Basic f MRI Physics In BOLD f MRI

Basic f. MRI Physics In BOLD f. MRI, we are measuring: Þ the inhomogeneities introduced into the magnetic field of the scanner… Þ as a result of the changing ratio of oxygenated: deoxygenated blood… Þ via their effect on the rates of dephasing of hydrogen nuclei. Ehhh? ? ?

History of MRI NMR = nuclear magnetic resonance nuclear: properties of nuclei of atoms magnetic: magnetic field required resonance: magnetic field x radio frequency NMR MRI: Why the name chang 1946: Block and Purcell atomic nuclei absorb and re-emit radio frequency energy 1992: Ogawa and colleagues first functional images using BOLD signal Bloch Purcell Ogawa most likely explanation: nuclear has bad connotations less likely explanation: NMR means Nouveau Mouvement Religieux

Necessary Equipment 3 T magnet RF Coil gradient coil (inside) Magnet Gradient Coil RF Coil Source: Joe Gati, photos

Recipe for MRI 1) Put subject in big magnetic field (leave him there) 2) Transmit radio waves into subject [about 3 ms] 3) Turn off radio wave transmitter 4) Receive radio waves re-transmitted by subject – Manipulate re-transmission with magnetic fields during this readout interval [10 -100 ms] 5) Store measured radio wave data vs. time – Now go back to 2) to get some more data 6) Process raw data to reconstruct images 7) Allow subject to leave scanner (this is optional) Source: Robert Cox’s web slides

The Big Magnet Main field = B 0 • Continuously on • Very strong : Earth’s magnetic field = 0. 5 Gauss / 1 Tesla (T) = 10, 000 Gauss 3 Tesla = 3 x 10, 000 0. 5 = 60, 000 x Earth’s magnetic field x 60, 000 = Source: www. spacedaily. com B 0 Robarts Research Institute 3 T

Safety The strength of the magnet makes safety essential : things fly – even big things! Source: www. howstuffworks. com Source: http: //www. simplyphysics. com/flying_objects. html Anyone entering the magnet must be metal free This subject was wearing a hair band with a ~2 mm copper clamp. Left: with hair band. Right: without. Source: Jorge Jovicich Develop screening strategies : do the metal macarena!

1 H aligns with B 0 Protons are abundant: high concentration in human body have high sensitivity: yields large signals Outside magnetic field • randomly oriented longitudinal axis transverse plane M=0 Inside magnetic field M Longitudinal magnetization • spins tend to align parallel or antiparallel to B 0 • net magnetization (M) along B 0 • spins precess with random phase • no net magnetization in transverse plane Source: Mark Cohen’s web slides • only 0. 0003% of protons/T align with Source: Robert Cox’s web slides field

Larmor equation : resonance frequency f = B 0/2π for 1 H = 42. 58 MHz/T Frequency (MHz) Larmor Frequency 127. 7 63. 8 1. 5 3. 0 Field Strength (Tesla) Turn your dial to 3 T f. MRI … … broadcasting at a frequency of 127. 7 Mhz

Radio-Frequency Excitation • transmission coil: apply magnetic field along B 1 (perpendicular to B 0 for ~3 ms) • oscillating field at Larmor frequency • frequencies in range of radio transmissions • B 1 is small: ~1/10, 000 T • tips M to transverse plane – spirals down Transverse • analogies: guitar string (Noll), swing (Cox)magnetization • final angle between B 0 and B 1 is the flip angle longitudinal axis Source: Robert Cox’s web slides

Relaxation and Receiving Receive Radio Frequency Field • receiving coil: measure net magnetization (M) • readout interval (~10 -100 ms) • relaxation: after RF field turned on and off, magnetization returns to normal longitudinal magnetization T 1 signal recovers (realignment) transverse magnetization T 2 signal decays (dephasing) Source: Robert Cox’s web slides

Why the dephasing ? • protons precess at slightly different frequencies because of (1) random fluctuations in the local field at the molecular level that affect both T 2 and T 2*; (2) larger scale variations in the magnetic field that affect T 2* only. • over time, the frequency differences lead to different phases between the molecules (clock analogy) • as the protons get out of phase, the transverse magnetization decays Source: Mark Cohen’s web slides

T 1 and TR T 1 = recovery of longitudinal magnetization (B 0) due to realignment of spins TR (time to repetition) = time to wait after excitation before sampling T 1 = time before next RF excitation ≈ M 0(1 -exp(t/T 1)) Source: Mark Cohen’s web slides

T 2 and TE T 2 = decay of transverse (B 1) magnetization due to dephasing of spins TE (time to echo) = time to wait before sampling T 2 (after refocusing of signal) ≈ exp(t/T 2)) Source: Mark Cohen’s web slides

T 1 and T 2 contrasts TISSUE T 1(s) T 2(s) grey matter 1. 0 0. 10 white matter 0. 7 0. 08 CSF 2. 0 0. 25 blood 1. 2 0. 25 water 4. 7 3. 50 Source: Mark Cohen’s web slides

T 2* relaxation • dephasing of transverse magnetization due to both: - microscopic molecular interactions (as for T 2) - spatial variations of the external main field B (tissue/air, tissue/bone interfaces) • exponential decay (T 2* 30 - 100 ms, shorter for higher Bo) Mxy Mo sin T 2* time Source: Jorge Jovicich

Spatial Coding: Gradients How can we encode spatial position? Frequency • Add a gradient to the main magnetic field • Excite only frequencies corresponding to slice plane • Use other tricks to get other two dimensions left-right: frequency encode top-bottom: phase encode Field Strength ~ z position Gradient switching – that’s what makes all the beeping & buzzing noises during imaging => EAR PLUGS !

Echos pulse sequence: series of excitations, gradient triggers and readouts Echos = refocusing of signal Spin echo (not shown) – measure T 2 (left-right) (top-bottom) t = TE/2 A gradient reversal at this point will lead to a recovery of transverse magnetization Gradient echo (shown) measure T 2* flip the gradient at t=TE/2 measure after refocusing at t=TE TE = time to wait to measure refocused spins Source: Mark Cohen’s web slides

A walk through the K-space (inverse Fourier transform) Source: Traveler’s Guide to K-space (C. A. Mistretta)

Susceptibility Adding a nonuniform object (like a person) to B 0 will make the total magnetic field nonuniform This is due to susceptibility: generation of extra magnetic fields in materials that are immersed in an external field Susceptibility Artifact - occurs near junctions between air and tissue sinuses, ear canals - spins become dephased so quickly (quick T 2*), no signal can be measured sinuses Source: Robert Cox’s web slides ear canals Susceptibility variations can also be seen around blood vessels where deoxyhemoglobin affects T 2* in nearby tissue

Hemoglobin A molecule to breathe with: - four globin chains - each globin chain contains a heme group - at center of each heme group is an iron atom (Fe) - each iron ion Fe 2+ can attach an oxygen molecule (O 2) - oxy-Hemoglobin (four O 2) is diamagnetic no B effects - deoxy-Hemoglobin is paramagnetic if [deoxy-Hgb] local B Source: http: //wsrv. clas. virginia. edu/~rjh 9 u/hemoglob. html, Jorge Jovicich

BOLD signal Blood Oxygen Level Dependent signal neural activity blood flow oxyhemoglobin T 2* MR signal Mxy Signal Mo sin T 2* task T 2* control Stask Scontrol S TEoptimum Source: Brief Introduction to f. MRI by Irene Tracey time Source: Jorge Jovicich

BOLD signal Source: Doug Noll’s primer

To take away Magnetic field Tissue protons align with magnetic field (equilibrium state) RF pulses Relaxation processes Protons absorb RF energy (excited state) Kwong et al. , 1992 Spatial encoding using magnetic field gradients Relaxation processes Protons emit RF energy (return to equilibrium state) NMR signal detection Repeat RAW DATA MATRIX Fourier transform IMAGE Source: Jorge Jovicich