Across‐vendor standardization of semi‐LASER for single‐voxel MRS at 3T

The semi‐adiabatic localization by adiabatic selective refocusing (sLASER) sequence provides single‐shot full intensity signal with clean localization and minimal chemical shift displacement error and was recommended by the international MRS Consensus Group as the preferred localization sequence at high‐ and ultra‐high fields. Across‐vendor standardization of the sLASER sequence at 3 tesla has been challenging due to the B1 requirements of the adiabatic inversion pulses and maximum B1 limitations on some platforms. The aims of this study were to design a short‐echo sLASER sequence that can be executed within a B1 limit of 15 μT by taking advantage of gradient‐modulated RF pulses, to implement it on three major platforms and to evaluate the between‐vendor reproducibility of its perfomance with phantoms and in vivo. In addition, voxel‐based first and second order B0 shimming and voxel‐based B1 adjustments of RF pulses were implemented on all platforms. Amongst the gradient‐modulated pulses considered (GOIA, FOCI and BASSI), GOIA‐WURST was identified as the optimal refocusing pulse that provides good voxel selection within a maximum B1 of 15 μT based on localization efficiency, contamination error and ripple artifacts of the inversion profile. An sLASER sequence (30 ms echo time) that incorporates VAPOR water suppression and 3D outer volume suppression was implemented with identical parameters (RF pulse type and duration, spoiler gradients and inter‐pulse delays) on GE, Philips and Siemens and generated identical spectra on the GE ‘Braino’ phantom between vendors. High‐quality spectra were consistently obtained in multiple regions (cerebellar white matter, hippocampus, pons, posterior cingulate cortex and putamen) in the human brain across vendors (5 subjects scanned per vendor per region; mean signal‐to‐noise ratio [less than] 33; mean water linewidth between 6.5 Hz to 11.4 Hz). The harmonized sLASER protocol is expected to produce high reproducibility of MRS across sites thereby allowing large multi‐site studies with clinical cohorts

Clinically Meaningful <scp>Magnetic Resonance</scp> Endpoints Sensitive to Preataxic Spinocerebellar Ataxia Types <scp>1</scp> and <scp>3</scp>

International audienceObjective: This study was undertaken to identify magnetic resonance (MR) metrics that are most sensitive to early changes in the brain in spinocerebellar ataxia type 1 (SCA1) and type 3 (SCA3) using an advanced multimodal MR imaging (MRI) protocol in the multisite trial setting. Methods: SCA1 or SCA3 mutation carriers and controls (n = 107) underwent MR scanning in the US-European READISCA study to obtain structural, diffusion MRI, and MR spectroscopy data using an advanced protocol at 3T. Morphometric, microstructural, and neurochemical metrics were analyzed blinded to diagnosis and compared between preataxic SCA (n = 11 SCA1, n = 28 SCA3), ataxic SCA (n = 14 SCA1, n = 37 SCA3), and control (n = 17) groups using nonparametric testing accounting for multiple comparisons. MR metrics that were most sensitive to preataxic abnormalities were identified using receiver operating characteristic (ROC) analyses

Results and interpretation of a fitting challenge for MR spectroscopy set up by the MRS study group of ISMRM.

PURPOSE Fitting of MRS data plays an important role in the quantification of metabolite concentrations. Many different spectral fitting packages are used by the MRS community. A fitting challenge was set up to allow comparison of fitting methods on the basis of performance and robustness. METHODS Synthetic data were generated for 28 datasets. Short-echo time PRESS spectra were simulated using ideal pulses for the common metabolites at mostly near-normal brain concentrations. Macromolecular contributions were also included. Modulations of signal-to-noise ratio (SNR); lineshape type and width; concentrations of γ-aminobutyric acid, glutathione, and macromolecules; and inclusion of artifacts and lipid signals to mimic tumor spectra were included as challenges to be coped with. RESULTS Twenty-six submissions were evaluated. Visually, most fit packages performed well with mostly noise-like residuals. However, striking differences in fit performance were found with bias problems also evident for well-known packages. In addition, often error bounds were not appropriately estimated and deduced confidence limits misleading. Soft constraints as used in LCModel were found to substantially influence the fitting results and their dependence on SNR. CONCLUSIONS Substantial differences were found for accuracy and precision of fit results obtained by the multiple fit packages

In vivo 1H MR spectroscopy with J‐refocusing

Purpose The goal of this study was to propose a novel localized proton MR spectroscopy (MRS) sequence that reduces signal loss due to J‐modulation in the rat brain in vivo. Methods Sprague‐Dawley rats were studied at 9.4 T. A semi‐LASER sequence with evenly distributed echo‐time (TE) was used, and a 90° J‐refocusing pulse was inserted at TE/2. Proton spectra were acquired at two TEs (30 and 68 ms), with and without the J‐refocused pulse. Data were processed in MATLAB and quantified with LCModel. Results The J‐refocused spectrum acquired at TE = 30 ms did not show any signal losses due to J‐modulation and had comparable spectral pattern to the one acquired with semi‐LASER using the minimum achievable TE. Higher signal amplitudes for glutamine, γ‐aminobutyric acid and glutathione led to more reliable quantification precision for these metabolites. The refocused signal intensities at TE = 68 ms were also unaffected by J‐modulation but were smaller than the signals at TE = 30 ms mainly due to transverse T2 relaxation of metabolites. Conclusion The proposed localized MRS sequence will be beneficial in both animal and human MRS studies when using ultra‐short TE is not possible while also providing more reliable quantification precision for J‐coupled metabolites

Rat brain MRI at 16.4T using a capacitively tunable patch antenna in combination with a receive array

For MRI at 16.4T, with a proton Larmor frequency of 698 MHz, one of the principal RF engineering challenges is to generate a spatially homogeneous transmit field over a larger volume of interest for spin excitation. Constructing volume coils large enough to house a receive array along with the subject and to maintain the quadrature symmetry for different loading conditions is difficult at this frequency. This calls for new approaches to RF coil design for ultra-high field MR systems. A remotely placed capacitively tunable patch antenna, which can easily be adjusted to different loading conditions, was used to generate a relatively homogeneous excitation field covering a large imaging volume with a transversal profile similar to that of a birdcage coil. Since it was placed in front of the animal, this created valuable free space in the narrow magnet bore around the subject for additional hardware. To enhance the reception sensitivity, the patch antenna was combined with an actively detunable 3-channel receive coil array. In addition to increased SNR compared to a quadrature transceive surface coil, we were able to get high quality gradient echo and spin-echo images covering the whole rat brain

The influence of cystathionine on neurochemical quantification in brain tumor in vivo magnetic resonance spectroscopy

International audiencePurpose: To evaluate the ability of the PRESS sequence (TE = 97 ms, optimized for 2-hydroxyglutarate detection) to detect cystathionine in gliomas and the effect of the omission of cystathionine on the quantification of the full neurochemical profile.Methods: Twenty-three subjects with a glioma were retrospectively included based on the availability of both MEGA-PRESS and PRESS acquisitions at 3T, and the presence of the cystathionine signal in the edited MR spectrum. In eight subjects, the PRESS acquisition was performed also in normal tissue. Metabolite quantification was performed using LCModel and simulated basis sets. The LCModel analysis for the PRESS data was performed with and without cystathionine.Results: All subjects with glioma had detectable cystathionine levels >1 mM with Cramér-Rao lower bounds (CRLB) <15%. The mean cystathionine concentrations were 3.49 ± 1.17 mM for MEGA-PRESS and 2.20 ± 0.80 mM for PRESS data. Cystathionine concentrations showed a significant correlation between the two MRS methods (r = 0.58, p = .004), and it was not detectable in normal tissue. Using PRESS, 19 metabolites were quantified with CRLB <50% for more than half of the subjects. The metabolites that were significantly (p < .0028) and mostly affected by the omission of cystathionine were aspartate, betaine, citrate, γ-aminobutyric acid (GABA), and serine.Conclusions: Cystathionine was detectable by PRESS in all the selected gliomas, while it was not detectable in normal tissue. The omission from the spectral analysis of cystathionine led to severe biases in the quantification of other neurochemicals that may play key roles in cancer metabolism

Voxel placement and data quality.

<p>(A) Images of human brains illustrating the position and size of the VOI. (B) Representative ¹H STEAM spectra (4 T, 27 mL, <i>T</i>_R = 4.5 s, number of averages = 4) measured at seven <i>T</i>_Es from the human occipital lobe in one young (left) and one elderly (right) subject. The vertical scale has been adjusted such that the NAA resonance detected at <i>T</i>_E = 10 ms for both young and elderly subjects has the same intensity. Horizontal dashed lines are visual guides to indicate that the intensity of NAA and tCr signals decrease faster in the elderly than the young subject. The faster signal decay reflects a shorter <i>T</i>₂ value. Spectra are shown without line broadening. NAA, N-acetylaspartate, tCr, total creatine = creatine (Cr) + phosphocreatine (PCr), tCho, total choline = choline containing compounds.</p

<i>T</i>₂ fits for metabolites in young and elderly subjects.

<p>Individual exponential fits (represented by decaying lines) of the experimentally measured data for (A) the NAA singlet at 2.01 ppm, (B) the tCr signal at 3.03 ppm, and (C) the tCho singlet at 3.2 ppm in one representative young and one representative elderly subject. The amplitude of all data sets was normalized by setting the first <i>T</i>_E point to unity for both young and elderly subjects. For all metabolites and all subjects, <i>T</i>₂s were fit with R² ≥ 0.918, with the lowest R² for tCho.</p

Abnormal response to cortical activation in early stages of Huntington disease.

International audienceBACKGROUND: We wished to identify noninvasive in vivo biomarkers of brain energy deficit in Huntington disease. METHODS: We studied 15 early affected patients (mean motor United Huntington Disease Rating Scale, 18 ± 9) and 15 age- and sex-matched controls. We coupled (31) phosphorus nuclear magnetic resonance spectroscopy with activation of the occipital cortex in order to measure the relative concentrations of adenosine triphosphate, phosphocreatine, and inorganic phosphate before, during, and after visual stimulation. RESULTS: In controls, we observed an 11% increase in the inorganic phosphate/phosphocreatine ratio (P = .024) and a 13% increase in the inorganic phosphate/adenosine triphosphate ratio (P = .016) during brain activation, reflecting increased adenosine diphosphate concentrations. Subsequently, controls had a return to baseline levels during recovery (P = .012 and .022, respectively). In contrast, both ratios were unchanged in patients during and after visual stimulation. CONCLUSIONS: (31) Phosphorus nuclear magnetic resonance spectroscopy could provide functional biomarkers of brain energy deficit to monitor therapeutic efficacy in Huntington disease