Activation Leads to: functional MRI: A primer CBF Increased +ΔR CBV Increased +ΔR (C+) O Utilization Increased slightly? Venous [O ] Increased -ΔR* Glucose Utilization Increased? Lactate BOLD R=/T R=/T 94 Signal Losses from Spin Dephasing 95 BOLD Inhomogeneous Magnetic Fields Within Voxels Result in Spin Dephasing and Signal Loss in Gradient Echo Sequences B Capillary Effect of blood CO level on BOLD contrast. (a) Coronal slice brain image showing BOLD contrast from a rat anesthetized with urethane. The gas inspired was % O. (b) The same brain but with 9% O/%CO as the gas inspired. BOLD contrast is greatly reduced. Gradients of several Gauss/cm may exist near deoxy-hb-filled capillaries. 96 S Ogawa, et al., PNAS, 87(4):9868,99 97
() () % 6% % % 5% % Under normal conditions oxygen diffuses down its concentration gradient from the capillary to the brain parenchyma % 98 5% % (3) 99 (4) 3% % 3% 3% % % % 5% % As oxygen s across the capillary lumen it is depleted in the capillary and no further oxygen can be delivered As the brain becomes more active, the oxygen consumption increases, increasing the transluminal oxygen gradient. % % % 3% % 5% % The vascular system responds by increasing blood so that more oxygenated blood is available throughout the capillary
(5) (6) % 3% % % % % 5% % % Because the blood is increased more oxygenated blood passes into the venous end of the capillary 6% 5% % Because the blood is increased more oxygenated blood passes into the venous end of the capillary 3 fmri BOLD Contrast & Field Strength BOLD Contrast arises from susceptibility differences explores intensity variations in MR signal The absolute field distortion (from BOLD) is proportional to the magnetic field strength The absolute change in MRI signal is proportional to both the field distortion and the signal strength. BOLD should go as kb intensity variations reflect venous 4 5
Ken Kwong Gradient-Recalled Echo Baseline Inversion Recovery TE=4 TR=3 TI = Thickness= Ken Kwong 6 Inversion Recovery 3 s 5 s OFF OFF s 3 s 7 s ON ON OFF 9 s 5 s 7 s OFF ON ON 7 Activation with Moving Visual Stimuli MT / V5 V Ken Kwong 8 9
Contrast Response Test.6% Motion Sensitivity Test 5% 6.3% 78% 8% MT MT 6 8 4 3 36 6 From R. Tootell 8 4 Stationary 3 36 Moving Moving V Stationary V From R. Tootell Hemifield Alternation Hemifield Alternation seconds right hemisphere left hemisphere Signal Intensity.75 left right.975.875 5 5 5 3 5 5 75 Image Numbers 5 5 3
Neurovascular Coupling and ƒmri latency Linear Systems Approach Pre-capillary Sphincters Hydraulic Delay x(t) 4 Response Latency vs. Stimulus Duration Average of recordings Stimulus Onset y(t) = ƒ[x(t)] In an LTI system, given two inputs A & B: ƒ(a + B) = ƒ(a) + ƒ(b) Potassium Slow Potentials? VIP release? NO release? Linear Time-Invariant (LTI) System 5 Impulse Response Light Flash ms light flash 5 Raw Data from R. Savoy 4 7 ms light flash 3 MR Signal (a.u.) sec light flash - -5 5 Data courtesy of Robert Savoy 5 sec 4 6 8 4 6 6 7
Convolution of Impulse Responses with Stimuli 4 3 % increase over baseline Linear Systems Approach Actual Response stim x(t) stim stim Linear Time-Invariant (LTI) System y(t) = ƒ[x(t)] - Convolution Model - In an LTI system, given two inputs A & B: ƒ(a + B) = ƒ(a) + ƒ(b) -3-4 4 6 8 8 Binocular vs Monocular Activation 9 Amplitude-weighted Linear Estimate Rate: 5 4 8 4 5 8 Binocular Monocular.5 - Bino minus Mono 4 Extrastriate activation 8 6 Residual Error baseline Signal and Estimate -.5 4
Crib Sheet Estimated vs. Actual ƒmri Response 8 pulse 975 95 Signal Intensity 95 9 875 85 -. 84 Longitudinal T - % Relaxation Rate (reaching equilibrium) Mz Signal T Frequency - Transverse Relaxation Rate (dephasing) (MHz) T* - observed T decay, made up of T, T, TD, etc... Magnetization 8 6 Signal Change (%) 4.5-5 3 4 3 Fat -. 4 between pulses.5 tr - Repetition time excitation = + + + te - Time aftertexcitation before forming images ω = γ X B = πf * T T ' T te CSF Static Magnetic Field: D T tr % S(t) = M (t) = M e T B xy ti - Time between inversion M (t) = Mand ( excitation e ) B Field γ - Gyromagnetic ratio - proportionality of Axis field and 84 6... frequency 3 4 te (milliseconds) Magnetic Field (Tesla) B - Static Magnetic Field B - Rotating Magnetic Field..4.6.8 Response Estimate 9 pulse.