Multinuclear MR Spectroscopy and Spatial Localization Techniques

1
Spatial Localization and Multinuclear MR
Spectroscopy Techniques
Navin Bansal, Ph.D. Associate Professor and
Director of MR Research
2
Proton MR Image

• MR images contain anatomical information based on
  the distribution of protons and the relative
  proton relaxation rates in various tissues
• MR images are based on proton signals from water
  and fat

3
MR Spectrum

• MR spectroscopy determines the presence of
  certain chemical compounds
• Stress, functional disorders, or diseases can
  cause the metabolite concentration to vary
• Metabolite concentrations are low, generating
  10,000 times less signal intensity than the
  water signal

4
Chemical Shift

• The electron cloud around each nuclei shields the
  external magnetic field
• Because of differences in electron shielding,
  identical nuclei resonate at different
  frequencies
• The resonance frequency in the presence of
  shielding ? is expressed as
• (1- ?)?Bo
• Where ? is the gyromagnetic ratio and Bo is the
  external magnetic field strength

1H MR spectra
-CH3
-OH
?, ppm
0
1
2
5
Chemical Shift

• The frequency shift increases with field
  strength. For example, shift difference between
  water and fat
• (?water - ?fat) at 1.5 T is 255 Hz at 3.0 T
  is 510 Hz
• ? (?water - ?fat) 106/?Bo, in ppm units
• ?water-fat is 3.5 ppm independent of field
  strength
• By convention
• Signals of weakly shielded nuclei with higher
  frequency are on the left
• Signals of more heavily shielded nuclei with
  lower frequency are on the right
• Chemical shift of water is set to 4.7 ppm at body
  temperature

6
MR Spectrum Peak Characteristics
7
1H MR Spectrum from Brain
Water Signal
Metabolite Signals
8
Spatial Localization

• Surface Coil Localization
• Simple surface coil acquisition
• Depth Resolved Surface Coil Spectroscopy, DRESS

• Single Volume Localization
• Image Selected In Vivo Spectroscopy, ISIS
• Point Resolved Spectroscopy, PRESS
• Stimulated Echo Acquisition Mode, STEAM

• Multiple Volume Acquisition
• Chemical Shift Imaging, CSI

9
Surface Coil Acquisition
A simple loop of wire and associated circuit
tuned to the desired frequency are placed
directly over the tissue of interest to obtain
spectra
A surface coil

• Advantages
• Easy to build and does not require specialized
  pulse sequence
• Superb SNR and filling factor
• Disadvantages
• Must be close to region of interest
• Changing ROI is difficult
• Inhomogeneous RF field

RF
Pulse-acquire sequence
10
Spin Echo Imaging Sequence
180
90
90
RF
G
z
G
y
G
x
TE
TR
11
Depth Resolved Surface Coil Spectroscopy, DRESS
A disk-shaped slice is excited parallel to the
surface coil with a frequency selective RF pulse
in the presence of a gradient.

• Advantages
• Relatively simple
• Suppresses signal from superficial tissue
• Multi-slice acquisition, SLIT-DRESS
• Disadvantages
• T2 loss
• Partial Localization

12
Single Volume Localization

• Localized spectra is obtained from a single
  volume of interest (VOI)
• Localization is achieved by sequential selection
  of three orthogonal slices
• The size and location of VOI can be easily
  controlled
• Anatomic 1H images are used for localizing the VOI

13
Single Volume Localization

• Image selected in vivo spectroscopy, ISIS
• Point resolved spectroscopy, PRESS
• Stimulated echo acquisition mode, STEAM

14
Image Selected In Vivo Spectroscopy ISIS
One Dimensional
180o
90o
RF
Two acquisitions with and without inversion of a
selected slice are obtained and subtracted
15
3D ISIS

• A set of eight pulse sequences with one, two, or
  three slice selective inversion pulses are used
• The signal is localized to a VOI by adding
  signals from sequences 1, 5, 6, and 7 and
  subtracting signals from 2, 3, 4, and 8.

16
Image Selected In Vivo Spectroscopy, ISIS

• Advantages
• No T2 loss 31P MRS
• Less sensitive to gradient imperfections
• Can be used with a surface coil
• Disadvantages
• Dynamic range
• Subtraction error due to motion

17
Point Resolved Spectroscopy, PRESS
180
180
90
RF
G
x
G
y
G
z
(TE1TE2)/2
TE1/2
TE2/2

• A slice-selective 90o pulse is followed by two
  slice-selective 180o refocusing pulses
• Achieves localization within a single acquisition
• Suitable for signals with long T2 1H MRS

18
Stimulated Echo Acquisition Mode, STEAM

• Three slice-selective 90o pulses form a
  stimulated echo from a single voxel.
• Achieves localization within a single acquisition
• Only half of the available signal is obtained
• Can achieve shorter TE than PRESS

19
Effects of MR Parameters on PRESS spectra

• Repetition Time, TR
• Number of Signal Averages
• Echo Time, TE
• Voxel Size

20
Effect of Repetition Time (TR)
TR 1500 ms
TR 5000 ms
21
Effect of Signal Averaging
8 Averages
64 Averages
256 Averages
22
Effect of Voxel Size
1 cc
2 cc
4 cc
8 cc
23
Effect of Echo Time, TE
TE 144 ms
TE 288 ms
24
Short TE 1H Brain Spectrum
Additional Peaks
Healthy volunteer
25
The Lactate Doublet
Tumor spectra showing no NAA, ? Cho, ? mI, ?
lactate
Lipids and lactate
Inverted lactate
Upright lactate
26
Single Voxel Spectroscopy Overview

• Simplicity
• Flexibility in voxel size and position
• Accurate definition of VOI
• Excellent shim and spectral resolution
• Many voxels within the same dataset

27
Chemical Shift Imaging

• Multiple localized spectra are obtained
  simultaneously from a set of voxels spanning the
  region of interest
• Uses same phase-encoding principles as imaging
• No gradient is applied during data collection, so
  spectral information is preserved

28
CSI Spectral Map

• Display of all spectra
• Underlying reference image shows voxel position
• Individual spectra can be displayed enlarged
• Spectral map can be archived together with the
  reference image and the CSI grid

29
CSI Data Analysis
Image showing voxel position
Spectrum from a voxel
30
Spectral Map and Metabolite Images
31
CSI Overview

• Advantages
• Acquisition of multiple voxels
• Metabolite images, spectral maps, peak
  information maps, and results table
• Many voxels within the same dataset
• Disadvantages
• Large volume more difficult to shim
• Voxel bleeding
• Large datasets

32
Multinuclear MR Spectroscopy
33
Important Nuclei for Biomedical MR
Nucleus Spin ?, MHz/T Natural Abundance Relative Sensitivity
1H 1/2 42.576 99.985 100
2H 1 6.536 0.015 0.96
3He 1/2 32.433 .00013 44
13C 1/2 10.705 1.108 1.6
17O 3/2 5.772 0.037 2.9
19F 1/2 40.055 100 83.4
23Na 3/2 11.262 100 9.3
31P 1/2 17.236 100 6.6
39K 3/2 1.987 93.08 .05
34
Important Nuclei for Biomedical MR

• 1H Neurotransmitters, amino acids, membrane
  constituents
• 2H Perfusion, drug metabolism, tissue and
  cartilage structure.
• 13C Glycogen, metabolic rates, substrate
  preference, drug metabolism, etc.
• 19F Drug metabolism, pH, Ca2 and other metal
  ion concentration, pO2, temperature, etc
• 23Na Transmembrane Na gradient, tissue and
  cartilage structure.
• 31P Cellular energetics, membrane
  constituents, pHi, Mg2, kinetics of creatine
  kinase and ATP hydrolysis.

35
1H MR Spectroscopy
36
1H MR Spectra of the Brain Short TE
NAA
Cr
Cho
Ins
Glx
Glx
Lipids
Cr
ppm
37
Important 1H Signals
N-Acetyl aspartate (NAA)

• NAA is a neuronal marker and indicates density
  and viability of neurons.
• It is decreased in glioma, ischemia and
  degenerative diseases.

2.02, CH3 2.52, CH2 2.70, CH2 4.40, CH
Creatine (Cr), phosphocreatine (PCr)

• Cr is a marker of aerobic energy metabolism
• Cr signal is constant even with pathologic
  changes and may be used as a control value
• However, isolated cases of Cr deficiency may
  occur in children

3.04, CH3 3.93, CH2
38
Important 1H Signals
Choline (Cho), choline compounds

• Cho compounds are involved in phospholipid
  metabolism of cell membrane.
• Increase Cho mark tumor tissue or multiple
  sclerosis plaques

3.24, CH3 3.56, CH2 4.07, CH2
Glutamate (Glu), glutamine (Gln)

• Glu is a neurotransmitter, Gln a regulator of Glu
  metabolism
• It is hardly possible to detect their signals
  sepratly. The signals are jointly designated
  Glx.

2.1, CH2 2.4, CH2 3.7, CH
39
Important 1H Signals
Lactate (Lac)

• Lactate is the final product of glycolysis
• It can be detected in ischemic/hypoxic tissue and
  tumors indicating lack of oxygen

1.33, CH3 4.12, CH
Taurine (Tau)

• Cells examination indicates taurine synthesis in
  astrocytes

3.27, NCH2 3.44, SCH2
NH2-CH2-CH2-S-OH
Myo-inositol (Ins)

• Ins marks glia cells in brain
• It is decreased in hepatic encephalopathy and
  elevated in Alzheimers disease.

3.56, CH
40
31P MR Spectroscopy
41
31P MR Spectra of Normal Tissue
4
2
3
1
6

• ?-ATP
• ?-ATP
• ?-ATP
• PCr
• PDE
• Pi
• PME

4
2
1
3
6
7
5
3
6
2
7
4
1
6
5
3
2
4
1
7
6
5
4
3
2
1
ppm
42
Important 31P Signals
Adenosine triphosphate (ATP)
ATP is the energy currency in living systems ?-
and ?-ATP have contributions from ADP, NAD and
NADH ?-ATP is uncontaminated and used for
quantification
-16.5 ?-ATP -7.8 ?-ATP -2.7 ?-ATP
Phosphocreatine (PCr)
PCr is used for storing energy and converting ADP
to ATP It is absent in liver, kideny and red
cells It is used as an internal reference for
chemical shift
0 PCr
43
Important 31P Signals
Inorganic Phosphate (Pi)

• Pi is generated from hydrolysis of ATP and
  increased in compromised tissue
• Its chemical shift is sensitive to pH

3.7 to 5.7 Pi
Phosphomonoester (PME)

• PME signal contains contribution from membrane
  constituents and glucose-6-phosphate and
  glycerol-3 phosphate.
• It is elevated in tumors

5.6 to 8.1 PME
Phosphodiester (PDE)

• PME signal contains contribution from membrane
  constituents

0.6 to 3.7 PDE
44
Measurement of pH by 31P MRS
H2PO4- ? HPO42- H pKa 6.75
ù
é
?
-
?
-
obs
ú
ê
PO
H


pH
log
4
2
?
-
?
ú
ê
û
ë
-
obs
2
HPO
4
45
Effect of Exercise on 31P MRS
46
Detection of myocardial infarctions by 31P-MR
spectroscopy
Beer et al., J Magn Reson Imaging.
200420798-802.
47
A Lesson from 31P MRS Tumor Microenvironment
Poor Vascularization and Perfusion
Tumors are expected to be acidic
Hypoxia
Aerobic Glycolysis
Anaerobic Glycolysis
Increased Acid Production
48
pH of Tumors and Normal Tissue Electrode
Measurements
A
pH
POT
Skeletal Muscle
Normal
Brain
Tissue
Skin
Glioblastomas
Astrocytomas
Meningiomas
Brain Metastases
Malignant Melanomas
Sarcomas
Mammary Ca.
Adenocarcenomas
Squamous
Cell Ca.
49
pH of Tumors and Normal Tissue MRS Measurements
B
pH
NMR
Skeletal Muscle
Normal
Brain
Skin
Tissue
Heart
Sarcomas
Squamous Cell Ca.
Mammary Ca.
Brain Tumors
Non-Hodgkin Lymp.
Misc Tumors
Bansal, et al.
50
23Na MR Spectroscopy and Imaging
51
Biological Importance of Sodium

• Sodium and other ions are inhomogeneously
  distributed across the cell membrane.
• A transmembrane sodium gradient reflects a
  dynamic equilibrium between Na-K ATPase versus
  passive or mediated flux.
• The sodium gradient may be altered in certain
  diseased states.

Bansal, et al.
52
Biomedical 23Na NMR

• 23Na is the second most sensitive nucleus for
  biomedical NMR.
• Intra- and extracellular sodium resonate at the
  same frequency.
• Two approaches to distinguish between different
  sodium pools
• Paramagnetic Shift Reagents
• Multiple Quantum Filters

Bansal, et al.
53
23Na Shift Reagents

• SRs are membrane impermeable negatively charged
  chelates of a lanthanide metal ion. They interact
  with extracellular Na, causing its signal to be
  shifted away from intracellular Na.

Nae
Nae
Nae
Nai
Nai
Nae
Nae
Nae
Nae
SR
SR
SR
54
Action of a Typical Shift Reagent
With SR
Nae
Nai
Without SR
Nai Nae
ppm
Bansal, et al.
55
23Na Shift Reagents
56
In Vivo 23Na Spectra after TmDOTP5- Infusion
Bansal, et al.
57
Nai in Perfused RIF-1 Tumor Cells
Significance p lt 0.01 (with vs without EIPA)
200
37 oC
37 oC
45 oC

• Hyperthermia produced a 60-70 increase in Nai.
• The increase in Nai is mainly due to an increase
  Na/H antiporter activity

180

w/o EIPA

160
with EIPA
140
Relative Nai Signal Intensity
120
EIPA
100
80
10
20
30
40
50
60
70
80
-20
-10
0
-10
Time, min
Bansal, et al.
58
Multiple-Quantum Filters
MQFs depend only on the relaxation properties of
23Na. Thus, they do not produce any known
physiological perturbation to the biological
system and cab be applied to humans.

• Disadvantages
• Low signal-to-noise ratio
• Some Nae contribution

Bansal, et al.
59
MQ Filtered 23Na NMR

• Transiently bound Na can pass through a MQ
  filter.

-3/2gt
SQ outer
-1/2gt
TQ
SQ inner
DQ
1/2gt
DQ
SQ outer
3/2gt

• Concentration of macromolecules within the
  cytoplasm is relatively high while the
  extracellular milieu is largely aqueous.

60
SQ and TQ Filtered 23Na Spectraof a Phantom
Aqueous
Agarose
40 mM TmDOTP5-
10 Agarose
SQ
TQ
Agarose
ppm
ppm
0
-50
50
0
-50
50
Bansal, et al.
61
Composition of Tissue Compartments
m Eq/L H2O
H2CO3
HCO3-
Cl -
Nonelectrolytes
H2CO3
Nonelectrolytes
H2CO3
HCO3-
HCO3-
HPO4-2
K
Na
SO4-2
Na
Cl -
Cl -
Na
HPO4-2
Protein
HPO4-2
SO4-2
SO4-2
Mg2
Organic acids
K
Organic acids
K
Ca2
Protein
Ca2
Mg2
Mg2
Protein
Intracellular fluid
Interstitial fluid
Blood plasma
62
3D MQF 23Na Imaging Pulse Sequence
RF
Readout
Phase Encoding 1
Phase Encoding 2
63
3D SQ and TQF 23Na MRI of a Live Rat Caronal
Sections
SQ
TQF
64
SQ and TQF 23Na MRI of Rat In vivo Effect of CCl4
Treatment
Control
CCl4 Treated
SQ
TQF