MR spectrosopy (MRS) allows tissue to be interrogated for the presence and concentration of various metabolites. Grossman and Yousem said "If you need this to help you, go back to page 1; everything except Canavan has low NAA, high Choline" 1. This is perhaps a little harsh, however it is fair to say that MRS often does not add a great deal to an overall MR study but does increase specificity, and may help in improving our ability to predict histological grade.
The basic principle that enables MR spectroscopy (MRS) is the fact that the electron cloud around an atom shields the nucleus from the magnetic field to a greater or lesser degree. This naturally therefore results in slightly resonant frequencies, which in turn return a slightly different signal.
When we see spectra in general radiology practice, this is usually of protons, although phosphorus can also be targeted to examine ATP.
If raw signal was processed then the spectra would be dominated by water, which would make all other spectra invisible. Water suppression is therefore part of any MRS sequence, either via Inversion Recovery or Chemical shift selective (CHESS). If water suppression is not successful then a general slope to the base line can be demonstrated, changing the relative heights of peaks.
Magnetic resonance spectroscopy (MRS) is performed with a variety of pulse sequences. The simplest sequence consists of a 90 degree RF pulse without any gradients with reception of the signal by the RF coil immediately after the single RF pulse. Many sequences used for imaging can be used for spectroscopy also (such as the spin echo sequence). The important difference between an imaging sequence and a spectroscopy sequence is that for spectroscopy, a read out gradient is not used during the time the RF coil is receiving the signal from the person or object being examined. Instead of using the frequency information (provided by the read out or frequency gradient) to provide spatial or positional information, the frequency information is used to identify different chemical compounds. This is possible because the electron cloud surrounding different chemical compounds shields the resonant atoms of spectroscopic interest to varying degrees depending on the specific compound and the specific position in the compound. This electron shielding causes the observed resonance frequency of the atoms to slightly different and therefore identifiable with MRS.
Magnetic resonance spectroscopy (MRS) of intact biological tissues was first reported by two groups: Moon and Richards using P-31 MRS to examine intact red blood cells in 1973, and Hoult et al. using P-31 MRS to examine excised leg muscle from the rat in 1974.
[Editor: These should all gradually migrate to their own articles]
- resonates at 2.0 ppm chemical shift
NAA is an acetylated amino acid which is found in high concentrations in neurons and is a marker of neuronal viability. It is therefore reduced in any process that destroys neurons.
For more information refer to the article: N-acetylaspartate
- resonates at 3.0 ppm chemical shift
Found in metabolically active tissues (brain, muscle, heart) important in storage and transfer of energy. Tends to be maintained at a relatively constant level, and is predominantly used as a convenient internal standard, although it also reduces in gliomas.
For more information refer to the article: Creatine
- resonates at 3.2 ppm chemical shift
Precursor to acetylcholine and cell membrane components. In MRS it is a marker of cellular membrane turnover, and is therefore elevated in neoplasms, demyelination and gliosis. In the setting of gliomas, choline will be elevated beyond the margins contrast enhancement in keeping with cellular infiltration.
For more information refer to the article: Choline
- resonates at 1.3 ppm chemical shift, with a characteristic double peak at long TEs. It is however superimposed on the lipid band, and using an intermediate TE (e.g. 144ms) will invert only lactate allowing it to be distinguished.
Marker of anaerobic metabolism (no peak is seen in normal spectra). It is therefore elevated in necrotic areas (eg: higher grade tumours) and infections / inflammatory infiltrates.
For more information refer to the article: Lactate
- resonates at 1.3 ppm chemical shift
Marker severe tissue damage with liberation of membrane lipids, as is seen in infarction.
For more information refer to the article: Lipids
Glutamine / GABA
- resonates at 2.2-2.4 ppm chemical shift
Neurotransmitters, seen in excess in gliomas.
- resonates at 1.48 ppm chemical shift
Seen in meningiomas.
For more information refer to the article: Alanine
- resonates at 3.5 ppm chemical shift
- seen in Progressive multifocal leukoencephalopathy (PML).
- reduced in Hepatic encephalopathy.
- elevated in Alzheimer's disease.
- elevated in Down syndrome 4.
- elevated in regions of gliosis 5, for example in congenital CMV infection.
For more information refer to the article: Myo-inositol
Spectra in specific conditions
MRS can help increase our ability to predict grade. As the grade increases NAA and creatine decrease and choline, lipids and lactate increase.
May be difficult but in general non-glial tumours will not have little if any NAA. In the setting of gliomas, choline will be elevated beyond the margins contrast enhancement in keeping with cellular infiltration.
Distinguishing radiation change and tumour recurrence can be problematic. In recurrent tumour choline will be elevated, whereas in radiation change, NAA, choline and creatine will all be low.
Ischaemia and infarction
Lactate will increase as the brain switches to anaerobic metabolism. When infarction takes place then lipids are released and peaks appear.
As in all processes which destroy normal brain tissue, NAA is absent. Within bacterial abscess cavities, lactate, alanine, cytosolic acid and acetate are elevated / present.
Of note choline is low or absent in toxoplasmosis, whereas it is elevated in lymphoma, helping to distinguish the two.
White matter diseases
- progressive multifocal leukoencephalopathy (PML) may demonstrate elevated myo-inositol.
- Canavan disease characteristically demonstrates elevated NAA.
Markedly reduced myo-inositol, and to a lesser degree choline. Glutamine is increased.
- Leigh syndrome : elevated choline, reduced NAA and occasionally elevated lactate.
My ChoCrNaaLa (think of a new chocolate energy bar or something)
- My : Myoinisitol 3.3
- Cho : Choline 3.2
- Cr : Creatine 3.0
- Naa : Naa 2.0
- L : Lactate 1.3
In prostate MRS, a citrate peak is looked for at 2.6 ppm. For more information go to: MR spectroscopy in prostate cancer
- MRI (introduction)
- MR physics
- MR hardware
- Signal processing
MRI pulse sequences (abbreviations | parameters)
- spin echo sequences
- inversion recovery sequences
- gradient echo sequences
- fat suppressed imaging sequences
- pulse sequence parameters
- diffusion weighted sequences
- saturation recovery sequences
- echo-planar pulse sequences
- metal artifact reduction sequence
- spiral pulse sequences
- MR angiography (and venography)
- MR spectroscopy (MRS)
- MR perfusion
- functional MRI
- MR hardware and room shielding
- MR software
- patient and physiologic motion
- tissue heterogeneity and foreign bodies
- Fourier transform and Nyqvist sampling theorem
MR contrast agents
- paramagnetic contrast agents
- contrast agent safety
- gastrointestinal MRI contrast agents
- intravenous MRI contrast agents
- intravascular (blood pool) MRI contrast agents
- tumor-specific MRI contrast agents
- hepatobiliary MRI contrast agents
- reticuloendothelial MRI contrast agents
- MR safety
- 1. Grossman RI, Yousem DM. Neuroradiology, the requisites. Mosby Inc. (2003) ISBN:032300508X. Read it at Google Books - Find it at Amazon
- 2. Al-okaili RN, Krejza J, Wang S et-al. Advanced MR imaging techniques in the diagnosis of intraaxial brain tumors in adults. Radiographics. 2006;26 Suppl 1 : S173-89. doi:10.1148/rg.26si065513 - Pubmed citation
- 3. Qayyum A. MR spectroscopy of the liver: principles and clinical applications. Radiographics. 2009;29 (6): 1653-64. doi:10.1148/rg.296095520 - Pubmed citation
- 4. Huang W, Alexander GE, Daly EM et-al. High brain myo-inositol levels in the predementia phase of Alzheimer's disease in adults with Down's syndrome: a 1H MRS study. Am J Psychiatry. 1999;156 (12): 1879-86. Am J Psychiatry (link) - Pubmed citation
- 5. Bradley WG, Bydder GM. Advanced MR imaging techniques. Informa HealthCare. (1997) ISBN:1853170240. Read it at Google Books - Find it at Amazon
Synonyms & Alternative Spellings
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