Test Details & Preparation
Proton magnetic resonance (MR) spectroscopy of the brain is a non-invasive, in vivo technique that allows investigation into regional chemical environments. Its complementary use with MR imaging sequences provides valuable insights into brain tumour characteristics, progression and response to treatment.
Additionally, its sensitivity to brain dysfunction in the presence of apparently normal structural imaging has increased interest in its use as a biomarker of neuro degenerative disorders such as Alzheimer's disease.
The technique is based on the same physical principles as magnetic resonance imaging (MRI) and the detection of energy exchange between external magnetic fields and specific nuclei within atoms. With MRI, this energy exchange, measured as a radiofrequency signal, is then translated into the familiar anatomic image by assigning different gray values according to the strength of the emitted signal. The principal difference between MRI and MRS is that in MRI, the emitted radiofrequency is based on the spatial position of nuclei, while MRS detects the chemical composition of the scanned tissue. The information produced by MRS is displayed graphically as a spectrum with peaks consistent with the various chemicals detected. MRS may be performed as an adjunct to MRI.
An MRI image is first generated, and then MRS spectra are developed at the site of interest, at the level of the voxel (3-dimensional volume X pixel). The voxel of interest (VOI) is typically a cube or rectangular prism with a dimensional pixel with a volume of 1 to 8 cm³. While an MRI provides an anatomic image of the brain, MRS provides a functional image related to underlying dynamic physiology. MRS can be performed with existing MRI equipment and modified with additional software and hardware.
There are 2 types of MRS data acquisition: single voxel or simultaneous multivoxel, also called chemical shift imaging. Reliable results are more difficult to obtain from some areas, eg, close to the brain surface or in children with smaller brains because of the lipid signal from the skull. Some techniques are used to deal with these issues various MRS techniques continue to be explored as well. A combination of MRS is often used with other MRI techniques, including diffusion-tensor imaging, susceptibility-weighted imaging, etc., and possibly other types of imaging such as positron emission tomography (PET).