Magnetic Resonance Spectroscopy By Saurabh Bhaskar Shaw Dwip Shah
What is Magnetic Resonance Spectroscopy? [1] Non invasive method to look at concentration of metabolites invivo. 2
Basics of MRS
Physics of MRS [1],[2] Nuclei have spin Some have magnetic moments. Net Magnetic Moment Net Magnetic Moment in B 0 4
Zeeman splitting [2] 2 Spin states slightly more in parallel than anti parallel state. 5
Physics of MRS [3] Precessional frequency given by Larmor Equation Structure of compound (σ CS ) causes changes in actual field experienced by the nuclei. 6
Physics of MRS [3],[2] The spectral composition of the signal is expressed in terms of frequency shift from frequency of reference compound (Standard for brain CH3 peak of NAA). Area under peak = Concentration 7
Methodology to collect data [4] Localize voxel Collect data from voxel Apply Time domain filters. Take Fourier Transform of the signal to get Frequency spectrum. Apply Frequency domain filters (LP, hamming) Analyze Processed spectrum. 8
Localization of Signal PRESS sequence Use the 3 Gradients with refocusing 180⁰ pulses to localize a voxel. Spectroscopic data from various voxels gives a spatial map of distribution of metabolites Magnetic Resonance Spectroscopic Imaging (MRSI) 9
Localization of Signal PRESS sequence [5] 10
Applications of MRS
Metabolites of Interest [2] 12
Applications Brain Tumor Epilepsy Multiple Sclerosis Aids Stroke TBI 13
Brain Tumor [6] Results show decrease in NAA, Cr, but increase in Cho Change in Cho levels can help determine level of malignancy Alternative to other invasive methods (Histological testing) to determine malignancy. 14
Lymphocytic Leukemia [6] 15
Epilepsy [6] Seizures are abnormal synchronized neuronal excitation, leading to neuronal loss. A decrease in NAA signal would correspond to neuronal loss in the voxel. Can be used to locate the focus of seizure 16
Stroke [6] Lactate increase can be seen without appearance on an MR image Ischemic Zones where potential blockage may occur Acute infarcts (blockage) have changes that can continue up to 1 week after Chronic Infarcts have decreases in NAA, Cr and Cho 17
Cerebral Abscess [6] 18
Multinuclear Spectroscopy
Multinuclear Spectroscopy [1] MRS applied to nuclei other than hydrogen 20
31 P Spectroscopy 31 P 100% isotopic abundance Stable Nuclear Spin of ½ Large spectral range 30 ppm Uses Metabolites involved with energy metabolism Intracellular ph 21
31P Brain Spectrum [1] 22
31P ph Curve [1] 23
31 P Research [1],[7] Mild Traumatic Brian Injuries(mTBI) very hard to distinguish changes using anatomical MRI. Need to have a quantitative measure of TBI. Experiments on mice show decrease in intracellular ph and PCr levels Loss of energy metabolism and secondary injury effects. 24
31 P Surface Coil 25
31 P Healthy Brain Spectra 26
31 P Expectations [7] Expect the spectra from the injured brain to have lower PCr and ATP from the corresponding animal studies. Could mean 2 things: 1. Mitochondrial energy metabolism pathways disrupted, or 2. Not enough glucose reaching the neurons. 27
13C Spectroscopy [1] Large spectral range Low natural abundance (1.108 %) Low gyromagnetic ratio (0.251) Hence, not very common Uses: Identifies important neurotransmitters (Glutamate and GABA). Other energy compounds (Glycogen, Glucose) 28
13 C Spectrum [1] 29
13 C Spectral Analysis Concentrations of Glucose required Curve fit and integrate (using LCModel). Need Basis sets for various compounds for curve fitting. E.g 1 H spectroscopy below 30
13 C Basis set simulation [9] These will be fed into LCModel to fit data and recognize the metabolites in sample spectra. 31 Glutamate Glucose - Beta
13 C Expectations [1],[7] A decrease in Glucose concentrations from normal would indicate issues with the Blood Brain Barrier. Changes in concentration of other energy compounds (Glycogen) may clarify the role of these in the progression of TBI. 32
Summary MRS uses same hardware as that of MRI Look at spectral composition of signal in MRS compared to reconstruction of image from spatial frequencies in MRI Useful in conjunction with other methods to quantify biochemical aspects of tissue. Useful in elucidation of poorly understood disease states and their progression. 33
References [1] Graaf. R.A, in vivo NMR Spectroscopy Principles and Techniques, 2 nd Edition, Wiley, 2007 [2] Bushberg.J.T, et al., "The Essential Physics of Medical Imaging," 2 nd Edition, Lippincott Williams & Wilkins, 2001 [3] Drost. D.J, et al.(2002);proton Magnetic Resonance Spectroscopy in the brain,med. Phys. 29 (9). [4] GE Medical Systems, MR - EXCITE HD Advanced and Multi-Nuclear Spectroscopy, Direction #5123076-100, Revision 1(10/04). [5] Sarty.G.E, MRI Physics for Life Science Researchers,2005 [6] Singh.A.K, et al.(2002); Magnetic Resonance Spectroscopy of the brain, Supplement to Applied Radiology, pg 58-65. [7] Vink.R, et al. (1987);Effects of TBI on Cerebral High Energy Phosphates and ph - 31P MRS study, Journal of Cerebral Blood Flow and Metabolism 7:563-571 [8] Alger. J.R, Basics of Magnetic Resonance Spectroscopy of the Practicing Clinician. [9] Brian.P, Vespa, Available online : http://scion.duhs.duke.edu/vespa/ 34