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