Transcript An Introduction to MRI

Topics
•
•
•
•
spatial saturation
TOF imaging
chemical saturation
magnetization transfer
Review: Relaxation
….
t=t0
t=t1
ML=0
900 RF

t=t2
ML=a
t=t3
ML=b
t=
ML=1
ML
t0
t1 t2 t3
t
Relaxation
t=t3
ML=b
t=t0
900 RF

t=t3+
ML=0
t=t4+
ML=0
t=t4
ML<b
900 RF
TR
t=t5
ML<<b
900 RF
TR
Equilibrium
RF in
• after 5 or so
repetitions, the
system reaches
equilibrium
• similar to water
flowing into a
leaky bucket
equilibrium
relaxation
T1 Relaxation
1
longer TR,
more recovery
of ML
0.9
0.8
0.7
0.6
long T1
M L 0.5
short T1
shorter TR,
less recovery
of ML
0.4
0.3
0.2
0.1
0
0
1000
2000
3000
m sec
4000
5000
TR and ML
• prolonged TRs allow for more
recovery of ML
• shorter TRs allow for less recovery
of ML
– condition referred to as “partial
saturation”
Saturation
• “total” magnetization
– application of additional RF pulses
has no effect on proton orientation
• saturation exists only briefly
– net magnetization recovers
longitudinal relaxation immediately
after protons are “saturated”
Types of Saturation
•
•
•
•
spatial
fat
water
magnetization transfer
(1st cousin)
Spatial Saturation
• application of an RF pulse
immediately prior to the imaging
sequence saturates all of the
protons under the influence of that
pulse
Spatial Saturation
purpose/advantages
• reduce motion artifacts in the phase
encoding direction
– swallowing
– CSF pulsation
– respiratory motion
• reduce signal from flowing blood
• facilitate angiography/venography
Spatial Saturation
disadvantages
• fewer slices per TR
– timing of saturation pulse prolongs
effective TR interval
• higher SAR
Spin Echo
gradient
frequency encode
 RF pulse
readout
 RF pulse
signal
FID
spin
echo
Saturation
 RF pulse
 RF pulse
signal
saturation pulse
additional time required
for single saturation pulse
no echo
Saturation Pulse
z
z
z
0
0 sat
pulse
y
x
t=t0
0 RF
y
x
t=t0+
ML=0
SATURATION
y
x
t=t0++
MXY=0
no signal
Saturation Pulse and Longitudinal Magnetization
0.9
0.8
0.7
ML
0.6
0.5
No Sat Pulse
0.4
Sat Pulse
0.3
0.2
0.1
0
time (msec)
SAT pulses
900 RF pulses
Spatial Saturation
saturation band
within
the FOV
superior saturation pulse
(arterial)
a
r
t
e
r
i
a
l
v
e
n
o
u
s
Spatial
Saturation
outside the FOV
stack of
slices
2D
acquisition
inferior saturation pulse
(venous)
fully magnetized
protons in arteries
arterial
flow
end slices may
have bright flow
in arteries or
veins
middle slices
usually have
“flow voids” in
vessels
partially saturated
protons in vessels
fully magnetized
protons in veins
venous
flow
Entry Slice
Phenomenon
s1 s2 s3
flow direction
blood
moves
downstream
s1 s2 s3
vessel
saturated
spins
unsaturated
spins
s1
900RF
MR Flow Void
s1
T=TE
bright flow,
entry slice
phenom
s2
900RF on
saturated
spins,
flow void
superior saturation pulse
(arterial)
a
r
t
e
r
i
a
l
v
e
n
o
u
s
stack of
slices
2D
acquisition
inferior saturation pulse
(venous)
Summary: Flow Effects
• entry slice phenomenon due to
unsaturated spins
• flow void due to saturation of
previous slice coupled with
downstream migration of spins
• spatial presaturation bands can
reduce (eliminate) signal from
flowing blood
Magnetic Resonance
Angiography
• exploits flow enhancement
of GR sequences
• saturation of venous flow
allows arterial visualization
• saturation of arterial flow
allows venous visualization
• no IV contrast is required
Magnetic Resonance
Angiography
AP projection
Lateral projection
right thigh
tumor
2D TOF Angiography
• anatomy imaged using a series of
gradient echo images
– each image is acquired separately
– all slices experience entry slice
phenomenon
• saturation pulse placed proximal for
venous imaging, distal for arterial
imaging
s1
s1
flow direction
vessel
presat
band
unsaturated
spins
s1
0RF
2D TOF
s1
T=TE
bright flow,
entry slice
phenom
2D TOF Angiography
• saturation band is located the same
distance from each slice to
maximize its effect
– “walking presat”
• vascular images reconstructed
using maximum intensity projection
technique
MIP
Reconstruction
.
.
.
lateral projection
AP projection
SPIRAL CT
ANGIOGRAPHY
2D TOF
• GR images used
– short TR (~ 20-40 msec)
– very short TE
• shortest TE times minimize intravoxel
dephasing resulting in maximum flow
effects
– small to medium flip angles
2D TOF Carotid Study
MIP
Chemical Saturation
• similar to spatial saturation
• narrow band RF pulse causes
selective saturation of water or fat
protons
– “chem sat”
– “fat sat”
• compatible with many imaging
sequences
Fat Sat
fat
water
frequency
220 Hz
1.5 T
fat
selective
bandwidth
Fat Saturation
 RF pulse
 RF pulse
signal
fat sat pulse
additional time required
for saturation pulse
echo from water only
FAT SAT
0.9
0.8
0.7
0.6
0.5
Water
0.4
Fat
ML
0.3
0.2
0.1
0
time (msec)
Fat Sat
examples
Fat Sat
advantages
• increase conspicuity of fluid on T2
weighted images
– widens dynamic range
• addresses FSE fat-fluid isointensity
problem
• post-gadolinium T1 weighted fat sat
• reduced respiratory motion artifact
Fat Sat
disadvantages
• fewer slices per TR
– timing of saturation pulse prolongs
effective TR interval
• higher SAR
• requires homogenous magnet
– shimming
Fat Sat
disadvantages
• requires uniformly shaped body part
– doesn’t work well at base of neck, crook
of ankle, etc.
• not recommended with FOV > 30 cms
– unreliable
• works poorly at lower fields
• S/N ratio drops
Fat Suppression and SNR
• non fat-suppressed image
– each image pixel comprised of signal
from water and fat in the imaging
voxel
• fat-suppression
– reduces total signal by suppression of
fat from the voxel
– reduces SNR
Fat Suppression
• without fat
suppresion
• high SNR
SI
water and fat
frequency
• with fat
suppression
• lower SNR
SI
water only
frequency
Magnetization Transfer
with MT
TR 550, TE 15.7, 45°
without MT
TR 450, TE 15.7, 45°
Magnetization Transfer
• first cousin of Fat Sat
• off-resonance RF pulse applied
similar to Fat Sat pulse
• “bound water” proton pool absorbs
the RF energy
– energy is transferred to “unbound”
proton pool
Magnetization Transfer
• think of as “tissue SAT”
• tissues high in proteins (brain, muscle)
become darker
– MT pulse causes a selective saturation effect
• tissues low in proteins relatively unaffected
– fat
– free fluid/water/edema
Magnetization Transfer
saturation
effect
energy
transfer
free
bound
frequency
MT pulse
~1000 kHz
off-resonance
Magnetization Transfer
 RF pulse
 RF pulse
signal
MT pulse
additional time required
for saturation pulse
echo
Magnetization Transfer
advantages
• generates T2-like weighting with
GR images
– good cartilage sequence
• suppresses background tissues
– improved TOF angiography
– increased contrast (gadolinium)
visualization
Magnetization Transfer
advantages
• magnetic field homogeneity not
critical
• generates images with new
contrast relationships
• compatible with many sequences;
also compatible with fat sat
Magnetization Transfer
disadvantages
• fewer slices per TR
– timing of saturation pulse prolongs
effective TR interval
• higher SAR
Magnetization Transfer
with MT
TR 550, TE 15.7, 45°
without MT
TR 450, TE 15.7, 45°