Single‐shot magnetic resonance spectroscopic imaging with partial parallel imaging

A magnetic resonance spectroscopic imaging (MRSI) pulse sequence based on proton–echo‐planar‐spectroscopic‐imaging (PEPSI) is introduced that measures two‐dimensional metabolite maps in a single excitation. Echo‐planar spatial–spectral encoding was combined with interleaved phase encoding and parallel imaging using SENSE to reconstruct absorption mode spectra. The symmetrical k‐space trajectory compensates phase errors due to convolution of spatial and spectral encoding. Single‐shot MRSI at short TE was evaluated in phantoms and in vivo on a 3‐T whole‐body scanner equipped with a 12‐channel array coil. Four‐step interleaved phase encoding and fourfold SENSE acceleration were used to encode a 16 × 16 spatial matrix with a 390‐Hz spectral width. Comparison with conventional PEPSI and PEPSI with fourfold SENSE acceleration demonstrated comparable sensitivity per unit time when taking into account g‐factor–related noise increases and differences in sampling efficiency. LCModel fitting enabled quantification of inositol, choline, creatine, and N‐acetyl‐aspartate (NAA) in vivo with concentration values in the ranges measured with conventional PEPSI and SENSE‐accelerated PEPSI. Cramer–Rao lower bounds were comparable to those obtained with conventional SENSE‐accelerated PEPSI at the same voxel size and measurement time. This single‐shot MRSI method is therefore suitable for applications that require high temporal resolution to monitor temporal dynamics or to reduce sensitivity to tissue movement. Magn Reson Med, 2009. © 2008 Wiley‐Liss, Inc.     

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.     

Sensitivity-encoded spectroscopic imaging.

Sensitivity encoding (SENSE) offers a new, highly effective approach to reducing the acquisition time in spectroscopic imaging (SI). In contrast to conventional fast SI techniques, which accelerate k-space sampling, this method permits reducing the number of phase encoding steps in each phase encoding dimension of conventional SI. Using a coil array for data acquisition, the missing encoding information is recovered exploiting knowledge of the distinct spatial sensitivities of the individual coil elements. In this work, SENSE is applied to 2D spectroscopic imaging. Fourfold reduction of scan time is achieved at preserved spectral and spatial resolution, maintaining a reasonable SNR. The basic properties of the proposed method are demonstrated by phantom experiments. The in vivo feasibility of SENSE-SI is verified by metabolic imaging of N-acetylaspartate, creatine, and choline in the human brain. These results are compared to conventional SI, with special attention to the spatial response and the SNR.     

Echo‐planar spectroscopic imaging (EPSI) of the water resonance structure in human breast using sensitivity encoding (SENSE)

High spectral and spatial resolution MRI, based on echo‐planar spectroscopic imaging, has been applied successfully in diagnostic breast imaging, but acquisition times are long. One way of increasing acquisition speed is to apply the sensitivity encoding algorithm for complex high spectral and spatial resolution data. We demonstrate application of a complex sensitivity encoding algorithm to high spectral and spatial resolution MRI data, in a phantom and human breast, with 7‐ and 16‐channel dedicated breast phased‐array coils. Very low g factors are obtained using the breast coils, and the signal‐to‐noise ratio (SNR) penalty for water resonance peak height and water resonance asymmetry images is small at acceleration factors of up to 6 and 4, respectively, as evidenced by high Pearson correlation factors between fully sampled and accelerated data. This is the first application of the sensitivity encoding algorithm to characterize the structure of the water resonance at high spatial resolution. Magn Reson Med 63:1557–1563, 2010. © 2010 Wiley‐Liss, Inc.     

Clinically constrained optimization of flexTPI acquisition parameters for the tissue sodium concentration bioscale

The rapid transverse relaxation of the sodium magnetic resonance signal during spatial encoding causes a loss of image resolution, an effect known as T₂‐blurring. Conventional wisdom suggests that spatial resolution is maximized by keeping the readout duration as short as possible to minimize T₂‐blurring. Flexible twisted projection imaging performed with an ultrashort echo time, relative to T₂, and a long repetition time, relative to T₁, has been shown to be effective for quantitative sodium magnetic resonance imaging. A minimized readout duration requires a very large number of projections and, consequentially, results in an impractically long total acquisition time to meet these conditions. When the total acquisition time is limited to a clinically practical duration (e.g., 10 min), the optimal parameters for maximal spatial resolution of a flexible twisted projection imaging acquisition do not correspond to the shortest possible readout. Simulation and experimental results for resolution optimized acquisition parameters of quantitative sodium flexible twisted projection imaging of parenchyma and cerebrospinal fluid are presented for the human brain at 9.4 and 3.0T. The effect of signal loss during data collection on sodium quantification bias and image signal‐to‐noise ratio are discussed. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Three-Dimensional Spectroscopic Imaging with Time-Varying Gradients

A spectroscopic imaging sequence with a time-varying readout gradient in the slice selection direction is used to image multiple contiguous slices. For a given voxel size, the imaging time and signal-to-noise ratio of the three-dimensional spectroscopic sequence are the same as for a single slice acquisition without the oscillating readout gradient. The data reconstruction employs a gridding algorithm in two dimensions to interpolate the nonuniformly sampled data onto a Cartesian grid, and a fast Fourier transform in four dimensions: three spatial dimensions and the spectral dimension. The method is demonstrated by in vivo imaging of NAA in human brain at 1.5 T with 10 slices of 16 x 16 pixels spectroscopic images acquired in a total scan time of 17 min.     

Out-and-in spiral spectroscopic imaging in rat brain at 7 T.

With standard spectroscopic imaging, high spatial resolution is achieved at the price of a large number of phase-encoding steps, leading to long acquisition times. Fast spatial encoding methods reduce the minimum total acquisition time. In this article, a k-space scanning scheme using a continuous series of growing and shrinking, or "out-and-in," spiral trajectories is implemented and the feasibility of spiral spectroscopic imaging for animal models at high B(0) field is demonstrated. This method was applied to rat brain at 7 T. With a voxel size of about 8.7 microl (as calculated from the point-spread function), a 30 x 30 matrix, and a spectral bandwidth of 11 kHz, the minimum scan time was 9 min 20 sec for a signal-to-noise ratio of 7.1 measured on the N-acetylaspartate peak.     

Accelerated short-TE 3D proton echo-planar spectroscopic imaging using 2D-SENSE with a 32-channel array coil.

MR spectroscopic imaging (MRSI) with whole brain coverage in clinically feasible acquisition times still remains a major challenge. A combination of MRSI with parallel imaging has shown promise to reduce the long encoding times and 2D acceleration with a large array coil is expected to provide high acceleration capability. In this work a very high-speed method for 3D-MRSI based on the combination of proton echo planar spectroscopic imaging (PEPSI) with regularized 2D-SENSE reconstruction is developed. Regularization was performed by constraining the singular value decomposition of the encoding matrix to reduce the effect of low-value and overlapped coil sensitivities. The effects of spectral heterogeneity and discontinuities in coil sensitivity across the spectroscopic voxels were minimized by unaliasing the point spread function. As a result the contamination from extracranial lipids was reduced 1.6-fold on average compared to standard SENSE. We show that the acquisition of short-TE (15 ms) 3D-PEPSI at 3 T with a 32 x 32 x 8 spatial matrix using a 32-channel array coil can be accelerated 8-fold (R = 4 x 2) along y-z to achieve a minimum acquisition time of 1 min. Maps of the concentrations of N-acetyl-aspartate, creatine, choline, and glutamate were obtained with moderate reduction in spatial-spectral quality. The short acquisition time makes the method suitable for volumetric metabolite mapping in clinical studies.     

Reproducibility study of whole‐brain 1H spectroscopic imaging with automated quantification

A reproducibility study of proton MR spectroscopic imaging (¹H‐MRSI) of the human brain was conducted to evaluate the reliability of an automated 3D in vivo spectroscopic imaging acquisition and associated quantification algorithm. A PRESS‐based pulse sequence was implemented using dualband spectral‐spatial RF pulses designed to fully excite the singlet resonances of choline (Cho), creatine (Cre), and N‐acetyl aspartate (NAA) while simultaneously suppressing water and lipids; 1% of the water signal was left to be used as a reference signal for robust data processing, and additional lipid suppression was obtained using adiabatic inversion recovery. Spiral k‐space trajectories were used for fast spectral and spatial encoding yielding high‐quality spectra from 1 cc voxels throughout the brain with a 13‐min acquisition time. Data were acquired with an 8‐channel phased‐array coil and optimal signal‐to‐noise ratio (SNR) for the combined signals was achieved using a weighting based on the residual water signal. Automated quantification of the spectrum of each voxel was performed using LCModel. The complete study consisted of eight healthy adult subjects to assess intersubject variations and two subjects scanned six times each to assess intrasubject variations. The results demonstrate that reproducible whole‐brain ¹H‐MRSI data can be robustly obtained with the proposed methods. Magn Reson Med 60:542–547, 2008. © 2008 Wiley‐Liss, Inc.     

2D-spatial/2D-spectral spectroscopic imaging of intracerebral gliomas in rat brain.

1H-MR spectroscopy in vivo is often hampered by poor spectral resolution. Spectral overlap can be avoided with two-dimensional spectroscopic techniques. Correlation peak imaging has been implemented to measure unambiguously the distribution of several metabolites in a rat brain glioma model. Acquisition-weighted spectroscopic imaging reduced the experimental time and provided excellent spatial localization. The choice of an appropriate spectral acquisition window granted good sensitivity. Spectroscopic images presenting a full two-dimensional spectrum in every image pixel were acquired in seven rats at 7 Tesla in 195 min, with a nominal voxel volume of 75 microl. Among other metabolites, the distribution of hypotaurine, phosphoethanolamine, alanine, and even glucose could be visualized both in the C6-glioma and in the unaffected brain.     

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.     

NAA-weighted imaging of the human brain using a conventional readout gradient.

An imaging sequence incorporating two complementary forms of water suppression was used in conjunction with conventional readout and phase-encoding gradients to acquire images of N-acetylaspartate (NAA) in human brain. The sequence consisted of CHESS water suppression pulses followed by dual echo sequence to select the imaging volume and frequency-selective refocusing pulses with asymmetric crushers to further reduce the water signal. Spectra and conventional spectroscopic images acquired with the sequence demonstrated robust suppression of water and other resonances down field from the 2.0-ppm NAA resonance with 98% of the brain signal resulting from the 3.0-ppm to 2.0-ppm region. The technique was found to provide NAA maps with a large (256 x 128) imaging matrix and to allow a flexible tradeoff between signal to noise ratio (SNR), matrix size, data acquisition window length, and imaging time. Since spectral information is not directly obtained, the length of the data acquisition window is not determined by the spectral resolution needed to resolve the components of the brain spectrum, allowing a significantly shorter readout period. The shorter acquisition window could potentially facilitate the acquisition of multi-echo or multi-slice brain NAA maps.     

Reduced phase encoding in spectroscopic imaging

The effect of different spatial-encoding (k-space) sampling distributions are evaluated for magnetic resonance spectroscopic imaging (MRSI) using Fourier reconstruction. Previously, most MRSI studies have used square or cubic k-space functions, symmetrically distributed. These studies examine the conventional k-space distribution with spherical distribution, and 1/2 k-space acquisition, using computer simulation studies of the MRSI acquisition for three spatial dimensions and experimental results. Results compare the spatial response function, Gibbs ringing effects, and signal contamination for different spatial-encoding distribution functions. Results indicate that spherical encoding, in comparison with cubic encoding, results in a modest improvement of the re sponse function with approximately equivalent spatial resolution for the same acquisition time. For spin-echo acquired data, reduced acquisition times can readily be obtained using 1/2 k-space methods, with a concomitant reduction in signal to noise ratio.     

Hybrid Three Dimensional (1D-Hadamard, 2D-Chemical Shift Imaging) Phosphorus Localized Spectroscopy of Phantom and Human Brain

A hybrid of two localized spectroscopy techniques, chemical shift imaging (CSI) and Hadamard spectroscopic imaging (HSI), is used to obtain an array of 16 x 16 x 4 (3 x3 x 3 cm³ voxels) proton-decoupled phosphorus (³¹P) spectra of human brain. For equal spatial resolution, this organ's oblate shape requires fewer axial than coronal or sagittal slices. These different spatial requirements are well suited to 1D, 4^th order, transverse HSI in the axial direction, combined with 2D 16 x 16 CSI in the other two orientations. The reduced localization matrix (16 x 16 x 4 over just the brain versus a cubic-16 x 16 x 16 matrix of equal resolution, over the entire head) may proportionally shorten data acquisition if the voxel size is not signal-to-noise limited. In addition, the use of Hadamard encoding can improve the intervoxel spectral isolation.     

Sensitivity and localization enhancement in multinuclear in vivo NMR spectroscopy by outer volume presaturation

Sampling of the entire brain tissue in ¹H NMR metabolic studies is complicated by the presence of intense pericranial lipid and tissue water resonances, which can obscure metabolite resonances. This study extends the concept of localization by outer volume suppression (OVS) to achieve fully conformal in viuo localization, sampling at least 95% of rat brain with complete elimination of pericranial lipids. This has permitted acquisition of lipid-free short echo time (7.5 ms), high spatial resolution 2D spectroscopic images exhibiting high sensitivity and information content in relatively short measurement time (68 min). Incorporation of spatially tailored OVS into existing methods (e.g., DRESS, STEAM, PRESS) extends their localization efficiency and performance. Two single spin-echo localization sequences have been described that attain much shorter echo times or achieve better water suppression than PRESS. One of these sequences allows editing and relaxo-metric studies of J-coupled spins. The methods described are suitable for in vivo localization of most tissues and can be applied with any biologically important nucleus.     

Rosette spectroscopic imaging: Optimal parameters for alias‐free,high sensitivity spectroscopic imaging

Purpose

To optimize the Rosette trajectories for fast, high sensitivity spectroscopic imaging experiments and to compare this acquisition technique with other chemical shift imaging (CSI) methods.

Materials and Methods

A framework for comparing the sensitivity of the Rosette Spectroscopic Imaging (RSI) acquisition to other spectroscopic imaging experiments is outlined. Accounting for hardware constraints, trajectory parameters that provide for optimal sampling and minimal artifact production are found. Along with an analytical expression for the number of excitations to be used in an RSI experiment that is provided, the theoretical precompensation weights used for optimal image reconstruction are derived.

Results

The spectral response function for RSI is shown to be approximately the same as the point spread function of standard Fourier reconstructions. While the signal‐to‐noise ratio (SNR) for an RSI experiment is reduced by the inherent nonuniform sampling of these trajectories, their circular k‐space support and speed of spatial encoding leads to greater SNR efficiency and improvements in the total data acquisition time relative to the gold standard CSI approach with square k‐space support and to similar efficiency to spiral CSI acquisitions. Numerical simulations and in vivo experimental data are presented to demonstrate the properties of this data acquisition technique.

Conclusion

This work demonstrates the use of Rosette trajectories and how to achieve improved efficiency for these trajectories in a two‐dimensional spectroscopic imaging experiment. J. Magn. Reson. Imaging 2009;29:1375–1385. © 2009 Wiley‐Liss, Inc.     

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.     

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

Recent advances in magnet technology have enabled the construction of ultrahigh‐field magnets (7T and higher) that can accommodate the human head and body. Despite the intrinsic advantages of performing spectroscopic imaging at 7T, increased signal‐to‐noise ratio (SNR), and spectral resolution, few studies have been reported to date. This limitation is largely due to increased power deposition and B₁ inhomogeneity. To overcome these limitations, we used an 8‐channel transceiver array with a short TE (15 ms) spectroscopic imaging sequence. Utilizing phase and amplitude mapping and optimization schemes, the 8‐element transceiver array provided both improved efficiency (17% less power for equivalent peak B₁) and homogeneity (SD(B₁) = ±10% versus ±22%) in comparison to a transverse electromagnetic (TEM) volume coil. To minimize the echo time to measure J‐modulating compounds such as glutamate, we developed a short TE sequence utilizing a single‐slice selective excitation pulse followed by a broadband semiselective refocusing pulse. Extracerebral lipid resonances were suppressed with an inversion recovery pulse and delay. The short TE sequence enabled visualization of a variety of resonances, including glutamate, in both a control subject and a patient with a Grade II oligodendroglioma. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Multiple-echo proton spectroscopic imaging using time domain parametric spectral analysis

A multiple-echo MR spectroscopic imaging (MRSI) method is presented that enables improved metabolite imaging in the presence of local field inhomogeneities and measurement of transverse relaxation parameters. Short echo spacing is used to maximize signal energy from inhomogeneously line-broadened resonances, and time domain parametric spectral analysis of the entire echo train is used to obtain sufficient spectral resolution from the shortened sampling periods. Optimal sequence parameters for ¹H MRSI are determined by computer simulation, and performance is compared with conventional single-echo acquisition using phantom studies at a field strength of 4.7 T. A preliminary example for use at 1.5 T is also presented using phantom and human brain MRSI studies. This technique is shown to offer improved performance relative to single-echo MRSI for imaging of metabolites with shortened T₂* values due to the presence of local field inhomogeneities. Additional advantages are the intrinsic measurement of metabolite T₂ values and determination of metabolite integrals without T₂ weighting, thereby facilitating quantitative metabolite imaging.     

Blipped‐controlled aliasing in parallel imaging for simultaneous multislice echo planar imaging with reduced g‐factor penalty

Simultaneous multislice Echo Planar Imaging (EPI) acquisition using parallel imaging can decrease the acquisition time for diffusion imaging and allow full‐brain, high‐resolution functional MRI (fMRI) acquisitions at a reduced repetition time (TR). However, the unaliasing of simultaneously acquired, closely spaced slices can be difficult, leading to a high g‐factor penalty. We introduce a method to create interslice image shifts in the phase encoding direction to increase the distance between aliasing pixels. The shift between the slices is induced using sign‐ and amplitude‐modulated slice‐select gradient blips simultaneous with the EPI phase encoding blips. This achieves the desired shifts but avoids an undesired “tilted voxel” blurring artifact associated with previous methods. We validate the method in 3× slice‐accelerated spin‐echo and gradient‐echo EPI at 3 T and 7 T using 32‐channel radio frequency (RF) coil brain arrays. The Monte‐Carlo simulated average g‐factor penalty of the 3‐fold slice‐accelerated acquisition with interslice shifts is <1% at 3 T (compared with 32% without slice shift). Combining 3× slice acceleration with 2× inplane acceleration, the g‐factor penalty becomes 19% at 3 T and 10% at 7 T (compared with 41% and 23% without slice shift). We demonstrate the potential of the method for accelerating diffusion imaging by comparing the fiber orientation uncertainty, where the 3‐fold faster acquisition showed no noticeable degradation. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.