High Field and Spin Echo: the Minnesota story

Spin Echo, high field 1Psy 8960, Fall ‘06

High Field and Spin Echo: the Minnesota story

• Preview: fMRI decision tree

• Why image at 7T?

Spin Echo, high field 2Psy 8960, Fall ‘06

Cheng et al. (2001)

Spin Echo, high field 3Psy 8960, Fall ‘06

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

100m

Spin Echo, high field 4Psy 8960, Fall ‘06

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

Spin Echo, high field 5Psy 8960, Fall ‘06

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, high field 6Psy 8960, Fall ‘06

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

Spin Echo, high field 7Psy 8960, Fall ‘06

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)

Spin Echo, high field 8Psy 8960, Fall ‘06

100m

Signal contributions: spin echo (T2)

Intravascular

Small venuole/capillary

Large venuoleField strength

Rel

ativ

e co

ntri

buti

on

Blood signal

Extravascular protons near small vessels

Spin Echo, high field 9Psy 8960, Fall ‘06

Other advantages of spin echo

• Refocusing of signal loss due to through-slice dephasing

• T2 instead of T2* contrast

SE

GE