Two-dimensional semi-LASER correlation spectroscopy with well-maintained cross peaks

Accelerated J-resolved 1 H-MRSI with limited and sparse sampling of ( k , t 1 , t 2 -space

PURPOSE

To accelerate the acquisition of J-resolved proton magnetic resonance spectroscopic imaging (¹ H-MRSI) data for high-resolution mapping of brain metabolites and neurotransmitters.

METHODS

The proposed method used a subspace model to represent multidimensional spatiospectral functions, which significantly reduced the number of parameters to be determined from J-resolved ¹ H-MRSI data. A semi-LASER-based (Localization by Adiabatic SElective Refocusing) echo-planar spectroscopic imaging (EPSI) sequence was used for data acquisition. The proposed data acquisition scheme sampled k , t 1 , t 2 -space in variable density, where t₁ and t₂ specify the J-coupling and chemical-shift encoding times, respectively. Selection of the J-coupling encoding times (or, echo time values) was based on a Cramer-Rao lower bound analysis, which were optimized for gamma-aminobutyric acid (GABA) detection. In image reconstruction, parameters of the subspace-based spatiospectral model were determined by solving a constrained optimization problem.

RESULTS

Feasibility of the proposed method was evaluated using both simulated and experimental data from a spectroscopic phantom. The phantom experimental results showed that the proposed method, with a factor of 12 acceleration in data acquisition, could determine the distribution of J-coupled molecules with expected accuracy. In vivo study with healthy human subjects also showed that 3D maps of brain metabolites and neurotransmitters can be obtained with a nominal spatial resolution of 3.0 × 3.0 × 4.8 mm³ from J-resolved ¹ H-MRSI data acquired in 19.4 min.

CONCLUSIONS

This work demonstrated the feasibility of highly accelerated J-resolved ¹ H-MRSI using limited and sparse sampling of k , t 1 , t 2 -space and subspace modeling. With further development, the proposed method may enable high-resolution mapping of brain metabolites and neurotransmitters in clinical applications.

Localized one-dimensional single voxel magnetic resonance spectroscopy without J coupling modulations

PURPOSE

To acquire single voxel localized one-dimensional ¹ H magnetic resonance spectroscopy (MRS) without J coupling modulations, free from amplitude and phase distortions.

METHODS

A pulse sequence, named PRESSIR, is developed for volume localized MRS without J modulations at arbitrary echo time (TE). The J coupling evolution is suppressed by the J-refocused module that uses a 90° pulse at the midpoint of a double spin echo.

RESULTS

The localization performance of the PRESSIR sequence was tested with a two-compartment phantom. The proposed sequence shows similar voxel localization accuracy as PRESS. Both PRESSIR and PRESS sequences were performed on MRS brain phantom and pig brain tissue. PRESS spectra suffer from amplitude and phase distortions due to J modulations, especially under moderate and long TEs, while PRESSIR spectra are almost free from distortions.

CONCLUSION

The PRESSIR sequence proposed herein enables the acquisition of single voxel in-phase MRS within a single scan. It allows an enhanced signal intensity of J coupling metabolites and reducing undesired broad resonances with short T2s while suppressing J modulations. Moreover, it provides an approach for direct measurement of nonoverlapping J coupling peaks and of transverse relaxation times T2s. Magn Reson Med 76:1661-1667, 2016. © 2015 International Society for Magnetic Resonance in Medicine.

Localized 2D COSY sequences: Method and experimental evaluation for a whole metabolite quantification approach

Two-dimensional spectroscopy offers the possibility to unambiguously distinguish metabolites by spreading out the multiplet structure of J-coupled spin systems into a second dimension. Quantification methods that perform parametric fitting of the 2D MRS signal have recently been proposed for resolved PRESS (JPRESS) but not explicitly for Localized Correlation Spectroscopy (LCOSY). Here, through a whole metabolite quantification approach, correlation spectroscopy quantification performances are studied. The ability to quantify metabolite relaxation constant times is studied for three localized 2D MRS sequences (LCOSY, LCTCOSY and the JPRESS) in vitro on preclinical MR systems. The issues encountered during implementation and quantification strategies are discussed with the help of the Fisher matrix formalism. The described parameterized models enable the computation of the lower bound for error variance--generally known as the Cramér Rao bounds (CRBs), a standard of precision--on the parameters estimated from these 2D MRS signal fittings. LCOSY has a theoretical net signal loss of two per unit of acquisition time compared to JPRESS. A rapid analysis could point that the relative CRBs of LCOSY compared to JPRESS (expressed as a percentage of the concentration values) should be doubled but we show that this is not necessarily true. Finally, the LCOSY quantification procedure has been applied on data acquired in vivo on a mouse brain.