Magnetic Resonance Spectroscopy (MRS)

Over the last decade, magnetic resonance spectroscopy (MRS) has become a rapidly developing tool in diagnosing, visualizing, monitoring, and evaluating a wide range of neurological diseases (Mandal, 2007; Gujar, Maheshwari, Björkman-Burtscher & Sundgren, 2005). This non-invasive in vivo diagnostic measure is acquired with a magnetic resonance imaging (MRI) scanner (Ross and Bluml, 2001). Where MRI is acquired for structural/ anatomical information, MRS provides information on the chemical environment of the brain (Currie et. al, 2012; Soares and Law, 2009). This in vivo technique can detect a variety of different atomic nuclei such as 1H (proton), 31P (phosphorus) and 23Na (sodium). Despite these possibilities 1H spectroscopy is preferred due to greater sensitivity, a higher single-to-noise ratio and abundance of hydrogen nuclei in the chemicals of the body. These chemicals include N-Acetyl Aspartate (NAA), total creatine (tCr, the sum of creatine and phosphocreatine), choline (Cho), glutamine (Gln), glutamate (Glu), gamma-aminobutyric acid (GABA), and glutamine + glutamate (Glx) (De Graaf & Rothman, 2001).

MRS

Magnetic Resonance Spectroscopy

NAA is the largest peak in the MRS spectra, and is known to be localised to the neurons, and as such is a sensitive marker of neuronal viability. The amino acid glutamate is a major excitatory neurotransmitter in the central nervous system (CNS) (Hertz, 2006). This abundant chemical messenger stimulates the entire brain and is involved in learning, memory acquisition, and a contributor to a number of neurodegenerative disorders, including Alzheimer’s disease (Carlson, 2007; Hu, Ondrejcak & Rowan, in Press). Glutamate is an essential excitatory stimulus and the NMDA receptor system plays a central role in the process of learning and memory (Riedel, Platt & Micheau, 2003).

The second major neurotransmitter in the CNS is gamma-aminoobutyric acid (GABA) (Carlson, 2007). This primary inhibitory neurotransmitter is an important metabolite for regulating brain functions and even though it is only present at millimolar levels it has captured the attention of the clinical and neuroscience community recently, which is eager for knowledge about the role of inhibitory processes in brain functioning (Puts & Edden, 2011).

Measurement of these chemicals has shown that alterations are present in dementia, with early reductions in NAA and Glutamate being the major predictors of poor outcome. Given the role of glutamate in neuronal signalling, and NAA’s location in neurons, reductions in these two chemicals are sensitive indicators of the neural changes and dysfunction that accompany dementia. Further study of the timing of these changes will provide novel and useful information in the quest to know more about the processes underlying dementias of all types.

References

Carlson, N.R. (2007). Physiology of behaviour. Boston, Massachusetts: Pearson

Currie, S., Hadjivassiliou, M., Craven, I. J., Wilkinson, I. D., Griffiths, P. D., & Hoggard, N. (2013). Magnetic resonance spectroscopy of the brain. Postgraduate medical journal, 89(1048), 94-106. doi:10.1136/postgradmedj-2011-130471

de Graaf, R. A. & Rothman, D. L. (2001), In vivo detection and quantification of scalar coupled 1H NMR resonances. Concepts Magn. Reson., 13, 32–76. doi: 10.1002/1099-0534(2001)13:1<32::AID-CMR4>3.0.CO;2-J

Gujar, S.K., Maheshwari, S., Björkman-Burtscher, I., & Sundgren, P. (2005) Magnetic resonance spectroscopy. J Neuroophthalmol, 25, 217-26.

Hu, N., Ondrejcak, T., & Rowan, M. (2012). Glutamate receptors in preclinical research on Alzheimer’s disease: Update on recent advances. Pharmacology Biochemistry and Behavior, 100(4), 855-862.

Mandal, P. K. (2007), Magnetic resonance spectroscopy (MRS) and its application in Alzheimer’s disease. Concepts Magn. Reson., 30, 40–64. doi: 10.1002/cmr.a.20072

Puts, N., & Edden, R., (2012). In vivo magnetic resonance spectroscopy of gaba: Amethodological review. Progress in Nuclear Magnetic Resonance Spectroscopy, 60, 29-41, doi: 10.1016/j.pnmrs.2011.06.001

Riedel, G., Platt, B., & Micheau,J. (2003). Glutamate receptor function in learning and memory. Behavioural Brain Research, 140(1–2), 1-47.

Ross, B. and Bluml, S. (2001), Magnetic resonance spectroscopy of the human brain. Anat. Rec., 265, 54–84. doi: 10.1002/ar.1058

Soares DP, & Law, M. (2009). Magnetic resonance spectroscopy of the brain: review of metabolites and clinical applications. Clin Radiol, 64, 12-21.