Advanced pulse sequences Signal Formula for SE M xy negligible TR gtgt T2 or spoiler gradient 90 180 M zA short pulse no T 1 relaxation between A and B or C and D ID: 384026
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Slide1
G16.4427 Practical MRI 1
Advanced pulse sequencesSlide2
Signal Formula for SE
M
xy
negligible
(TR >> T2, or
spoiler gradient)
= 90°
= 180°
M
zA
short pulse (no T
1
relaxation
between A and B, or C and D)
Bernstein et al
. (2004) Handbook
of MRI
Pulse SequencesSlide3
Multi-Echo SE
The transverse magnetization can be repeatedly refocused into subsequent
SEs
by playing additional RF refocusing pulse
The series of echoes is called an echo train
Each echo number fits its own independent k-spaceThe length of the echo train is limited by T2
decayIn most cases we are interested in 2 echoes (an early and a late one).
Question: if TR is long, what contrast will have the 2 resulting images?Slide4
Multi-Echo SE
The transverse magnetization can be repeatedly refocused into subsequent
SEs
by playing additional RF refocusing pulse
The series of echoes is called an echo train
Each echo number fits its own independent k-spaceThe length of the echo train is limited by T2
decayIn most cases we are interested in 2 echoes (an early and a late one).
if TR is long, the two images will be PD- and T2-weighted, respectivelySlide5
Example of Dual-Echo SE Acquisition
Proton density-weighted
TE/TR = 17/2200 ms
T
2
-weighted
TE/TR = 80/2200 msSlide6
Dual-Echo SE
Bernstein et al
. (2004) Handbook
of MRI
Pulse SequencesSlide7
T2-Mapping
It is a common application of acquiring longer echo trains (otherwise more than two echoes per TR are rarely acquired in MRI)
In theory we can acquire long echo train of
SEs
and fit the signal intensity at each pixel to calculate T
2In practice there are systematic errors that make it difficult to fit a monoexponential decay curve
Variable flip angle across slice profileStimulated echoes can introduce unwanted T
1-weighting variations into the echo-train signalsIf magnitude reconstruction is used, the noise floor has nonzero mean leading to incorrectly larger T2 valuesSlide8
Paper DiscussionSlide9
Advanced pulse sequencesSlide10
Echo Planar Imaging (EPI)
EPI is one of the fastest MRI pulse sequences
2D image in few tens of milliseconds
Allowed developing challenging MR applications
Diffusion, perfusion, cardiac imaging, etc.
EPI uses a gradient-echo trainTypical to produce ~100 gradient echoes to produce a low-resolution image from a single RF excitationMore prone to a variety of artifactsGhosting along phase-encoded directionSlide11
Ghosting Artifacts
Not caused by motion, but by eddy currents, imperfect gradients, field non-uniformities, or a mismatch between the timing of the even and odd echoes
Which results in
mis
-registration of alternating lines of k-space
Phase errors may result
from the multiple positive
a
nd negative passes through k-spaceGhost artifacts in the
phase directionSlide12
GRE vs. EPI
In a simple GRE pulse sequence the transverse magnetization decays as
M
xy
(t
) = Mxy(0)e-t/T2*Half lifetime of
Mxy is T
2*ln2A very small fraction of the lifetime is actually used for data acquisition in GREEPI maximally uses the transverse magnetization without additional RF excitationsBipolar oscillating readout gradient produces a series of echoes, each individually phase-encodedSlide13
GRE and EPI
N
etl
= echo train length = number of echoes following RF excitation
t
esp
= echo spacing (typically echoes are evenly spaced)
A series of echoes
is produced before
M
xy decays away due
to T2
* relaxation
Bernstein et al. (2004) Handbook
of MRI Pulse SequencesSlide14
EPI Readout Gradient
Starts with a
prephasing
gradient that position the
k
-space trajectory to k
x,min
followed by a series of readout gradient lobes with alternating polarity. Question: what is the area of the prephasing gradient lobe Gx,p?
Bernstein et al
. (2004) Handbook
of MRI
Pulse SequencesSlide15
EPI Readout Gradient
Starts with a
prephasing
gradient that position the
k
-space trajectory to k
x,min
followed by a series of readout gradient lobes with alternating polarity. Answer: Half the area of the first readout gradient area.
The second half of
each lobe serves as
prephasing
for the
following gradient
(that is why the polaritymust alternate)
Bernstein et al
. (2004) Handbook of MRI Pulse SequencesSlide16
EPI Phase-Encoding Gradient
Constant phase-encoding gradient throughout the entire readout echo train (
k
y
varies linearly with time)
Gridding is needed before reconstruction
A series of blips with the same polarity and identical area, each played before the acquisition of an echo
Bernstein et al
. (2004) Handbook
of MRI
Pulse SequencesSlide17
Gradient-Echo EPI
Bernstein et al
. (2004) Handbook
of MRI
Pulse SequencesSlide18
Spin-Echo EPI
What is different compared to GRE-EPI?Slide19
Phase Contrast (PC) Imaging
A method to image moving magnetization by applying flow-encoding gradients
Image flow in blood vessels and CSF, track motion
The flow-encoding gradients translates velocity into the phase of the image
Bipolar gradient, as it produces linear proportionality
The axis of the gradient determines the direction of flow sensitivityNormally applied to only one axis at a timeTypically performed with GRE pulse
sequences, adding phase-encoding
gradients. Why?Slide20
Typical PC Pulse Sequence
Typically a bipolar gradient is added to only one of the three logical axes at a time
Toggling of the bipolar gradient (dotted lines) varies the gradient first moment and introduces flow sensitivity along that axis
Bernstein et al
. (2004) Handbook
of MRI
Pulse SequencesSlide21
PC Acquisition and Reconstruction
Two complete sets of image are acquired varying only the 1
st
moment of the bipolar gradient
The amount of such operator-selected variation determines the amount of velocity encoding
The phases of the two images are subtracted on a pixel-by-pixel basis in image domainAllows to quantify flow direction, flow velocity and volume flow ratePhase-difference or complex-difference reconstruction methods are in common useSlide22
Diffusion Imaging
In the presence of a magnetic field gradient, diffusion of water molecules causes a phase dispersion of the transverse magnetization
The degree of signal loss depends on tissue type, structure, physical and physiological state
Diffusion imaging is a family of techniques
E.g. DWI, DTI, DKI
All diffusion pulse sequences contain diffusion-weighting gradientsDWI typically employs a single b
-value, other quantitative mapping methods at least 2 b-valuesSlide23
Diffusion-Weighting Gradients
Typically consist of two lobes with equal area
Amplitude is the maximum allowed
Pulse width is larger than most imaging gradients
When used, water diffusion can cause an attenuation in proton MRI signals
Degree of attenuation depends on the product between the diffusion coefficient D and the b-valueb
-value is analogous to TE in T2-weighted sequencesIncreasing gradient amplitude, separation of its lobes, or pulse width of each lobe results in a higher
b-value. How does diffusion-weighting change consequently?Slide24
Diffusion Weighting in GRE and SE
Spin Echo
Gradient Echo
Bernstein et al
. (2004) Handbook
of MRI
Pulse SequencesSlide25
Single-Shot Spin-Echo EPI
The most prevalent sequence due to high acquisition speed (e.g. < 100 ms per image)
A pair of identical gradient lobes on either side of the refocusing pulse
Gradient direction can be controlled by varying its vector components along the 3 axes
To minimize TE the max amplitude is used to achieve the desired
b-valueWhat is another way to minimize TE?Slide26
Single-Shot Spin-Echo EPI
The most prevalent sequence due to high acquisition speed (e.g. < 100 ms per image)
A pair of identical gradient lobes on either side of the refocusing pulse
Gradient direction can be controlled by varying its vector components along the 3 axes
To minimize TE the max amplitude is used to achieve the desired
b-valueMaximizing SR also reduces TE, but can cause peripheral nerve stimulationSlide27
Diffusion-Weighted Single-Shot SE
Bernstein et al
. (2004) Handbook
of MRI
Pulse SequencesSlide28
Diffusion-Weighted (DW) Imaging
In the presence of a gradient, molecular diffusion attenuates the MRI signal exponentially:
Tissue with fast diffusion experiences more signal loss
low intensity in the DW image
To remove the patient-orientation dependence, 3 DW images can be obtained with a DW gradient applied along the three orthogonal directionsIf same
b-value then isotropic DW image
(S and S
0
are the voxel signal intensity with and without diffusion)Slide29
Example: White Matter InfarctSlide30
Quantitative Apparent Diffusion Coefficient (ADC) Mapping
A series of DW images are acquired with multiple
b
-values:
A linear fit between ln(S
0/Si) and
bi is performed on a pixel-by-pixel basis to find
DThe contrast of the ADC map is inverted compared to a DW imageTo keep a constant TE in all DW images, b-values are typically changed by varying the diffusion-gradient amplitude instead of its durationContribution from imaging gradients should be included in b-value calculation to avoid overestimating ADC
…Slide31
DW Image vs. ADC MapSlide32
Fat Suppression
Because of its short T
1
, the bright appearance of fat is a problem for T
1
-weighted images with short TR and short TEFat is a main contributor to chemical shift artifactsThere are several methods for fat suppressionSpectrally selective RF pulsesWhat are the drawbacks?Slide33
Fat Suppression
Because of its short T
1
, the bright appearance of fat is a problem for T
1
-weighted images with short TR and short TEFat is a main contributor to chemical shift artifactsThere are several methods for fat suppressionSpectrally selective RF pulsesB
1 and B0 inhomogeneities, not good at low field strengths
Short TI recovery (STIR)B1 inhomogeneities, long scan, signal from other tissuesSlide34
Two-Point Dixon Pulse Sequence
In conventional spin echo
Δ
= 0
Question:
what happens if
Δ
≠ 0
Bernstein et al
. (2004) Handbook
of MRI Pulse SequencesSlide35
Two-Point Dixon Pulse Sequence
In conventional spin echo
Δ
= 0
If
Δ
≠ 0, spins with different chemical shifts will be out of phase at
k
x = 0 (unless phase shift happens to be a multiple of 2π)
If the 180° pulse is delayed or advanced by Δ/2, the RF spin echo is delayed or advanced by
Δ
relative to where k
x = 0
Consider a voxel with a water and a fat spin having a frequency difference f
cs and assume there are no B0
inhomogeneitiesIn the 2-point Dixon technique, we acquire two SE images:
Δ = 0 and normal acquisitionFat and water in-phase at k
x = 0 (in-phase image)
Δ = 1/(2fcs) and RF pulse advanced or delayed by Δ/2 = 1/(4f
cs)
Fat and water 180° out-of-phase at k
x = 0 (out-of-phase image)
ϕ = 2π
fcsΔ at
kx
= 0 Slide36
Two-Point Dixon Technique
Because image contrast is heavily determined by the peak signal amplitude that occurs
at
k
x
= 0, the resulting complex images are approximately given by:
I
0 = W + F I
1 = W – F
Separate images of the water and fat magnetization can be reconstructed from:
W
= (1/2) (
I
0
+
I
1
) F = (1/2) (I0
- I1)
The water image W
can be used as a fat suppressed image, whereas W and F separately provide information about the relative water and fat content of tissuesSlide37
Limits of Two-Point Dixon
The 2-point Dixon technique assumes perfect
B
0
homogeneity which is almost never true
Due to ΔB0 fat and water have accumulated an additional phase shift Δ
ϕ = γ(ΔB
0)Δ at kx = 0 in I1Although fat and water spins in any given voxel are still anti-parallel in the opposed-phase image, they might no longer be parallel or anti-parallel to the fat and water spins in the in-phase image
(example of 90° phase shift caused by
B
0
inhomogeneities)
Question:
what is wrong with the combined images?Slide38
Example: Healthy Liver
AJR April 2010, vol. 194(4),
p
. 964-971Slide39
Example: Fatty Liver
AJR April 2010, vol. 194(4),
p
. 964-971Slide40
Any questions?Slide41
See you next Thursday!