Download Steady State Free Precession Imaging

M R I Physics Course
Some Body Techniques/Protocols
Nathan Yanasak, Ph.D.
Jerry Allison, Ph.D.
Tom Lavin, M.S.
Department of Radiology
Medical College of Georgia
References:
1) The Physics of Clinical MR, Focusing on the Abdomen,
Taught Through Images
AUTHORS: VAL M. RUNGE1 MD, WOLFGANG R. NITZ2 PHD, STUART H.
SCHMEETS2 BS, RT, WILLIAM H. FAULKNER, JR.3 BS, RT, NILESH K. DESAI1
MD
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Some Techniques Relevant to Body
Imaging
The trick of Body Imaging usually is “reduce motion”.
Specific Absorption Rate (SAR) can also be an issue in the large
volume of abdominal tissue.
Two different ways to go fast:
• HASTE – rapid abdominal imaging (spin echo)
• TrueFISP – rapid abdominal imaging (gradient echo)
•
•
Motion Correction – patient restlessness/chest movement
Spectral Fat Suppresion – differentiation, delineation of tissues.
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HASTE
(Half Acquisition Single-Shot Fast Spin Echo)
Advancements in gradient efficiency and RF
systems  assist sequences that decrease scan
time and minimize the impact of patient motion.
Example: HASTE (Half Acquisition Single-shot
Turbo spin Echo). This approach combines halfFourier and fast spin echo imaging.
GE name: SSFSE – single shot fast spin echo
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HASTE
(Half Acquisition Single-Shot Fast Spin Echo)
Component #1: With HASTE, each slice is acquired (and
often reconstructed) before acquiring the next slice.
How? The echo train will consist of the required number
of phase encoding steps for one slice. So, somewhat like
a “spin-echo” EPI sequence.
This differs from normal turbo (fast) spin echo in which
phase encoding lines from multiple slices are acquired
throughout the examination.
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HASTE
(Half Acquisition Single-Shot Fast Spin Echo)
Component #2: The HASTE method employs the half Fourier
technique:
the symmetry of k-space is used to synthesize approximately
50% of the data for each slice.
Long echo trains for spin echo = many 180o pulses  ( SAR).
Filling ½ of k-space reduces SAR issue.
Reduced SNR, but same spatial resolution.
Images with HASTE are usually acquired in 2 seconds or less per
slice
very good for reducing patient motion artifacts.
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HASTE
(Half Acquisition Single-Shot Fast Spin Echo)
Sampling bandwidth issues:
Low  longer times between echoes
T2 image blurring (A,C).
High  smaller echo spacing
 lower SNR (B,D).
However, the SNR reduction is typically less
of an effect than the image blurring (i.e.
choose high bandwidth).
HASTE finds clinical applicability in
particular for the upper abdomen
(sometimes, for rapid brain imaging), with
excellent depiction of tissue morphology.
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Steady State Free Precession Imaging
(TrueFISP)
Fully coherent steady-state free precession imaging (SSFP) is a
fast, gradient echo based imaging method.
Unlike many GRE sequences, SSFP sequences rephase, instead of
spoiling, the transverse magnetization after multiple, rapid
excitations.
Examples: true fast imaging with steady state precession (trueFISP)
balanced fast field encoding (balanced-FFE)
fast imaging employing steady-state acquisition
(FIESTA))
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Steady State Free Precession Imaging
(TrueFISP)
In fast imaging, some residual amount of transverse
magnetization remains before the next repetition of rf
pulses is applied (i.e., before the next TR). What should
be done?
If nothing is done, residual
magnetization (with residual
coherence) from one TR period can
cause spurious echoes occurring at a
later times, unpredictably.
With grad echo, these can both be
observed during later readouts.
(from “Carl’s Roost” of artifacts:
http://chickscope.beckman.uiuc.edu/roosts/carl/artifacts.html )
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Steady State Free Precession Imaging
(TrueFISP)
What can we do?
Spoiled-gradient echo – destroy the transverse
magnetization before the next repetition. (e.g., SPGR–
spoiled gradient recall). A.k.a., “Steady-state
incoherent”.
Recalled-gradient echo – “rewind” (or rephase) the
magnetization before the next repetition. trueFISP
belongs to this group. A.k.a., “Steady-state coherent”.
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Both achieve a long-term steady-state magnetization state.
Steady State Free Precession Imaging
(TrueFISP)
Ordinary 2D gradient echo sequence (note that the gradient pulse
shapes are simplified for this discussion):
Echo location
rf
Slice select
Phase-encode
Readout grad
ADC Readout
1. Spins are excited in plane.
2. Phase-encode gradient
increments phase of spins
as a function of space.
3. Readout gradient encodes
spins with different
frequency, and data is read
out.
4. Spoiling (not shown) with
large gradient before next
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excitation.
Steady State Free Precession Imaging
(TrueFISP)
Example of a simple Steadystate Coherent sequence :
1. Spins are excited in plane.
2. Phase-encode gradient
increments phase of spins
as a function of space.
3. Readout gradient applied
during readout of data.
4. Equal but opposite phaseencode gradient rewinds
the spins.
Although longitudinal mag.
increases slightly during a
TR, this sequence attempts
to wind the phases back to12
what they were at the start.
Steady State Free Precession Imaging
(TrueFISP)
Motion will affect this technique. The echo times are kept short
(via larger bandwidths, etc), decreasing sensitivity to artifacts
from moving spins such as blood and CSF.
SSFP is excellent for abdominal and cardiac applications.
The SSFP technique has increased sensitivity to off-resonance
effects  ruin the ability to rewind the phases.
Therefore, shimming, similar to that done prior to spectral fatsaturation, is carried out before the measurement to improve
overall B0 field homogeneity.
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Steady State Free Precession Imaging
(TrueFISP)
TrueFISP provides the highest signal intensity of
all steady state sequences.
Tissue contrast for longer TR values (~ .5 – 2 sec)
is a complicated function of the T1/T2 ratio.
When acquired with short TR and short TE,
trueFISP images are primarily T2 weighted, with
very high signal intensity for all types of fluid
(including CSF, flowing blood).
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Steady State Free Precession Imaging
(TrueFISP)
Coronal trueFISP imaging of
the upper abdomen (A)
without and (B) with fat
suppression. Bowel, liver, and,
in particular, the vascular
system are well defined.
Scan time: ~ 1 second per image.
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Steady State Free Precession Imaging
(TrueFISP)
Example: (A) Axial trueFISP with fat suppression at the kidneys,
with high signal intensity from CSF, fluid within the small bowel,
the vascular structures, and urine within the collecting system. (B)
Sagittal trueFISP. The gall bladder and hepatic vasculature are
depicted with high signal intensity.
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HASTE vs TrueFISP
TrueFISP is more technically complicated
• Poor scanner performance can ruin quality (B homogeneity,
gradients)
• Motion is a problem
TrueFISP has higher SNR, but more complicated T1/T2 contrast.
Somewhat like T1, but not exactly.
HASTE is typically T2-weighted, due to readout of multiple echoes.
HASTE will always be SAR-limited.
MCG: Contrast-enhanced image: TrueFISP, for T1-like image
T2W image: HASTE
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Abdomen – Motion Correction
Magnetic resonance provides an excellent tool
for imaging of the internal structures of the thorax
and abdomen. But…physiologic motion (e.g.,
respiration and cardiac pulsation, squirming) can
lead to artifacts reducing the diagnostic quality of
the final image.
Most MR scanners offer hardware, software, and
sequence-based options designed to minimize or
eliminate the effects of physiologic motion during
scan acquisition.
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Abdomen – Motion Correction
Motion artifacts: caused by translations or shifts in the
position of the imaged structure during the acquisition of
data.
Routine spin echo and gradient echo sequences encode and
acquire data to fill k-space in a line-by-line fashion:
each line collected at a different time (TR).
In fast spin echo imaging, several lines are collected with
each TR.
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Abdomen – Motion Correction
In-plane motion (2D or 3D):
Changes in the location of anatomic structures between TR periods
can lead to misalignment in spatial encoding.
Fourier transform of the lines of k space data acquired during motion
results in blurring or ghosting in the final image.
Often, “duplicates” occurs along the phase-encoded direction.
Phase Encoding
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Abdomen – Motion Correction
Through-plane motion (for 2D imaging):
Motion along the slice-select direction result in the anatomy
changing in a slice for each TR.
For T1W imaging, steady state transverse magnetization relies on
repeated excitation of the SAME slice at periodic intervals (i.e.,
TR).
Through-plane motion disrupts this steady state, resulting in
unexpected changes in signal intensity between TRs. Image is
generally blurred.
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Abdomen – Motion Correction
How to reduce motion (non-abdomen, e.g., brain):
1) Tell patient to lie still.
2) Use restraints of some sort.
Usually not much of a problem for standard
patients. Children, patients with pain, seizures,
Parkinson’s, etc… can create problems even for
imaging other than the abdomen.
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Abdomen – Motion Correction
Respiratory method #1: breath holds (simplest). Rapidly
acquired T1- and T2-weighted gradient echo sequences are
used to collect data during suspended respiration, thereby
minimizing motion artifacts.
Problem: scan time < 25 seconds for the average patient,
limiting spatial resolution and number of slices.
The use of this approach is also restricted by the patients’
health and mental status, as well as their age.
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Abdomen – Motion Correction
Method #2: Single-slice techniques such as HASTE,
trueFISP, and echo planar imaging (EPI) acquire the data
for one entire slice before beginning the next slice.
Parallel imaging techniques may also help.
In most cases, the acquisition is rapid enough to freeze
respiratory motion, greatly minimizing artifacts within
each slice.
These don’t account for through-plane motion.
The addition of techniques such as gating or breath-holding
is necessary to assure consistent and complete anatomic
coverage.
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Abdomen – Motion Correction
Method #3: Special sequences allow for
reconstruction of images with movement (e.g.,
PROPELLER).
Acquire strips in k-space, in
short periods of time.
Each strip will sample
very little motion, and
strips can be aligned with
each other in k-space.
(from Pipe, J., MRM 1999, 42: 963-969)
Each strip stores “complete”
image info along a
particular direction in
the
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image.
Propeller Example
(from Pipe, J.,
MRM 1999,
42: 963-969)
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Abdomen – Motion Correction
Method #4: Respiratory gating incorporates the
use of a bellows device placed around the
patient’s chest to track respiratory motion.
Data is acquired during a specific portion of the
respiratory cycle defined in the sequence
parameters during set up.
This method can reduce respiratory artifacts
(most of the time); however, scan times are
lengthened (significantly at times: ~3x
increase), sometimes to the point of futility.
Not frequently used for humans—o.k. for mice.
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Abdomen – Motion Correction
Method #5: Navigator echoes do not require either
additional hardware or patient cooperation
Act as a measure of the respiratory cycle, using MRI
info as a “bellows”.
A simple, 1D navigator: additional RF pulses in the
sequence track superior to inferior translational motion
of abdominal structures based on the diaphragm position.
Information from the navigator can be used to trigger data
(“gate”) acquisition during a specified portion of the
respiratory cycle.
Higher order navigator echoes also exist to adjust for 2D
translation in cardiac imaging and 3D translation in the
brain during functional MRI studies.
studies
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Abdomen – Motion Correction
Advantages: Incorporating navigators to reduce the time
required for breath-holding will break up the large,
multi-slice measurements into smaller groups of slices,
reducing the breath-hold duration of each group by a
corresponding amount.
Differences in the diaphragmatic position between
measurements due to varying levels of inspiratory
volume are corrected by gradient system adjustments,
reducing the chance for overlapping of slices or large
gaps in slice coverage.
Some vendors call this approach PACE (prospective
acquisition correction).
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Abdomen – Motion Correction
For the navigator echo, a
"rod of tissue" is
excited, placed through
the dome of the liver
with a 1D craniocaudal
extension (A). The 1D
information is read out
in parallel to the
imaging sequence with
the motion of the liverlung interface serving as
an indicator for
One defines whether data acquisition should be activated
breathing (B).
close to the inspiratory or expiratory portion of the
respiratory cycle. A tolerance in mm of liver excursion is
usually taken as indication whether the data acquisition is
to be switched on or off.
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Fat Suppression
(Spectral Saturation)
The suppression of signal produced
by adipose tissue in MR imaging can
be helpful in the delineation and
differentiation of certain tissues and
pathologies.
One method: spectral fat saturation
(or “fat sat”).
Axial T1-weighted images of the upper
abdomen, (A) without and (B) with
spectral fat sat.
Note that the pancreas (arrow) is well
delineated on the image with fat sat,
leading to widespread use of this
technique for pancreatic imaging.
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Fat Suppression
(Spectral Saturation)
What is Spectral Fat Saturation?
Water and fat protons resonate at slightly different frequencies in a
magnetic field. Frequency separation increases with field
strength:
At 1.5 T, ∆ω~220 Hz.
A special 90o RF pulse is applied prior to ordinary excitation, at
the specific resonance frequency of fat fat is excited.
Then, a spoiler gradient is applied to dephase this signal
 fat tissue remains saturated yet dephased during the spin
excitation of water
 fat does not contribute to the resulting echo and image
formation.
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Fat Suppression
(Spectral Saturation)
Differences in magnetic susceptibility (e.g., metal objects
in tissue, or variations in tissue shape in neck/chest) can
lead to a change in the specific resonant frequency of fat
in localized areas
(∆ω would be different, and ωfat pulse would be wrong).
So, shimming can be critical.
Greater uniformity of the magnetic field gives less change
for incomplete or inconsistent fat saturation.
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Fat Suppression
(Spectral Saturation)
Consequently, users should make an extra effort to assure
that all metal objects (e.g., buttons and jewelry), are
removed prior to the exam to improve the final spectral
fat saturation.
In general, we only allow rings to be left on, for any
protocol. For abdomen, we usually specify that patients
are in a gown. Body piercings can be a problem.
Other than for safety (ferrous material), metal can distort an
image depending on how massive it is, how far it is from
the isocenter, and motion of the object.
Moral of the story: less metal is better.
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Spectral vs. Spatial Saturation
Spatial Saturation – can also saturate materials in a
particular region by applying a 90o RF pulse prior to the
spin preparation excitation. When applied with the
proper gradients
switched on, spins in a
localized area can be
saturated prior to
excitation of desired
region. This is useful
for eliminating flow
effects in the image.
(from IUPUI School of Medicine Rad. Lectures)
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Saturation vs. STIR
Let’s review our options re: fat suppresion
1) Spatial saturation – saturation of everything in a region
(like an extra slice-select before the initial excitation).
 Good saturation in a region, but does not discriminate
between tissue.
2) “Short Tau Inversion Recovery” – T1-based approach.
 less sensitive to magnetic inhomogeneity, but some loss
of SNR.
3) Spectral saturation – chemical shift-based approach
 better SNR, but more dependent on shimming. So, the
amount of saturation may vary spatially if shimming isn’t
good.
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Fat Suppression
(Spectral Saturation)
Spectral fat saturation is often also
employed in the lumbar spine
to facilitate the detection of
lesions within the marrow on
T2-weighted scans.
Sagittal T2-weighted images of the
lumbar spine (A) without and
(B) with spectral fat saturation.
Note the improved visualization of
disk hydration (and the loss of
hydration at L5-S1, arrow) in
the image with fat saturation.
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Fat Suppression
(In-Phase, Opposed-Phase)
Two main populations of magnetization: adipose (fat) and
water-containing tissue.
The adipose resonance frequency is about 3.5 ppm lower
than that of water (~ -220 Hz on a 1.5 T system, as
previously noted). After an initial excitation, the
transverse magnetization within adipose tissue will fall
behind the transverse magnetization within water.
Useful technique: the time when the MR signal is observed
can be chosen so that the transverse magnetizations from
fat and water are either opposed-phase or in-phase. 38
Fat Suppression
(In-Phase, Opposed-Phase)
The duration of the acquisition window in a pulse
sequence is inversely proportional to the bandwidth of
the sequence (remember this from the SNR lecture?).
If the bandwidth is large enough ( the acquisition
window is very short), it is possible to acquire opposedphase and in-phase images simultaneously with a double
echo gradient echo sequence (Figure 2). This approach is
typically employed with TE, TR, and tip angle chosen to
provide T1-weighting.
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Fat Suppression
(In-Phase, Opposed-Phase)
Thus voxels containing fat are high signal intensity and
those containing water low signal intensity.
However, in voxels in which there is both fat and water,
there is a cancellation (loss) of signal on opposed-phase
images, due to the transverse magnetization from fat and
water being of opposite phase and contained in the same
voxel. This leads to signal loss at the interface between
fat and water containing structures, for example at the
margin of the liver (or spleen) and adjacent intraabdominal fat.
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Figure 2 (from Ref #1)
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Fat Suppression
(In-Phase, Opposed-Phase)
Example: Nonhyperfunctioning adrenal adenoma,
on (A) in-phase and (B) opposed-phase images
(reprinted with permission from Clinical
Magnetic Resonance Imaging, VM Runge
(editor), W.B. Saunders Company, Philadelphia,
2002).
A round, sharply demarcated, homogenous lesion
of the left adrenal gland is noted (arrow).
In-phase: the lesion is nearly isointense with
normal liver parenchyma.
Opposed-phase: the lesion is markedly
hypointense.
80% of adrenal adenomas contain sufficient lipid to
show a marked signal intensity reduction on
opposed-phase imaging, which is not seen for
the other major lesions considered in differential
diagnosis – metastasis, pheochromocytoma, and
adrenal carcinoma.
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