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Statistical Parametric Mapping
Lecture 4 - Chapter 7
Spatial and temporal resolution of 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
Spatial and Temporal Resolution
Issues
• Spatial Resolution
– Spatial sampling and aliasing
– Partial volume averaging alters strength of response
based on voxel size and size of responding region
• Temporal Resolution
– Temporal sampling and filtering
– Would like to sample electrical activity which happens
earlier than BOLD
– Order and timing of events would improve modeling
capabilities
Spatial Resolution Issues
• Excitatory and Inhibitory neural activity are both
energy consuming, but inhibited neurons
produce less neuronal activity.
• Need to include all regions of brain involved in
the designed brain tasks (whole brain preferred).
– Activity could be weaker due to partial volume effects
in smaller components of a system level activated
brain network.
– Need to improve task induced change and reduce
partial volume averaging.
• Position errors due to motion, veins,
macroscopic susceptibility, etc.
Impact of Spatial Resolution
• Extent of BOLD response (rb) is related to the extent of neurovascular response (rv) and the imaging spatial resolution extent (rs).
• General relationship
• rb2 = rv2 + rs2
• BOLD signal is variable due to partial volume averaging
• When rv < rs (voxel larger than signal region)
• rb ~ rs
• Bold signal is reduced by partial volume averaging
• When rv > rs (voxel smaller than signal region)
• rb ~ rv
• BOLD signal minimally affected by rs
Based on classical linear system where
output(x,y,z) = input(x,y,z)  PSF(x,y,z)
But?
Two Main Focus Points
• Responding well to changing hemodynamics
– Initial dip in BOLD response more spatially specific to activated
brain area than later hypoxic rise in response, but later phase
response is larger and needed for fMRI.
– Late hyperoxic response more broadly distributed spatially.
• Techniques to eliminate unwanted contributions to signal
(increase contrast to noise ratio - CNR).
– Short duration stimuli seem to be more narrowly distributed spatially
than long duration stimuli in BOLD studies.
– Higher B0 appears to improve microvascular signals more than
interfering signals
– Better RF coils improve SNR
– Improved motion correction improves CNR
– Multi-shot EPI to reduce T2* blurring supports smaller voxels
Neuro-Vascular Signalling
Neural activity
Signalling
Vascular response
Vascular tone (reactivity)
Autoregulation
Synaptic signalling
BOLD signal
Blood flow,
oxygenation
and volume
arteriole
B0 field
glia
Metabolic signalling
end bouton
dendrite
venule
• If signalling is mediated by diffusion then densely packed vasculature such
as in V1 would show faster BOLD response.
• If signalling is mediated through membrane potentials on glia then different
areas within brain would have similar BOLD response timing.
Temporal Hemodynamics
Arterial inflow effects
Venous outflow effects
Figure 8.1. from textbook.
Figure 7.3 from textbook.
3.6
3
2
initial
dip
overshoot
post stimulus
undershoot
1
0
fMRI response ratio
BOLD response, %
positive
BOLD response
3.2
2.8
2.4
2.0
stimulus
time
1.6
0
4
8
12
16
Stimulus duration (s)
20
Response extent
• Initial dip – localized response (low signal)
• Overshoot next in extent (high signal)
• Plateau has greatest extent (high signal)
•
•
fMRI response ratio drops off with
stimulus duration
Dilution of signal into larger extent
seems to be dominant effect
Brodmann’s Functional Map
Visual Field Mapping
• Visual information from right
visual field sent to left V1
area in occipital lobe
• Vice-versa for left visual field
• Right and left eye views
used to form 3D images
through stereo effect
(overlapping central visual
area)
V1 – Primary visual cortex
Anatomy of the Visual System
Both eyes project to each visual cortex, but at the primary visual area
(BA17), they remain largely segregated into ocular dominance columns.
RIGHT VISUAL FIELD MAPPING
Occular Dominance Column Imaging
4
Corresponding eye
stimulation
Percent signal change
3
2
Other eye
stimulation
1
0
0
Figure 7.1 from textbook.
4
8
12
16
20
24
28
Time (s)
Figure 7.2 from textbook.
• Figure 7.1
–Blue is right eye response when corresponding (right eye right visual field) stimulated.
Red is left eye (right visual field) stimulation.
–Note similarity in columnar (connected) organization for each eye’s response.
• Figure 7.2 shows timing of visual stimulus and BOLD response. TR = 1sec.
• Note the spatial detail in figure 7.1. Short duration stimuli used.
Typical Paradigm
Task
Behavior
• Instruction
• Presentation
– stimulation
– timing
• Processing
– sensing
– decision
• Response
– plan
– motor
Behaviour
Presentation Response
Trial #1
Trial #2
Onset and Width of BOLD
response as temporal measures.
---- Not time to peak ---Figure 7.4 from textbook.
fMRI responses
0
time (s)
5
0
5
time (s)
• BOLD signal time course
• presentation (black)
• processing (light grey)
• response (dark grey)
Estimating Neural Processing Time From
BOLD Response Onset
(b)
M1
SMA
V1


Figure 7.5 from textbook.
300
BOLD onset difference (ms)
fMRI response ampitude
(a)
time
250

200
150

100
50
0
-50
150
200
250
300
kinematic RT (ms)
Task – use joystick to move cursor from start box to target box as rapidly and accurately as
possible (10 trials in multiple subjects).
BOLD response – V1 (primary visual cortex), SMA (supplementary motor area), M1
(primary motor area)
Analysis –  but not  increases with increasing reaction time (RT).
Conclusion – Delay in reaction time from planning rather than execution of movement.
350
Estimating Neural Processing Time From from
BOLD Response Width
(b)
fMRI
(a)
Task
fMRI signal change from
SPL
1. 03
Trial B
(more angular
disparity)
1. 02
1. 01
Trial A
1. 00
RT(A) RT(B)
0. 99
0. 9 8
5
10
15
Time after presentation (s)
0
(c)
Normalized width of BOLD response (s)
Task – determine if one object could be rotated to
match a second. Rotation angle varied by design.
Press button yes or no.
BOLD response – Superior Parietal Lobule (SPL)
Analysis – Normalized width of BOLD response
correlated with reaction time (RT).
Conclusion – SPL intimately involved in mental
rotation of object.
20
16
12
8
4
00
4
8
12
Reaction Time(s)
16
Figure 7.6 from textbook.
Forward Connections
Parietal
Temporal
Visual overview.pdf
Parietal Lobe
Mango and Anatomy
• Talairach Daemon (TD)
– Anatomical/functional labels
– 5 hierarchical levels
•
•
•
•
•
Hemispheres
Lobes
Gyri
Tissue
Cellular
• Spatial Normalization
– Supports x-y-z coordinate lookup of
anatomical/functional labels using the TD
Talairach Daemon Atlas Sections at Z = +1
Hemisphere Level
Lobe Level
Gyrus Level
Tissue Level
Cell Level