Spectroscopic imaging with multidimensional pulses for excitation: SIMPLE

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.

A minimum-phase Shinnar-Le Roux spectral-spatial excitation RF pulse for simultaneous water and lipid suppression in 1H-MRSI of body extremities

PURPOSE

To develop a spectral-spatial (SPSP) excitation RF pulse for simultaneous water and lipid suppression in proton (1H) magnetic resonance spectroscopic imaging (MRSI) of body extremities.

METHODS

An SPSP excitation pulse is designed to excite Creatine (Cr) and Choline (Cho) metabolite signals while suppressing the overwhelming water and lipid signals. The SPSP pulse is designed using a recently proposed multidimensional Shinnar-Le Roux (SLR) RF pulse design method. A minimum-phase spectral selectivity profile is used to minimize signal loss from T2⁎ decay.

RESULTS

The performance of the SPSP pulse is evaluated via Bloch equation simulations and phantom experiments. The feasibility of the proposed method is demonstrated using three-dimensional, short repetition-time, free induction decay-based 1H-MRSI in the thigh muscle at 3T.

CONCLUSION

The proposed SPSP excitation pulse is useful for simultaneous water and lipid suppression. The proposed method enables new applications of high-resolution 1H-MRSI in body extremities.

Constrained Source Space MR Spectroscopy: Multiple Voxels, No Gradient Readout

BACKGROUND AND PURPOSE

Our goal was to develop a novel technique for measuring a small number of localized spectra simultaneously and in a time-efficient manner.

MATERIALS AND METHODS

Using appropriate radiofrequency pulses, the magnetization from multiple voxels is excited simultaneously and then separated (reconstructed) by using the individual coil-sensitivity profiles from a multichannel receiver coil. Because no gradients are used for k-space encoding, constrained source space MR spectroscopy provides a time advantage over conventional spectroscopic imaging and an improved signal-to-noise ratio per square root of unit time over single-voxel spectroscopy applied at each successive location. In the present work, we considered prototype application of constrained source space MR spectroscopy for 2 voxels.

RESULTS

Experimental data from healthy volunteers and simulation results showed that constrained source space MR spectroscopy is effective at extracting 2 independent spectra even in the challenging scenario of the voxels being closely spaced. Also, from 6 patients with various types of brain cancer we obtained 2-voxel constrained source space MR spectroscopy data, which showed spectra of clinical quality in half the time required to perform successive single-voxel MR spectroscopy.

CONCLUSIONS

Constrained source space MR spectroscopy provides clinical quality spectra and could be used to probe multiple voxels simultaneously in combination with Hadamard encoding for further scan-time reductions.

Removal of nuisance signals from limited and sparse 1H MRSI data using a union-of-subspaces model

PURPOSE

To remove nuisance signals (e.g., water and lipid signals) for (1) H MRSI data collected from the brain with limited and/or sparse (k, t)-space coverage.

METHODS

A union-of-subspace model is proposed for removing nuisance signals. The model exploits the partial separability of both the nuisance signals and the metabolite signal, and decomposes an MRSI dataset into several sets of generalized voxels that share the same spectral distributions. This model enables the estimation of the nuisance signals from an MRSI dataset that has limited and/or sparse (k, t)-space coverage.

RESULTS

The proposed method has been evaluated using in vivo MRSI data. For conventional chemical shift imaging data with limited k-space coverage, the proposed method produced "lipid-free" spectra without lipid suppression during data acquisition at 130 ms echo time. For sparse (k, t)-space data acquired with conventional pulses for water and lipid suppression, the proposed method was also able to remove the remaining water and lipid signals with negligible residuals.

CONCLUSION

Nuisance signals in (1) H MRSI data reside in low-dimensional subspaces. This property can be utilized for estimation and removal of nuisance signals from (1) H MRSI data even when they have limited and/or sparse coverage of (k, t)-space. The proposed method should prove useful especially for accelerated high-resolution (1) H MRSI of the brain.

An analytic framework for the evaluation of coil configurations for parallel transmission MRI with subsampled cartesian excitation k-space

The use of multiple independent simultaneous radio-frequency (RF) transmitters and coils, known as parallel transmission, has the potential to make multidimensional excitation applicable to a wide range of magnetic resonance imaging applications. The sensitivity profile of the RF coils in a parallel transmission system determines the performance of the system. We present a theoretical framework, allowing the evaluation of the performance of a coil array for parallel transmission. We show through theoretical analysis and Monte Carlo simulation that the proposed framework predicts the fidelity of excitation that can be achieved by a given coil configuration in the presence of noise in the measured coil sensitivity profiles. We evaluate the fidelity of excitation achieved by four candidate coil configurations for a four-channel parallel transmission system with noisy coil sensitivity estimates. Theoretical results are confirmed with Monte Carlo simulation. The results give insight into the design of coil configurations for parallel transmission. In particular, optimal fidelity of excitation for subsampled Cartesian excitation k -space is achieved with a coil sensitivity profile having uniform amplitude and increasing linear phase for each channel. Such sensitivity profiles may be achieved with twisted birdcage coil designs.

A noniterative method to design large-tip-angle multidimensional spatially-selective radio frequency pulses for parallel transmission

Recently, theoretical and experimental work has shown that parallel transmission of RF pulses can be used to shorten the duration of multidimensional spatially-selective pulses and compensate for B(1) field inhomogeneity. However, all the existing noniterative methods can design only excitation pulses for parallel transmission with a small flip angle (e.g., 30 degrees , or at most 90 degrees ) and cannot design large-tip-angle inversion/refocusing pulses, because these methods are based on the small-tip-angle (STA) approximation of the Bloch equation. In this work, a method to design large-tip-angle multidimensional spatially-selective pulses for parallel transmission is proposed, based on an extension of the single-channel linear-class large-tip-angle (LCLTA) theory. Design examples of 2D refocusing and inversion parallel transmit pulses and magnetization profiles from Bloch equation simulations demonstrate the strength of the proposed method. A 2D spin-echo parallel transmission experiment on a slab phantom using a 180 degrees refocusing pulse with an eight-channel transmit-only array further validates the effectiveness of the proposed method.

Recent advances in magnetic resonance neurospectroscopy

Over the past two decades, proton magnetic resonance spectroscopy (proton MRS) of the brain has made the transition from research tool to a clinically useful modality. In this review, we first describe the localization methods currently used in MRS studies of the brain and discuss the technical and practical factors that determine the applicability of the methods to particular clinical studies. We also describe each of the resonances detected by localized solvent-suppressed proton MRS of the brain and discuss the metabolic and biochemical information that can be derived from an analysis of their concentrations. We discuss spectral quantitation and summarize the reproducibility of both single-voxel and multivoxel methods at 1.5 and 3-4 T. We have selected three clinical neurologic applications in which there has been a consensus as to the diagnostic value of MRS and summarize the information relevant to clinical applications. Finally, we speculate about some of the potential technical developments, either in progress or in the future, that may lead to improvements in the performance of proton MRS.

Spatial localization in nuclear magnetic resonance spectroscopy

The ability to select a discrete region within the body for signal acquisition is a fundamental requirement of in vivo NMR spectroscopy. Ideally, it should be possible to tailor the selected volume to coincide exactly with the lesion or tissue of interest, without loss of signal from within this volume or contamination with extraneous signals. Many techniques have been developed over the past 25 years employing a combination of RF coil properties, static magnetic field gradients and pulse sequence design in an attempt to meet these goals. This review presents a comprehensive survey of these techniques, their various advantages and disadvantages, and implications for clinical applications. Particular emphasis is placed on the reliability of the techniques in terms of signal loss, contamination and the effect of nuclear relaxation and J-coupling. The survey includes techniques based on RF coil and pulse design alone, those using static magnetic field gradients, and magnetic resonance spectroscopic imaging. Although there is an emphasis on techniques currently in widespread use (PRESS, STEAM, ISIS and MRSI), the review also includes earlier techniques, in order to provide historical context, and techniques that are promising for future use in clinical and biomedical applications.

Cortical NAA deficits in HIV infection without dementia: influence of alcoholism comorbidity

Alcoholism comorbidity is highly prevalent in individuals infected with human immunodeficiency virus (HIV). Each condition is known to affect brain structure, function, and metabolism, but the combined effects on the brain have only recently been considered. Single-voxel, proton MR spectroscopy (MRS) has yielded sensitive measures of early brain deterioration in the progression of HIV, but has limited coverage of neocortex, whereas MRS imaging (MRSI) can simultaneously interrogate large regions of cortex. Included were 15 men with HIV+alcoholism, nine men with HIV alone, eight men with alcoholism alone (abstinent for 3-17 months), and 23 controls. The two HIV groups were matched in T-cell count and were not demented; the two alcoholism groups were relatively matched in lifetime alcohol consumption. We used MRSI with a variable-density spiral sequence to quantify major proton metabolites--N-acetylaspartate (NAA), creatine (Cr), and choline (Cho)-in the superior parietal-occipital cortex. Metabolites were expressed in absolute units and as the NAA/Cr ratio. Significant group effects were present for NAA and Cr. Only the HIV+alcoholism group was significantly affected, exhibiting a 0.8 SD deficit in NAA and a 1.0 SD deficit in Cr. The deficits were not related to highly active antiretroviral therapy (HAART) status. Neither HIV infection nor alcoholism independently resulted in parietal-occipital cortical metabolite abnormalities, yet each disease carried a liability that put affected individuals at a heightened risk of neuronal compromise when the diseases were compounded. Further, the use of absolute measures revealed deficits in NAA and Cr that would have gone undetected if these metabolites were expressed as a ratio.

Iterative RF pulse design for multidimensional, small-tip-angle selective excitation

The excitation k-space perspective on small-tip-angle selective excitation has facilitated RF pulse designs in a range of MR applications. In this paper, k-space-based design of multidimensional RF pulses is formulated as a quadratic optimization problem, and solved efficiently by the iterative conjugate-gradient (CG) algorithm. Compared to conventional design approaches, such as the conjugate-phase (CP) method, the new design approach is beneficial in several regards. It generally produces more accurate excitation patterns. The improvement is particularly significant when k-space is undersampled, and it can potentially shorten pulse lengths. A prominent improvement in accuracy is also observed when large off-resonance gradients are present. A further boost in excitation accuracy can be accomplished in regions of interest (ROIs) if they are specified together with "don't-care" regions. The density compensation function (DCF) is no longer required. In addition, regularization techniques allow control over integrated and peak pulse power.

Dualband spectral-spatial RF pulses for prostate MR spectroscopic imaging

Although MR spectroscopic imaging (MRSI) of the prostate has demonstrated clinical utility for the staging and monitoring of cancer extent, current acquisition methods are often inadequate in several aspects. Conventional 180 degrees pulses can suffer from chemical shift misregistration, and have high peak-power requirements that can exceed hardware limits in many prostate MRSI studies. Optimal water and lipid suppression are also critical to obtain interpretable spectra. While complete suppression of the periprostatic lipid resonance is desired, controlled partial suppression of water can provide a valuable phase and frequency reference for data analysis and an assessment of experimental success in cases in which all other resonances are undetectable following treatment. In this study, new spectral-spatial RF pulses were developed to negate chemical shift misregistration errors and to provide dualband excitation with partial excitation of the water resonance and full excitation of the metabolites of interest. Optimal phase modulation was also included in the pulse design to provide 40% reduction in peak RF power. Patient studies using the new pulses demonstrated both feasibility and clear benefits in the reliability and applicability of prostate cancer MRSI.

Volume selective detection by weighted averaging of constant tip angle scans

We propose a method to improve the sensitivity in volume selective detection based on the CARVE excitation sequence (I. Sersa and S. Macura, J. Magn. Reson. 135, 466-477 (1998)) which consists of signal acquisition with constant tip angle excitation and a short phase-encoding gradient pulse. Volume selectivity is achieved using the weighted average of a number of scans whose weights and gradient steps are determined by the shape of the excitation profile. The method is particularly useful for broadband volume selective detection of insensitive spins where the volume selection can be merged with the standard signal averaging process, without compromising the excitation bandwidth or sensitivity.

Excitation of complicated shapes in three dimensions

We experimentally verified a recently proposed technique for the excitation of a complicated three-dimensional profile (CARVE, completely arbitrary regional volume excitation). CARVE is based on a generalized DANTE RF pulse sequence and a synchronous string of gradient steps. Provided there is no limitation in the number of pulses, CARVE can generate an excitation profile of any shape with any resolution. However, hardware limitations and sample properties restrict the number of RF pulses and gradient steps and, thus, limit attainable resolution of the excitation profile. We theoretically and experimentally showed that spatial resolution can be increased by distributing a long sequence among several CARVE experiments and summing up their signals. This is particularly important for three-dimensional excitation profiles where an n-fold increase in resolution requires an n3-fold increase of the number of events in the sequence. The potential use of three-dimensional CARVE might be in spectroscopic imaging where the excitation profile can be tailored to match the shape of a selected organ or body part.

Spatial localization combining projection presaturation with a two-dimensional excitation pulse

Single-shot spatial localization of short T1 nuclei was achieved by outer volume suppression with projection presaturation followed by selective excitation of the desired volume with a two-dimensional (2D) pulse. After the projection, presaturation with a 2D pulse avoided signal contamination from magnetization regrowth in the outer volume, whereas preceding the 2D pulse with projection presaturation reduced any unwanted excitation in the outer volume caused by the 2D pulse. The improvement in outer volume suppression achieved by combining these two techniques was demonstrated by images collected after the application of projection presaturation alone, a 2D pulse alone, and the two combined.

Fast 1H spectroscopic imaging using a multi-element head-coil array

Fast proton magnetic resonance spectroscopic imaging (MRSI) using a multi-element head-coil array is examined with respect to three aspects: the coil design, the use of an appropriate signal combination method, and the design of the MRSI pulse sequence itself. An eight-element head-coil array has been developed to increase the signal-to-noise ratio (SNR) of MRSI in the human brain. The flexible wraparound design optimally fits different head sizes and thus provides high sensitivity. The signal combination of the individual coil elements is based on the approach proposed by Roemer et al. (Magn. Reson. Med. 16, 192 (1990)). An additional short prescan is performed to provide a good estimate of the complex coil sensitivity profiles, which are used in the signal combination procedure to correct the spectroscopic imaging data for the spatially varying intensity. The use of coil arrays in MRSI has some effect on the requirements for both water and lipid suppression. These techniques and a MRSI pulse sequence that provides a high spectroscopic resolution are described and discussed. Experimental results at 1.5 T show that metabolite maps of N-acetylaspartate (NAA), choline (Cho), phosphocreatine (PCr)/creatine (Cr) can be obtained within a 5-min acquisition time.

Three-dimensional spectral-spatial excitation

A method of three-dimensional spectral-spatial excitation is presented which is selective simultaneously in two spatial dimensions and in the spectral or chemical shift dimension. This method can be used to create spectral passbands whose center frequency varies as a function of spatial location within an imaging plane. This variation of passband center frequency may be specified by an acquired main field (B0) map; the resulting excitations compensate for inhomogeneity of the B0 field. In vivo images are presented in which three-dimensional spectral-spatial excitation allows selective water-only imaging in the presence of large B0 inhomogeneity where conventional spectrally selective imaging falls. Phantom studies give a detailed profile of the performance of three-dimensional spectral-spatial pulses suitable for water-only or fat-saturation imaging. These pulses may also be useful for fat and water suppression in spectroscopic imaging. Performance constraints imposed by limited gradient slew rates are analyzed and quantified.

Short echo time proton magnetic resonance spectroscopic imaging of macromolecule and metabolite signal intensities in the human brain

A novel approach is presented for imaging macromolecule and metabolite signals in brain by proton magnetic resonance spectroscopic imaging. The method differentiates between metabolites and macromolecules by T1 weighting using an inversion pulse followed by a variable inversion recovery time before localization and spectroscopic imaging. In healthy subjects, the major macromolecule resonances at 2.05 and 0.9 ppm were mapped at a nominal spatial resolution of 1 x 1 x 1.5 cm3 and were demonstrated to be highly reproducible between subjects. In subacute stroke patients, a highly elevated macromolecule resonance at 1.3 ppm was mapped to infarcted brain regions, suggesting potential applications for studying pathological conditions.

High speed 1H spectroscopic imaging in human brain by echo planar spatial-spectral encoding

We introduce a fast and robust spatial-spectral encoding method, which enables acquisition of high resolution short echo time (13 ms) proton spectroscopic images from human brain with acquisition times as short as 64 s when using surface coils. The encoding scheme, which was implemented on a clinical 1.5 Tesla whole body scanner, is a modification of an echo-planar spectroscopic imaging method originally proposed by Mansfield Magn. Reson. Med. 1, 370-386 (1984), and utilizes a series of read-out gradients to simultaneously encode spatial and spectral information. Superficial lipid signals are suppressed by a novel double outer volume suppression along the contours of the brain. The spectral resolution and the signal-to-noise per unit time and unit volume from resonances such as N-acetyl aspartate, choline, creatine, and inositol are comparable with those obtained with conventional methods. The short encoding time of this technique enhances the flexibility of in vivo spectroscopic imaging by reducing motion artifacts and allowing acquisition of multiple data sets with different parameter settings.

Magnetic resonance spectroscopic imaging of ethanol in the human brain: a feasibility study

The in vivo distribution of ethanol in normal human brain following the consumption of a moderate amount of alcohol was measured using magnetic resonance spectroscopic imaging. Three subjects were studied, and the spatial distribution of brain ethanol, 60-min postingestion and measured at a spatial resolution of 1.5 cm, was found to be highly nonuniform with the relative ethanol signal in cerebral spinal fluid, gray matter, and white matter determined to be 1.00, 0.72, and 0.37, respectively. These spectroscopic imaging results indicate that whereas in vivo magnetic resonance studies of ethanol are feasible, quantitative studies of alcohol need to account carefully for the various tissue types within the observed volume.

Fast proton spectroscopic imaging of human brain using multiple spin-echoes

We introduce a multi-echo multi-slice MR proton spectroscopic imaging method, which allows for a dramatic reduction of the measurement time by acquiring multiple spin-echoes within a single repetition time. In the multi-echo multi-slice experiment discussed in this paper, a threefold reduction in measurement time is obtained by sacrificing some spectral resolution. Signal-to-noise ratio and spatial resolution are preserved. Metabolite images of N-acetyl aspartate, and total choline+total creatine from multiple slices through the human brain are presented and compared with images obtained with a conventional single-echo multi-slice method.

Echo-planar spin-echo and inversion pulses

The echo-planar k-space trajectory can be used as the basis for any two-dimensional selective pulse. The main application is spectral-spatial pulses, which must be based on the echo-planar trajectory. In this paper we show how echo-planar spin-echo (EPSE) pulses may be designed.

A three-dimensional spin-echo or inversion pulse

In theory, multidimensional pulses can be designed to be selective in any number of dimensions. In practice, available gradient power has enforced a limit to two dimensions. We show here that three-dimensional pi pulses are feasible on commercial imaging machines provided that the range of off-resonance frequencies are limited.

Lipid-suppressed single- and multisection proton spectroscopic imaging of the human brain

Spectroscopic images of the brain have great potential in disease diagnosis and treatment monitoring. Unfortunately, interfering lipid signals from subcutaneous fat and poor water suppression due to magnetic field inhomogeneities can make such images difficult to obtain. A pulse sequence that uses inversion recovery for lipid suppression and a spectral-spatial refocusing pulse for water suppression is introduced. In contrast to methods that eliminate fat signal by restricting the excited volume to lie completely within the brain, inversion-recovery techniques allow imaging of an entire section without such restrictions. In addition, the spectral-spatial pulse was designed to provide water suppression insensitive to a reasonable range of B0 and B1 inhomogeneities. Several data processing algorithms have also been developed and used in conjunction with the new pulse sequence to produce metabolite maps covering large volumes of the human brain. Images from single- and multisection studies demonstrate the performance of these techniques.