Promises, limitations, and challenges of MR spectroscopy at ultra-high field
Malgorzata Marjanska1
1Center for Magnetic Resonance Research, Department of Radiology, University of Minnesota, United States

Synopsis

This presentation will explore the role and the effects of static magnetic field on magnetic resonance spectroscopy. It will focus on promises, limitations and challenges of MR spectroscopy at ultra-high field (≥ 7 T) and will consider physical properties, such as chemical shift, J-coupling, linewidth, T1 and T2 relaxation time constants, and technical requirements, such as inhomogeneity of the static magnetic field, inhomogeneous transmit and receive B1, and limited radiofrequency amplifier power.

Abstract

The first 7-T magnet for human imaging was introduced two decades ago (1) and research at this ultra-high magnetic field has demonstrated unique advantages for numerous applications in MRI of the human body including magnet resonance spectroscopy (MRS) (2-5). 7-T MRI scanners are more widely available, and there is growing interest in moving significantly beyond 7 T.

MRS provides a non-invasive way to quantify metabolites in vivo and is unique among imaging modalities because signals from several metabolites are measured noninvasively during a single examination period (6). Each observable metabolite can provide distinctive information about intracellular processes since metabolites are primarily located in the intracellular compartment of the brain. More than 20 metabolites have been identified and quantified in humans and animals and their chemical shifts and J-coupling constants have been reported (7,8).

In my talk, I will focus on promises, limitations and challenges of MRS at ultra-high field. I will mostly focus on brain MRS and 1H, but will provide examples of other nuclei, e.g., 31P and 2H (9). The promises and limitations will be discussed based on the physical properties such as chemical shift, J-coupling, linewidth, T1 and T2 relaxation time constants. The signal-to-noise ratio (SNR) in the time domain increases linearly with the magnetic field strength (10,11). The spectral resolution improves due to increased spectral dispersion and simplification of the J-coupled spectral patterns. The T1 relaxation time constants of metabolites in the human brain slightly increase with the field strength between 3 T and 7 T, and remain almost constant between 7 T and 9.4 T (12-14). However, the T2 relaxation time constants of metabolites in the human brain at 7 T are substantially shorter than at 1.5 T and 3 T (13,15), and even shorter at 9.4 T (12). The linewidths are broader due to shorter T2 relaxation and increased B0 susceptibility and increase linearly with B0 above 1.5 T (12). All of those physical properties influence what is possible at the ultra-high field, but increased sensitivity and spectral resolution at ultra-high field substantially improve the precision of metabolite quantification and expand the potential to detect and reliably quantify weakly represented metabolites (16). The challenges will be discussed taking into consideration technical requirements such as inhomogeneity of the static magnetic field, inhomogeneous transmit and receive B1, and limited radiofrequency amplifier power.

A comprehensive overview of the advantages, challenges and advances in ultra-high field MRS in human brain and spinal cord at field strength of 7 T and 9.4 T was published recently (17).

Acknowledgements

No acknowledgement found.

References

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Proc. Intl. Soc. Mag. Reson. Med. 30 (2022)