Download - Origin of Negative BOLD fMRI Signals
Origin of Negative BOLD fMRI Signals
Norm Harel at al
Presented by Hyunggoo Kim
History• 1930 Dia/paramagnetic property of Oxy/deoxy
heamoglobin• 1952 Discovered NMR• 1971 nuclear magnetic relaxation times of tis
sues and tumors differed• 1982 blood oxygenation lead to decrease in T
2, T2* NMR relaxation time of blood• 1989 in vivo imaging in rat• 1992, BOLD contrast by ogawa
TERM
• CBF: cerebral blood flow• CBV: cerebral blood volumne• CMR: cerebral metabolic rate
MRI (1)• NMR(Nuclear magnetic resonance) Imaging te
chnology• Proton, neutron spins along strong magnetic fi
eld• Spin frequency V = rB, r is specific for each at
om• Using H (H2O is abundant in body)• High/low energy spin state• Emiting electomagnetic wave (High to low)
MRI (2)• Decaying time(T1,T2,T2*) varies along to environment
(skeleton, tissue, etc)• Detecting signal using coil by electomagnetic Inductio
n• Using constrast of signal strength for converting signa
l strength to visual graphic• Various technique about
– How to evoke signal– How to detect signal– How to analyze data
fMRI
• Functional <-> anatomical• Detect signal constrast between specific
task• Spatio-temporal difference of signal• Correlate function achiving the task and
the region of brain area
BOLD fMRI (1)• Dia/Paramagnetic stuff change relaxation time• Oxygen + Fe (in Hb) : Diamagnetic• (Deoxy) + Fe (in Hb): Paramagnetic• Paramagnetic makes magnetic field inhomogeneous• Decay time prolonged in inhomogeneous M.F.• What cause deoxy ?
– C6H12O6 + 6O2 -> 6CO2 + 6H20 (Glycolysis)– mainly in neurotransmitter metabolism– So, Neural activity is related– But much argument about precise mechanism– CBV, CBF, etc..
time
strength
BOLD fMRI (2)
• 1st phase– More deoxy -> more paramagnetic -> inhomogeneous magnetic field
-> T2 time shortened -> signal-• 2st phase
– CBF >> oxygen demand -> T2 prolonged -> signal+• 3st phase
– CBF return, oxygen demand return, signal return
Experiment Overview• Objective: Clarify BOLD mechanism• Question: Negative BOLD mean decreas
ed neuroal activity ?• Input: square-wave grating stimulus• Output: BOLD constrast• Analysis: Cross-correlation method
Positive / Nagative BOLD
• V1 positive, Higher area negative
BOLD – CBV relation
• V1: TCBV < TBOLD
• Higher: TBOLD < TCBV
Discussion (1)
• V1 & Higher area are highly related
• At lower PCO2, BOLD response faster and no initial dip
Discussion (2)
• Common arterial branch share blood flow• More active region can ‘steel’ blood flow from
relatively less active region• CBF may not explain all neuronal activation• Despite of Increased neuronal activity, these r
egion may not escape ‘initial dip’ state• TCBV < TBOLD explains arterial expansion -> he
modynamic change
Conclusion
• Caution should be taken when interpreting negative BOLD signal