Psy 8960, Fall ‘06 Spin Echo, high field1 High Field and Spin Echo: the Minnesota story Preview: fMRI decision tree Why image at 7T?

Psy 8960, Fall ‘06 Spin Echo, high field2 Cheng et al. (2001)

Psy 8960, Fall ‘06 Spin Echo, high field3 Harrison, Harel et al., Cerebral Cortex 12:225 (2002) 100  m

Psy 8960, Fall ‘06 Spin Echo, high field4 Duvernoy et al., (1981) Brain Res. Bull. 7:518

Psy 8960, Fall ‘06 Spin Echo, high field5 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

Psy 8960, Fall ‘06 Spin Echo, high field6 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

Psy 8960, Fall ‘06 Spin Echo, high field7 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)

Psy 8960, Fall ‘06 Spin Echo, high field8 100  m Signal contributions: spin echo (T 2 ) Intravascular Small venuole/capillary Large venuole Field strength Relative contribution Blood signal Extravascular protons near small vessels

Psy 8960, Fall ‘06 Spin Echo, high field9 Other advantages of spin echo Refocusing of signal loss due to through-slice dephasing T 2 instead of T 2 * contrast SE GE