Functional MRI and BOLD EffectSeminar on Physical Fundamentals on Medical Imaging
Jonas Botz
17.06.2019
Table of content
• Introduction into functional MRI
• Methods for measuring brain activity
• Basics of Physiology and Physics
• BOLD Effect
• NMR Basics of BOLD-fMRI
• BOLD-fMRI in Practice
• BOLD-fMRI vs. PET
• Applications and Research 1
Introduction into functional MRI
• non invasive imaging technique to visualise ‘active’ brain areas
• test functions of brain areas
• study brain’s physiology
• most common realised by Blood Oxygenated Level Dependent fMRI (BOLD-fMRI)
[1]2
Methods for measuring brain activity
Action potential
Ion flux (Na+,K+,C2+)
Metabolism
Cerebral blood flow (CBF)
Cerebral blood volume (CBV)
OxyHb/DeOxyHb
EEG, MEG
MRI
PET, SPECT
PET, SPECT,MRI
PET,MRI
BOLD fMRI 3
Basics of Physics and Physiology
Local brain metabolism:
“There exists a coupling between neuronalactivity and local blood flow, so that
increased neuronal stimulation leads to alocal increase in blood flow.” *
• Constant delivery of oxygen and clearance of carbon dioxide by blood flow
• Oxygen is binded to haemoglobin in the lungs and released in the capillary
*ROY, C. W. and SHERINGTON, C. S. “ On the regulation of the blood-supply of the brain”. J. Physiol. (Lond.) 11: 85-108, 1890.4
Basics of Physics and Physiology
• Oxygenated Haemoglobin (OxyHb) is diamagnetic➡ no effect on the magnetic field
• Deoxygenated Haemoglobin (DeOxyHb) is paramagnetic➡ disturbs the magnetic field (natural contrast agent)➡ T2* constant decreases
*The Magnetic Properties and Structure of Hemoglobin, Oxyhemoglobin and Carbonmonoxyhemoglobin Proc Natl Acad Sci U S A. 1936 Apr; 22(4): 210–216; Linus Pauling and Charles D. Coryell
*
μso ∝ n(n + 2)
[2]
n = number of unpaired electrons
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BOLD Effect
(1) Stimulus
(2) Initial Dip
(3) ca. 5s to peak
(4) poststimulus undershoot1
2
3
4
small change of 0.5% to 15%
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𝑂𝑥𝑦𝐻𝑏𝐷𝑒𝑜𝑥𝑦𝐻𝑏
𝑂𝑥𝑦𝐻𝑏𝐷𝑒𝑜𝑥𝑦𝐻𝑏
𝑂𝑥𝑦𝐻𝑏𝐷𝑒𝑜𝑥𝑦𝐻𝑏
NMR Basics of BOLD fMRI
1. B0 alligns all spins
2. RF-Pulse flips the spins into xy-plane
3. Spins interact with each other Free Induction Decay (FID) Usually decays with time constant T2 Due to field inhomogenities the real time is T2* ca. ms
➡Use EPI for filling k-space
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BOLD signal for Gradient Echo sequence
S = S0 ⋅ e−TE⋅R*2 R*2 = R*2 (0) + R
=1
T*2
;
S0
R*2R
R*2 (0) > > R
TE
- Signal at TE=0 -> no dephasing
- Value for transverse relaxation
- Relaxation due to DeOxyHb
- Echo time
NMR Basics of BOLD fMRI
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NMR Basics of BOLD fMRI
Echo Planar Imaging (EPI):
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NMR Basics of BOLD fMRI
Echo Planar Imaging (EPI):
etc.
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NMR Basics of BOLD fMRI
Spatial resolution:
• Whole brain image requires about 200 x 180 x 180 mm3 volume coverage (cuboid)
• Typical spatial and temporal resolution of 3 x 3 x 3 mm3 and 3s
• Can be improved if not whole brain is covered (usually the case)with parallel imaging using multi-channel coils 1 x 1 x 2 mm3 ; 1.5 s
• Improved spatial resolution and decreased acquisition time reduces the SNR
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NMR Basics of BOLD fMRI
Noise:
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NMR Basics of BOLD fMRI
Noise:
fMRI data is very noisy ➡ have to repeat the experiment many times
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NMR Basics of BOLD fMRI
Noise:
fMRI data is very noisy ➡ have to repeat the experiment many times
physiological noise: • cardiac circuit ≈ 0.9 Hz • respiration ≈ 0.3 Hz • CSF movement • change in vessel diameter
} avoid tasks with the same frequency &
tell the patient not to take big breaths
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fMRI in Practice
What do you need for a fMRI experiment?
• as always: MRI scanner and patient
• goggles or mirror and screen
• response grips, joystick or similar
• headphones
• sync-box
• paradigm [2] [2]
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fMRI in Practice
What is a Paradigm?
• task the patient has to perform inside the scanner
• if possible one that ‘isolates a sense’
Examples:
• flickering checkerboard (vision); hand movement (motor)
• memory game (prefrontal cortex); ball game (lymbic system)
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rest
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fMRI in Practice
activation
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fMRI in Practice
activation
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fMRI in Practice
activation
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fMRI in Practice
activation
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fMRI in Practice
activation
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fMRI in Practice
activation
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fMRI in Practice
activation
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fMRI in Practice
activation
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fMRI in Practice
activation
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fMRI in Practice
activation
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fMRI in Practice
activation
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fMRI in Practice
activation
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fMRI in Practice
rest
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fMRI in Practice
Stimulation
rest rest rest restactivation activationactivation
(10-40)s (10-40)s (10-40)s (10-40)s(5-20)s (5-20)s (5-20)s
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fMRI in Practice
fMRI in Practice
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fMRI in Practice
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BOLD-fMRI vs. PETBOLD-fMRI PET
• Non invasive
• No contrast agents
• No radioactive isotopes
• Patient has to lie still
• Spatial resolution: ca. 2 mm3
• Noisy (acoustic)
• Cannot trace paths of chemicals (DTI)
• “Non invasive“
• Contrast agents
• Radioactive isotopes
• Small movement allowed
• Spatial resolution: ca. 5 mm3
• No noise
• Traces paths of chemicals
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Applications and Research
Applications:
• Localisation and test of cognitive functions
• Brain tumor surgery
• Care and treatment of epilepsy
• Diagnoses and management of Alzheimer‘s disease
• Diagnoses and management of psychiatric disease
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Applications and Research
Research:
• High field fMRI (7T)
• High resolution fMRI with EPIK
• Realtime fMRI and neurofeedback
• combination of fMRI with MEG or EEG
• combination of fMRI and PET
• Combination of fMRI, PET and MEG or EEG
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Summary
• BOLD-fMRI is used to visualise the active brain regions
• Deoxygenated haemoglobin is paramagnetic and thus disturbs the magnetic field
• non active brain regions have a shorter T2*
• the signal will decay faster
• with paradigms different brains’ parts can be activated and the response can be measured
• fMRI experiments are very noisy and have to be repeated several times
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Thank you for your attention!
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Bibliography• https://askabiologist.asu.edu/brain-regions [1]
• https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Supplemental_Modules_(Inorganic_Chemistry)/Crystal_Field_Theory/Magnetic_Moments_of_Transition_Metals [2]
• https://www.nordicneurolab.com/en/research/hardware [3]
• https://www.mriquestions.com/index.html
• https://www.fz-juelich.de/inm/inm-4/DE/Forschung/MR-Physik /TeamFmriDE/fmri&PET&EEG/_node.html
• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3787513/
• lecture on fMRI Live Eikenes NTNU Trondheim 2018
• The physics of functional magneticresonance imaging (fMRI), Richard B BuxtonDepartment of Radiology, University of California, San Diego, 04.09.2013
• Physiological measurements using ultra-high field fMRI: a review, Sue Francis and Rosa Sanchez Panchuelo, University of Nottingham, 13.08.2014
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