MRS imaging

Magnetic Resonance (MR) spectroscopy is a noninvasive diagnostic test for measuring biochemical changes in the brain, especially the presence of tumors. While magnetic resonance imaging (MRI) identifies the anatomical location of a tumor, MR spectroscopy compares the chemical composition of normal brain tissue with abnormal tumor tissue. This test can also be used to detect tissue changes in stroke and epilepsy. MR spectroscopy is conducted on the same machine as conventional MRI. The MRI scan uses a powerful magnet, radio waves, and a computer to create detailed images. Spectroscopy is a series of tests that are added to the MRI scan of your brain or spine to measure the chemical metabolism of a suspected tumor. MR spectroscopy analyzes molecules such as hydrogen ions or protons. Proton spectroscopy is more commonly used. There are several different metabolites, or products of metabolism, that can be measured to differentiate between tumor types:

• Amino acids
• Lipid
• Lactate
• Alanine
• N-acetyl aspartate
• Choline
• Creatine
• Myoinositol

The frequency of these metabolites is measured in units called parts per million (ppm) and plotted on a graph as peaks of varying height. By measuring each metabolite’s ppm and comparing it to normal brain tissue, the neuroradiologist can determine the type of tissue present. MR spectroscopy can be used to determine tumor type and aggressiveness, and distinguish between tumor recurrence and radiation necrosis. Different metabolites can indicate:

• Glioma: lower than normal N-acetyl aspartate levels, elevated choline and lipid levels, and lactate peaks
• Radiation necrosis: does not have elevated choline levels
• Meningioma: elevated alanine levels

The basic principle that enables MR spectroscopy (MRS) is that the distribution of electrons within an atom cause nuclei in different molecules to experience a slightly different magnetic field. This results in slightly different resonant frequencies, which in turn return a slightly different signal. The technique is identical to that of nuclear magnetic resonance (NMR) as used in analytical chemistry, but the community commonly refers to in vivo NMR as MRS to avoid confusion. 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) techniques. If water suppression is not successful then a general slope to the baseline can be demonstrated, changing the relative heights of peaks. (MRS) is performed with a variety of pulse sequences. The simplest sequence consists of a 90 degree radiofrequency (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. MRS can help increase our ability to predict grade. As the grade increases, NAA and creatine decrease and choline, lipids and lactate increase. In the setting of gliomas, choline will be elevated beyond the margins of contrast enhancement in keeping with cellular infiltration. While single-voxel MRS can be performed quickly and easily in most parts of the human brain, it provides no information on the spatial variations of metabolites and is generally limited to one or two brain regions in most clinical studies. Since MRS can measure chemical contents, it is a proper option for evaluating metabolism. This ability stems from the fact that the chemical properties and its surrounding environment determine its location in the MR spectrum. It can be concluded that the peak position on the spectrum corresponds to a specific metabolite and the constituent nuclei of each metabolite. The adaptability of MRS provides a technique that can probe a wide range of metabolic usages across different tissues. Although MRS is mostly applied for brain tissue, it can be used for detection, localization, staging, tumour aggressiveness evaluation, and tumour response assessment of breast, prostate, hepatic, and other cancers. MRS is used for brain studies, in order to provide precise information about the metabolites of different regions of brain tumours. This information can help in tumour grading, differentiation of neoplastic and nonneoplastic legions, prediction of tumour response to the treatment, demonstration of active and invasive parts of tumour and even radiation treatment planning, etc. MRS techniques have also been employed to detect and evaluate cerebral diseases via monitoring the metabolic changes.