Statistical Parametric MappingStatistical 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 ResolutionSpatial Resolution– Spatial sampling and aliasing– Partial volume averaging alters strength of response

based on voxel size and size of responding region

• Temporal ResolutionTemporal 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 neuro-

vascular 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 hemodynamicsResponding 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 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

• 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.

Neural activity Signalling Vascular response

Vascular tone (reactivity)Autoregulation

Metabolic signalling

BOLD signal

glia

arteriole

venule

B0 field

Synaptic signalling

Blood flow,oxygenationand volume

dendriteend bouton

Temporal Hemodynamics

Arterial inflow effectsVenous outflow effects

• fMRI response ratio drops off with stimulus duration

• Dilution of signal into larger extent seems to be dominant effect

1.6

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Figure 7.3 from textbook.

time

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initialdip

positiveBOLD response

post stimulusundershootovershoot

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stimulus

• Initial dip – localized response (low signal)• Overshoot next in extent (high signal)• Plateau has greatest extent (high signal)

Response extent

Figure 8.1. from textbook.

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 SystemBoth 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

• 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.

Figure 7.1 from textbook.

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Time (s)

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Other eyestimulation

Figure 7.2 from textbook.

Typical Paradigm

• Instruction• Presentation

– stimulation– timing

• Processing– sensing– decision

• Response– plan– motor

fMRI responses

time (s)

Trial #1

Trial #2

Presentation Response

Behaviour

time (s) 0 5 0 5

Figure 7.4 from textbook.

• BOLD signal time course• presentation (black)• processing (light grey)• response (dark grey)

Task

Behavior

Onset and Width of BOLD response as temporal measures.---- Not time to peak ----

Estimating Neural Processing Time From BOLD Response Onset

V1

SMA

M1

time

fMR

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kinematic RT (ms)

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Figure 7.5 from textbook.

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.

Estimating Neural Processing Time From from BOLD Response Width

fMR

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gnal

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from

S

PL

Time after presentation (s)

0. 98

0. 99

1. 00

1. 01

1. 02

1. 03fMRI

(b)

20151050

Trial A

Trial B (more angular

disparity)

RT(A) RT(B)

Task

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(c)

16128400

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Reaction Time (s)

Figure 7.6 from textbook.

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.

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

Cell Level

Tissue Level

Gyrus Level

Hemisphere Level

Lobe Level

Talairach Daemon Atlas Sections at Z = +1Talairach Daemon Atlas Sections at Z = +1