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Basic MR Imaging(MRI) and MR

spectroscopy (MRS)

Basic MR Imaging(MRI) and MR

spectroscopy (MRS)

Xiaojuan Li, PhD

Dept of Radiology, UCSF

PT210, March 1st, 2005

Xiaojuan Li, PhD

Dept of Radiology, UCSF

PT210, March 1st, 2005

MRI OverviewMRI Overview

QuestionsQuestions

ow do we get MR signals?

ow do we reconstruct MR images?

ow do we interpret MR images?

ow do we get MR signals?

ow do we reconstruct MR images?

ow do we interpret MR images?

Protons constantly spins around an axis

The positive charge attached with proto

also moves

Electrical current magnetic field

Protons constantly spins around an axis

The positive charge attached with proto

also moves

Electrical current magnetic field

SpinsSpins

MR Signals -- SpinsMR Signals -- Spins

N

S

N

S

N

S

PrecessionPrecession

0 = B 0Larmor frequency

N

S

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R Signals -- Magnetic VectorsR Signals -- Magnetic Vectors

m Status

d with B0

y

M0

Signal released

during relaxation

M0 back to Equ.

status

x y

zM

0

Apply B1| B0

M0 titled to xy

plane

x y

zM

0

B1

Relaxation -- T1Relaxation -- T1

T1 relaxation

(longitudinal relaxation)T1 relaxation

(longitudinal relaxation)

t=T1

M0

0.63M0

Spin-lattice relaxation

x y

zM

0

Energy change btw spi

surroundings;

T1 time depends on tiss

composition, structure a

surroundings

Relaxation -- T2Relaxation -- T2

axation (transverse

relaxation)axation (transverse

relaxation)

t=T2

Spin-spin relaxation

x y

zM

0

Loss of phase coherence due to:

1) Molecular interactions;

2) Inhomogeneity of B0

1/T2* = 1/T2 + 1/T2inhomo

Spin EchoSpin Echo

In Phase Out of Phase

after time due to different

rotating freq

Spins flip 1800

around y axis

In ph

again

time

1800 RF pulse

LocalizationLocalization

ice selection

hase encoding

equency encoding

ice selection

hase encoding

equency encoding

Slice SelectionSlice Selection

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Phase encodingPhase encoding

Turn the gradient on for a period of time then off

Frequency EncodingFrequency Encoding

Spins with the sam

phase have differe

frequencies

Spins with the sam

frequencies have

different phases

Spins with the sam

phase have differe

frequencies

Spins with the sam

frequencies have

different phases

The gradient is on during

data acquisition

Image ReconstructionImage Reconstruction

Spatial

Decoding

K-space Spatial domain

Pulse SequencePulse Sequence

RF

Gz

Gx

Gy

DA

TE

TR

Echo

TE: time of echo; TR: time of repetition

900 1800

Image Contrast - T1 weightImage Contrast - T1 weight

1B T1A

TRTR

rt TR give T1 weight. Tissues with short T1 are brighter

T2B

M0

0.37M0

T2A

Image Contrast - T2 weightImage Contrast - T2 weight

TE TE

Long TE gives T2 weight. Tissues with long T2 are br

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Image ContrastImage Contrast

T2ProtonDensityong TR

T1, T2T1hort TR

Long TEShort TE

normally have longest T1 and T2, so it appears darker

weighted images and brighter in T2-weighted images.

reful to images with fluid suppression though!)

Factors affecting MR signalsFactors affecting MR signals

PD: Proton density at location (x,y,z)

T1: Longitudinal relaxation time

T2: Transverse relaxation time

TR: Time of RepetitionTE: Time of Echo

TI: Time of Inversion

FA: Flip angle

Pulse sequence, flow, contrast medium etc

Contrast agentContrast agent

Post-contrast T1-weighted imagentrast T1-weighted image

contrast c

cc

c ccc c

c c

cc

c

cc

cc

c

cc

c c

c c

c

c

c

c

cc

c

c

c

c

Blood-Brain-Barrier

Normal Tissues

Tumors

MR SpectroscopyMR Spectroscopy

MRS is a powerful, non-invasive, non-

destructive tool to study chemical compositio

and metabolic processes.

Proton MRS detects signal from protons from

metabolites other than water.

MRS is a powerful, non-invasive, non-

destructive tool to study chemical compositio

and metabolic processes.

Proton MRS detects signal from protons from

metabolites other than water.

Chemical ShiftChemical Shift

0

0 = B 0

i = B0(1i)

In presence of B0, the electrons

surrounding atoms will also

interact with the field.

The I depends on molecularenvironment a spin experiences.

To get rid of B0 dependency,

this chemical shift is defined as

parts per million (ppm):

= i ref

ref10

6

1-H MRS for brain tissue1-H MRS for brain tissue

5.0 4.0 3.0 2.0 1.0

ppm

CholineCreatine

N-acetyl aspartate

(lactate/

lipid)

water

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MR spectroscopic imagingMR spectroscopic imaging

Tumor

Cho

Necrosis

Normal

NAA

Crt

R spectroscopy provides a unique biochemicaldow to study cellular metabolism non-invasively,ch helps to improve the sensitivity and specificity forecting active tumor.

R spectroscopy provides a unique biochemicaldow to study cellular metabolism non-invasively,ch helps to improve the sensitivity and specificity forecting active tumor.

Comparison with MRIComparison with MRI

No frequency encoding during acquisitio

Long acquisition time

Low SNR and low resolution due to low

concentration of metabolites of interest Provides metabolic or functional

information that may help to improve

diagnosis and treatment monitoring

No frequency encoding during acquisitio

Long acquisition time

Low SNR and low resolution due to low

concentration of metabolites of interest

Provides metabolic or functional

information that may help to improve

diagnosis and treatment monitoring

Grade 3

Grade 4

+Lip

Lac+Lip

17

e 4 vs. Grade 3 of brain tumorse 4 vs. Grade 3 of brain tumors

ReferencesReferences

MR Made Easy, GE and BERLEX

http://www.cis.rit.edu/htbooks/mri/

http://www.cis.rit.edu/htbooks/nmr/

MR Made Easy, GE and BERLEX

http://www.cis.rit.edu/htbooks/mri/

http://www.cis.rit.edu/htbooks/nmr/