Magnetic resonance spectroscopy

Magnetic resonance spectroscopy (MRS) complements magnetic resonance imaging (MRI) as a noninvasive means for the characterization of tissue. While MRI uses the signal from hydrogen protons to form anatomic images, proton MRS uses this information to determine the concentration of brain metabolites such as N-acetyl aspartate (NAA), choline (Cho), creatine (Cr) and lactate in the tissue examined. The most widely used clinical application of MRS has been in the evaluation of central nervous system disorders. MRS has its limitations and is not always specific but, with good technique and in combination with clinical information and conventional MRI, can be very helpful in diagnosing certain entities. For example, a specific pattern of metabolites can be seen in disorders such as Canavan’s disease, creatine deficiency, and untreated bacterial brain abscess. MRS may also be helpful in the differentiation of high grade from low grade brain tumors, and perhaps in separating recurrent brain neoplasm from radiation injury. (J Neuro-Ophthalmol 2005;25: 217–226) Magnetic resonance spectroscopy (MRS), like magnetic resonance imaging (MRI), is based on the principle of nuclear magnetic resonance (NMR). The technique of spectroscopy has been widely applied in chemistry for the analysis of compounds in solution. Although MRS can theoretically be performed in almost any tissue of the human body, the brain has been the major organ of interest for clinical MRS studies. This is due to the rather homogeneous tissue structure of the brain, its easy accessibility to MRS, and limited motion artifacts. MRS provides a non-invasive diagnostic tool for the biochemical characterization of pathophysiological processes in the brain. MRS of the brain parenchyma (neurospectroscopy) is of interest to physicians treating primary brain diseases and may also contribute to the diagnostic work-up of systemic diseases involving the central nervous system. MRS TECHNIQUE AND OTHER CONSIDERATIONS Clinical MRS became feasible with the development of a rapid, inexpensive, and automated technique that could be easily integrated with the MRI exam. As a clinical tool, MRS received approval of the United States Food and Drug Administration in 1995 (1). Though MRS can be performed using a variety of nuclei such as carbon (C), nitrogen (N), fluorine (F), and sodium (Na), only the nuclei phosphorus (P) and hydrogen (H) exist in vivo in concentrations high enough for routine clinical evaluation. Proton (H) MRS studies have become popular due to the high natural abundance of protons and their high absolute sensitivity to magnetic manipulation, better spatial resolution, and relative simplicity of technique, as well as a shift of interest to areas of metabolism that lack phosphorylated metabolites (2–5). MRS is performed in a fashion similar to MRI with the addition of a few steps before the data acquisition, a change not discernible to the patient undergoing the examination (1). The first step used to obtain diagnostic data is ensuring the homogeneity of the magnetic field (1). This is accomplished by ‘‘shimming,’’ an automated process that on occasion may have to be performed manually (2,6). The concentration of water exceeds the concentration of metabolites of interest by a factor of 10,000 or more. This means that the dominant resonance (the resonance peak) in a hydrogen spectrum will represent protons from the water molecule, dwarfing the millimolar concentrations of other metabolites (1,2,4,6). Hence, suppression of the water signal becomes a necessary step in MRS, achieved by the addition of water suppression pulses (1,6). Commonly used spectroscopic techniques include the single voxel spectroscopy (SVS), with a spatial resolution in the order of one to eight cm (2), and the multi voxel technique, also called ‘‘chemical shift imaging’’ (CSI) or magnetic resonance spectroscopic imaging (MRSI), Department of Radiology (SG, PS), University of Michigan Health System, Ann Arbor, MI 48109, USA, Department of Imaging (SM), Jaslok Hospital and Medical Research Center, 15, Dr. G. Deshmukh road, Mumbai 400026, India. Center for Medical Imaging and Physiology (IB-B), Department of Diagnostic Radiology, Lund University, 22185