Michal Považan

In vivo 31P magnetic resonance spectroscopy of the human liver at 7 T: an initial experience

Phosphorus (31P) MRS is a powerful tool for the non‐invasive investigation of human liver metabolism. Four in vivo 31P localization approaches (single voxel image selected in vivo spectroscopy (3D‐ISIS), slab selective 1D‐ISIS, 2D chemical shift imaging (CSI), and 3D‐CSI) with different voxel volumes and acquisition times were demonstrated in nine healthy volunteers. Localization techniques provided comparable signal‐to‐noise ratios normalized for voxel volume and acquisition time differences, Cramer–Rao lower bounds (8.7u2009±u20093.3%1D‐ISIS, 7.6u2009±u20092.5%3D‐ISIS, 8.6u2009±u20094.2%2D‐CSI, 10.3u2009±u20092.7%3D‐CSI), and linewidths (50u2009±u200924 Hz1D‐ISIS, 34u2009±u200910 Hz3D‐ISIS, 33u2009±u200910 Hz2D‐CSI, 34u2009±u200911 Hz3D‐CSI).

Mapping of brain macromolecules and their use for spectral processing of (1)H-MRSI data with an ultra-short acquisition delay at 7 T.

Long echo time (TE) MR spectroscopy (MRS) sequences are sensitive only to metabolites of low molecular weight. At shorter TE, significantly more metabolite signals are detectable, including broad signals of high-molecular-weight macromolecules (MMs). Although the presence of MM resonances can bias metabolite quantification at short TE, proper quantification of MMs is important since MMs themselves may serve as potentially valuable biomarkers for many pathologies. We have therefore developed an FID-based 2D-MR Spectroscopic Imaging (2D-MRSI) sequence to map MMs in healthy brain tissue at 7 T within a scan time of ~17 min and a repetition time of 879 ms. This 2D-MRSI technique provides MM maps over a whole slice (i.e., including cortical gray matter) at an ultra-short acquisition delay of 1.3 ms, using double inversion for efficient nulling of low-molecular-weight metabolites. The optimal sequence parameters were estimated using Bloch simulations, phantom testing, and in vivo validation. The acquired in vivo MM spectra (n=6) included nine distinct MM peaks in the range of ~0.9-3.7 ppm. The measured average MM spectrum was incorporated into the LCModel basis set and utilized for further quantification of MRSI data sets without metabolite nulling, which were acquired in five additional volunteers. The quantification results for two basis sets, one including the MMs and one without MM spectrum, were compared. Due to the high spectral resolution and full signal detection provided by the FID-MRSI sequence, we could successfully map five important brain metabolites. Most quantified metabolite signal amplitudes were significantly lower since the inclusion of MMs into the basis set corrected the overestimation of metabolite signals. The precision of fit (i.e., Cramér Rao lower bounds) remained unchanged. Our MM maps show that the overall MM contribution was higher in gray matter than in white matter. In conclusion, the acquired MM spectrum improved the accuracy of metabolite quantification and allowed the acquisition of high spatial resolution maps of five major brain metabolites and also MMs.

Lipid suppression via double inversion recovery with symmetric frequency sweep for robust 2D-GRAPPA-accelerated MRSI of the brain at 7 T.

This work presents a new approach for high‐resolution MRSI of the brain at 7u2009T in clinically feasible measurement times. Two major problems of MRSI are the long scan times for large matrix sizes and the possible spectral contamination by the transcranial lipid signal. We propose a combination of free induction decay (FID)‐MRSI with a short acquisition delay and acceleration via in‐plane two‐dimensional generalised autocalibrating partially parallel acquisition (2D‐GRAPPA) with adiabatic double inversion recovery (IR)‐based lipid suppression to allow robust high‐resolution MRSI. We performed Bloch simulations to evaluate the magnetisation pathways of lipids and metabolites, and compared the results with phantom measurements. Acceleration factors in the range 2–25 were tested in a phantom. Five volunteers were scanned to verify the value of our MRSI method in vivo. GRAPPA artefacts that cause fold‐in of transcranial lipids were suppressed via double IR, with a non‐selective symmetric frequency sweep. The use of long, low‐power inversion pulses (100u2009ms) reduced specific absorption rate requirements. The symmetric frequency sweep over both pulses provided good lipid suppression (>90%), in addition to a reduced loss in metabolite signal‐to‐noise ratio (SNR), compared with conventional IR suppression (52–70%). The metabolic mapping over the whole brain slice was not limited to a rectangular region of interest. 2D‐GRAPPA provided acceleration up to a factor of nine for in vivo FID‐MRSI without a substantial increase in g‐factors (<1.1). A 64 × 64 matrix can be acquired with a common repetition time of ~1.3u2009s in only 8u2009min without lipid artefacts caused by acceleration. Overall, we present a fast and robust MRSI method, using combined double IR fat suppression and 2D‐GRAPPA acceleration, which may be used in (pre)clinical studies of the brain at 7u2009T.

Ultra-high resolution brain metabolite mapping at 7 T by short-TR Hadamard-encoded FID-MRSI ☆

ABSTRACT MRSI in the brain at ≥7 T is a technique of great promise, but has been limited mainly by low B0/B1+‐homogeneity, specific absorption rate restrictions, long measurement times, and low spatial resolution. To overcome these limitations, we propose an ultra‐high resolution (UHR) MRSI sequence that provides a 128×128 matrix with a nominal voxel volume of 1.7×1.7×8 mm3 in a comparatively short measurement time. A clinically feasible scan time of 10–20 min is reached via a short TR of 200 ms due to an optimised free induction decay‐based acquisition with shortened water suppression as well as parallel imaging (PI) using Controlled Aliasing In Parallel Imaging Results IN Higher Acceleration (CAIPIRINHA). This approach is not limited to a rectangular region of interest in the centre of the brain, but also covers cortical brain regions. Transversal pulse‐cascaded Hadamard encoding was able to further extend the coverage to 3D‐UHR‐MRSI of four slices (100×100×4 matrix size), with a measurement time of 17 min. Lipid contamination was removed during post‐processing using L2‐regularisation. Simulations, phantom and volunteer measurements were performed. The obtained single‐slice and 3D‐metabolite maps show the brain in unprecedented detail (e.g., hemispheres, ventricles, gyri, and the contrast between grey and white matter). This facilitates the use of UHR‐MRSI for clinical applications, such as measurements of the small structures and metabolic pathologic deviations found in small Multiple Sclerosis lesions. HIGHLIGHTSUltra‐high resolution MRSI (128×128 in‐plane matrix) at 7 T.Parallel imaging and short TR of 200 ms make UHR‐MRSI clinically feasible (10–20 min).Pulse‐cascaded Hadamard encoding provides 3D‐MRSI coverage.

(2 + 1)D-CAIPIRINHA accelerated MR spectroscopic imaging of the brain at 7T.

To compare a new parallel imaging (PI) method for multislice proton magnetic resonance spectroscopic imaging (1H‐MRSI), termed (2u2009+u20091)D‐CAIPIRINHA, with two standard PI methods: 2D‐GRAPPA and 2D‐CAIPIRINHA at 7 Tesla (T).

One-dimensional image-selected in vivo spectroscopy localized phosphorus saturation transfer at 7T.

To evaluate the feasibility of a one‐dimensional image‐selected in vivo spectroscopy (1D‐ISIS) saturation transfer (ST) sequence at 7T for localized in vivo measurements of energy metabolism in different tissues in clinically reasonable examination times.

Flip-angle mapping of 31P coils by steady-state MR spectroscopic imaging

Phosphorus (31P) MR spectroscopic imaging (MRSI) is primarily applied with sensitive, surface radiofrequency (RF) coils that provide inhomogeneous excitation RF field (B1+) and rough localization due to their B1+ and sensitivity (B1−) profiles. A careful and time‐consuming pulse adjustment and an accurate knowledge of flip angle (FA) are mandatory for quantification corrections.

Simultaneous mapping of metabolites and individual macromolecular components via ultra-short acquisition delay 1H MRSI in the brain at 7T

Short‐echo‐time proton MR spectra at 7T feature nine to 10 distinct macromolecule (MM) resonances that overlap with the signals of metabolites. Typically, a metabolite‐nulled in vivo MM spectrum is included in the quantification`s prior knowledge to provide unbiased metabolite quantification. However, this MM model may fail if MMs are pathologically altered. In addition, information about the individual MM peaks is lost. In this study, we aimed to create an improved MM model by parameterization of the in vivo MM spectrum into individual components, and to use this new model to quantify free induction decay MR spectroscopic imaging (FID‐MRSI) data.

Spatial variability and reproducibility of GABA‐edited MEGA‐LASER 3D‐MRSI in the brain at 3 T

The reproducibility of gamma‐aminobutyric acid (GABA) quantification results, obtained with MRSI, was determined on a 3 T MR scanner in healthy adults. In this study, a spiral‐encoded, GABA‐edited, MEGA‐LASER MRSI sequence with real‐time motion–scanner‐instability corrections was applied for robust 3D mapping of neurotransmitters in the brain. In particular, the GABA+ (i.e. GABA plus macromolecule contamination) and Glx (i.e. glutamate plus glutamine contamination) signal was measured. This sequence enables 3D‐MRSI with about 3 cm3 nominal resolution in about 20 min. Since reliable quantification of GABA is challenging, the spatial distribution of the inter‐subject and intra‐subject variability of GABA+ and Glx levels was studied via test–retest assessment in 14 healthy volunteers (seven men–seven women).

Density-weighted concentric circle trajectories for high resolution brain magnetic resonance spectroscopic imaging at 7T: DW-CONCEPT for High Resolution Brain MRSI at 7T

Full‐slice magnetic resonance spectroscopic imaging at ≥7 T is especially vulnerable to lipid contaminations arising from regions close to the skull. This contamination can be mitigated by improving the point spread function via higher spatial resolution sampling and k‐space filtering, but this prolongs scan times and reduces the signal‐to‐noise ratio (SNR) efficiency. Currently applied parallel imaging methods accelerate magnetic resonance spectroscopic imaging scans at 7T, but increase lipid artifacts and lower SNR‐efficiency further. In this study, we propose an SNR‐efficient spatial‐spectral sampling scheme using concentric circle echo planar trajectories (CONCEPT), which was adapted to intrinsically acquire a Hamming‐weighted k‐space, thus termed density‐weighted‐CONCEPT. This minimizes voxel bleeding, while preserving an optimal SNR.