HERMES: Hadamard encoding and reconstruction of MEGA-edited spectroscopy

Area of Science:

• Magnetic Resonance Spectroscopy
• Neuroimaging
• Metabolomics

Background:

• Simultaneous quantification of N-acetyl aspartate (NAA) and N-acetyl aspartyl glutamate (NAAG) is challenging due to overlapping signals.
• Accurate measurement of NAA and NAAG is crucial for understanding brain metabolism and neurological conditions.

Purpose of the Study:

• To introduce and evaluate a novel Hadamard-encoded spectral editing scheme, HERMES (Hadamard Encoding and Reconstruction of MEGA-Edited Spectroscopy).
• To assess the performance of HERMES for simultaneous quantification of NAA and NAAG at 3 Tesla magnetic resonance imaging (MRI).

Main Methods:

• Utilized density-matrix simulations for optimizing the HERMES pulse sequence.
• Validated the method using phantom experiments at 3T.
• Acquired in vivo data from the centrum semiovale of 12 healthy subjects.
• Modeled in vivo data to determine the NAA:NAAG concentration ratio and assessed potential coediting.

Main Results:

• HERMES demonstrated excellent signal segregation for NAA and NAAG with minimal crosstalk in simulations and phantom studies.
• In vivo data showed good agreement with simulated multiplet patterns.
• The in vivo NAA:NAAG concentration ratio was determined to be 4.22:1, consistent with literature values.
• Simulations indicated minor coediting from aspartate and glutathione.

Conclusions:

• The HERMES technique enables simultaneous and separable detection of NAA and NAAG at 3T.
• This method offers a viable solution for quantifying otherwise overlapping metabolites in magnetic resonance spectroscopy.