Magnetic Resonance Spectroscopy

1
Magnetic Resonance Spectroscopy

• Douglas L. Rothman, Ph.D.
• Magnetic Resonance Research Center
• Yale School of Medicine

2
Basics of MRS
MRS works on the same physical principles as
MRI, and uses similar equipment However MRS
also allows the mapping of the distribution of
metabolites and flux rates in the body MRS is
able to distinguish chemicals based on
differences in resonance frequency
3
Example MRS Spectrum of GABA
3.01 ppm C-4
2.31 ppm C-2
1.91 ppm C-3
4.0
3.5
3.0
2.5
2.0
1.5
1.0
PPM
4
13C MRS for Measuring Fluxes

• Measurement of fluxes by MRS is based upon the
  enrichment of carbon positions of metabolic
  precursors by 13C
• 13C is a stable isotope that is only 1.1 natural
  abundant
• Using selectively 13C enriched metabolic
  precursors in combination 13C MRS the fluxes of
  specific metabolic pathways can be measured

5
13C MRS Spectrum from Human Cerebral Cortex of
GABA, Glutamate, and Glutamine Labeling during
1-13C Glucose Infusion
13C MRS Spectrum
60 mins averaging, 81 ml
Glu-C4
GABA-C2
Glu-C2
Glu-C3
GABA-C4
Gln-C2
Asp-C3
Gln-C3

NAA-C3

Asp-C2
NAA (methyl)
6
Metabolic Imaging with MRS

• The combination of MRS with MRI methods allows
  images of metabolite levels and flux rates to be
  obtained.
• Metabolic imaging puts the highest demand on
  equipment performance

7
Identification of epileptogenic regions with NAA
MRS Metabolic Imaging
Hetherington et al. Neurology 48 1018-1024, 1997
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Why High Field Magnets?
Most metabolites are present in low
concentration Differences in resonance
frequencies are on the order of 0.1-1.0
ppm Resolution and sensitivity are proportional
to magnetic field strength
9
MRRC - MRS Equipment
State of the art 4.0T Bruker MRS system for
human studies State of the Art 4T, 9.4T, and
11.7 T Bruker MRS/MRI systems for animal
studies Modern 500, and 600 MHz high resolution
MRS spectrometers for biochemical analysis and
protein structure determination.
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4T Human MRS/MRI System

11
MRRC - MRS Research
Development of novel MRS methods to measure
metabolic pathways Development of MRS technology
Control and Regulation of Metabolism Interdisci
plinary Research (Biomedical engineering, Lab
Medicine, Medicine, Pediatrics, Neurobiology,
Neurosurgery, Neurology, Psychiatry
12
MRRC - MRS Research
Development of novel MRS methods to measure
metabolic pathways Development of MRS technology
Control and Regulation of Metabolism Interdisci
plinary Research (Biomedical engineering, Lab
Medicine, Medicine, Pediatrics, Neurobiology,
Neurosurgery, Neurology, Psychiatry, Radiology)
13
Strategy for Developing MRS Methods for Studying
Metabolic Pathways

• Develop optimum tracers for the target
  metabolic pathway
• Develop a metabolic model of the pathway
• Validate the measurement in an animal model
• Test the tracer in human studies, and validate
  when possible

14
Example Development of 13C MRS Methods to
Measure the Glutamate/GlutamineCycle
Magistretti et al, Science 283496,1999
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Strategy for Developing MRS Methods for Studying
Metabolic Pathways

• Develop optimum tracers for the target
  metabolic pathway
• Develop a metabolic model of the pathway
• Validate the measurement in an animal model
• Test the tracer in human studies, and validate
  when possible

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13C Metabolic Model of Neuronal Glutamate
Astroglial Gln by Glutamate/Glutamine Cycle

Blood
Brain
Gln4
Gln4ECF
Gln4
Vefflux
Vcycle
Vcycle
Glu4
Glu4 ECF
Glu4
Vcycle
aKG4
KG4
VTCA
VTCA
TCAA
TCAN
OAA
PDH
Astrocyte
Neuron
1-13CGlucose
Pyr3
Pyr3
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Measurement of Brain Glu/Gln Cycle by 13C MRS in
Humans

• Subject placed in MRI scanner
• Region selected from scout MRI
• IV infusion of 13C glucose
• Localized 13C MRS spectra
• Metabolic modeling to calculate rates

proton
proton
carbon
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13C MRS of Labeling of Neuronal Glutamate
Astroglial Gln from 1-13C Glucose
Glu-C4
Glucose-C1
Gln-C4
b
a
99 lt t lt 117
81 lt t lt 99
63 lt t lt 81
45 lt t lt 63
time (mins)
27 lt t lt 45
9 lt t lt 27
-9 lt t lt 9
100
90
60
50
40
30
20
PPM
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Metabolic modeling of 13C labeling time courses
yields Metabolic rates
glutamate C4
best fit
glutamine C4
best fit
30
VTCAn 0.76
20
adjusted FE ()
10
Vglu/gln 0.32
0
time (min)
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Strategy for Developing MRS Methods for Studying
Metabolic Pathways

• Develop optimum tracers for the target
  metabolic pathway
• Develop a metabolic model of the pathway
• Validate the measurement in animal models
• Test the tracer in human studies, and validate
  when possible

21
Validation Strategy Alter Synaptic Activity and
Measure Associated Change in Vcycle
Magistretti et al, Science 283496,1999
22
Validation in an Animal Model Rates of the
Glu/Gln Cycle as a Function of Electrical Activity
23
Strategy for Developing MRS Methods for Studying
Metabolic Pathways

• Develop optimum tracers for the target
  metabolic pathway
• Develop a metabolic model of the pathway
• Validate the measurement in animal models
• Test the tracer in human studies, and validate
  when possible

24
Validation Strategy Compare Fluxes Determined
Using Alternate Tracers
Acetate C2
Acetate C2
MCT
NEURON
GLIA
BLOOD
Vac
GlngC4
GlnnC4
AcCoA
Vcycle
Vcycle
Cit
GlugC4
GlunC4
OAA
VTCAg
VTCAn
?KG
?KG
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Validation in Human Cerebral Cortex Comparison
of Fluxes Calculated from Independent Tracers
2-13C acetate (n8)
1-13C glucose (n6)
26
MRRC - MRS Research
Development of novel MRS methods to measure
metabolic pathways Development of MRS technology
Control and Regulation of Metabolism Interdisci
plinary Research (Biomedical engineering, Lab
Medicine, Medicine, Pediatrics, Neurobiology,
Neurosurgery, Neurology, Psychiatry, Radiology)
27
Development of Metabolic Imaging Metabolic
Imaging of the Neuronal TCA Cycle(Hyder et al.
2000)
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Improvement of Bo homogeneity
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Simultaneous Multi Voxel MRS
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MRRC - MRS Research
Development of novel MRS methods to measure
metabolic pathways Development of MRS technology
Control and Regulation of Metabolism Interdisci
plinary Research (Biomedical engineering, Lab
Medicine, Medicine, Pediatrics, Neurobiology,
Neurosurgery, Neurology, Psychiatry, Radiology)