Spatial Localization and Multinuclear MR

Spectroscopy Techniques

Navin Bansal, Ph.D.Associate Professor and Director of MR

Research

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

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

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

012, ppm

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

o Signals of weakly shielded nuclei with higher frequency are on the left

o 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

MR Spectrum: Peak Characteristics

1H MR Spectrum from Brain

Water Signal Metabolite Signals

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

Surface Coil Acquisition

A surface coil

RF

Pulse-acquire sequence

A simple loop of wire and associated circuit tuned to the desired frequency are placed directly over the tissue of interest to obtain spectra

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

180°

RF

90°

Gy

TE

Gx

Gz

TR

90°

Spin Echo Imaging Sequence

Depth Resolved Surface Coil Spectroscopy, DRESS

RF

Gslice

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

Single Volume Localization

RF

Gx

RF

Gy

RF

Gz

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

Image selected in vivo spectroscopy, ISIS

Point resolved spectroscopy, PRESS

Stimulated echo acquisition mode, STEAM

Single Volume Localization

Image Selected In Vivo Spectroscopy ISIS

Two acquisitions with and without inversion of a selected slice are obtained and subtracted

RF

Gslice

180o 90o

Slice inversion

No inversion

Subtraction

One Dimensional

3D ISIS

T1

Gx

Gy

Gz

180°

T1T1

1

180°180°

180° 180°

180°180°

180°180°

180°

180°

180°

90°

90°

90°

90°

90°

90°

90°

90°

3

2

4

5

7

6

RF

RF

RF

RF

RF

RF

RF

RF8

+

-

-

-

+

+

+

-

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.

Advantages

No T2 loss – 31P MRS

Less sensitive to gradient imperfections

Can be used with a surface coil

DisadvantagesDynamic range

Subtraction error due to motion

Image Selected In Vivo Spectroscopy, ISIS

(TE1+TE2)/2

RF

TE1/2

Gx

Gy

Gz

90°180°

TE2/2

180°

Point Resolved Spectroscopy, PRESS

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

TE/2

90° 90° 90°

TE/2TM

RF

Gx

Gy

Gz

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

Effects of MR Parameters on PRESS spectra

Repetition Time, TR

Number of Signal Averages

Echo Time, TE

Voxel Size

NAA

Cr/PCrCho

Cr/PCr

TR = 1500 ms TR = 5000 ms

Effect of Repetition Time (TR)

Effect of Signal Averaging

8 Averages 64 Averages 256 Averages

Effect of Voxel Size

1 cc 2 cc 4 cc 8 cc

Effect of Echo Time, TE

TE = 144 ms TE = 288 ms

Short TE 1H Brain Spectrum

Glx 2.05-2.45 ppm3.6 - 3.8 ppm

mI 3.56 ppm

Glucose 3.43 ppm 3.8 ppm

And more

Additional PeaksHealthy volunteer

The Lactate Doublet

Tumor spectra: showing no NAA, Cho, mI, lactate

Lipids and lactate

Inverted lactate

Upright lactate

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

RF

Gslice

Gy

Gz

90°

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

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

CSI Spectral Map

CSI Data Analysis

Image showing voxel position

Spectrum from a voxel

NAA NAA/Cho

Spectral Map and Metabolite Images

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

Multinuclear MR Spectroscopy

Nucleus Spin , MHz/T Natural Abundanc

e

Relative Sensitivity

1H 1/2 42.576 99.985 1002H 1 6.536 0.015 0.96

3He 1/2 32.433 .00013 4413C 1/2 10.705 1.108 1.617O 3/2 5.772 0.037 2.919F 1/2 40.055 100 83.4

23Na 3/2 11.262 100 9.331P 1/2 17.236 100 6.639K 3/2 1.987 93.08 .05

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.

Important Nuclei for Biomedical MR

1H MR Spectroscopy

NAA

Glx

Cr

ChoInsGlx

CrLipids

1.04.5 2.53.0 2.0 1.53.5 0.5ppm

1H MR Spectra of the BrainShort TE

•NAA is a neuronal marker and indicates density and viability of neurons.

•It is decreased in glioma, ischemia and degenerative diseases.

N-Acetyl aspartate (NAA)

CH3-C-NH-CH-CH2-COOH

O

CH2-COOH

2.02, CH3

2.52, CH2

2.70, CH2

4.40, CH

Creatine (Cr), phosphocreatine (PCr)

NH2-C-N-CH2-COOH

CH3

NH3.04, CH3

3.93, CH2

•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

Important 1H Signals

•Cho compounds are involved in phospholipid metabolism of cell membrane.

•Increase Cho mark tumor tissue or multiple sclerosis plaques

Choline (Cho), choline compounds

3.24, CH3

3.56, CH2

4.07, CH2

Glutamate (Glu), glutamine (Gln)

2.1, CH2

2.4, CH2

3.7, CH

•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”.

CH3-N-CH2-CH2-OH

CH3

CH3

HOOC-CH2-CH2-CH-COOH

NH2

NH2-CH2-CH2-CH-COOH

NH2

Important 1H Signals

•Lactate is the final product of glycolysis

•It can be detected in ischemic/hypoxic tissue and tumors indicating lack of oxygen

Lactate (Lac)

1.33, CH3

4.12, CH

Taurine (Tau)

3.27, NCH2

3.44, SCH2

•Cells examination indicates taurine synthesis in astrocytes

CH3-CH-COOH

OH

NH2-CH2-CH2-S-OH

PO4-

PO4-

PO4- PO4

-

PO4-

PO4-

Myo-inositol (Ins)

3.56, CH

•Ins marks glia cells in brain

•It is decreased in hepatic encephalopathy and elevated in Alzheimer’s disease.

Important 1H Signals

31P MR Spectroscopy

31P MR Spectra of Normal Tissue

7 6

5

4

17

6

4

3 2 1

6

3 2 1

6 54

3 21

5

4

21

7 63

4

3 2

10 0 -10 -20ppm

Muscle

Heart

Liver

Kidney

Brain

1. -ATP

2. -ATP

3. -ATP

4. PCr

5. PDE

6. Pi

7. PME

Adenosine triphosphate (ATP)

-16.5 -ATP-7.8 -ATP-2.7 -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

Phosphocreatine (PCr)

0 PCr

PCr is used for storing energy and converting ADP to ATPIt is absent in liver, kideny and red cellsIt is used as an internal reference for chemical shift

Important 31P Signals

Phosphomonoester (PME)

5.6 to 8.1 PME

•PME signal contains contribution from membrane constituents and glucose-6-phosphate and glycerol-3 phosphate.

•It is elevated in tumors

Phosphodiester (PDE)

0.6 to 3.7 PDE•PME signal contains contribution from membrane constituents

Important 31P Signals

Inorganic Phosphate (Pi)

3.7 to 5.7 Pi

•Pi is generated from hydrolysis of ATP and increased in compromised tissue

•Its chemical shift is sensitive to pH

Measurement of pH by 31P MRS

Shift, ppm30 20 10 0 -10 -20

PCr ATP

Pi

PME

H2PO4- HPO4

2- + H+ pKa = 6.75

-

--

-

obsHPO

POHobs

24

42log= apkpH

Effect of Exercise on 31P MRS

Beer et al., J Magn Reson Imaging. 2004;20:798-802.

Detection of myocardial infarctions by 31P-MR spectroscopy

Aerobic Glycolysis

Poor Vascularization and Perfusion

Hypoxia

Anaerobic Glycolysis

Increased Acid Production

Tumors are expected

to be acidic

A Lesson from 31P MRSTumor Microenvironment

GlioblastomasAstrocytomasMeningiomasBrain Metastases

Malignant MelanomasSarcomasMammary Ca.AdenocarcenomasSquamous Cell Ca.

A: pHPOT

Skeletal MuscleBrain

pH 5.6 6.0 6.4 6.8 7.2 7.6

Skin

NormalTissue

pH of Tumors and Normal TissueElectrode Measurements

Bansal, Bansal, et al.et al.

B: pHNMR

Sarcomas

Non-Hodgkin Lymp.

Squamous Cell Ca.Mammary Ca.Brain Tumors

Misc Tumors

pH 5.6 6.0 6.4 6.8 7.2 7.6

Skeletal MuscleBrainSkinHeart

NormalTissue

pH of Tumors and Normal TissueMRS Measurements

23Na MR Spectroscopy and Imaging

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, Bansal, et al.et al.

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, Bansal, et al.et al.

SRSR SRSR SRSR

NaNa++ee

NaNa++ee

NaNa++ee

NaNa++ee

NaNa++ee

NaNa++eeNaNa++

ee

NaNa++ii

NaNa++ii

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+.

23Na Shift Reagents

Action of a Typical Shift Reagent

Bansal, Bansal, et al.et al.

With SRWith SR

Without SRWithout SRNaNaii + Na + Naee

001010 ppmppm

NaNaii

NaNaee

001010 ppmppm

23Na Shift Reagents

O

N N

O

OO

N

O

N

O

O

OO

O

O

O

Dy

P

O

O

OP

OO

OOP

OO

O

Dy

P

O

O

O P

OO

OOP

OO

O

N

N N N

PPP

P Tm

O

O

OO

O

O

O

O

O

O

O

O

Dy(PPP)Dy(PPP)227-7- DyTTHADyTTHA3-3- TmDOTPTmDOTP5-5-

Muscle

Heart

Liver

Brain

Kidney

Ext

Int

x 5Urine

9L Glioma

40 30 20 10 0 -10ppm

In VivoIn Vivo 2323Na Spectra after TmDOTPNa Spectra after TmDOTP5- 5-

InfusionInfusion

Bansal, Bansal, et al.et al.

80

100

120

140

160

180

200

EIPA

****

** 37 oC 37 oC 45 oC

Time, min

10 20 30 40 50 60 70 80-20 -10 0-10

Rela

tive N

ai S

ignal In

tensi

ty

Significance: ** p < 0.01 (with vs without EIPA)

with EIPA

w/o EIPA

Nai in Perfused RIF-1 Tumor Cells

Hyperthermia produced a 60-70% increase in Nai

+.

The increase in Nai

+ is mainly due to an increase Na+/H+ antiporter activity

Bansal, Bansal, et al.et al.

Multiple-Quantum FiltersMQFs 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, Bansal, et al.et al.

MQ Filtered 23Na NMR “Transiently bound” Na+ can pass through a MQ filter.

|3/2>

|1/2>

|-1/2>

|-3/2>

SQ outer

SQ inner

SQ outer

DQ

DQ TQ

“Free” Na+ “Transiently Bound” Na+

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

SQ and TQ Filtered 23Na Spectraof a Phantom

00 -50-505050ppmppm 00 -50-505050ppmppm

SQSQ TQTQ

Agarose

Agarose

Aqueous

40 mMTmDOTP5-

10%Agarose

Bansal, Bansal, et al.et al.

200

180

160

140

120

100

80

60

40

20

0

NonelectrolytesH2CO3

Na+

K+

Ca+2Mg+2

Organicacids

SO4-2

HPO4-2

HCO3-

Cl -

Protein

H2CO3

HCO3-

Cl -

HPO4-2

Na+

K+

SO4-2

Protein

Intracellularfluid

OrganicacidsCa+2

Mg+2

K+

Protein

SO4-2

HPO4-2

Na+

Cl -

HCO3-

H2CO3

Nonelectrolytes

Interstitialfluid

Bloodplasma

m Eq/L H2O

Mg+2

Composition of Tissue Compartments

3D MQF 23Na Imaging Pulse Sequence

PD(100 ms)

DELTA ()(3 µs)

TAU ()(3 ms)

TE(4 ms)

RF

Readout

PhaseEncoding 1

PhaseEncoding 2

3D SQ and TQF 3D SQ and TQF 2323Na MRI of a Live RatNa MRI of a Live RatCaronal Sections

TQFTQF

SQSQ

SQ

TQF

Control CCl4 Treated

Liver

bladder

kidneyheart

lungsaline

agarose

SQ and TQF SQ and TQF 2323Na MRI of Rat Na MRI of Rat In vivoIn vivoEffect of CClEffect of CCl44 Treatment Treatment