MRS Glossary & Jargon

Frequency & Shift

Up-field: Refers to the right side of the NMR spectrum (lower ppm values). Nuclei in this region are more shielded by electrons, effectively feeling a lower magnetic field. Example: Methyl protons (-CH₃).
Down-field: Refers to the left side of the NMR spectrum (higher ppm values). Nuclei in this region are de-shielded (deshielded), often due to proximity to electronegative atoms. Example: Aldehyde protons.
ppm (Parts Per Million): A frequency-independent unit for chemical shift. It allows spectra acquired at different field strengths (e.g., 1.5T vs 3.0T) to be directly compared. Calculated as (observed frequency - reference frequency) / spectrometer frequency.

X-nuclei: Any NMR-visible nucleus other than the proton (¹H). Common X-nuclei in clinical MRS include ³¹P, ¹³C, ²³Na, and ¹²⁹Xe. They typically have lower sensitivity (gyromagnetic ratio) and natural abundance than protons.
NOE (Nuclear Overhauser Effect): The transfer of nuclear spin polarization from one population of spin-active nuclei (e.g., ¹H) to another (e.g., ¹³C or ³¹P) via cross-relaxation. This enhancement mechanism is crucial for boosting the signal of low-sensitivity X-nuclei.
Decoupling: The application of continuous or pulsed RF irradiation (usually to protons) during the acquisition of an X-nucleus signal to remove the splitting caused by J-coupling (interaction between spins). This simplifies the spectrum (collapsing multiplets into singlets) and increases signal-to-noise ratio.
Inhomogeneity: Variations in the magnetic field strength (B0) or Radiofrequency field (B1) across the sample.
B0 Inhomogeneity: Causes spectral line broadening, requiring "shimming."
B1 Inhomogeneity: Causes variations in flip angle across the volume, especially with surface coils.
Shimming: The process of correcting B0 inhomogeneities using small adjustment coils (shim coils) to make the magnetic field as uniform as possible. Critical for spectral resolution.