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7/31/2019 Basic MRI Spect Xiaojuan Li PhD
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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/