Comparison of k-space sampling schemes for multidimensional MR spectroscopic imaging

For clinical ³¹P MR spectroscopic imaging (MRSI) studies, where signal averaging is necessary, some improvement of sensitivity and spatial response function may be achieved by acquiring data over a spherical k-space volume and varying the number of averages acquired in proportion to the desired spatial filter. Eight different k-space sampling schemes are compared through simulations that provide graphs of the spatial response functions (SRF), and tabulations of voxel volumes, relative signal-to-noise ratios (SNR), and relative data collection efficiencies (SNR per unit volume over the same time). All schemes were based on practical experiments, each of which could be implemented in the same length of time. The results show that in comparison with cubic k-space sampling with the same number of signal averages at each point, spherical and acquisition-weighted k-space sampling can be used to achieve reduced Gibbs ringing along the principal axes directions, and thus reduced contamination from adjacent tissue in these directions, without degradation of voxel volume or SNR.     

Fast proton spectroscopic imaging using the sliced k-space method

The use of one-shot imaging methods for proton spectroscolpic imaging (¹H-SI) is examined. In particular the acquisition of K_x × K_y × N_t data points by means of N_t excitations, each acquiring a K_x, × K_y, k-space slice, is advocated. A number of strategies for realising this experiment, and combining it with water suppression and volume-selection are proposed. The practical implementation at 4.7 T for ¹H-Sl of the rat brain is described. Experimental results from a 32 × 32 spatial matrix with N_t = 64 are presented. Spectra obtained from volumes as low as 3.5 μl and within measuring times of as little as 3.8 min are shown. In these choline, creatine/phosphocreatine and N-acetylaspartate are all clearly visible.     

In-plane motion correction for MR spectroscopic imaging

A motion-detection method is described that is specifically suited for MR spectroscopic imaging (MRSI) studies. Information on in-plane rotation and translation of the subject was obtained using external spatial reference markers that are uniquely identified via their chemical shift. The marker locations were obtained directly from the acquired data at each encoding step, and no additional data acquisition was required. This method was applied to brain ¹H MRSI studies that include subcutaneous lipid signals, which otherwise result in enhanced sensitivity to subject motion.     

Fast proton spectroscopic imaging employing k-space weighting achieved by variable repetition times

A k-space weighted spectroscopic imaging (SI) method is presented that allows a reduction in the total data acquisition time by up to 55% compared with standard SI. The k-space weighting is achieved by varying the repetition time, thus realizing an inherent apodization that corresponds to a circularly symmetric generalized Hamming filter. The flip angle is varied with the repetition time to enhance the signal-to-noise ratio. These techniques were employed using a short echo time of 10 ms. In vivo measurements on healthy rat brain at 4.7 T were conducted, obtaining two-dimensional spectroscopic imaging data from a 25 × 25 circularly reduced k-space area in as little as 5 min. The signal-to-noise ratio is sufficiently high to detect J-coupled resonances such as myo-inositol or glutamate/glutamine, demonstrating the ability to combine short acquisition times with comprehensive metabolic information. The T₁ dependency of the apodization and the corresponding point spread function was evaluated by computer simulations. The achievable signal-to-noise ratio per unit time was compared with standard SI giving a parameter-dependent advantage of approximately 20% of the standard SI method.     

Unaliasing lipid contamination for MR spectroscopic imaging of gliomas at 3T using sensitivity encoding (SENSE).

3D magnetic resonance spectroscopic imaging (MRSI) has been successfully employed to extract information about brain tumor metabolism, such as cell membrane breakdown, cellular energetics, and neuronal integrity, through its ability to differentiate signals coming from choline (Cho), creatine (Cr), and N-acetyl aspartate (NAA) molecules. The additional presence of lipids within subregions of the tumor may indicate cellular membrane breakdown due to cell death. Another potential source of lipids is subcutaneous fat, which may be excited with point-resolved spectroscopy (PRESS) volume selection and aliased into the spectral field of view (FOV) due to the chemical shift artifact and the low bandwidth of the selection pulses. The purpose of our study was to employ a postprocessing method for unaliasing lipid resonances originating from in-slice subcutaneous lipids from the 3D MRSI of gliomas at 3T, using an eight-channel phased-array coil and sensitivity encoding (SENSE).     

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.     

Double-echo multislice proton spectroscopic imaging using Hadamard slice encoding

Simultaneous multislice proton spectroscopic imaging (SI) is presented using a pulse sequence with multifrequency selective RF excitation and Hadamard encoding in the slice direction, and conventional Fourier phase encoding in the in-plane directions. Double-echo data acquisition is used to increase the spectral information of the experiment. Tests on a phantom demonstrate the quality of the slice selection. Results of in vivo measurements on the healthy rat brain show that spectra with a high signal-to-noise ratio can be acquired from four slices within 32 min. The measurements were performed at 4.7 T using a field of view of 32 × 32 mm², a slice thickness of 3 mm, and a voxel size of 12 μl. The proposed method is a useful alternative to sequential multislice SI and 3D SI. Furthermore, it is possible to combine sequential and simultaneous multi-slice SI.     

Reduced k-space encoding for dual-contrast TrueFISP imaging.

Dual-contrast TrueFISP imaging relies on the use of two RF pulses with different RF flip-angles for enhancing image contrast and performing automatic tissue classification based on multispectrum clustering. The original technique, however, involves an extended imaging time, which limits its clinical application. The purpose of this study is to compare the applicability of two reduced k-space sampling techniques, reduced imaging using generalized series reconstruction (RIGR) and key-hole imaging, for minimizing the imaging time required. The performance of both approaches was evaluated theoretically and practically using 3D cine datasets acquired from eight asymptomatic subjects.     

Optimizing the efficiency of high-field multivoxel spectroscopic imaging by multiplexing in space and time.

A new strategy to yield information from the maximum number of voxels, each at the optimum signal-to-noise ratio (SNR) per unit time, in MR spectroscopic imaging (MRSI) is introduced. In the past, maximum acquisition duty-cycle was obtained by multiplexing in time several single slices each repetition time (TR), while optimal SNR was achieved by encoding the entire volume of interest (VOI) each TR. We show that optimal SNR and acquisition efficiency can both be achieved simultaneously by multiplexing in space and time several slabs of several slices, each. Since coverage of common VOIs in 3D proton MRSI in the human brain typically requires eight or more slices, at 3 T or higher magnetic fields, two or more slabs can fit into the optimum TR (approximately 1.6 s). Since typically four or less slices would then fit into each slab, Hadamard encoding is favored in that direction for slice profile reasons. It is demonstrated that per fixed examination length, the new method gives, at 3 T, twice as many voxels, each of the same SNR and size, compared with current 3D chemical shift imaging techniques. It is shown that this gain will increase for more extensive spatial coverage or higher fields.     

Investigation of the PSF‐choice method for reduced lipid contamination in prostate MR spectroscopic imaging

The purpose of this work was to evaluate a previously proposed approach that aims to improve the point spread function (PSF) of MR spectroscopic imaging (MRSI) to avoid corruption by lipid signal arising from neighboring voxels. Retrospective spatial filtering can be used to alter the PSF; however, this either reduces spatial resolution or requires extending the acquisition in k‐space at the cost of increased imaging time. Alternatively, the method evaluated here, PSF‐choice, can modify the PSF localization to reduce the contamination from adjacent lipids by conforming the signal response more closely to the desired MRSI voxel grid. This is done without increasing scan time or degrading SNR of important metabolites. PSF‐choice achieves improvements in spatial localization through modifications to the radiofrequency excitation pulses. An implementation of this method is reported for MRSI of the prostate, where it is demonstrated that, in 13 of 16 pilot prostate MRSI scans, intravoxel spectral contamination from lipid was significantly reduced when using PSF‐choice. Phantom studies were also performed that demonstrate, compared with MRSI with standard Fourier phase encoding, out‐of‐voxel signal contamination of spectra was significantly reduced in MRSI with PSF‐choice. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Evaluation of variable line-shape models and prior information in automated 1H spectroscopic imaging analysis.

Analysis of in vivo short TE 1H spectra is complicated by broad baseline signal contributions and resonance line-shape distortions. Although the assumptions of ideal metabolite resonance line-shapes and slowly varying baseline signals can be used to separate these signals, the presence of broad or asymmetric line-shapes can invalidate this model. More complex line-shape models are computationally expensive or difficult to constrain, particularly for the low signal-to-noise commonly found for in vivo MR spectroscopic imaging applications. In this study, two time-domain models for fitting variable spectral line-shapes are examined, one using B-splines and another using summed sinusoids. The methods were verified using both phantom and human data, and Monte Carlo simulations were used to evaluate variations in calculated metabolite amplitudes due to interactions between the baseline and line-shape estimations. Additional studies investigated the use of prior line-shape information, obtained from either a water MRSI measurement or calculations from B(0) maps, to determine parameter starting values or optimization constraints. Both line-shape models showed the ability to fit the variety of line-shapes present in both the phantom and human MRSI data, with similar or improved accuracy over a Gaussian line-shape model; however, this improvement resulted in only minor improvement for the high-SNR phantom data and moderate improvements in regions with asymmetry for the fitted in vivo metabolite images. The use of prior line-shape information was of most benefit when applied toward setting optimization constraints but was of limited benefit when used to define initial starting values.     

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.     

Mapping fiber orientation in human muscle by proton MR spectroscopic imaging.

Proton magnetic resonance spectroscopic imaging ((1)H-MRSI) was used to determine muscle fiber orientations in human calf muscles. The method is based on the fact that some resonances show orientation-dependent dipolar splitting, caused by incomplete motional averaging. This leads to proton spectra that depend strongly on the angle between muscle fibers and the magnetic field B(0). The orientation-dependent dipolar splittings were mapped using a fit with a basis set of predefined coupling patterns reflecting the fiber orientation. The fitted coupling patterns were displayed as images and assigned to different muscles based on segmented MR images. They showed gross differences in fiber orientation between some muscles, including m. soleus and m. tibialis anterior, for all subjects. In addition, smaller but significant differences between subjects were detected, which could be due to localization differences or real interindividual differences. Since dipolar splitting affects metabolite intensities, it is important to take this effect into account when calculating metabolite concentrations from MR spectra in muscle tissue. Spatial maps of the MR signals of trimethyl-ammonium groups and creatine/ phosphocreatine revealed significant differences in intensity between muscles.     

Frequency resolved single-shot MR imaging using stochastic k-space trajectories

A new single-shot stochastic imaging technique with a random k-space path that provides very selective filtering with respect to chemical shift or off-resonance signals of the investigated tissue is proposed. It is demonstrated that in stochastic imaging only on-resonance compartments are visible whereas frequency shifted compartments cancel to noise that is distributed over the whole image. This method can be used as a single-shot chemical shift selective imaging technique and allows to calculate frequency resolved spectra for each spatial position of the image based on a single signal aquisition. The single-shot stochastic imaging sequence makes high demands on the gradient system and the theoretical k-space trajectory is distorted by imperfect gradient performance. Therefore an additional k-space guided imaging technique that uses the true, measured k-space trajectory to correct artifacts generated by eddy currents and delay times of the rapid switched gradients is presented. In vitro and in vivo measurements demonstrate the successful implementation of single-shot stochastic imaging on a conventional MR scanner with unshielded gradient systems.     

Quantitative mapping of total choline in healthy human breast using proton echo planar spectroscopic imaging (PEPSI) at 3 Tesla

Purpose:

To quantitatively measure tCho levels in healthy breasts using Proton‐Echo‐Planar‐Spectroscopic‐Imaging (PEPSI).

Materials and Methods:

The two‐dimensional mapping of tCho at 3 Tesla across an entire breast slice using PEPSI and a hybrid spectral quantification method based on LCModel fitting and integration of tCho using the fitted spectrum were developed. This method was validated in 19 healthy females and compared with single voxel spectroscopy (SVS) and with PRESS prelocalized conventional Magnetic Resonance Spectroscopic Imaging (MRSI) using identical voxel size (8 cc) and similar scan times (～7 min).

Results:

A tCho peak with a signal to noise ratio larger than 2 was detected in 10 subjects using both PEPSI and SVS. The average tCho concentration in these subjects was 0.45 ± 0.2 mmol/kg using PEPSI and 0.48 ± 0.3 mmol/kg using SVS. Comparable results were obtained in two subjects using conventional MRSI. High lipid content in the spectra of nine tCho negative subjects was associated with spectral line broadening of more than 26 Hz, which made tCho detection impossible. Conventional MRSI with PRESS prelocalization in glandular tissue in two of these subjects yielded tCho concentrations comparable to PEPSI.

Conclusion:

The detection sensitivity of PEPSI is comparable to SVS and conventional PRESS‐MRSI. PEPSI can be potentially used in the evaluation of tCho in breast cancer. A tCho threshold concentration value of ～0.7 mmol/kg might be used to differentiate between cancerous and healthy (or benign) breast tissues based on this work and previous studies. J. Magn. Reson. Imaging 2012;36:1113–1123. © 2012 Wiley Periodicals, Inc.     

Automated spectral analysis III: Application to in Vivo proton MR Spectroscopy and spectroscopic imaging

An automated method for analysis of in vivo proton magnetic resonance (MR) spectra and reconstruction of metabolite distributions from MR spectroscopic imaging (MRSI) data is described. A parametric spectral model using acquisition specific, a priori information is combined with a wavelet-based, nonparametric characterization of baseline signals. For image reconstruction, the initial fit estimates were additionally modified according to a priori spatial constraints. The automated fitting procedure was applied to four different examples of MRS data obtained at 1.5 T and 4.1 T. For analysis of major metabolites at medium TE values, the method was shown to perform reliably even in the presence of large baseline signals and relatively poor signal-to-noise ratios typical of in vivo proton MRSI. identification of additional metabolites was also demonstrated for short TE data. Automated formation of metabolite images will greatly facilitate and expand the clinical applications of MR spectroscopic imaging.     

MR spectroscopic imaging: Principles and recent advances

MR spectroscopic imaging (MRSI) has become a valuable tool for quantifying metabolic abnormalities in human brain, prostate, breast and other organs. It is used in routine clinical imaging, particularly for cancer assessment, and in clinical research applications. This article describes basic principles of commonly used MRSI data acquisition and analysis methods and their impact on clinical applications. It also highlights technical advances, such as parallel imaging and newer high‐speed MRSI approaches that are becoming viable alternatives to conventional MRSI methods. Although the main focus is on ¹H‐MRSI, the principles described are applicable to other MR‐compatible nuclei. This review of the state‐of‐the‐art in MRSI methodology provides a framework for critically assessing the clinical utility of MRSI and for defining future technical development that is expected to lead to increased clinical use of MRSI. Future technical development will likely focus on ultra‐high field MRI scanners, novel hyperpolarized contrast agents using metabolically active compounds, and ultra‐fast MRSI techniques because these technologies offer unprecedented sensitivity and specificity for probing tissue metabolic status and dynamics. J. Magn. Reson. Imaging 2013;37:1301–1325. © 2012 Wiley Periodicals, Inc.