- Email: [email protected]
Magnetic Resonance Spectroscopy
Magnetic Resonance Spectroscopy J Neil, Washington University School of Medicine, St. Louis, MO, USA JJH Ackerman, Washington University, St. Louis, MO, USA r 2014 Elsevier Inc. All rights reserved. This article is a revision of the previous edition article by Jeff Neil, Jonathan Sehy, volume 3, pp 13–16, r 2003, Elsevier Inc.
Basics Analysis of the specific frequencies at which radiofrequency (RF) energy is absorbed (or emitted) by a nuclear spin system, the ‘resonance frequencies,’ can provide considerable information about the system. The parent resonance frequency (o0) of a given nuclide is determined by two factors: the magnetogyric ratio (g), a property of the atomic nucleus, which is unique for a particular nuclide, and the magnetic field (B0) in which it resides at the time of the magnetic resonance (MR) experiment. This relationship can be written as o0 ¼ g B0
½1
In a magnetic field of 11.7 T, for example, the parent resonance frequency of 1H is 500 MHz (where 1 MHz is 1 106 Hz or 1 106 cycles/s). Thus, an MR study of 1H in an 11.7-T magnet would be done using RF excitation and detection at a parent frequency of 500 MHz (approximately five times that of a typical FM radio station). However, to study deuterium (2H) in the same magnetic field, one would use a parent frequency of 76.8 MHz because 2H has its own characteristic value for g. For reference, a magnetic field of 11.7 T is nearly 250 000 times that of the Earth’s magnetic field. (1 T is 104 G. The Earth’s field is about half a gauss.)
Chemical Shift If eqn [1] were the whole story, MR spectroscopy would not be very interesting. However, soon after the initial demonstration of the MR phenomenon, it was discovered that the signal obtained from an MR sample contains finely resolved (‘high-resolution’) small variations (‘shifts’) in the resonance frequencies for particular atoms in a given molecule. For example, adenosine triphosphate has three shifted 31P resonance frequencies, one for each of the three chemically distinct sites of phosphorus atoms in the molecule. This frequency shift phenomenon is due to the fact that the magnetic field experienced by a nucleus is reduced (‘shielded’) by the electrons that orbit the nucleus and make up the chemical bonds of the molecule. This reduction in magnetic field is very small, measured in parts per million (ppm), and some atoms are relatively more shielded from the applied magnetic field than others. Thus, for the ith atomic site in a molecule, eqn [1] must be modified slightly to account for this shielding phenomenon. This is done by introducing the unitless shielding parameter for that site, si o0,i ¼ g B0 ð1 si Þ ¼ g Beff
Encyclopedia of the Neurological Sciences, Volume 2
½2
where siooo1 and Beff is the effective field experienced by the nucleus at the ith atomic site in the molecule. This frequency shift is referred to as the ‘chemical shift,’ because the degree of shielding is characteristic of the details of the underlying electron wave-functions making up the chemical bonding network of the molecule. Chemical shifts are extremely small relative to the parent resonance frequency. For example, the spectrum of ethanol (HO–CH2–CH3) shown in Figure 1 was obtained at a parent frequency of 500 MHz. In this example, the abscissa of the spectrum is the resonance (RF signal) frequency and the ordinate is the resonance (RF signal) amplitude. The resonance frequency is typically expressed in ppm units so as to be independent of magnetic field strength. In Figure 1, 1 ppm corresponds to 1 millionth of 500 MHz or 500 Hz. As shown in the spectrum, 1H atoms at different positions in ethanol have resonance frequencies spread over approximately 2500 Hz or 5 ppm.
Scalar Coupling In addition to chemical shift, the RF signals from each 1H group are split into closely spaced resonance frequencies known as ‘multiplets.’ The splitting is a consequence of magnetic interactions between adjacent 1H nuclei in the molecule, interactions that further modify Beff. One way of explaining this phenomenon is to regard each 1H nucleus as possessing a small magnetic dipole that generates its own, extremely small, magnetic field. This small field alters the Beff of nearby 1H atoms. Quantum mechanics requires the magnetic dipole of a given 1H atom to be oriented either parallel or antiparallel to the applied strong magnetic field (the field generated by the large magnet into which the sample was placed). This orientation determines whether the Beff for neighboring 1H nuclei is increased or decreased by a tiny amount, thus either slightly increasing or decreasing the resonance frequency of those nuclei. This phenomenon is known in liquids as throughbond or ‘scalar’ coupling because the magnetic interactions between coupled nuclei are transmitted via the chemical bonding network. (In solids, direct through-space interactions are important.) For example, the two 1H atoms of the –CH2– group in a particular ethanol molecule may be oriented in three possible configurations: both parallel to the field, both antiparallel, or one parallel and one antiparallel (and vice versa, thus twice the probability relative to the other spin states). As a result there are three possible states to affect the 1 H atoms of the neighboring –OH and –CH3 groups. In the first state the resonance of adjacent nuclei is shifted to a slightly higher resonance frequency, in the second to a lower frequency, and in the third it is unchanged because the small fields generated by the two dipoles cancel. When the spectrum from all the ethanol molecules in the sample together is
doi:10.1016/B978-0-12-385157-4.00199-8
973
974
Magnetic Resonance Spectroscopy
CH3
HO
6
CH2
5
4
3
2
1 ppm
0
1
Figure 1 A H MR spectrum of dry ethanol. The abscissa is the chemical shift in parts per million. The ordinate represents the relative resonance amplitude. Each resonance line contains a multiplet structure that is expanded as an inset. The multiplet at 5.2 ppm corresponds to the –OH group, the one at 3.5 ppm to the –CH2– group, and the one at 1.1 ppm to the –CH3 group. Spectrum provided by Dr. Andre´ d’Avignon.
measured, the resulting –OH and –CH3 resonances are 1:2:1 ratio triplets representing the signal from molecules with 1H atoms of the –CH2– groups in each of the three configurations. These triplets are enlarged and shown as insets in Figure 1. The presence of multiplets can be useful for inferring molecular structure by providing information about how the 1H nuclei in the spectrum are positioned relative to one another. Note that it is possible to have ‘multiplets of multiplets’ as a result of scalar coupling. The –CH2– resonance of ethanol is split into a 1:3:3:1 ratio quartet through scalar coupling with 1H atoms of the –CH3 group and simultaneously split into a 1:1 ratio doublet through scalar coupling with the 1H atom of the –OH group. This results in a multiplet with 2 4 or eight components as shown in the inset of Figure 1. MR spectra also contain information about the relative number of nuclei contributing to each resonance. The total amplitude of a resonance multiplet is proportional to the relative number of equivalent nuclei in the molecule that contribute to it. In the example given, the –OH, –CH2–, and –CH3 resonances (the component amplitudes of each multiplet added together) have relative amplitudes of 1:2:3 because there are 1, 2, and 3 atoms at each position. Thus, it is possible to use MR spectroscopy of brain metabolites to estimate their concentration in the brain.
Clinical Application Although MR spectroscopy can be used to elegantly evaluate the chemical structure of a sample contained in a test tube, its application to intact human brain introduces a number of complexities and challenges. First, the many molecules of
the human brain are detected simultaneously, with their resonances overlapping throughout the spectrum. Second, many of the molecules are considerably larger than ethanol, and their spectra are correspondingly more intricate. Third, the metabolites detected from human brain are typically present in low concentrations (on the order of millimoles per liter), and provide correspondingly lower signal. Finally, there are technical limitations to the quality of spectra that can be obtained from the human brain at this time. One technical limitation is related to the magnetic field strength at which the spectra are obtained. It is desirable to obtain MR spectra at high magnetic field strength for two reasons. First, spectral dispersion increases as field strength increases, and thus the resonances are spread out over a wider range of frequencies. For example, two 1H resonances separated by a 1 ppm chemical shift difference are 500 Hz apart at a field strength of 11.7 T (o0 ¼500 MHz) but only 60 Hz apart at 1.5 T (o0 ¼ 60 MHz). Thus, signals that tend to overlap at lower magnetic field strengths may not overlap at higher field strengths. This improvement in spectral dispersion simplifies spectral analysis. Second, MR signals are stronger at higher magnetic fields, providing a better signalto-noise ratio in the spectra. (A full description of this phenomenon is beyond the scope of this discussion.) At present, high-resolution MR spectroscopy of small samples – those that can be contained in a small glass tube – is typically done at magnetic fields of greater than 7 T (spectrometers operating at 21 T, 1H o0 ¼900 MHz, are commercially available). Because of difficulties in constructing a high field magnet large enough to fit a human inside, as well as other technical difficulties, studies of humans are generally done at fields less than 7 T. Thus, spectra obtained from live humans suffer from poorer signalto-noise ratio and spectral dispersion than spectra obtained at high fields.
Magnetic Resonance Spectroscopy
A wide variety of methods are available for obtaining MR spectra from the human brain. The primary task is to obtain RF signal from the brain region of interest while avoiding that arising from other parts of the ‘sample’ (i.e., other brain areas, scalp, bone, etc.). The methods in use today can be divided into two major categories: single-voxel methods and chemical shift imaging (CSI). In single-voxel methods, a single region of interest is chosen (e.g., the occipital lobe) and a spectrum is obtained only from it. Examples of single-voxel methods are point-resolved spectroscopy and stimulated echo acquisition mode. One advantage of single-voxel methods is that signal may be acquired relatively quickly, but only from a single region of interest. For CSI, signal is obtained from a ‘slab’ of tissue. This slab is typically more than a centimeter thick (as compared with typical imaging ‘slices,’ which are generally less than 5-mm thick). Once the data have been obtained, the slab can be analyzed as a two-dimensional grid and spectra can be displayed from individual regions of interest from the slab. CSI offers greater coverage of the brain than single-voxel methods but it can be more time consuming to obtain the data. The method used depends on the information desired in a given situation and differences in signal-to-noise ratio between the methods as a function of acquisition time and region of interest size. No matter which method is used for acquisition of spectra, detection of 1H metabolite signals from the brain faces a unique hurdle as compared with detection of other nuclides such as 31P or 23Na. When 1H spectra are obtained there is an enormous signal from 1H in brain water with which to contend. The concentration of equivalent 1H nuclei in water is 110 M (approximately 80 M in brain tissue). This is an extraordinary advantage in standard MR imaging, which is based on detection of 1 H signal from tissue water. For spectroscopy, however, the 1H signal from water interferes with detection of signal from metabolites of interest, which are present in concentrations of tens of millimoles or less (approximately 10 000 times less than the
975
water 1H concentration). The problem arises because all signals are detected simultaneously, and the massive signal from water 1 H overwhelms the smaller signal from metabolites. The RF signal detected is generally digitized by an analog-to-digital converter (ADC). If a full water signal is present, most of the range of the ADC is applied to digitizing the water signal, leaving a much smaller dynamic range over which to digitize the metabolite signal. As a result, most 1H spectroscopy of human brain employs some sort of water signal suppression. This commonly involves excitation of the 1H resonance of water alone and then suppressing the signal by magnetic-fieldgradient induced dephasing of the water magnetization. With present-day technology, 1H spectra from human brain typically show three to four readily detected resonances. As shown in Figure 2, these are choline, creatine (both creatine and phosphocreatine as a single resonance), N-acetylcontaining compounds (NAA), and lactate. The reason that these resonances are more easily detected than others is related to the fact that the particular resonances have relatively less splitting due to scalar coupling than others and that these metabolites are present in relatively higher concentrations. Choline serves as a component of membranes and is also a constituent of the neurotransmitter acetylcholine. Creatine, when phosphorylated, stores energy in the form of phosphate bonds. Phosphocreatine levels reflect the cellular energy state, but its detection requires 31P nuclear MR (NMR), as creatine and phosphocreatine are indistinguishable using 1H NMR. The precise role of NAA in brain metabolism is unclear, though it is widely believed that NAA is found primarily in neurons and not glia. As a result, changes in NAA level may reflect changes in the number of neurons present in a given region of interest. Lactate is an intermediary of energy metabolism. Its levels may be increased under a variety of conditions. For example, if brain tissue is alive but receiving inadequate oxygen for aerobic glycolysis, lactate levels may increase due to anaerobic glycolysis. Lactate levels may also be
Choline
NAA Creatine
Lactate
ppm
ppm 4
3
2
1
4
3
2
1
Figure 2 1H MR spectra obtained from a human brain at 3.0 T. NAA, N-acetyl-containing compounds. The spectrum on the left shows a relatively large lactate doublet related to asphyxia. The spectrum on the right, obtained 9 days later, shows resolution of the lactate doublet and a reduction in the NAA resonance (arrow) relative to the choline and creatine resonances due to neuronal loss.
976
Magnetic Resonance Spectroscopy
increased by the presence of inflammatory cells, which often utilize anaerobic glycolysis. Note that the 1H signal from the methyl group of lactate is a doublet due to scalar coupling with the 1H atom on the a carbon of lactate. As mentioned earlier MR is a quantitative technique and the amplitude of an MR resonance can be used to measure metabolite concentration. However, the absolute amplitude of a resonance signal depends on many factors, including the particular RF antenna coil used, its tuning, coil loading by the sample, and the gain of the receiver amplifiers. Because absolute resonance amplitudes are difficult to relate directly to concentrations, resonance amplitudes are often presented as ratios, with the understanding that most of the factors affecting resonance amplitude are the same for all resonances in the spectrum. For example, ratios of various resonance amplitudes relative to creatine are often used (e.g., NAA/creatine), assuming that creatine levels are relatively constant in the brain. A more sophisticated approach is to use an external standard. This is a small amount of a compound that is not found in the brain but has an easily detectable resonance. The standard, containing a known concentration of the reference compound, is placed within the sensitive volume of the RF coil during data acquisition. The resultant spectrum contains the resonances of interest plus the reference resonance. The absolute concentration of brain metabolites can be calculated by taking the ratio of the metabolite resonance amplitude to that of the standard, the concentration of which is known. A third
approach is to obtain a non-water-suppressed spectrum from the same region of interest as the water-suppressed 1H spectrum. The ratio of metabolite resonance amplitude detected on the water-suppressed spectrum to water resonance amplitude detected on the nonsuppressed spectrum can be used to calculate metabolite concentration as the ratio of metabolite to water, giving concentration in millimoles per liter. Overall, MR spectroscopy has the potential to provide a wealth of information regarding metabolite levels in living human brain. At present, human spectroscopy studies are just beginning to find a place in everyday patient care. With time, it is likely that improvements in magnetic field strength and MR equipment, coupled with advances in pulse sequence design (spectral editing), will enable routine detection of lower-amplitude resonances in spectra from human brain.
See also: Magnetic Resonance Imaging. Magnetic Resonance (MR); Overview
Further Reading Becker ED (1993) A brief history of nuclear magnetic resonance. Analytical Chemistry 65: 295A–302A.
Basics Analysis of the specific frequencies at which radiofrequency (RF) energy is absorbed (or emitted) by a nuclear spin system, the ‘resonance frequencies,’ can provide considerable information about the system. The parent resonance frequency (o0) of a given nuclide is determined by two factors: the magnetogyric ratio (g), a property of the atomic nucleus, which is unique for a particular nuclide, and the magnetic field (B0) in which it resides at the time of the magnetic resonance (MR) experiment. This relationship can be written as o0 ¼ g B0
½1
In a magnetic field of 11.7 T, for example, the parent resonance frequency of 1H is 500 MHz (where 1 MHz is 1 106 Hz or 1 106 cycles/s). Thus, an MR study of 1H in an 11.7-T magnet would be done using RF excitation and detection at a parent frequency of 500 MHz (approximately five times that of a typical FM radio station). However, to study deuterium (2H) in the same magnetic field, one would use a parent frequency of 76.8 MHz because 2H has its own characteristic value for g. For reference, a magnetic field of 11.7 T is nearly 250 000 times that of the Earth’s magnetic field. (1 T is 104 G. The Earth’s field is about half a gauss.)
Chemical Shift If eqn [1] were the whole story, MR spectroscopy would not be very interesting. However, soon after the initial demonstration of the MR phenomenon, it was discovered that the signal obtained from an MR sample contains finely resolved (‘high-resolution’) small variations (‘shifts’) in the resonance frequencies for particular atoms in a given molecule. For example, adenosine triphosphate has three shifted 31P resonance frequencies, one for each of the three chemically distinct sites of phosphorus atoms in the molecule. This frequency shift phenomenon is due to the fact that the magnetic field experienced by a nucleus is reduced (‘shielded’) by the electrons that orbit the nucleus and make up the chemical bonds of the molecule. This reduction in magnetic field is very small, measured in parts per million (ppm), and some atoms are relatively more shielded from the applied magnetic field than others. Thus, for the ith atomic site in a molecule, eqn [1] must be modified slightly to account for this shielding phenomenon. This is done by introducing the unitless shielding parameter for that site, si o0,i ¼ g B0 ð1 si Þ ¼ g Beff
Encyclopedia of the Neurological Sciences, Volume 2
½2
where siooo1 and Beff is the effective field experienced by the nucleus at the ith atomic site in the molecule. This frequency shift is referred to as the ‘chemical shift,’ because the degree of shielding is characteristic of the details of the underlying electron wave-functions making up the chemical bonding network of the molecule. Chemical shifts are extremely small relative to the parent resonance frequency. For example, the spectrum of ethanol (HO–CH2–CH3) shown in Figure 1 was obtained at a parent frequency of 500 MHz. In this example, the abscissa of the spectrum is the resonance (RF signal) frequency and the ordinate is the resonance (RF signal) amplitude. The resonance frequency is typically expressed in ppm units so as to be independent of magnetic field strength. In Figure 1, 1 ppm corresponds to 1 millionth of 500 MHz or 500 Hz. As shown in the spectrum, 1H atoms at different positions in ethanol have resonance frequencies spread over approximately 2500 Hz or 5 ppm.
Scalar Coupling In addition to chemical shift, the RF signals from each 1H group are split into closely spaced resonance frequencies known as ‘multiplets.’ The splitting is a consequence of magnetic interactions between adjacent 1H nuclei in the molecule, interactions that further modify Beff. One way of explaining this phenomenon is to regard each 1H nucleus as possessing a small magnetic dipole that generates its own, extremely small, magnetic field. This small field alters the Beff of nearby 1H atoms. Quantum mechanics requires the magnetic dipole of a given 1H atom to be oriented either parallel or antiparallel to the applied strong magnetic field (the field generated by the large magnet into which the sample was placed). This orientation determines whether the Beff for neighboring 1H nuclei is increased or decreased by a tiny amount, thus either slightly increasing or decreasing the resonance frequency of those nuclei. This phenomenon is known in liquids as throughbond or ‘scalar’ coupling because the magnetic interactions between coupled nuclei are transmitted via the chemical bonding network. (In solids, direct through-space interactions are important.) For example, the two 1H atoms of the –CH2– group in a particular ethanol molecule may be oriented in three possible configurations: both parallel to the field, both antiparallel, or one parallel and one antiparallel (and vice versa, thus twice the probability relative to the other spin states). As a result there are three possible states to affect the 1 H atoms of the neighboring –OH and –CH3 groups. In the first state the resonance of adjacent nuclei is shifted to a slightly higher resonance frequency, in the second to a lower frequency, and in the third it is unchanged because the small fields generated by the two dipoles cancel. When the spectrum from all the ethanol molecules in the sample together is
doi:10.1016/B978-0-12-385157-4.00199-8
973
974
Magnetic Resonance Spectroscopy
CH3
HO
6
CH2
5
4
3
2
1 ppm
0
1
Figure 1 A H MR spectrum of dry ethanol. The abscissa is the chemical shift in parts per million. The ordinate represents the relative resonance amplitude. Each resonance line contains a multiplet structure that is expanded as an inset. The multiplet at 5.2 ppm corresponds to the –OH group, the one at 3.5 ppm to the –CH2– group, and the one at 1.1 ppm to the –CH3 group. Spectrum provided by Dr. Andre´ d’Avignon.
measured, the resulting –OH and –CH3 resonances are 1:2:1 ratio triplets representing the signal from molecules with 1H atoms of the –CH2– groups in each of the three configurations. These triplets are enlarged and shown as insets in Figure 1. The presence of multiplets can be useful for inferring molecular structure by providing information about how the 1H nuclei in the spectrum are positioned relative to one another. Note that it is possible to have ‘multiplets of multiplets’ as a result of scalar coupling. The –CH2– resonance of ethanol is split into a 1:3:3:1 ratio quartet through scalar coupling with 1H atoms of the –CH3 group and simultaneously split into a 1:1 ratio doublet through scalar coupling with the 1H atom of the –OH group. This results in a multiplet with 2 4 or eight components as shown in the inset of Figure 1. MR spectra also contain information about the relative number of nuclei contributing to each resonance. The total amplitude of a resonance multiplet is proportional to the relative number of equivalent nuclei in the molecule that contribute to it. In the example given, the –OH, –CH2–, and –CH3 resonances (the component amplitudes of each multiplet added together) have relative amplitudes of 1:2:3 because there are 1, 2, and 3 atoms at each position. Thus, it is possible to use MR spectroscopy of brain metabolites to estimate their concentration in the brain.
Clinical Application Although MR spectroscopy can be used to elegantly evaluate the chemical structure of a sample contained in a test tube, its application to intact human brain introduces a number of complexities and challenges. First, the many molecules of
the human brain are detected simultaneously, with their resonances overlapping throughout the spectrum. Second, many of the molecules are considerably larger than ethanol, and their spectra are correspondingly more intricate. Third, the metabolites detected from human brain are typically present in low concentrations (on the order of millimoles per liter), and provide correspondingly lower signal. Finally, there are technical limitations to the quality of spectra that can be obtained from the human brain at this time. One technical limitation is related to the magnetic field strength at which the spectra are obtained. It is desirable to obtain MR spectra at high magnetic field strength for two reasons. First, spectral dispersion increases as field strength increases, and thus the resonances are spread out over a wider range of frequencies. For example, two 1H resonances separated by a 1 ppm chemical shift difference are 500 Hz apart at a field strength of 11.7 T (o0 ¼500 MHz) but only 60 Hz apart at 1.5 T (o0 ¼ 60 MHz). Thus, signals that tend to overlap at lower magnetic field strengths may not overlap at higher field strengths. This improvement in spectral dispersion simplifies spectral analysis. Second, MR signals are stronger at higher magnetic fields, providing a better signalto-noise ratio in the spectra. (A full description of this phenomenon is beyond the scope of this discussion.) At present, high-resolution MR spectroscopy of small samples – those that can be contained in a small glass tube – is typically done at magnetic fields of greater than 7 T (spectrometers operating at 21 T, 1H o0 ¼900 MHz, are commercially available). Because of difficulties in constructing a high field magnet large enough to fit a human inside, as well as other technical difficulties, studies of humans are generally done at fields less than 7 T. Thus, spectra obtained from live humans suffer from poorer signalto-noise ratio and spectral dispersion than spectra obtained at high fields.
Magnetic Resonance Spectroscopy
A wide variety of methods are available for obtaining MR spectra from the human brain. The primary task is to obtain RF signal from the brain region of interest while avoiding that arising from other parts of the ‘sample’ (i.e., other brain areas, scalp, bone, etc.). The methods in use today can be divided into two major categories: single-voxel methods and chemical shift imaging (CSI). In single-voxel methods, a single region of interest is chosen (e.g., the occipital lobe) and a spectrum is obtained only from it. Examples of single-voxel methods are point-resolved spectroscopy and stimulated echo acquisition mode. One advantage of single-voxel methods is that signal may be acquired relatively quickly, but only from a single region of interest. For CSI, signal is obtained from a ‘slab’ of tissue. This slab is typically more than a centimeter thick (as compared with typical imaging ‘slices,’ which are generally less than 5-mm thick). Once the data have been obtained, the slab can be analyzed as a two-dimensional grid and spectra can be displayed from individual regions of interest from the slab. CSI offers greater coverage of the brain than single-voxel methods but it can be more time consuming to obtain the data. The method used depends on the information desired in a given situation and differences in signal-to-noise ratio between the methods as a function of acquisition time and region of interest size. No matter which method is used for acquisition of spectra, detection of 1H metabolite signals from the brain faces a unique hurdle as compared with detection of other nuclides such as 31P or 23Na. When 1H spectra are obtained there is an enormous signal from 1H in brain water with which to contend. The concentration of equivalent 1H nuclei in water is 110 M (approximately 80 M in brain tissue). This is an extraordinary advantage in standard MR imaging, which is based on detection of 1 H signal from tissue water. For spectroscopy, however, the 1H signal from water interferes with detection of signal from metabolites of interest, which are present in concentrations of tens of millimoles or less (approximately 10 000 times less than the
975
water 1H concentration). The problem arises because all signals are detected simultaneously, and the massive signal from water 1 H overwhelms the smaller signal from metabolites. The RF signal detected is generally digitized by an analog-to-digital converter (ADC). If a full water signal is present, most of the range of the ADC is applied to digitizing the water signal, leaving a much smaller dynamic range over which to digitize the metabolite signal. As a result, most 1H spectroscopy of human brain employs some sort of water signal suppression. This commonly involves excitation of the 1H resonance of water alone and then suppressing the signal by magnetic-fieldgradient induced dephasing of the water magnetization. With present-day technology, 1H spectra from human brain typically show three to four readily detected resonances. As shown in Figure 2, these are choline, creatine (both creatine and phosphocreatine as a single resonance), N-acetylcontaining compounds (NAA), and lactate. The reason that these resonances are more easily detected than others is related to the fact that the particular resonances have relatively less splitting due to scalar coupling than others and that these metabolites are present in relatively higher concentrations. Choline serves as a component of membranes and is also a constituent of the neurotransmitter acetylcholine. Creatine, when phosphorylated, stores energy in the form of phosphate bonds. Phosphocreatine levels reflect the cellular energy state, but its detection requires 31P nuclear MR (NMR), as creatine and phosphocreatine are indistinguishable using 1H NMR. The precise role of NAA in brain metabolism is unclear, though it is widely believed that NAA is found primarily in neurons and not glia. As a result, changes in NAA level may reflect changes in the number of neurons present in a given region of interest. Lactate is an intermediary of energy metabolism. Its levels may be increased under a variety of conditions. For example, if brain tissue is alive but receiving inadequate oxygen for aerobic glycolysis, lactate levels may increase due to anaerobic glycolysis. Lactate levels may also be
Choline
NAA Creatine
Lactate
ppm
ppm 4
3
2
1
4
3
2
1
Figure 2 1H MR spectra obtained from a human brain at 3.0 T. NAA, N-acetyl-containing compounds. The spectrum on the left shows a relatively large lactate doublet related to asphyxia. The spectrum on the right, obtained 9 days later, shows resolution of the lactate doublet and a reduction in the NAA resonance (arrow) relative to the choline and creatine resonances due to neuronal loss.
976
Magnetic Resonance Spectroscopy
increased by the presence of inflammatory cells, which often utilize anaerobic glycolysis. Note that the 1H signal from the methyl group of lactate is a doublet due to scalar coupling with the 1H atom on the a carbon of lactate. As mentioned earlier MR is a quantitative technique and the amplitude of an MR resonance can be used to measure metabolite concentration. However, the absolute amplitude of a resonance signal depends on many factors, including the particular RF antenna coil used, its tuning, coil loading by the sample, and the gain of the receiver amplifiers. Because absolute resonance amplitudes are difficult to relate directly to concentrations, resonance amplitudes are often presented as ratios, with the understanding that most of the factors affecting resonance amplitude are the same for all resonances in the spectrum. For example, ratios of various resonance amplitudes relative to creatine are often used (e.g., NAA/creatine), assuming that creatine levels are relatively constant in the brain. A more sophisticated approach is to use an external standard. This is a small amount of a compound that is not found in the brain but has an easily detectable resonance. The standard, containing a known concentration of the reference compound, is placed within the sensitive volume of the RF coil during data acquisition. The resultant spectrum contains the resonances of interest plus the reference resonance. The absolute concentration of brain metabolites can be calculated by taking the ratio of the metabolite resonance amplitude to that of the standard, the concentration of which is known. A third
approach is to obtain a non-water-suppressed spectrum from the same region of interest as the water-suppressed 1H spectrum. The ratio of metabolite resonance amplitude detected on the water-suppressed spectrum to water resonance amplitude detected on the nonsuppressed spectrum can be used to calculate metabolite concentration as the ratio of metabolite to water, giving concentration in millimoles per liter. Overall, MR spectroscopy has the potential to provide a wealth of information regarding metabolite levels in living human brain. At present, human spectroscopy studies are just beginning to find a place in everyday patient care. With time, it is likely that improvements in magnetic field strength and MR equipment, coupled with advances in pulse sequence design (spectral editing), will enable routine detection of lower-amplitude resonances in spectra from human brain.
See also: Magnetic Resonance Imaging. Magnetic Resonance (MR); Overview
Further Reading Becker ED (1993) A brief history of nuclear magnetic resonance. Analytical Chemistry 65: 295A–302A.