Correction of local B0 shifts in 3D EPSI of the human brain at 4 T.
TL;DR: The approach provided a high yield of voxels with good spectral quality for 3D EPSI, resulting in improved brain coverage, and the combination of HR and local B(0)-shift correction to limit line broadening was evaluated at 4 T.
About: This article is published in Magnetic Resonance Imaging.The article was published on 2007-04-01 and is currently open access. It has received 16 citations till now.
Citations
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TL;DR: This review highlights the developments of the last five years and puts them into the context of earlier MRSI acceleration techniques.
Abstract: Over more than 30 years in vivo MR spectroscopic imaging (MRSI) has undergone an enormous evolution from theoretical concepts in the early 1980s to the robust imaging technique that it is today. The development of both fast and efficient sampling and reconstruction techniques has played a fundamental role in this process. State-of-the-art MRSI has grown from a slow purely phase-encoded acquisition technique to a method that today combines the benefits of different acceleration techniques. These include shortening of repetition times, spatial-spectral encoding, undersampling of k-space and time domain, and use of spatial-spectral prior knowledge in the reconstruction. In this way in vivo MRSI has considerably advanced in terms of spatial coverage, spatial resolution, acquisition speed, artifact suppression, number of detectable metabolites and quantification precision. Acceleration not only has been the enabling factor in high-resolution whole-brain 1 H-MRSI, but today is also common in non-proton MRSI (31 P, 2 H and 13 C) and applied in many different organs. In this process, MRSI techniques had to constantly adapt, but have also benefitted from the significant increase of magnetic field strength boosting the signal-to-noise ratio along with high gradient fidelity and high-density receive arrays. In combination with recent trends in image reconstruction and much improved computation power, these advances led to a number of novel developments with respect to MRSI acceleration. Today MRSI allows for non-invasive and non-ionizing mapping of the spatial distribution of various metabolites' tissue concentrations in animals or humans, is applied for clinical diagnostics and has been established as an important tool for neuro-scientific and metabolism research. This review highlights the developments of the last five years and puts them into the context of earlier MRSI acceleration techniques. In addition to 1 H-MRSI it also includes other relevant nuclei and is not limited to certain body regions or specific applications.
66 citations
TL;DR: A feasibility study of an echo-planar spectroscopic imaging (EPSI) using a short echo time (TE) that trades off sensitivity, compared with other short-TE methods, to achieve whole brain coverage using inversion recovery and spatial oversampling to control lipid bleeding is presented in this paper.
Abstract: Purpose
A feasibility study of an echo-planar spectroscopic imaging (EPSI) using a short echo time (TE) that trades off sensitivity, compared with other short-TE methods, to achieve whole brain coverage using inversion recovery and spatial oversampling to control lipid bleeding.
Methods
Twenty subjects were scanned to examine intersubject variance. One subject was scanned five times to examine intrasubject reproducibility. Data were analyzed to determine coefficients of variance (COV) and intraclass correlation coefficient (ICC) for N–acetylaspartate (NAA), total creatine (tCr), total choline (tCho), glutamine/glutamate (Glx), and myo-inositol (mI). Regional metabolite concentrations were derived by using multi-voxel analysis based on lobar-level anatomic regions.
Results
For whole-brain mean values, the intrasubject COVs were 14%, 15%, and 20% for NAA, tCr, and tCho, respectively, and 31% for Glx and mI. The intersubject COVs were up to 6% higher. For regional distributions, the intrasubject COVs were ≤ 5% for NAA, tCr, and tCho; ≤ 9% for Glx; and ≤15% for mI, with about 6% higher intersubject COVs. The ICCs of 5 metabolites were ≥ 0.7, indicating the reliability of the measurements.
Conclusion
The present EPSI method enables estimation of the whole-brain metabolite distributions, including Glx and mI with small voxel size, and a reasonable scan time and reproducibility. Magn Reson Med 73:921–928, 2015. © 2014 Wiley Periodicals, Inc.
44 citations
TL;DR: In this paper, the authors combine the dual-density approach and the lipid-basis penalty to estimate the high-resolution lipid image from 2-average k-space data to incur minimal increase on the scan time.
Abstract: Mapping 1 H brain metabolites using chemical shift imaging is hampered by the presence of subcutaneous lipid signals, which contaminate the metabolites by ringing due to limited spatial resolution. Even though chemical shift imaging at spatial resolution high enough to mitigate the lipid artifacts is infeasible due to signal-to-noise constraints on the metabolites, the lipid signals have orders of magnitude of higher concentration, which enables the collection of high-resolution lipid maps with adequate signal-to-noise. The previously proposed dual-density approach exploits this high signal-to-noise property of the lipid layer to suppress truncation artifacts using high-resolution lipid maps. Another recent approach for lipid suppression makes use of the fact that metabolite and lipid spectra are approximately orthogonal, and seeks sparse metabolite spectra when projected onto lipid-basis functions. This work combines and extends the dual-density approach and the lipid-basis penalty, while estimating the high-resolution lipid image from 2-average k-space data to incur minimal increase on the scan time. Further, we exploit the spectral-spatial sparsity of the lipid ring and propose to estimate it from substantially undersampled (acceleration R 5 10 in the peripheral k-space) 2-average in vivo data using compressed sensing and still obtain improved lipid suppression relative to using dual-density or lipid-basis penalty alone. Magn Reson Med 000:000–000, 2012. V C 2012 Wiley Periodicals, Inc.
36 citations
TL;DR: 3D EPSI data suggests a heterogeneous etiology of extrahippocampal spectroscopic metabolic abnormalities in TLE and reduces their value for focus lateralization, while statistical parametric mapping identifies regions of significantly decreased NAA/(Cr+Cho) in Tle groups and in individual patients.
Abstract: MR spectroscopy has demonstrated extrahippocampal NAA/(Cr+Cho) reductions in medial temporal lobe epilepsy with (TLE-MTS) and without (TLE-no) mesial temporal sclerosis. Because of the limited brain coverage of those previous studies, it was, however, not possible to assess differences in the distribution and extent of these abnormalities between TLE-MTS and TLE-no. This study used a 3D whole brain echoplanar spectroscopic imaging (EPSI) sequence to address the following questions: (1) Do TLE-MTS and TLE-no differ regarding severity and distribution of extrahippocampal NAA/(Cr+Cho) reductions? (2) Do extrahippocampal NAA/(Cr+Cho) reductions provide additional information for focus lateralization? Forty-three subjects (12 TLE-MTS, 13 TLE-no, 18 controls) were studied with 3D EPSI. Statistical parametric mapping (SPM2) was used to identify regions of significantly decreased NAA/(Cr+Cho) in TLE groups and in individual patients. TLE-MTS and TLE-no had widespread extrahippocampal NAA/(Cr+Cho) reductions. NAA/(Cr+Cho) reductions had a bilateral fronto-temporal distribution in TLE-MTS and a more diffuse, less well defined distribution in TLE-no. Extrahippocampal NAA/(Cr+Cho) decreases in the single subject analysis showed a large inter-individual variability and did not provide additional focus lateralizing information. Extrahippocampal NAA/(Cr+Cho) reductions in TLE-MTS and TLE-no are neither focal nor homogeneous. This reduces their value for focus lateralization and suggests a heterogeneous etiology of extrahippocampal spectroscopic metabolic abnormalities in TLE.
31 citations
Cites methods from "Correction of local B0 shifts in 3D..."
...Fig. 1). A binary map containing only voxels of acceptable quality (QC map) was generated using linewidth (between 3‐21.3 Hz) and fit accuracy (fits with residual sum squares outside the 95 percentile distribution of residuals from all fits were rejected) [ 18 , 20] as main criteria....
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...The water reference data was then zero-filled from 50 9 50 9 18 to 64 9 64 9 24 points, and a spatial apodization using a Gaussian filter was applied before performing of a four-dimensional spectral FT and B0 correction [ 18 ]....
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01 Jul 2012
TL;DR: The spectral‐spatial sparsity of the lipid ring is exploited and it is proposed to estimate it from substantially undersampled 2‐average in vivo data using compressed sensing and still obtain improved lipid suppression relative to using dual‐density or lipid‐basis penalty alone.
Abstract: Mapping 1 H brain metabolites using chemical shift imaging is hampered by the presence of subcutaneous lipid signals, which contaminate the metabolites by ringing due to limited spatial resolution. Even though chemical shift imaging at spatial resolution high enough to mitigate the lipid artifacts is infeasible due to signal-to-noise constraints on the metabolites, the lipid signals have orders of magnitude of higher concentration, which enables the collection of high-resolution lipid maps with adequate signal-to-noise. The previously proposed dual-density approach exploits this high signal-to-noise property of the lipid layer to suppress truncation artifacts using high-resolution lipid maps. Another recent approach for lipid suppression makes use of the fact that metabolite and lipid spectra are approximately orthogonal, and seeks sparse metabolite spectra when projected onto lipid-basis functions. This work combines and extends the dual-density approach and the lipid-basis penalty, while estimating the high-resolution lipid image from 2-average k-space data to incur minimal increase on the scan time. Further, we exploit the spectral-spatial sparsity of the lipid ring and propose to estimate it from substantially undersampled (acceleration R 5 10 in the peripheral k-space) 2-average in vivo data using compressed sensing and still obtain improved lipid suppression relative to using dual-density or lipid-basis penalty alone. Magn Reson Med 000:000–000, 2012. V C 2012 Wiley Periodicals, Inc.
29 citations
References
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TL;DR: Variations of image intensity of up to 28% at 4 T, and up to 7% at 1.5 T, were observed in primary visual cortex, corresponding to an increase of blood oxygenation in regions of increased neural activity.
Abstract: The effects of photic stimulation on the visual cortex of human brain were studied by means of gradient-echo echo-planar imaging (EPI). Whole-body 4 and 1.5 T MRI systems, equipped with a small z axis head gradient coil, were used. Variations of image intensity of up to 28% at 4 T, and up to 7% at 1.5 T, were observed in primary visual cortex, corresponding to an increase of blood oxygenation in regions of increased neural activity. The larger effects at 4 T are due to the increased importance of the susceptibility difference between deoxygenated and oxygenated blood at high fields.
459 citations
"Correction of local B0 shifts in 3D..." refers background in this paper
...At higher magnetic field strengths (B0 > 3T), the problems with poor spectral quality are exacerbated, since regional B0 inhomogeneities scale with the magnetic field strength [3]....
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TL;DR: The automated fitting procedure was applied to four different examples of MRS data obtained at 1.5 T and 4.1 T and 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.
Abstract: 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.
261 citations
"Correction of local B0 shifts in 3D..." refers methods in this paper
...Data were processed once with and once without the correction for local B0-shifts, and fitted using an automated spectral analysis procedure [5] to extract metabolite peak areas and linewidths....
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TL;DR: A 3D echo‐planar MRSI technique has been implemented without volume preselection to provide sufficient spatial resolution with maximum coverage of the brain and the resultant spectral quality and the extent of viable data in human brain was assessed.
Abstract: For many clinical applications of proton MR spectroscopic imaging (MRSI) of the brain, diagnostic assessment is limited by insufficient coverage provided by single- or multislice acquisition methods as well as by the use of volume preselection methods. Additionally, traditional spectral analysis methods may limit the operator to detailed analysis of only a few selected brain regions. It is therefore highly desirable to use a fully 3D approach, combined with spectral analysis procedures that enable automated assessment of 3D metabolite distributions over the whole brain. In this study, a 3D echo-planar MRSI technique has been implemented without volume preselection to provide sufficient spatial resolution with maximum coverage of the brain. Using MRSI acquisitions in normal subjects at 1.5T and a fully automated spectral analysis procedure, an assessment of the resultant spectral quality and the extent of viable data in human brain was carried out. The analysis found that 69% of brain voxels were obtained with acceptable spectral quality at TE = 135 ms, and 52% at TE = 25 ms. Most of the rejected voxels were located near the sinuses or temporal bones and demonstrated poor B0 homogeneity and additional regions were affected by stronger lipid contamination at TE = 25 ms.
92 citations
"Correction of local B0 shifts in 3D..." refers background or methods in this paper
...3 Hz (4 T), were calculated [6], and the relative number of acceptable voxels compared for data reconstructed with and without B0-shift correction....
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...As pointed out previously [6], Cramer-Rao bounds and confidence intervals [7] have limits to evaluate fitting accuracy....
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TL;DR: A modified acquisition strategy for volumetric echo-planar spectroscopic imaging (3D EPSI) is presented that extends the region of the brain that can be observed and provides considerable reduction of spectral linewidths in many problematic brain regions, though with a reduction in signal-to-noise ratio.
79 citations
"Correction of local B0 shifts in 3D..." refers background or methods or result in this paper
...The B0 field map was derived from the water reference EPSI data [1], which was acquired either sequentially (1....
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...6 for the matrix sizes used [1]....
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...Note that since in this study high-resolution EPSI data without B0-shift correction were used as opposed to “standard resolution” EPSI data acquired in our previous study [1], results for 1....
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...Previously, a modified acquisition strategy for volumetric echo-planar spectroscopic imaging (3D EPSI) was presented to recover spectral quality in these brain regions [1]....
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...5 T (Siemens Magnetom Vision, Siemens Medical Solutions, Erlangen, Germany) and 4 T (Bruker MedSpec, Bruker BioSpin, Ettlingen, Germany) have been described previously ([1] and [4], respectively)....
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TL;DR: It is shown that while the cubic voxels' dimensions were all halved, reducing their volume eightfold, their metabolites' SNR decreased only fourfold, due to their Δυ*s' twofold decrease, and both spatial and spectral resolutions were doubled at a significantly, ×2, smaller‐than‐expected SNR loss.
Abstract: It is commonly accepted that the signal-to-noise ratio (SNR = peak-signal/RMS-noise) per-unit-time of proton MR spectroscopy ( 1 H-MRS) is linearly proportional to the voxel volume. Consequently, with a headcoil and 30-min acquisition, 1 cm 3 is considered the SNR-limited spatial resolution barrier in the human brain. However, since local linewidths, Δυ* = (πT* 2 ) -1 , at high magnetic fields (B 0 ), are dominated by regional inhomogeneities (ΔB 0 ), i.e., T* 2 « T 2 , reducing the voxel dimensions may increase T* 2 . This could compensate, in part, for signal loss with volume decrease. It is shown that for two cubic voxels of sides I 1 and I 2 , I 1 > I 2 , as the volume decreases by (I 1 /I 2 ) 3 , their SNR ratio is reduced by only (I 1 /I 2 ) 2 due to a commensurate T * 2 increase of I 1 /I 2 . This is demonstrated in a phantom and the brains of volunteers, with 3D 1 H-MRS in a headcoil at 4 T. It is shown that while the cubic voxels' dimensions were all halved, reducing their volume eightfold, their metabolites' SNR decreased only fourfold, due to their Δυ*s' two-fold decrease. In other words, both spatial and spectral resolutions were doubled at a significantly, x2, smaller-than-expected SNR loss. This advantage was exploited to produce quality high spatial resolution, 0.75 x 0.75 x 0.75 cm 3 , metabolic maps in a 27-min acquisition.
56 citations
Additional excerpts
...[2] that B0-field variations primarily determine spectral...
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