1. Origin of Negative BOLD fMRI Signals
Norm Harel at al
Presented by Hyunggoo Kim

2. History 1930 Dia/paramagnetic property of Oxy/deoxyheamoglobin
1952 Discovered NMR
1971 nuclear magnetic relaxation times of tissues and tumors differed
1982 blood oxygenation lead to decrease in T2, T2* NMR relaxation time of blood
1989 in vivo imaging in rat
1992, BOLD contrast by ogawa

3. TERM CBF: cerebral blood flow CBV: cerebral blood volumne
CMR: cerebral metabolic rate

4. MRI (1) NMR(Nuclear magnetic resonance) Imaging technology
Proton, neutron spins along strong magnetic field
Spin frequency V = rB, r is specific for each atom
Using H (H2O is abundant in body)
High/low energy spin state
Emiting electomagnetic wave (High to low)

5. MRI (2)
Decaying time(T1,T2,T2*) varies along to environment (skeleton, tissue, etc)
Detecting signal using coil by electomagnetic Induction
Using constrast of signal strength for converting signal strength to visual graphic
Various technique about
How to evoke signal
How to detect signal
How to analyze data

6. 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

7. 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

8. BOLD fMRI (2) 1st phase 2st phase 3st 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

9. Experiment Overview Objective: Clarify BOLD mechanism
Question: Negative BOLD mean decreased neuroal activity ?
Input: square-wave grating stimulus
Output: BOLD constrast
Analysis: Cross-correlation method

10. Positive / Nagative BOLD
V1 positive, Higher area negative

11. BOLD – CBV relation
V1: TCBV < TBOLD
Higher: TBOLD < TCBV

12. Discussion (1) V1 & Higher area are highly related
At lower PCO2, BOLD response faster and no initial dip

13. 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 region may not escape ‘initial dip’ state
TCBV < TBOLD explains arterial expansion -> hemodynamic change

14. Conclusion
Caution should be taken when interpreting negative BOLD signal