INTRO TO BOLD FMRI FRANZ JOSEPH GALL ( ) OUTLINE. MRI & Fast MRI Observations Models Statistical Detection

Transcription

1 INTRO TO BOLD FMRI 2014 M.S. Cohen all rights reserved OUTLINE FRANZ JOSEPH GALL ( ) MRI & Fast MRI Observations Models Statistical Detection

2 PAUL BROCA ( ) WILLIAM JAMES (1890) We must suppose a very delicate adjustment whereby the circulation follows the needs of the cerebral activity. Blood very likely may rush to each region of the cortex according as it is most active, but of this we know nothing. BRAIN ACTIVATION LEADS TO: CBF Increased +ΔR1 CBV O Utilization 2 Increased +ΔR2 (C+) Increased slightly? Venous [O 2 ] Increased -ΔR2* BOLD Glucose Utilization Increased? Lactate R1=1/T1 R2=1/T2

3 SIGNAL LOSSES FROM SPIN DEPHASING NMR Signal (%) Darkness Flashing Lights injection Inhomogeneous Magnetic Fields Within Voxels Result in Spin Dephasing and Signal Loss in Gradient Echo Sequences B Time (seconds) Capillary Jack Belliveau mscohen@ucla.edu 2014 M.S. Cohen all rights reserved BOLD 2014 M.S. Cohen all rights reserved Gradients of several Gauss/cm may exist near deoxy-hb-filled capillaries. mscohen@ucla.edu FMRI explores intensity variations in MR signal Effect of blood CO2 level on BOLD contrast. (a) Coronal slice brain image showing BOLD contrast from a rat anesthetized with urethane. The gas inspired was O2. (b) The same brain but with 90% O2/10%CO2 as the gas inspired. BOLD contrast is greatly reduced. S Ogawa, et al., intensity variations reflect venous [O2] PNAS, 87(24):9868, M.S. Cohen all rights reserved mscohen@ucla.edu 2014 M.S. Cohen all rights reserved 12 mscohen@ucla.edu

4 WHY DOES VENOUS O 2 INCREASE? (1) [O 2 ] 98% 60% 65% WHY DOES VENOUS O 2 INCREASE? (2) [O 2 ] 98% 30% 65% 0% flow Artery Brain Vein 0% Artery Brain Vein flow 0% 50% 0% 50% Under normal conditions oxygen diffuses down its concentration gradient from the capillary to the brain parenchyma As the brain becomes more active, the oxygen consumption increases, increasing the transluminal oxygen gradient WHY DOES VENOUS O 2 INCREASE? (3) WHY DOES VENOUS O 2 INCREASE? (4) 98% 98% 30% 32% 30% 65% [O 2 ] [O 2 ] 0% Artery Brain Vein flow 0% Artery Brain Vein flow 0% 50% 0% 50% As oxygen flows across the capillary lumen it is depleted in the capillary and no further oxygen can be delivered The vascular system responds by increasing blood flow so that more oxygenated blood is available throughout the capillary 15

5 WHY DOES VENOUS O 2 INCREASE? (5) [O 2 ] Because the blood flow is increased more oxygenated blood passes into the venous end of the capillary 0% Artery Brain Vein flow 98% 0% 50% 30% 65% WHY DOES VENOUS O 2 INCREASE? (6) [O 2 ] Because the blood flow is increased more oxygenated blood passes into the venous end of the capillary 0% Artery Brain Vein flow 98% 0% 50% 60% 65% GRADIENT-RECALLED ECHO Ken Kwong Baseline 30 s 50 s INVERSION RECOVERY TE=42 TR=3000 TI = 1100 THICKNESS=10 OFF OFF 110 s 130 s 170 s ON ON OFF 190 s 250 s 270 s Ken Kwong OFF ON ON Seiji Ogawa Ken Kwong

6 A Chink in the Armor HEMIFIELD ALTERNATION right hemisphere left hemisphere Signal Intensity Time (seconds) Cohen 2013 Mark Cohen, all rights reserved 22 ACTIVATION WITH MOVING VISUAL STIMULI CONTRAST RESPONSE TEST 1.6% 6.3% 25% 78% 82% MT V1 MT / V5 V Time (seconds) From R. Tootell

7 MOTION SENSITIVITY TEST MT TRADITIONAL MRI ANALYSIS - MODEL DRIVEN Task Timing V1 Moving Stationary Moving Stationary Observed Signals Time (seconds) From R. Tootell TRADITIONAL MRI ANALYSIS - MODEL DRIVEN Task Model Signal Model z=5 z=1.5 Hemodynamic Response Model CONVOLUTION OF IMPULSE RESPONSES WITH STIMULI % increase over baseline stim stim stim Time (seconds) Actual Response Convolutio n Model

8 AMPLITUDE-WEIGHTED LINEAR ESTIMATE Rate: Signal and Estimate Time (seconds) Residual Error TRADITIONAL MRI ANALYSIS - MODEL DRIVEN Task Model Signal Model z=5 z=1.5 Hemodynamic Response Model Observed noise = MRI instrument noise + Scanner instabilities + Thermal noise from subjects + Subject physiological fluctuations + Subject motion + Interference from devices + other subject factors White Noise Pink (1/f) Noise

9 Loudness 60 Hz b a a b a - Scott Joplin piano rags b - Classical Radio c - Rock Radio d - News and Talk Radio c c d Pitch ad R. F. Voss and J. Clarke, Nature 258 (1975) 317. Human EEG 1kΩ Well-Behaved Noise: Scanner Noise Subject Thermal Noise 1kΩ BW =16000 Hz, T = 300 K, R 1000Ω N = BW E 23 T R N BW E V 0.5µV

10 10 0 Amplitude 10 BADLY BEHAVED NOISE Interference from Devices Scanner Instabilities thermal noise Frequency 10 0 quantization noise Subject physiological fluctuations Subject motion Amplitude 10 Amplitude 10 other subject factors Frequency Frequency

11 SCANNER PROBLEMS MODEL-FREE MRI ANALYSIS INDEPENDENT COMPONENTS ANALYSIS (ICA) Time Location (space) Scan #k fmri Image Data Spatial ICA for fmri # ICs Location (space) # ICs Time IC Spatial Maps ICA EXPOSES FUNCTIONAL NETWORKS data are decomposed into a set of spatially-independent maps and a set of time courses. impose spatial indepedence

12 BRAIN READING MACHINE LEARNING IN FMRI Postulate: All interesting behavioral, affective, mental or cognitive states are the expression of, or reflected in, neural activity Resulting maps are difficult to interpret. Haxby, et al., Science 293:2426