Optimizing MRI Protocols
7/27/2005
Overview
Optimizing MRI Protocols
Clinical Practice & Compromises
Geoffrey D. Clarke, Radiology Department
University of Texas Health Science Center at San Antonio
• Pulse timing parameters for adjusting
image contrast
• Effects of spatial resolution and
imaging speed on signal-to-noise ratio
• Effects of magnetic field strength
• Adapting MRI pulsing protocols to
physiology under investigation
Image Contrast
• Basic image contrast is effected by the
amplitude and timing of the RF pulses
used to excite the spin system.
• More advanced methods may use gradient
pulses (to modulate motion) and alter
tissue properties with exogenous contrast
agents
Tissue Parameters
• Tissue Water Density
– Proton Density (PD)
• Longitudinal Relaxation Time (T1)
Morphology & Physiology
• Chemical Shift (water vs. fat)
• Blood motion (macroscopic & microscopic)
• Diffusion of water
• Gross motion (peristalsis, respiration)
• Tissue Susceptibility
User Selectable Parameters
• Magnetic Field Strength (Bo)
• RF Pulse timing (TR, TE, TI)
• RF pulse amplitude - flip angles (α)
• Gradient Amplitude & Timing (b-value)
• Transverse Relaxation Time (T2)
• Magnetization transfer rate constants
G.D. Clarke, UT HSC San Antonio
• RF pulse excitation frequency & bandwidth
• Receiver bandwidth (BW)
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Optimizing MRI Protocols
7/27/2005
Contrast-to-Noise Ratio
Relaxation Times
Contrast relationships are determined by the RF pulse timings used
and the relaxation properties of tissues under investigation.
• T1: longitudinal relaxation time defines recovery
of potential for next signal
CNR = n
(I a − I b )
• T2: transverse relaxation time defines rate of
dephasing of MRI signal due to microscopic
processes
σ
σ = standard deviation of the signal (the noise)
n = number of signals acquired
Ia,Ib = signal intensities from tissues “a” and “b”
T1, T2 and for Various Tissues
Tissue
T1(ms)
Liver
675+142
Kidney
559+10
Muscle
1123+119
Gray Matter 1136+91
White Matter 889+30
• T2*: transverse relaxation time with Bo
inhomogeneity effects added; defines rate of
dephasing of MRI signal due to macroscopic
and microscopic processes
Manipulating Contrast
T2(ms)
• The “weighting” of image contrast is related to
delay times, TR (repetition time) & TE (echo time)
54+8
84+8
43+4
87+15
86+1.5
• Spin Echo – manipulates image contrast with 180o
refocusing pulses (insensitive to Bo inhomogeneities)
• Gradient Echo – manipulates image contrast by
varying the excitation flip angle (fast scans)
• Inversion Recovery – manipulates image contrast
with 180o inversion pulses
Akber,
Akber, 1996 (at 63 MHz)
Pulse Sequence Classifications
Spin Echo - Rules of Thumb
• TE controls T2 dependence
RF
Pulses
Contrast
Weighting
Application
Two or
more
T1, PD or
T2
Conventional
Gradient
Echo
One
T1 or T2*
Fast imaging
(3DFT)
“PD Weighted” = long TR, short TE
Inversion
Recovery
Three
T1 and T2
Exclude
certain tissues
“T2 Weighted” = long TR, long TE
Name
Spin Echo
• TR controls T1 dependence
G.D. Clarke, UT HSC San Antonio
• T1 competes with T2 and proton density
“T1 Weighted” = short TR, short TE
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Spin-Echo Imaging Sequence
Multi-Echo Acquisitions
RF2=180o
RF1=90o
Image 1
TX
Spin
Echo
RX
time
Image 2
time
Image 3
Gsl
time
Gro
time
Gpe
time
SNR and Imaging
Parameters
SNR ∝
Matched Bandwidth
FOVro FOVph
NSA
⋅ ∆z ⋅
M ro M ph
BWrx
FOV = field of view
M = matrix size
1
2
NSA = number of signals averaged
BW rx = receiver bandwidth ro = read out (frequency encoding) direction
∆z = slice thickness
ph = phase encoding direction
Mugler & Brookeman, Magn Reson Imag 1989; 7:487-493.
Matched Bandwidth
• Bandwidth on ADC1 is limited by finite Tmax
& desire to minimize TE1.
Matched Bandwidth =
Unmatched Distortion
T1W
T2W
• T1 weighting and SNR is sacrificed more
by T2 decay than by BW
• T2-weighted image - lots of time for ADC2,
poorer SNR. It would be nice to decrease
BW (increase Tmax)
Mugler & Brookeman, Magn Reson Imag 1989; 7:487-493.
G.D. Clarke, UT HSC San Antonio
• Smaller BW on T2W image leads to increased
image distortion
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Optimizing MRI Protocols
7/27/2005
SE – Effect of TR
MR Brain Imaging
TR = 63 ms, NSA = 16
AX T1W
FLAIR
AX T2W
TR =125 ms, NSA = 8
TR = 250 ms, NSA = 4
TR = 500 ms, NSA = 2
Diffusion Gadolinium
• Gray-white matter
contrast
• Depicts white matter
lesions
Bo = 1.5 T
• See inside bony
structures
• Evaluate cerebral
blood flow
st = 4 mm
FOV = 230 mm
256 x 256
TE = 15 ms
• High spatial resolution
TR = 1 s, NSA = 1
TR = 2 s, NSA = 1
TR = 4 s, NSA = 1
Vlaardingerbroek & den Boer, 1999
TR (s)
4
Multiecho
Image
Matrix
Paramagnetism
2
1.5
Most T1 Weighted
1.2
Most Proton
Density Weighted
0.9
• Metal ions with unpaired electrons
resulting in positive susceptibility
- Fe
Most T2 Weighted
0.6
TE 30
• Oxygen
60
90
- Gd
- Mg
• Paramagnetism has less than 1/1000th
effect of ferromagnetism
• Shortens T1 and T2 times
120 150 180 ms
Gd-DTPA
• Typical dose of ~0.1 mM/kg
• 550-600 Dalton
• seven unpaired electrons
– high magnetic moment
– unpaired electrons react with
protons in adjacent water molecules
shortening their relaxation time
G.D. Clarke, UT HSC San Antonio
Gadolinium
• shortens T1 relaxation of water
– high SI on T1 weighted images in lower
concentrations
• shortens T2 relaxation of water
– lowers SI on T1 weighted images in high
concentrations
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Optimizing MRI Protocols
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Gadolinium Contrast Agents
Gadolinium
• Nonuniform distribution of Gd-DTPA
increases magnetic susceptibility
differences
– Decreases MR signal on T2*-weighted
images
Meningioma
Normal
With Gd Contrast
T1-weighted Images
Commercial Contrast Agents
FLAIR
(FLuid Attenuated Inversion Recovery)
Mz
Brain
Inversion Recovery
Multiple Sclerosis
Proton Density
T2-Weighted
FLAIR
Mxy
Lesion
Lesion
0
time
CSF
Brain
TI
Teff
time
TR = 2350
TE = 30
TR=2350
TE= 80
Rydberg JN et al. Radiology 1994; 193:173-180
G.D. Clarke, UT HSC San Antonio
TR=2600
TE=145
TI= 11,000
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Optimizing MRI Protocols
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FSE Pulse Sequence
FSE Point Spread
Function Depends on T2
Effective TE
ETL = Echo Train Length
Effect of Echo Spacing
Spin Echo vs. Fast Spin Echo
TE = 30 ms
T1-weighted
(TR = 500)
T2-weighted
(TR= 2000)
Signal-to-noise decreases for short T2 tissues (gray & white
matter) leading to a decrease in spatial resolution
Vlardengerbrook & den Boer, 1999
Inversion Recovery FSE
TR =1400
TI = 280
Spin Echo
Fast Spin Echo (Echo Train Length = 4)
FLAIR Imaging
TR =2000
TI = 280
Modulus
Phase
Compensated
TR =1400
TI = 100
TE = 120 ms
T2W-FSE
TE/TR = 98/3500ms,
Slice 5/1.5mm, ET:8 (split)
256x224, 1 NEX,
20x20 cm FOV, 3:23
FLAIR-FSE
TI/TE/TR = 2200/147/10000ms,
Slice 5/1.5mm,
256x160, 1 NEX,
20x20cm FOV, 3:40
Vlaardingerbroek & den Boer, 1999
G.D. Clarke, UT HSC San Antonio
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Optimizing MRI Protocols
7/27/2005
Magnetization Transfer
Magnetization Transfer Contrast
Multislice
FSE:
Magnetization
Transfer
Contrast
Enhances T2Weighted
Appearance
Attenuation Due to Diffusion
δ
A(TE ) = A(0) exp[−γ 2G 2 Dappδ 2α 2 (∆ − )]
4
Diffusion Weighted Imaging:
Prototype Pulse Sequence
Where: α=π/2;
G is amplitude of diffusion sensitive gradient
pulse;
δ is duration of diffusion sensitive gradient;
∆ is time between diffusion sensitive gradient
pulses;
Dapp is the apparent diffusion coefficient
Hahn E., Phys Rev 1950
Diffusion Echo-Planar Imaging
(
b = γ 2G 2δ 2 ∆ − δ
3
)
Stejskal EO & Tanner JE, 1965. 42: 288-292
The b-value
• Controls amount of diffusion weighting
in image
• The greater the b-value the greater the
area under the diffusion-weighted
gradient pulses
•Signal has to be acquired in time ~T2
•Images in less than 100 ms but poor spatial resolution ( ~ 3mm x 3mm pixel)
– longer TE
– stronger and faster ramping the gradients
•Requires very good Bo homogeneity Æ big susceptibility artifacts
G.D. Clarke, UT HSC San Antonio
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Optimizing MRI Protocols
7/27/2005
Spine Imaging
Diffusion Imaging of Infarcts
SE Echo Planar
TE = 80 ms
SE Echo Planar
b = 1205 s mm-1
Gradient-Echo Imaging
•
•
•
•
•
Partial (<90o) flip angle keeps all
longitudinal magnetization from being
used up
Short TE values preserve SNR
Short TR values allow for fast scanning
(~ 1 sec/image)
Poor T2-weighted contrast
Sensitive to susceptibility
Magnetic Susceptibility Effects
SAG T1W
SAG T2W
STIR
Left:
Coronal
T2W
(one slice)
Right:
MIP
Gadolinium
• See inside bony
structures
• Surface coil almost
always used
• Evaluate nerve root
compression
Spoiled Gradient Echo
Also called spoiled GRASS, fast field echo, FLASH, etc.
Gradient-Echo Imaging
The susceptibility-induced artifacts in GRE
images:
• Increase with TE - limiting the utility of GRE
T2*-weighted images in many cases.
• Are worst for tissue/air interfaces, but
noticeable at tissue/bone interfaces.
• Are usually a detriment, but are useful in some
circumstances (e.g., blood-sensitive imaging,
BOLD contrast functional imaging, etc.).
G.D. Clarke, UT HSC San Antonio
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Optimizing MRI Protocols
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Gradient-Echo Imaging
Spatial
Saturation
Spatial presaturation
pulses (slabs) can be
applied in oblique
coronal planes and are
used to suppress
signal intensities of
tissue adjacent to
spine and reduce
breathing artifacts
Reference: Wehrli, Fast-Scan Magnetic Resonance. Principles and Applications
Phased Array Surface Coils
STIR
(Short TI Inversion Recovery)
Mz
www.toshiba-europe.com
Fat
0
time
Liver
TI
www.medical.philips.com
Spine Imaging
•
SAG T2W
Right: Saggital T1W
image of T11–L1
demonstrates minimal
wedging of the vertebral
bodies associated with
intravertebral disc
herniations (arrows)
STIR
Gadolinium
T1--weighted
T1
T1-weighted
SAG T1W
G.D. Clarke, UT HSC San Antonio
Spine Imaging
SAG T1W
MRI
SAG T2W
CT
STIR
Gadolinium
T2-weighted
T2-weighted
Surface receiver coils are used to
maximize signal from regions of
interest while minimizing noise from
volumes outside of the region of
investigation.
•
Heavily T2weighted
images show
CSF as bright,
creating
myelographic
effect
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Optimizing MRI Protocols
7/27/2005
Spine Imaging
Top: Sag T1W
Bottom: STIR
•Short
•TI
•Inversion
•Recovery
Gadolinium
SAG T1W
Left – Axial T1W image
of S1 (note nerve root)
Right – Gd-enhanced
image confirms abnormal
soft tissue is scar
Liver Imaging
AX T2W
AX EPI
STIR
SAG T2W
AX T1W
Gadolinium
Contrast
Agent
STIR
SAG T2W
STIR
SAG T1W
Spine Imaging
Chemical Shift Artifact
Ferumoxide
• Occurs in
– Readout direction
• Conventional SE
Body MR Imaging requires:
Advantages:
• Control of respiratory an
dother motion artiafcts
•
Soft tissue contrast
•
High degree of contrast
manipulation
• Identification and/or
elimination of fat signals
• Avoidance of wrap-around
(aliasing) artifact
•
Lesion characterization
•
High sensitivity to iron
– Phase encode
direction
• Echo-Planar
• Controlled by
– Fat Pre-Saturation
– STIR sequence
Chemical Shift
Chemical shift artifact
seen in axial image of
kidney
CHESS
• Chemical shift (fat-water) ~3.5 ppm
– At 1.5T:
∆f fat − water = 3.5 × 10 −6 ⋅
42.6 MHz
⋅1.5T ≅ 220 Hz
T
– At 0.5T:
∆f fat − water = 3.5 × 10 −6 ⋅
42.6 MHz
⋅ 0.5T ≅ 73 Hz
T
• ↓ field strength …. ↓ chemical shift
G.D. Clarke, UT HSC San Antonio
•
This is typically accomplished by preceding a SE or
FSE sequence with a 90o pulse that is frequency, not
spatially, selective.
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Optimizing MRI Protocols
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Liver Imaging
Liver Imaging
Chemical Shift
A
AX FSE
C
AX T1W
Ferumoxide
•
T2-Weighted
FAST SPIN
ECHO is often
used to reduce
motion artifact
& scan time
T2W
B
AX EPI
A. In phase-spoiled FFE image w/ TE=
4 ms
B. Out of phase spoiled FFE images w/
TE = 2 ms
C. T2 breath-hold FSE with fatsat pulse
http://www.users.on.net/~vision/papers/abdomen/abdominal-mri.htm
Liver Imaging
AX T2W
Echo planar is the fastest
imaging sequence and can
be used to minimize
motion artifacts
Long TE Gradient Echoes
produce T2* contrast to
identify tumors
Ferumoxide
Increasing the
number of shots
increases imaging
time but decreases
geometric
distortions
8 Shot
4 Shot
AX EPI
AX T1W
AX T1W
2 Shot
PDW
/T1W
SGE
1 Shot
Ferumoxide Super
Ferumoxide
Very fast (EPI or Gradient
Echo) T1 weighted images
allow effective
management of
respiratory motion.
Medhi P-A et al. Radiographics 2001; 21:767
Paramagnetic Iron
Oxide is the
contrast agent of
choice in liver
imaging
Contrast
agents
T2W Fast Spin Echo
AX EPI
photo
Liver Imaging
AX T2W
AX T2W
PD/T1W
•
AX T1W
Echo Planar
•
T2W EPI
Liver Imaging
Changes in MRI with
Bo Field-Strength
The Good
The Bad
The Ugly
Increased
signal
Poorer T1W
contrast due to
longer T1
Increased
absorption of
RF power
Increased
chemical shift?
Increases in
magnetic
susceptibility
Less uniform
images
T2*W Gradient Echo
G.D. Clarke, UT HSC San Antonio
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Optimizing MRI Protocols
7/27/2005
Contrast Agents
& Bo Strength
• Due to increases in tissue T1’s, Gdbased contrast agents are more
effective at 3T compared to 1.5T
• Use less contrast agent to get same
tissue contrast
What is SAR?
• The patient is in an RF magnetic field that causes spin
excitation (the B1 field)
• The RF field can induce small currents in the
electrically conductive patient which result in energy
being absorbed.
or
• The RF power absorbed by the body is called the
specific absorption rate (SAR)
• Achieve much higher tissue contrast
for the same dose
• SAR has units of watts absorbed per kg of patient
• If the SAR exceeds the thermal regulation capacity
the patient’s body temperature will rise.
Nobauer-Huhmann IM, Invest Radiol 2002; 37:114-119
Scan Parameters Effecting SAR
Parallel Imaging
• Patient size: SAR increases as the patient size
increases – directly related to patient radius
• Uses spatial information obtained from arrays of
RF coils
• Resonant frequency: SAR increases with the square
of the Larmor frequency (ωo) – therefore ↑ with Bo2
• Information is used to perform some portion of
spatial encoding usually done by gradient fields
and RF pulses
• RF pulse flip angle: SAR increases as the square of
the flip angle (α2)
• Multiplies imaging speed
– without needing faster-switching gradients
• Number of RF pulses: SAR increases with the
number of RF pulses in a given time
Generalized Projections
3 RF coils
– without additional RF power deposited
SENSE Imaging
(SENSitivity Encoding)
SENSE
X-ray CT
Parallel MRI
Parallel imaging can be thought of as being analogous to x-ray CT…
Data acquired from each PA coil element goes into
reconstruction of whole image – reduces imaging time, reduces
SNR, reduces uniformity.
http://www.mr.ethz.ch/sense/
G.D. Clarke, UT HSC San Antonio
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Optimizing MRI Protocols
7/27/2005
Image Acceleration
Conventional
breath-hold
cardiac MRI
Requires 14
heartbeats.
• MR Perfusion Imaging
• Dynamic contrast uptake
• Presaturating inversion
slabs (FAIR)
SENSE breathhold cardiac MRI
Requires 3
heartbeats.
• Functional MRI
• EchoEcho-planar imaging
• Blood oxygen level
dependent contrast
http://www.mr.ethz.ch/sense/sense_application.html
Summary
Resolution
Signal-to-Noise
Contrast
FOV & matrix size FOV & matrix size Relaxation times
Slice thickness
RF Pulse flip
angles & timing
RF Pulse flip
angles & timing
FSE inter-echo
spacing
Bo field strength
Preparation
pulses
Motion artifact
Receiver
bandwidth
RF coil sensitivity
Gradient timing
(b-value)
Magnetization
transfer
Chemical shift
artifact
What We Haven’t Covered
• MR Angiography
• InIn-flow enhancement
• Magnetization transfer
contrast
G.D. Clarke, UT HSC San Antonio
http://www.mr.ethz.ch/3t/patient_5.html
Suggested Reading
(In order of increasing complexity)
• MRI: From Picture to Proton McRobbie DW, Moore
EA, Graves MJ & Prince MR. Cambridge Univ.
Press, 2003; ISBN: 0521523192
• Magnetic Resonance Imaging 3rd ed.
Vlaardingerbroek MT, den Boer JA, Luiten A.
Springer 2002; ISBN: 3540436812
• Handbook of MRI Pulse Sequences Bernstein MA,
King KF, Zhou XJ. Elsevier, 2004; ISBN:
0120928612
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