Magnetic resonance spectroscopy (MRS) is a non-invasive technique that is able to measure the concentrations of different chemical components in tissue; it can be used to measure tissue metabolism.

An MR spectrum shows peaks at different frequencies, because each metabolite has a slightly different resonance frequency. This difference in resonance frequency is the result of the electron shielding effect. Electrons rotate around the nucleus and produce their own small local magnetic field (B_ind) in the presence of the main magnetic field (B₀). The effective magnetic field (B_effective) experienced by the nucleus is B₀ minus B_ind. The electron shielding effect depends on the molecular structure of a metabolite and therefore each metabolite experiences a slightly different B_effective. The resonance frequency of metabolite is then determined by the Larmor equation ($$$\omega_{0}=\gamma B_{effective}$$$).

The resonance frequency and shielding effect are field dependent, which makes it difficult to compare spectra between different field strength. This issue is overcome by using the chemical shift instead of the resonance frequency. Chemical shift is the difference in resonance frequency of a metabolite relative to a reference compound and expressed in parts per million. For ¹H MRS, the chosen reference compound tetramethylsilane (TMS) and consequently water will always resonate at 4.7 ppm, independent of field strength.

This lecture will also discuss the following factors that affect the size and shape of peak in the MR spectrum:
1) Concentration of a nucleus in the measured sample determines the height of the peak
2) T1 and T2 relaxation times of the metabolite in combination with the chosen repetition time (TR) and echo time (TE). The TR and TE affect the height of the peak via the T1 and T2 relaxation times of the metabolite. Furthermore, the T2 relaxation time also affects the linewidth of a peak, longer T2 relaxation times give narrower peaks.
3) Magnetic inhomogeneity will reduce the T2* and lead to a broadening of the peaks in a spectrum. To minimize this effect shimming is critical.
4) Presence of hidden/underlying peaks can alter the shape of a peak.
5) Processed like J-coupling can make the peak appear as multiplet instead of a singlet.

Further focus of this lecture will be on an overview of the detectable metabolites per nucleus (¹H, ³¹P, ¹³C and ²³NA).This will include the recently published consensus papers on how to reliably measure metabolites detectable with ¹H MRS and ³¹P MRS. Metabolites to be discussed will for example be intramyocellular lipids, carnosine, acetylcarnitine, phosphocreatine, inorganic phosphate and ATP.

Acknowledgements

No acknowledgement found.