6 ) MR Spectroscopy
Basic Principles:
MR spectroscopy is a separate acquisition that generates spectra of various metabolites (an in vivo version of proton NMR that you may have studied prior to residency) The atoms neighboring the hydrogens (protons) in a metabolite determine the amount of electron shielding or deshielding of the protons. The electron cloud shields the proton from the applied field (H) (i.e. creates a magnetic field opposed to the applied field). e.g. the oxygen in water is electronegative. This pulls electrons toward the oxygen, with resulting deshielding of the proton. The proton therefore experiences a realtively stronger local field (B).The Larmour equation determines the frequency/location of the peak/peaks of the metabolite. Radiologists tend to use B when we are really talking about H. For imaging it probably doesn't matter. For spectroscopy it makes a fundamental difference.
This difference in frequency is small and is typically described as a part per million (ppm) difference with respect to the standard tetramethylsilane (TMS).
Indications:
Tumor:
MR spectroscopy can be useful for differentiating tumor (elevated choline 3.2 ppm, decreased NAA 2.0 ppm) from non tumor. CMS presumably is not impressed with the indications, since they will not reimburse for this study.
Hunter’s angle: the angle formed by normal peaks of choline (3.2), creatine (3.0) and NAA (2.0) on a normal STEAM spectra (short TE)
0 ppm TMS (see below)
0.3 silicone
1.3 ppm lactate (aneorobic glycolysis)
1.3 ppm "fat"
2.0 ppm NAA N-acetyl-asparate (molecule present in healthy neurons)
3.0 ppm Creatine/phosphocreatine (energy metabolism molecules)
3.2 ppm Choline (marker in the synthesis and breakdown of cell membranes)
4.7 ppm water
Q: What does ppm mean?
A: It is the difference in frequency of the chemical in question as compared to a standard (tetramethyl silane (TMS) shown below)
http://en.wikipedia.org/wiki/Tetramethylsilane
Q: Why is ppm used instead of frequency?
A: ppm is used to standardize results of MR spectroscopy no matter what field strength is used. Frequency would vary depending on the strength of the magnet used.
References:
http://spinwarp.ucsd.edu/neuroweb/Text/mrs-TXT.htm
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1064986/
MRS Learning Modules
http://www.imaios.com/en/e-Courses/e-MRI/Magnetic-Resonance-Spectroscopy-MRS
I have to thank my organic chemistry professor, George Whitesides, who later moved his lab to a small liberal arts school up the river from MIT. Apologies for an oversimplification, although from his TED talk he is an advocate of simplification.
http://en.wikipedia.org/wiki/George_M._Whitesides
http://www.ted.com/talks/george_whitesides_toward_a_science_of_simplicity