Statistical Parametric Mapping Lecture 5 - Chapter 6 Selection of the optimal pulse sequence for...
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Transcript of Statistical Parametric Mapping Lecture 5 - Chapter 6 Selection of the optimal pulse sequence for...
Statistical Parametric MappingStatistical Parametric Mapping
Lecture 5 - Chapter 6Selection 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 Disadvantages
BOLD 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 Disadvantages
Volume 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)
Venous outflow
Perfusion
NoVelocityNulling
VelocityNulling
ASLTI
Time/secs 1 2 40 3
Venous outflow
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.
Arteries Arterioles Capillaries Venules Veins
GE-BOLD
NoVelocityNulling
VelocityNulling
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 capillary-venous 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.
Arteries Arterioles Capillaries Venules Veins
Arterial inflow(BOLD TR < 500 ms)
Time/secs 1 2 40 3
SE-BOLD
NoVelocityNulling
VelocityNulling
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.
Arteries Arterioles Capillaries Venules Veins
Arterial inflow(BOLD TR < 500 ms)
Time/secs 1 2 40 3
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.
Gradient-echo
RF
Gx
Gz
Gy
90°
TEASLpulse
TISpin-echo
180° TE
RF
Gx
Gz
Gy
ASLpulse
TI
90°
spin-echo
180° TE
RF
Gx
Gz
Gy
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
R2 = (1/T2) -0.2 s-1 -0.4 s-1
R2* = (1/T2*) -0.8 s-1 -1.6 s-1
R2’ = (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 (R2s) indicate potential for
signal production
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MRI signal
(ms)
7090110130
Spin-echo time (ms)
3 T1.5 T
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MRI signal
TE (ms)
3 T1.5 T
Gradient - echo Asymmetricspin - echo
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.
0
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-80 -40 0 40 80Per cent change
(ms)
1301109070
Spin-echo time (ms)
3 T1.5 T
0
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0 20 40 60 80 100
Per cent change
TE (ms)
3 T1.5 T
Gradient - echo Asymmetricspin - echo
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.
0
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-80 -40 0 40 80
Difference
(ms)
7090110130
Spin-echo time (ms)
3 T1.5 T
0
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0.05
0 20 40 60 80 100
Difference
TE (ms)
3 T1.5 T
Gradient - echo Asymmetricspin - echo
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.
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