fMRI: Biological Basis and Experiment DesignLecture 14: Localization I

• Spin echo BOLD• Experiment design

1 light year = 5,913,000,000,000 miles?

Harrison, Harel et al., Cerebral Cortex 12:225 (2002)

100m

Duvernoy et al., (1981) Brain Res. Bull. 7:518

BOLD fMRI is differentially sensitive to large and small vessels

Spin echo sequences refocus dephasing caused by susceptibility-induced gradients near large veins

In both cases magnitude of field perturbation depends on:- field strength- deoxyhemoglobin concentration

Dynamic averaging regime: diffusion of water molecule is large compared to field gradient

Static averaging regime: diffusion of water molecule is small compared to field gradient

Spin Echo

90 deg.180 deg.

time (ms)

M

T2*

T2

Spin echo does not form – BOLD contrast is measured

Spin echo forms – BOLDcontrast is erased

Signal contributions: gradient echo (T2*)

100m

Intravascular

Small venuole/capillary

Large venuoleField strength

Extravascular protons near large vessels

Extravascular protons near small vessels

Rel

ativ

e co

ntri

buti

on

Blood signal

Harrison, Harel et al., Cerebral Cortex 12:225 (2002)

100m

Signal contributions: spin echo (T2)

Intravascular

Small venuole/capillary

Large venuoleField strength

Extravascular protons near small vessels

Rel

ativ

e co

ntri

buti

on

Blood signal

BOLD response: 1st approximation

~0.75 mmincreased blood flow

Signal near smallveins and venuoles

~2.0mm

capillarysignal

Neural activity (column)

2.5 mm FWHM, GE

1.5mm FWHM, SE

GE BOLD, 2.5 mm PSF:

SE BOLD, 1.5 mm PSF:

Cortical “columns”:

Experiment design practice

• Pick a system to study• Make up a task• Predict a neural response• Predict a BOLD response

What can you measure? What can't you measure?