis of pure k-space trajectories, are presented.

Efficient high-resolution RF pulse design applied to simultaneous multi-slice excitation

RF pulse design via optimal control is typically based on gradient and quasi-Newton approaches and therefore suffers from slow convergence. We present a flexible and highly efficient method that uses exact second-order information within a globally convergent trust-region CG-Newton method to yield an improved convergence rate. The approach is applied to the design of RF pulses for single- and simultaneous multi-slice (SMS) excitation and validated using phantom and in vivo experiments on a 3T scanner using a modified gradient echo sequence.

Slice-selective adiabatic magnetization T2 -preparation (SAMPA) for efficient T2 -weighted imaging at ultrahigh field strengths

PURPOSE

At high field, T₂ -weighted (T₂ w) imaging is limited by transmit field inhomogeneity and increased radiofrequency power deposition. In this work, we introduce SAMPA (Slice-selective Adiabatic Magnetization T₂ PrepAration) and demonstrate its use for efficient brain T₂ w imaging at 7 Tesla (T).

METHODS

SAMPA was designed by subsampling an optimized B₁ insensitive rotation (BIR4) waveform with small tip angle linear subpulses. To perform T₂ w imaging, SAMPA was inserted before a fast gradient echo acquisition. The off-resonance behavior, B₁ robustness, and slice selectivity of the novel T₂ preparation module were analyzed using Bloch simulations. The performance of SAMPA for T₂ w imaging was demonstrated in phantom experiments as well as in the brains of healthy volunteers at 7T.

RESULTS

Based on simulations, the proposed design operates at peak B₁ of 15 μT and higher, within a 400 Hz bandwidth. T₂ values were in strong agreement with spin echo-based T₂ mapping in phantom experiments. Whole brain, interleaved multislab three-dimensional imaging could be acquired with 0.8 mm³ isotropic resolution in 5:36 min per T₂ weighting.

CONCLUSION

Compared with previous adiabatic T₂ preparation techniques, SAMPA allows for slice-selectivity, which can lead to efficient and robust acquisitions for T₂ w imaging at high field. Magn Reson Med 76:1741-1749, 2016. © 2015 International Society for Magnetic Resonance in Medicine.

A semiadiabatic spectral-spatial spectroscopic imaging (SASSI) sequence for improved high-field MR spectroscopic imaging

PURPOSE

MR spectroscopic imaging (MRSI) benefits from operation at 7T due to increased signal-to-noise ratio (SNR) and spectral separation. The 180° radiofrequency (RF) pulses used in the conventional MRSI sequences are particularly susceptible to the variation in the transmitted RF (B1 ) field and severe chemical shift localization errors at 7T. RF power deposition, as measured by specific absorption rate (SAR), also increases with field strength. Adiabatic 180° RF pulses may mitigate the effects of B1 variation. We designed and implemented a semiadiabatic spectral-spatial spectroscopic imaging (SASSI) pulse sequence to provide more uniform spectral data at 7T with reduced SAR.

METHODS

The adiabatic Shinnar-Le Roux algorithm was used to generate a high bandwidth 180° adiabatic spectral-spatial (SPSP) pulse that captured a spectral range containing the main metabolites of interest. A pair of 180° SPSP pulses was used to refocus the signal excited by a 90° SPSP pulse in order to select a 3D volume of interest in the SASSI sequence.

RESULTS

The SASSI pulse sequence produced spectra with more uniform brain metabolite SNR when compared with the conventional nonadiabatic MRSI sequence.

CONCLUSION

SASSI achieved comparable SNR to the current adiabatic alternative, semi-LASER, but with 1/3 of the SAR. Magn Reson Med 76:1071-1082, 2016. © 2015 Wiley Periodicals, Inc.

Ultrahigh field single-refocused diffusion weighted imaging using a matched-phase adiabatic spin echo (MASE)

PURPOSE

To improve ultrahigh field diffusion-weighted imaging (DWI) in the presence of inhomogeneous transmit B1 field by designing a novel semi-adiabatic single-refocused DWI technique.

METHODS

A 180° slice-selective, adiabatic radiofrequency (RF) pulse of 4 ms duration was designed using the adiabatic Shinnar-Le Roux algorithm. A matched-phase slice-selective 90° RF pulse of 8 ms duration was designed to compensate the nonlinear phase of the adiabatic 180° RF pulse. The resulting RF pulse combination, matched-phase adiabatic spin echo (MASE), was integrated into a single-shot echo planar DWI sequence. The performance of this sequence was compared with single-refocused Stejskal-Tanner (ST), twice-refocused spin echo (TRSE) and twice-refocused adiabatic spin echo (TRASE) in simulations, phantoms, and healthy volunteers at 7 Tesla (T).

RESULTS

In regions with inhomogeneous B1 , MASE resulted in increased signal intensity compared with ST (up to 64%). Moderate increase in specific absorption rate (35-39%) was observed for adiabatic RF pulses. MASE resulted in higher signal homogeneity at 7T, leading to improved visualization of measures derived from diffusion-weighted images such as white matter tractography and track density images.

CONCLUSION

Efficient adiabatic SLR pulses can be adapted to single-refocused DWI, leading to substantially improved signal uniformity when compared with conventional acquisitions.

A SEmi-Adiabatic matched-phase spin echo (SEAMS) PINS pulse-pair for B1 -insensitive simultaneous multislice imaging

PURPOSE

Simultaneous multislice (SMS) imaging is a powerful technique that can reduce image acquisition time for anatomical, functional, and diffusion weighted magnetic resonance imaging. At higher magnetic fields, such as 7 Tesla, increased radiofrequency (RF) field inhomogeneity, power deposition, and changes in relaxation parameters make SMS spin echo imaging challenging. We designed an adiabatic 180° Power Independent of Number of Slices (PINS) pulse and a matched-phase 90° PINS pulse to generate a SEmi-Adiabatic Matched-phase Spin echo (SEAMS) PINS sequence to address these issues.

METHODS

We used the adiabatic Shinnar Le-Roux (SLR) algorithm to generate a 180° pulse. The SLR polynomials for the 180° pulse were then used to create a matched-phase 90° pulse. The pulses were sub-sampled to produce a SEAMS PINS pulse-pair and the performance of this pulse-pair was validated in phantoms and in vivo.

RESULTS

Simulations as well as phantom and in vivo results, demonstrate multislice capability and improved B1 -insensitivity of the SEAMS PINS pulse-pair when operating at RF amplitudes of up to 40% above adiabatic threshold.

CONCLUSION

The SEAMS PINS approach presented here achieves multislice spin echo profiles with improved B1 -insensitivity when compared with a conventional spin echo.

Design of multidimensional Shinnar-Le Roux radiofrequency pulses

PURPOSE

To generalize the conventional Shinnar-Le Roux method for the design of multidimensional radiofrequency pulses.

METHODS

Using echo-planar gradients, the multidimensional radiofrequency pulse design problem was converted into a series of one-dimensional polynomial design problems. Each of the one-dimensional polynomial design problems was solved efficiently. B0 inhomogeneity compensation and design of spatial-spectral pulses were also considered.

RESULTS

The proposed method was used to design two-dimensional excitation and refocusing pulses. The results were validated through Bloch equation simulation and experiments on a 3.0 T scanner. Large-tip-angle, equiripple-error, multidimensional excitation was achieved with ripple levels closely matching the design specifications.

CONCLUSION

The conventional Shinnar-Le Roux method can be extended to design multidimensional radiofrequency pulses. The proposed method achieves almost equiripple excitation errors, allows easy control of the tradeoff among design parameters, and is computationally efficient.

New insights on human skeletal muscle tissue compartments revealed by in vivo t2 NMR relaxometry

The spin-spin (T2) relaxation of (1)H-NMR signals in human skeletal muscle has been previously hypothesized to reveal information about myowater compartmentation. Although experimental support has been provided, no consensus has yet emerged concerning the attribution of specific anatomical compartments to the observed T2 components. Potential application of a noninvasive tool that might offer such information urges the quest for a definitive answer to this question. The purpose of this work was to obtain new information that might help elucidate the mechanism of T2 distribution in muscle. To do so, in vivo T2 relaxation data was acquired from the soleus of eight healthy volunteers using a localized Carr-Purcell-Meiboom-Gill technique. Each acquisition contained 1000 echoes with an interecho spacing of 1 ms. Data were acquired from each subject under different vascular filling preparations expected to change exclusively the extracellular water fraction. Two exponential components were systematically observed: an intermediate component (T2 ~ 32 ms) and a long component (100 < T2 < 210 ms). The relative fraction and T2 value characterizing the long component systematically increased after progressive augmentation of extracellular water volume. Characteristic relaxation behavior for each vascular filling condition was analyzed with a two-site exchange model and a three-site two-exchange model. We show that a two-site exchange model can only predict the observations for small exchange rates, much more representative of transendothelial than transcytolemmal exchange regimes. The three-site two-exchange model representing the intracellular, interstitial, and vascular spaces was capable of precisely predicting the observations for realistic transcytolemmal and transendothelial exchange rates. The estimated intrinsic relative fractions of each of these compartments corroborate with estimations from previous works and strongly suggest that the T2 relaxation from water within the intracellular and interstitial spaces is described by the intermediate component, whereas the long component represents water within the vascular space.

Ultra-High-Field MR Neuroimaging

At ultra-high magnetic fields, such as 7T, MR imaging can noninvasively visualize the brain in unprecedented detail and through enhanced contrast mechanisms. The increased SNR and enhanced contrast available at 7T enable higher resolution anatomic and vascular imaging. Greater spectral separation improves detection and characterization of metabolites in spectroscopic imaging. Enhanced blood oxygen level-dependent contrast affords higher resolution functional MR imaging. Ultra-high-field MR imaging also facilitates imaging of nonproton nuclei such as sodium and phosphorus. These improved imaging methods may be applied to detect subtle anatomic, functional, and metabolic abnormalities associated with a wide range of neurologic disorders, including epilepsy, brain tumors, multiple sclerosis, Alzheimer disease, and psychiatric conditions. At 7T, however, physical and hardware limitations cause conventional MR imaging pulse sequences to generate artifacts, requiring specialized pulse sequences and new hardware solutions to maximize the high-field gain in signal and contrast. Practical considerations for ultra-high-field MR imaging include cost, siting, and patient experience.

Semi-adiabatic Shinnar-Le Roux pulses and their application to diffusion tensor imaging of humans at 7T

The adiabatic Shinnar-Le Roux (SLR) algorithm for radiofrequency (RF) pulse design enables systematic control of pulse parameters such as bandwidth, RF energy distribution and duration. Some applications, such as diffusion-weighted imaging (DWI) at high magnetic fields, would benefit from RF pulses that can provide greater B1 insensitivity while adhering to echo time and specific absorption rate (SAR) limits. In this study, the adiabatic SLR algorithm was employed to generate 6-ms and 4-ms 180° semi-adiabatic RF pulses which were used to replace the refocusing pulses in a twice-refocused spin echo (TRSE) diffusion-weighted echo planar imaging (DW-EPI) sequence to create two versions of a twice-refocused adiabatic spin echo (TRASE) sequence. The two versions were designed for different trade-offs between adiabaticity and echo time. Since a pair of identical refocusing pulses is applied, the quadratic phase imposed by the first is unwound by the second, preserving the linear phase created by the excitation pulse. In vivo images of the human brain obtained at 7Testa (7T) demonstrate that both versions of the TRASE sequence developed in this study achieve more homogeneous signal in the diffusion-weighted images than the conventional TRSE sequence. Semi-adiabatic SLR pulses offer a more B1-insensitive solution for diffusion preparation at 7T, while operating within SAR constraints. This method may be coupled with any EPI readout trajectory and parallel imaging scheme to provide more uniform coverage for diffusion tensor imaging at 7T and 3T.

Sequence design and evaluation of the reproducibility of water-selective diffusion-weighted imaging of the breast at 3 T

Diffusion measurements derived from breast MRI can be adversely affected by unwanted signals from abundant fatty tissues if they are not suppressed adequately. To minimize this undesired contribution, we designed and optimized a water-selective diffusion-weighted imaging (DWI) sequence, which relies on spectrally selective excitation on the water resonance, obviating the need for fat suppression. As this method is more complex than standard DWI methods, we also report a test-retest study to evaluate its reproducibility. In this study, a spectrally selective Gaussian pulse on water resonance was combined with a pair of slice-selective adiabatic refocusing pulses for water-only DWI. Field map-based shimming and manual determination of the center frequency were used for water selection. The selectivity of the excitation pulse was optimized by a spectrally selective spectroscopy sequence based on the same principles. A test-retest study of 10 volunteers in two separate visits was used to evaluate its reproducibility. Our results from all subjects showed high-quality diffusion-weighted images of the breast without fat contamination. Mean apparent diffusion coefficients for b = 0, 600 s/mm(2) and b = 50, 600 s/mm(2) all showed good reproducibility, as 95% confidence intervals of the apparent diffusion coefficients were 4 × 10(-5)  mm(2) /s and 5 × 10(-5)  mm(2) /s and repeatability values were 1.09 × 10(-4) and 1.31 × 10(-4) , respectively. In conclusion, water-selective DWI is a feasible alternative to standard methods of DWI based on fat suppression. The added complexity of the method does not compromise the reproducibility of diffusion measurements in the breast.

Improved slice-selective adiabatic excitation

PURPOSE

The purpose of this work is to design an improved Slice-selective Tunable-flip AdiaBatic Low peak-power Excitation (STABLE) pulse with shorter duration and increased off-resonance immunity to make it suitable for use in a greater range of applications and at higher field strengths. An additional aim is to design a variant of this pulse to achieve B1 -insensitive, fat-suppressed excitation.

METHODS

The adiabatic SLR algorithm was used to generate a more uniform spectral pulse envelope for this improved radiofrequency pulse for adiabatic slice-selective excitation, called STABLE-2. Pulse parameters were adjusted to design a version of STABLE-2 with a spectral null centered on lipids.

RESULTS

In vivo images obtained of the human brain at 3 and 7 T demonstrate that STABLE-2 provides robust, uniform, slice-selective excitation over a range of B1 values. Phantom and in vivo knee images obtained at 3 T demonstrate the effectiveness of STABLE-2 for fat suppression.

CONCLUSIONS

STABLE-2 achieves B1 -insensitive slice-selective excitation while providing greater off-resonance immunity and a shorter pulse duration, when compared to the original STABLE pulse. In particular, the 9.8-ms STABLE-2 pulse provides slice selectivity over 120 Hz whereas the 21-ms STABLE pulse is limited to 80 Hz off-resonance. B1 -Insensitive fat-suppressed excitation may also be achieved by using a variant of this pulse.

Slice-selective excitation with B₁⁺-insensitive composite pulses

Spatially selective excitation pulses have been designed to produce uniform flip angles in the presence of the RF and static field inhomogeneities typically encountered in MRI studies of the human brain at 7 T. Pulse designs are based upon non-selective, composite pulses numerically optimized for the desired performance over prescribed ranges of field inhomogeneities. The non-selective pulses are subsequently transformed into spatially selective pulses with the same field-insensitive properties through modification of the spectral composition of the individual sub-pulses which are then executed in conjunction with an oscillating gradient waveform. An in-depth analysis of the performance of these RF pulses is presented in terms of total pulse durations, slice profiles, linearity of in-slice magnetization phase, sensitivity to RF and static field variations, and signal loss due to T(2) effects. Both simulations and measurements in phantoms and in the human brain are used to evaluate pulses with nominal flip angles of 45° and 90°. Target slice thickness in all cases is 2mm. Results indicate that the described class of field-insensitive RF pulses is capable of improving flip-angle uniformity in 7 T human brain imaging. There appears to be a subset of pulses with durations ≲10 ms for which non-linearities in the magnetization phase are minimal and signal loss due to T(2) decay is not prohibitive. Such pulses represent practical solutions for achieving uniform flip angles in the presence of the large field inhomogeneities common to high-field human imaging and help to better establish the performance limits of high-field imaging systems with single-channel transmission.

Nonuniform and multidimensional Shinnar-Le Roux RF pulse design method

The Shinnar-Le Roux (SLR) radiofrequency (RF) pulse design algorithm is widely used for designing slice-selective RF pulses due to its intuitiveness, optimality, and speed. SLR is limited, however, in that it is only capable of designing one-dimensional pulses played along constant gradients. We present a nonuniform SLR RF pulse design framework that extends most of the capabilities of classical SLR to nonuniform gradient trajectories and multiple dimensions. Specifically, like classical SLR, the new method is a hard pulse approximation-based technique that uses filter design relationships to produce the lowest power RF pulse that satisfies target magnetization ripple levels. The new method is validated and compared with methods conventionally used for nonuniform and multidimensional large-tip-angle RF pulse design.

Self-refocused adiabatic pulse for spin echo imaging at 7 T

Spin echo pulse sequences are used to produce clinically important T(2) contrast. However, conventional 180° radiofrequency pulses required to generate a spin echo are highly susceptible to the B(1) inhomogeneity at high magnetic fields such as 7 Tesla (7 T), resulting in varying signal and contrast over the region of interest. Adiabatic 180° pulses may be used to replace conventional 180° pulses in spin echo sequences to provide greater immunity to the inhomogeneous B(1) field at 7 T. However, because the spectral profile of an adiabatic 180° pulse has nonlinear phase, pairs of these pulses are needed for proper refocusing, resulting in increased radiofrequency power deposition and long minimum echo times. We used the adiabatic Shinnar Le-Roux method to generate a matched-phase adiabatic 90°-180° pulse pair to obviate the need for a second adiabatic 180° pulse for phase refocusing. The pulse pair was then reformulated into a single self-refocused pulse to minimize the echo time, and phantom and in vivo experiments were performed to validate pulse performance. The self-refocused adiabatic pulse produced transmit profiles that were substantially more uniform than those achieved using a conventional spin echo sequence.