Clinical Relevance
Natural abundance Carbon-13 MRS is the gold standard for non-invasive measurement of hepatic glycogen
concentrations in humans. Unlike biopsy, it assesses a larger volume of tissue and can be repeated
frequently to monitor metabolic changes (e.g., post-prandial glycogen storage, effects of insulin, or
exercise).
Technical Challenges at 3T
Sensitivity Issues
13C has a natural abundance of only 1.1% and a gyromagnetic ratio one-quarter that of
protons. This results in very low signal-to-noise ratio (SNR), necessitating:
- Surface Coils: Essential for maximizing sensitivity near the liver surface.
- Proton Decoupling: Required to collapse multiplet splitting and enhance signal
via the Nuclear Overhauser Effect (NOE).
- Long Scan Times: Often 15-30 minutes of signal averaging is required for
sufficient SNR.
SAR Limitations
Broadband proton decoupling deposits significant RF energy into the patient. At 3T, keeping within
Specific Absorption Rate (SAR) limits is the primary constraint. Power-optimized decoupling schemes
(e.g., WALTZ-4, WALTZ-16, or adiabatic pulses) and duty cycling are critical.
Spectral Features
[Spectra Graphic: Glycogen C1 Peak at ~100.5 ppm]
The primary target is the Glycogen C1 resonance at approximately 100.5
ppm.
- C1 (100.5 ppm): The most distinct peak, well-separated from lipid resonances.
- Lipids (Fatty Acids): Huge peaks in the 10-40 ppm range (methylene/methyl carbons)
and ~130 ppm (unsaturated carbons). These can dominate the spectrum but are far from the glycogen
region.
- C2-C6 Glycogen (~60-80 ppm): Often obscured by glycerol backbone signals of lipids.
Acquisition Protocol (Typical)
- Hardware: 13C/1H surface coil placed over the right liver
lobe.
- Localization: Often simple pulse-acquire (non-localized within the sensitivity
profile of the coil) or ISIS (Image Selected In Vivo Spectroscopy) if organ avoidance is critical.
- Sequence: Block pulse or Adiabatic Half Passage (to compensate for B1 inhomogeneity
of surface coil) followed by acquisition with 1H decoupling.
- TR: Short TR (e.g., < 200ms) optimizes SNR per unit time for Glycogen due to its
short T1.