1. Jody Culham
Brain and Mind Institute
Department of Psychology
Western University
fMRI Physics in a Nutshell Understanding WTF your MR physicist is talking about
Last Update: September 14, 2017
Last Course: Psychology 9223, F2017

2. 1C: Basics of fMRI and BOLD
Putting the f in fMRI

3. Susceptibility Artifacts
Hair tie
Hair tie
removed

4. You have air in your head
sinuses
ear canals
trabecular (spongey) bone

5. Susceptibility Artifacts
artifacts in phase encode direction T1
T2*
sinuses
ear
canals

6. Why Susceptiblity? spins become dephased very quickly weak T2* signal
Time to Echo = TE (ms)
Mxy
spins become dephased very quickly
weak T2* signal
one solution to getting signal in susceptibility zones can be to reduce TE, but then you may have suboptimal contrast for other types of tissue you want to differentiate
types of tissue we want to differentiate
regions of susceptibility

7. Why do regions of susceptibility dephase so fast?
The magnetic field is distorted by materials
many metals (e.g., hair clip) are very paramagnetic (ferromagnetic)
most tissue is slightly diamagnetic
O2 is slightly paramagnetic
Distortions in the local magnetic field  faster T2* decay

8. Hemoglobin Hemoglogin (Hgb): can attach up to four oxygen atoms (O2)
oxy-Hgb (four O2) is weakly diamagnetic
deoxy-Hgb is paramagnetic
Source: Jorge Jovicich

9. Deoxygenated Blood  Signal Loss
Seiji Ogawa
Oxygenated blood?
Weakly diamagnetic
Doesn’t distort surrounding magnetic field
No signal loss…
rat breathing pure oxygen
rat breathing normal air (less than pure oxygen)
Deoxygenated blood?
Paramagnetic
Distorts surrounding magnetic field
Signal loss !!!
Images from Huettel, Song & McCarthy, 2004, Functional Magnetic Resonance Imaging
based on two papers from Ogawa et al., 1990, both in Magnetic Resonance in Medicine

10. Aha!

11. History of fMRI fMRI -1990: Ogawa observes BOLD effect with T2*
blood vessels became more visible as blood oxygen decreased
-1991: Belliveau observes first functional images using a contrast agent
-1992: Ogawa et al. and Kwong et al. publish first functional images using BOLD signal
Seiji Ogawa

12. First Functional Images
Flickering Checkerboard
OFF (60 s) - ON (60 s) - OFF (60 s) - ON (60 s)
Source: Kwong et al., 1992

13. The Paradox neurons fire, requiring oxygen (and glucose)
this should lead to more deoxyHb in active regions
thus active regions should show a drop in signal intensity
BUT active regions show a rise in signal intensity?!!!!

14. Paradox Explained At Rest: Active: Blood Oxygen Level Dependent signal
neural activity   blood flow   oxyhemoglobin   T2*   MR signal
At Rest:
Mxy
Signal
Mo sin
T2* task
T2* control
Stask S
Scontrol
Active:
time
TEoptimum
Source: Jorge Jovicich
Figure Source: Huettel, Song & McCarthy, 2004, Functional Magnetic Resonance Imaging

15. BOLD Time Course Blood Oxygenation Level-Dependent Signal
Positive BOLD response
Initial Dip
Overshoot
Post-stimulus Undershoot 1 2 3
BOLD Response (% signal change)
Time
Stimulus

16. Evolution of BOLD Response
Hu et al., 1997, MRM

17. Perhaps it should be BDLD? Blood DE-oxygenation level-dependent signal?
Technically, “BOLD” is a misnomer
The fMRI signal is dependent on deoxygenation rather than oxygenation per se
The more deoxy-Hb, the lower the signal
Stimulus
“BOLD” Signal
looks like inverse of DeoxyHb
Signal Intensity
Time
Time
OxyHb
“BOLD” Signal
Relative Concentration
Time
DeoxyHb
Time
“BDLD” idea from Bruce Pike, MNI

18. loss in signal because of air-tissue interface
Interim Summary
susceptibility
local inhomogeneities  rapid dephasing in transverse plane  drop in T2* intensity
susceptibility caused by metal and air is a bad thing – it leads to signal dropout
susceptibility caused by deoxyhemoglobin is a good thing – it enables fMRI
loss in signal because of air-tissue interface
gain in signal because deoxyHb flushed out by compensatory blood response

19. Interim Summary: BOLD Time Course
Positive BOLD response
compensatory response flushes out hemoglobin
big response, fairly consistent across individuals
Initial Dip
too small/short!
Overshoot
may be neural (transients; adaptation) or vascular
Post-stimulus Undershoot
things get wonky because different aspects of hemodynamics return to normal at different rates
very inconsistent across individuals 1 2 3
BOLD Response (% signal change)
Time
Stimulus