fMRI: Biological Basis and Experiment Design Lecture 5: non-BOLD MRI

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fMRI Biological Basis and Experiment
DesignLecture 5 non-BOLD MRI

• Equilibrium and excitation
• Relaxation rates
• Image contrast
• TE
• TR

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Equilibrium (classical picture)
In free space, these spins have no preferred
orientation, but in a magnetic field, they
like to be aligned with the field.
No net magnetization in free space
Sample has net equilibrium magnetization, M0, in
"magnet".
B0
M0
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Excitation (classical picture)
Equilibrium
Excitation
B0
B0
?
M0
M
M0 is rotated away from positive z-axis by
excitation pulse, through a flip angle, ?.
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Excitation (classical picture)
Excitation
M
M
B0
?
M?
Magnetization vector is always described by 2
components, longitudinal and transverse.
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Relaxation rates

• T1 longitudinal (spin-lattice) relaxation
• Exchange of energy between protons and their
  environment
• Time-scale around 1s

M
time
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Relaxation rates

• T1 longitudinal (spin-lattice) relaxation
• T2 transverse (spin-spin) relaxation
• Exchange of energy between protons
• Time-scale 50ms

M?
time
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Relaxation rates

• T1 longitudinal (spin-lattice) relaxation
• T2 transverse (spin-spin) relaxation
• T2 transverse relaxation in the presence of
  magnetic field inhomogeneities
• Bread and butter of BOLD fMRI

B0
"B0" is never perfectly uniform - macroscopic
variation - microscopic variation
?
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The diverse environment in a voxel
Imaging signal comes from protons on water
molecules.
Sensitive to microscopic variations in B0.
375 ?m
Reina de la Torre et al (1998) Anatomical Record
25187
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Paramagnetic deoxygenated blood is different from
diamagnetic tissue
Protons surrounded by paramagnetic environment
will resonate at a slightly higher frequency
(near veins with deoxyhemoglobin)
Protons surrounded by diamagnetic environment
will resonate at a lower frequency (when tissue
and/or blood is fully oxygenated)
375 ?m
Reina de la Torre et al (1998) Anatomical Record
25187
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Heterogeneous magnetic field environments result
in short T2 decay times
375 ?m
Reina de la Torre et al (1998) Anatomical Record
25187