Transcript Statistical Parametric Mapping

Statistical Parametric Mapping
Lecture 5 - Chapter 6
Selection of the optimal pulse
sequence for fMRI
Textbook: Functional MRI an introduction to methods, Peter Jezzard, Paul
Matthews, and Stephen Smith
Many thanks to those that share their MRI slides online
Advantages
BOLD
Disadvantages
Highest activation contrast 2x-4x over
perfusion
complicated non-quantitative signal
easiest to implement
no baseline information
multislice trivial
susceptibility artifacts
can use very short TR
Perfusion
unique and quantitative information
low activation contrast
baseline information
longer TR required
easy control over observed vasculature
multislice is difficult
non-invasive
slow mapping of baseline
information
no susceptibility artifacts
Table 6.1a. Summary of practical advantages and disadvantages of pulse
sequences (derived from textbook)
Advantages
Volume
Disadvantages
unique information
invasive
baseline information
susceptibility artifacts
multislice trivial
requires separate run for each task
rapid mapping of baseline information
CMRO2
unique and quantitative information
semi-invasive
extremely low activation contrast
susceptibility artifacts
processing intensive
multislice is difficult
longer TR required
Table 6.1b. Continued summary of practical advantages and disadvantages of
pulse sequences (derived from textbook)
Time/secs
0
1
2
3
4
Perfusion
Venous outflow
No
Velocity
Nulling
Velocity
Nulling
Arteries
Arterioles Capillaries Venules
Veins
TI
ASL
Figure 6.1a Signal is detected from water spins in the arterial-capillary region of the
vasculature and from water in tissues surrounding the capillaries. Relative sensitivity
controlled by adjusting TI and by incorporating velocity nulling gradients (also known as
diffusion weighting). Nulling and TI~1 sec makes ASL sensitive to capillaries and surrounds.
Time/secs
Arterial inflow
(BOLD TR < 500 ms)
0
1
2
3
4
GE-BOLD
No
Velocity
Nulling
Velocity
Nulling
Arteries
Arterioles Capillaries Venules
Veins
Figure 6.1b Gradient Echo BOLD is sensitive to susceptibility perturbers of all sizes, and
are therefore sensitive to all intravasculature and extravascular effects in the capillaryvenous portions of the vasculature. If a very short TR is used may show signal from arterial
inflow, which can be removed by using a longer TR and/or outer volume saturation.
Time/secs
Arterial inflow
(BOLD TR < 500 ms)
0
1
2
3
4
SE-BOLD
No
Velocity
Nulling
Velocity
Nulling
Arteries
Arterioles Capillaries Venules
Veins
Figure 6.1c Spin Echo BOLD is sensitive to susceptibility perturbers about the size of a red
blood cell or capillary, making it predominantly sensitive to intravascular water spins in vessels
of all sizes and to extravascular (tissue) water surrounding capillaries. Velocity nulling reduces
the signals from larger vessesl.
TI
TI
ASL
pulse
Gradient-echo
TE
90°
ASL
pulse
Spin-echo
TE
180°
90°
RF
RF
Gx
Gx
Gy
Gy
Gz
Gz
spin-echo
180°
Figure 6.2 Pulse sequence diagrams of (a)
gradient echo, (b) spin echo, and (c)
asymmetric spin echo EPI. The TE is shown
at the center of 9-line k-space (typically 64
or more lines).  is the offset from center of
k-space to echo. Additional pulses needed
for ASL are indicated schematically.
90
RF
Gx
Gy
Gz
/2
TE

Approximate GM Relaxation And Activation
Induced Rexalation Rate Changes
1.5T
3T
T2
100 ms
80 ms
T2*
60 ms
50 ms
T2’
150 ms
133.3 ms
DR2 = D(1/T2)
-0.2 s-1
-0.4 s-1
DR2* = D(1/T2*)
-0.8 s-1
-1.6 s-1
DR2’ = D(1/T2’)
-0.6 s-1
-1.2 s-1
• T2, T2* and T2’ (from ASE) of GM decrease with increasing field strength
• During activation relaxation rates decrease (T2 increase) slightly
• Activation induced changes in relaxation rates (DR2s) indicate potential for
signal production
Asymmetric
spin - echo
Gradient - echo
1
1
0.8
MRI signal
MRI signal
0.8
0.6
0.4
0.2
0
-20
Spin-echo time (ms)
3T
1.5 T
0
20
40
60
TE (ms)
80
100
0.6
0.4
3T
1.5 T
70
90
110
130
0.2
0
-80
-40
0
 (ms)
40
80
Figure 6.3a Signal intensity for GE, SE, and ASE for approximate relaxation rates of
grey matter at 1.5T and 3T. SE sequence corresponds to ASE at  = 0. Signal decays
more rapidly since T2 and T2* is shorter at 3T.
Asymmetric
spin - echo
Gradient - echo
16
14
Per cent change
Per cent change
16
3T
1.5 T
12
10
8
6
4
2
0
Spin-echo time (ms)
14
12
10
8
3T
1.5 T
130
110
90
70
6
4
2
0
0
20
40
60
TE (ms)
80
100
-80
-40
0
 (ms)
40
80
Figure 6.3b Percent signal change for approximated activation-induced relaxation rate
changes (using Table 6.2). Note linear increase for GE and for ASE with |  |>0. Also, 3T
shows larger change than 1.5T for all three.
Asymmetric
spin - echo
Gradient - echo
Spin-echo time (ms)
0.05
0.04
3T
1.5 T
Difference
Difference
0.05
0.03
0.02
0.04
0.03
0.01
0
0
20
40
60
TE (ms)
80
100
3T
1.5 T
0.02
0.01
0
70
90
110
130
-80
-40
0
 (ms)
40
Figure 6.3c Signal difference or contrast with brain activation. Peak contrast for GE
when TE~T2* and ASE when  ~T2*. SE has lowest contrast.
80
Maximizing Signal
• Field Strength and sequence parameters
– Higher B means higher SNR but more susceptibility
issues
– TE ~ T2* (30-40 msec @ 3T) for best activation contrast
– TR large enough to cover volume of interest, sampling
time consistent with experiment, >500 msec
recommended, T1 increases with increasing B
• RF coils
– Larger coil for transmit
– Smaller coil for receive
– RF inhomogeneity increases with B
• Voxel size
– Match to volume of smallest desired functional area
– 1.5x1.5x1.5 suggested as optimal (Hyde et al., 2000)
– T2* increase and activation signal increase with small
voxels if shim is poor
Maximizing Signal
• Reducing physiological fluctuations
– Cardiac and breathing artifacts (sampling
issues)
– Filtering to remove artifactual frequencies from
time signal, breathing easier to manage by
filtering
– Pulse sequence strategies
• Snap shot (EPI) each image in 30-40 msec
reduces impact of artifacts
• Multi-shot ghosting (spiral imaging, navigator
pulses, retrospective correction)
– Gating
• Acquiring image at consistent phase of cardiac
cycle or respiration
• Problems (changing heart rate, wasted time)
Minimizing Temporal Artifacts
• Brain activation paradigm timing
– On-off cycles usually > 8 seconds
– Maximum number of cycles and maximum
contrast between
– Cycling activations no longer than 3-4 minutes
• Post processing
– Motion correction
• Real time fMRI
– Monitoring immediately and repeat if artifacts
are excessive
– Tuning of slice location
Minimizing Temporal Artifacts
• Physical restraint
– Limited success
– Cooperative subject helps
• Pulse sequence strategies
– Clustered acquisition (auditory stimulation 4-6
seconds before acquisition)
– Set phase encode direction to minimize overlap
with brain areas of interest
– Select image plane with most motion to
minimize between plane motion artifacts
– Crusher gradients to minimize inflow artifacts
Issues of Resolution and Speed
• Acquisition speed
– Echo planar sequence preferred for fMRI
– Multi-shot imaging used for anatomy
• Image resolution
– Higher resolution takes more time and T2* leads
to low signal for later k-space lines
• multi-shot EPI
• Partial k-space acquisition
• Brain Coverage
– Full brain coverage desirable
– Uniform response throughout brain also needed
Structural and Functional Image Quality
• Functional time series image quality
– Warping
– Signal dropout
• High resolution structural image quality
– 3D sub-millimeter possible
– Matching functional to structural