Simultaneous Concentration and T₂ Mapping of Brain Metabolites by Fast Multi-Echo Spectroscopic Imaging.

Area of Science:

• Neuroimaging
• Magnetic Resonance Imaging
• Spectroscopy

Background:

• Magnetic Resonance Spectroscopic Imaging (MRSI) is crucial for non-invasively quantifying brain metabolites.
• Traditional MRSI methods often require long scan times, limiting clinical applicability.
• Accurate T2 relaxation time mapping provides insights into tissue microstructure and metabolite properties.

Purpose of the Study:

• To develop and validate a novel multi-echo single-shot MRSI (MESS-MRSI) technique for producing metabolite-specific T2 and concentration maps.
• To achieve these maps within a clinically compatible timeframe.
• To assess the performance of MESS-MRSI compared to traditional methods.

Main Methods:

• A multi-echo single-shot MRSI (MESS-MRSI) experiment was designed using truncated and partially sampled multi-echo trains.
• Simultaneous multiparametric model fitting was employed.
• The method was tested in vivo on five healthy subjects, with performance evaluated using Cramér-Rao lower bounds (CRLB).
• Comparisons were made against traditional multi-echo multi-shot (MEMS) MRSI and a mocked version of MESS acquisition.

Main Results:

• MESS-MRSI successfully generated simultaneous concentration and T2 maps with a nominal voxel size of ~2 cm³ in a 7-minute scan.
• Estimated metabolite concentrations and T2 values showed good agreement with a threefold longer MEMS acquisition.
• MESS-MRSI demonstrated improved CRLB performance over traditional MEMS, with 17-45% improvement for concentrations and 10-23% for T2 values.
• Tissue-type and regional distributions of 16 metabolite concentrations aligned with existing literature.
• Preliminary findings suggest correlations between T2 relaxation times and gray/white matter fractions, indicating tissue-type-dependent microstructural changes.

Conclusions:

• The novel MESS-MRSI acquisition strategy, combined with multiparametric fitting, enables simultaneous 2D concentration and T2 mapping in clinically feasible scan times.
• This approach reliably estimates metabolite concentrations and T2 times by leveraging prior knowledge from a fully sampled echo.
• MESS-MRSI offers a significant reduction in scan time compared to traditional methods, making advanced MRSI techniques more accessible for clinical use.
• The technique holds potential for further improvements in speed, coverage, or resolution by incorporating advanced MRSI principles.