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 neuro vascular response (rv) and the imaging spatial resolution extent (rs).

• General relationship • rb 2 = rv 2 + rs 2 • 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 B 0 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 BOLD signal Synaptic signalling Vascular tone (reactivity) Autoregulation Blood flow, oxygenation and volume end bouton dendrite glia Metabolic signalling arteriole venule B 0 field • 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.

positive BOLD response 1 0 3 2 initial dip overshoot stimulus post stimulus undershoot time Response extent • Initial dip – localized response (low signal) • Overshoot next in extent (high signal) • Plateau has greatest extent (high signal)

Figure

7.3 from textbook.

3.6

3.2

2.8

2.4

2.0

1.6

0 4 8 12

Stimulus duration (s)

16 20 • • 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 3 Corresponding eye stimulation 2 Other eye stimulation 1 0

Figure

7.1 from textbook.

0 4 8 12 16

Time (s)

20

Figure

7.2 from textbook.

24 28 • 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 0 time (s) 5 Onset and Width of BOLD response as temporal measures.

---- Not time to peak ----

Figure

7.4 from textbook.

0 fMRI responses 5 time (s) • BOLD signal time course • presentation (black) • processing (light grey) • response (dark grey)

Estimating Neural Processing Time From BOLD Response Onset

(a) V1 SMA M1 (b) 300 250 200 150 100 50 0 -50 150    

Figure

7.5 from textbook.

time 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

(a) Task 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.

(b) fMRI 1. 03 1. 02 1. 01 1. 00 0. 99 0. 98 0 (c) RT(A) RT(B) Trial A Trial B (more angular disparity) 5 10 15 Time after presentation (s) 20 16 12 8 4 0 0 4 8 12 Reaction Time(s) 16

Figure

7.6 from textbook.

Forward Connections

Parietal

Visual overview.pdf

Temporal

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 Cell Level Hemisphere Level Lobe Level Gyrus Level Tissue Level