fMRI: Biological Basis and Experiment Design Lecture 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) 100  m

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M T2*T2* T2T2 Spin echo does not form – BOLD contrast is measured Spin echo forms – BOLD contrast is erased

Signal contributions: gradient echo (T 2 *) 100  m Intravascular Small venuole/capillary Large venuole Field strength Extravascular protons near large vessels Extravascular protons near small vessels Relative contribution Blood signal Harrison, Harel et al., Cerebral Cortex 12:225 (2002)

100  m Signal contributions: spin echo (T 2 ) Intravascular Small venuole/capillary Large venuole Field strength Extravascular protons near small vessels Relative contribution Blood signal

BOLD response: 1 st approximation ~0.75 mm increased blood flow Signal near small veins and venuoles ~2.0mm capillary signal 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?