MR Spectroscopy PDF

# Spectroscopy ## Magnetic Resonance Spectroscopy - Magnetic Resonance (MR) spectroscopy is a noninvasive diagnostic test for measuring biochemical changes in the brain, especially the presence of tumors. - Magnetic resonance imaging (MRI) identifies the anatomical location of a tumor. - MR spect...

# Spectroscopy ## Magnetic Resonance Spectroscopy - Magnetic Resonance (MR) spectroscopy is a noninvasive diagnostic test for measuring biochemical changes in the brain, especially the presence of tumors. - 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. ## How Does MR Spectroscopy Work? - MR spectroscopy is conducted on the same machine as conventional MRI. - The MRI scan uses a powerful magnetic field, radio waves, and a computer to create detailed images. - Spectroscopy adds 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. ## What Are the Metabolites? - Metabolites are products of metabolism that can be measured to differentiate between tumor types. - Amino acids - Lipids - Lactate - Alanine - N-acetyl aspartate - Choline - Creatine - Myoinositol ## How Are the Metabolites Measured? - 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. ## What Can MR Spectroscopy Reveal About a Tumor? - 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. ## What are the Risks of MR Spectroscopy? - MRI and MR spectroscopy are very safe. - There are no known health risks associated with the magnetic field or the radio waves used by the machine. - Some people are sensitive to the contrast agent and may develop an allergic reaction. - All contrast agents are FDA-approved and safe. ## What are Important Precautions for MR Spectroscopy? - It's important to tell your doctor if any of the following apply to you: - Cardiac pacemaker or artificial heart valve - Metal plate, pin, or other metallic implant - Intrauterine device, such as Copper-7 IUD - Insulin or other drug pump - Aneurysm clips - Previous gunshot wound - Cochlear implant or other hearing device - Employment history as a metalworker (had metal in eye) - Permanent (tattoo) eye-liner - Any metallic substance on your body can affect the quality of the images and values obtained. - It can also cause discomfort or injury to you when placed in the magnetic field, and may exclude you from the exam. ## Localization Techniques in MRS - In initial days, localization of volume of interest was done by the surface coil. - The area (volume) covered by the coil was the volume of interest from which metabolite information was obtained. - Currently, four methods are commonly used for the localization of volume of interest: STEAM, PRESS, ISIS & CSI (MRSI). - STEAM, PRESS & ISIS are used for single voxel spectroscopy (SVS). - CSI is a multivoxel (MVS) technique. ## Important Terms Used in MR Spectroscopy - **Orthogonal planes**: The 3 basic orthogonal slice orientations are: transversal (T), sagittal (S) and coronal (C). ... The orientation of single oblique slices can be specified by rotating a slice in one of the basic orientations toward one of the other two basic orthogonal planes about an axis defined by the intersection of the 2 planes. - **Voxel**: A voxel is a volume element (volumetric and pixel) representing a value in the three dimensional space, corresponding to a pixel for a given slice thickness. Voxels are frequently used in the visualization and analysis of medical data. ## Types of MR Spectroscopy - **31P**: Phosphorus-31 - **Single-voxel spectroscopy (SVS)**: This technique is the simplest to acquire and interpret, and hence is the most widely used. They provide high signal-to-noise in a relatively short scan time. Because the imaged region is compact, excellent shimming can be obtained with resultant high-quality spectra suitable for quantitative analysis. - **Multi-voxel Chemical Shift Imaging (CSI):** This technique offers two potential advantages over SVS: 1) a larger total coverage area (since the size of the entire multivoxel slab is greater), and 2) higher spatial resolution (since the individual voxels are smaller). A wide coverage area is important for large, heterogeneous lesions like the brain tumor shown above, where the SVS technique provides data from only a small portion of the mass. ## Specific Techniques: STEAM, PRESS, ISIS, & CSI - **STEAM**: stimulated echo acquisition method. The volume of interest is excited by three 90 degrees pluses in three orthogonal planes. Since the echo is stimulated, the signal is weak. STEAM is used for short TE (20-30 ms) spectroscopy. - **PRESS**: Point resolved spectroscopy. In PRESS, one 90 degree and two 180 degree pulses are applied along three orthogonal planes. The signal is strong with better SNR, hence PRESS is used for longer TE (135,270 ms) spectroscopy. - **ISIS**: Image selected in vivo spectroscopy. In ISIS, the frequency selective inversion pluses are applied in presences of the orthogonal gradients. The fourth non-selective pulse is used for the observation of signal. ISIS is used in 31P spectroscopy. - **CSI**: chemical shift imaging. CSI is used for multivoxel spectroscopy, where a large area is covered and divided into multiple voxels. CSI is also called as Magnetic resonance spectroscopy imaging (MRSI) as it combines features of both imaging and spectroscopy. Spatial localization is done by phase encoding in one, two or three directions to get one, two or three dimensional spectroscopy respectively. Metabolite maps or metabolic ratio can be seen overlaid over the image.