Basis of the BOLD signal

Basis of the BOLD signal

Zephyr/Science Photo Library Basis of the BOLD signal Maria Molina Sanchez Ivan Ezquerra Romano MRI BASICS fMRI - BOLD SIGNAL 2 The MRI Scanner

Earths magnetic field: 0.00003 Tesla 3 Clinical MRI: 1.5 - 7 Tesla Z B0 longitudinal axis X


5 Gradient coils: x, y, z 6 Alter the strength of the primary magnetic field Change the precession frequency between slices Allow Spatial encoding for MRI images Hydrogen atoms

Human tissue contains water molecules with hydrogen atoms. Hydrogen atoms are positively charged. They are tiny magnets. NUCLEAR SPIN 8 Normally protons are aligned in a random fashion.

When they are exposed to a strong external magnetic field (B0) they align in only two ways: parallel or antiparallel. Net magnetitation parallel to the magnetic field (Z) PRECESSIO N The axis of the spinning top circles forming a cone shape.

How fast the protons precess? Precession frequency depends upon the strength of the magnetic field. Larmor Equation 10 Precession frequency is calculated by the Larmor equation:

0 = 0 0 resonant frequency (e.g., 63.87 Hz for 1.5 T scanner) gyromagnetic ratio (42.58Hz for Hygegon) 0 magnetic field strength (1.5-7 T) Normally: Magnetic fields are

randomly aligned In magnetic field of MRI: Spinning nuclei align to B0 field WHAT HAPPENS WHEN WE APPLY A RF PULSE? Spinning nuclei align

to B field(Z-axis) RF "kick" the nuclei to an excitation state Nuclei flip back RF is applied The protons get in RESONANCE with the RF pulse. Magnetization rotates away from the longitudinal direction. RF is over protons release the energy they absorbed.

12 WHAT HAPPENS WHEN THE RF PULSE CESES? 1. Longitudinal relaxation: Precession back towards B0 longitudinal field (T1)

2. Transversal relaxation: Dephasing of spins (T2) 13 Relaxation time Switch off RF pulse

Receive signal 14 T1 recovery Different tissue types have different T1 and T2 dinamics. This creates contrast in imaging. T2 decay Different tissue types have different T1 and T2 characteristics. This creates contrast in imaging.

16 T2 and T2* T2 = the true natural decay time of the tissues (perfect homogeneous environment) T2* = dephasing is faster than T2 due to inhomogeneities, which affect spin dephasing, and lead to signal decrease. In BOLD fMRI:

T2* changes in relation to brain activity. 18 The BOLD Signal 19

Stimulus to BOLD Source: Arthurs & Boniface, 2002, Trends in Neurosciences 20 Neurons and neural networks 21 Stimulus to BOLD

Source: Arthurs & Boniface, 2002, Trends in Neurosciences 22 Capillary network and blood flow Blood flow = volume of moving blood per unit time Blood flow is proportional to: the Pressure Gradient divided by the Vessel Flow

Resistance Vessel resistance depends primarily on vessel diameter Vessels 23 Mechanisms of blood flow control

24 Neurovascular unit Neurovascular unit = neuron + astrocyte + pericyte and endothelium Neural activation Astrocyte Ca2+ influx Vasoactive substance release 25 The Haemodynamic Response

26 Resting state Oxygenated = Deoxygenated Hb - MR (T2*) Signal loss Active state Oxygenated > Deoxygenated Hb - Less MR (T2*) Signal loss

Blood-Oxygen-Level-Dependent signal Deoxyhaemoglobin (dHb) -> Paramagnetic Oxyhaemoglobin (Hb) -> Diamagnetic 27 Hemoglobin magnetic properties DeoxyHb paramagnetic Fast dephasing

strong field inhomogeneities Fast T2* OxyHb diamagnetic Slower dephasing weak field

inhomogeneities slower T2* T2 and T2* Blood with oxygen -> slower decay Blood without oxygen -> faster decay

Source: - Figure. 29 Brain vs. Vessels Large vessels produce BOLD activation further from the true site of activation Large vessels line the sulci and make it hard to tell which bank of a sulcus the activity arises from The % signal change in large vessels can be considerably higher than in small vessels (e.g., 10% vs. 2%) Activation in large vessels occurs up to 3 s later than in small ones(time lag)

Source: Ono et al., 1990, Atlas of the Cerebral Neurophysiology Typical haemodynamic response to single short stimulus Fast response: increase in metabolic consumption Main BOLD response: increased local blood flow Post-stimulus undershoot: ballon model or increased metabolic consumption with low blood

31 Neurophysiology 32 Stimulus to BOLD Source: Arthurs & Boniface, 2002, Trends in Neurosciences 33

Oxygenated Hb B0 voxel Vessel Tissue Deoxygenated Hb 34

Dr. Samira Kazan 34 Oxygenated Hb B0 voxel Vessel Tissue

Deoxygenated Hb 35 Dr. Samira Kazan 35 Oxygenated Hb B0

voxel Vessel Tissue Deoxygenated Hb 36 Dr. Samira Kazan

36 How does this relate to neural activity? T2* decay is quicker with dHbO Brain areas with increased blood flow there is an increase in HbO (during main signal phase) Inhomogeneities in the field due to O2 signal Mxy Signal

Stas S k control Take-home message: Mo sin T2* task

T2* control S TEoptimum tim e BOLD is a T2*-weighted contrast We are measuring a signal from hydrogen but

the signal we get from hydrogen atoms is weaker when less oxygen (Oxyheamoglobin) is present Stimulus to BOLD What we want to measure but cant What we actually measure with fMRI

Source: Arthurs & Boniface, 2002, Trends in Neurosciences 38 Comparing Electrophysiology and BOLD Disbrow et al., 2000, PNAS 39 BOLD Correlations

24 s stimulus Local Field Potentials (LFP) 12 s stimulus reflect post-synaptic potentials Multi-Unit Activity (MUA)

reflects action potentials Logothetis et al. (2001) 4 s stimulus found that BOLD activity is more closely related to LFPs than MUA Source: Logothetis et al., 2001,

Nature 40 What does the BOLD signal represent? gray matter (dendrites, cell bodies & synapses) white matter (axons)

Lower tier area (e.g., thalamus) Will BOLD activation from the blue voxel reflects: Middle tier area (e.g., V1, primary visual cortex)

output of the black neuron (action potentials)? excitatory input (green synapses)? Higher tier area (e.g., V2, secondary visual cortex) inhibitory input (red synapses)?

inputs from the same layer (which constitute ~80% of synapses)? feedforward projections (from lower-tier areas)? feedback projections (from higher-tier areas)? Advantages of BOLD High spatial resolution Non-invasive

Increasing availability Enables visualising of entire brain areas/ networks engaged in specific activities 42 Disadvantages of BOLD Indirect-indirect measure of neural activity (T2* weighted decay blood oxygenation neural activity) Measuring Local Field Potential and not

Spiking activity Cant differentiate between excitation/inhibition and neuromodulation (all need oxygen and glucose) Signal intensity does not accurately differentiate between: Different brain regions Different tasks within the same region 43

Thanks! Any questions? 44 Overview: What are we measuring with BOLD? the inhomogeneities introduced into the magnetic field of the scanner

changing ratio of oxygenated:deoxygenated blood... via their effect on the rates of dephasing of hydrogen nuclei Should it 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 there is the lower the signal Time fMRI Signal Amount of deoxy-Hb

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