HC5 Structural Neuroimaging Flashcards
MRI and fMRI
- MRI: anatomy
- fMRI: brain function
- You need to take a MRI before taking a fMRI as you have to know the anatomy before being able to make a FMRI scan.
- Same machine used for both scan types, based on the same principle
Magnets produce magnetic fields
o Strong visible attraction and repulsion effects on range of materials
o MRI safety: life or death
▪ NEVER bring anything metal into a MRI room!
o Less obvious effect on human body: influence on nuclei
Less obvious effect on human body: influence on nuclei
→ nuclear magnetic resonance imaging (NMR)
▪ = MRI
▪ Nuclear because of the effect on nuclei it has
▪ ≠ Radioactivity
▪ Nausea (because of influence of magnet on the nuclei in the brain, if you get up quickly)
▪ No consequent short- or long-term illness
Most relevant element for brain imaging
Hydrogen
o Human body is made of ~ 70% water, the brain ~ 75%
o Water molecules contain hydrogen atoms
o Hydrogen atoms only contain one single proton
o Hydrogen could take another electron in its innermost shell
o Atoms with uneven number of protons act as dipoles. Dipoles have an positive and a negative side, two “poles”. Basically, they are small magnets.
Nuclei with odd number of protons/neutrons spin at the Larmor frequency
o Depends linearly upon magnetic field strength of the magnet (For 1H: 1.5T = 63.76 MHz, 3T = 127.7 MHz, 7T = 298.0 MHz)
o A stronger magnetic field causes the protons/neutrons to spin faster
Nuclear MAGNETIC resonance imaging
Part of nuclei align with direction of the magnetic field
Nuclear magnetic RESONANCE imaging
If additional magnetic field oscillates with the Larmor frequency, nuclei absorb energy from the field.
Nuclei resonate with the additional magnetic field you add
Static magnetic field
Atoms align with direction magnetic field
Oscillating magnetic field
= radio frequency (RF) pulse
o Spins individual atoms so they get in phase
o Atoms flip and take direction of oscillating field
▪ Increase in energy state
When RF pulse is no longer applied
o Dephasing of atoms
o Realigning to static magnetic field (flip back)
▪ emits energy = small signal in radio
frequency range -
▪ This energy you measure in the MRI
Small signals over all the re-aligning nuclei integrate
o The less dephasing happened, the stronger this signal is
o The time it takes to go back to equilibrium depends on the kind of tissue
Gradients of field strength
- Remember: Larmor frequency depends upon field strength
- Gradients: as linear as possible, stationary and of short duration
- Static magnetic field differs across space
Three orthogonal gradients of field strength on top of static magnetic field
Static magnetic field differs across space
o Nuclei in different locations have a different Larmor frequency
→ RF pulse only affects the nuclei with matching Larmor frequency
Three orthogonal gradients of field strength on top of static magnetic field
o Slice selection gradient (Z): applied at time of RF pulse
o Phase encoding gradient (Y): use of dephasing after RF pulse
o Frequency encoding gradient (X): applied at time of read out of signal
Slice slection gradient
Applied during RF pulse
RF pulse only affects nuclei that experience a total field strength with matching Larmor frequency
Slice: volume of excited nuclei
Every slice has a different frequency
One slice per RF pulse if 2D image
Interleaved slice acquisition: to minimize cumulative effects due to cross-slice excitation
The excited nuclei (the slice) are affected by the 2 other gradients
RF pulse only affects nuclei that experience a total field strength with matching Larmor frequency
3T (B0) & 127.7 MHz RF pulse & slice selection gradient G from
inferior to superior → RF pulse only affects nuclei in horizontal
slice where summed field strength = 3T
One slice per RF pulse if 2D image
→ scanning a full 3D image requires as many RF pulses as number of slices needed
Interleaved slice acquisition: to minimize cumulative effects due to cross-slice excitation
Less spill over, keeps the slices separate
Phase encoding gradient
- Applied after RF pulse
- Change spin resonance frequency of excited nuclei depending on their location in the gradient, causing dephasing
- When removed, resonance frequencies are the same again, but differences in phase persist
- All nuclei at a certain position in the gradient have same phase, thus phase is informative about position
Frequency encoding gradient
- Applied during data acquisition
- Frequency encoding gradient = the read-out direction
- All nuclei at a certain position in gradient have same resonance frequency, thus frequency at read-out is informative about position
.
Gradient-echo echo planar imaging
- Gradient reversals un-do effect of initial gradient
o signal consists of a series of echo’s elicited by the reversals
Sufficient echo’s: all combinations of phases and frequencies are characterized
o use Fourier analysis to reconstruct image
>< Spin-echo sequence: reverses the RF pulse to create echo
Voxels
- Unit of space = voxel
- The shorter the time in which an image has to be taken, the lower the number of slices that can be imaged
- Number of voxels per row/column in the slice relates back to number of steps of phase encoding gradient
o 128 steps per dimension with field of view of 256 mm → resolution of 2 mm
How do these physical principles give rise to an image with anatomical structure?
- Emitted signal decays over time
- Signal intensity depends upon several (biological) factors
- Factors are different in different tissues, resulting in signal contrast
- Pulse sequence and parameter choice determine which factor has most weight
