Jody Culham Brain and Mind Institute Department of

Jody Culham Brain and Mind Institute Department of Psychology Western University http: //www. fmri 4 newbies. com/ f. MRI Physics in a Nutshell Understanding WTF your MR physicist is talking about Last Update: September 14, 2017 Last Course: Psychology 9223, F 2017

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

Susceptibility Artifacts Hair tie removed

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

Susceptibility Artifacts T 1 artifacts in phase encode direction T 2* sinuses ear canals

Why Susceptiblity? • spins become dephased very quickly • weak T 2* 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 Mxy types of tissue we want to differentiate regions of susceptibility Time to Echo = TE (ms)

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 – O 2 is slightly paramagnetic • Distortions in the local magnetic field faster T 2* decay

Hemoglobin Hemoglogin (Hgb): • can attach up to four oxygen atoms (O 2) • oxy-Hgb (four O 2) is weakly diamagnetic • deoxy-Hgb is paramagnetic Source: http: //wsrv. clas. virginia. edu/~rjh 9 u/hemoglob. html, Jorge Jovicich

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 & Mc. Carthy, 2004, Functional Magnetic Resonance Imaging based on two papers from Ogawa et al. , 1990, both in Magnetic Resonance in Medicine

History of f. MRI -1990: Ogawa observes BOLD effect with T 2* 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

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

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

Paradox Explained Blood Oxygen Level Dependent signal neural activity blood flow oxyhemoglobin T 2* MR signal At Rest: Mxy Signal Mo sin T 2* task T 2* control Stask Scontrol Active: S TEoptimum time Source: Jorge Jovicich Figure Source: Huettel, Song & Mc. Carthy, 2004, Functional Magnetic Resonance Imaging

BOLD Time Course Blood Oxygenation Level-Dependent Signal BOLD Response (% signal change) Positive BOLD response 3 2 Overshoot 1 Initial Dip 0 Post-stimulus Undershoot Time Stimulus

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

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

Interim Summary • susceptibility – local inhomogeneities rapid dephasing in transverse plane drop in T 2* 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 f. MRI loss in signal because of air -tissue interface gain in signal because deoxy. Hb flushed out by compensatory blood response

Interim Summary: BOLD Time Course BOLD Response (% signal change) Positive BOLD response • compensatory response flushes out hemoglobin • big response, fairly consistent across individuals 3 2 1 0 Initial Dip • too small/short! Overshoot • may be neural (transients; adaptation) or vascular Stimulus Post-stimulus Undershoot • things get wonky because different aspects of hemodynamics return to normal at different rates • very inconsistent across individuals Time