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Brain Imaging Methods > Structual Neuroimaging > Flashcards

Structual Neuroimaging Flashcards

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1
Q

MRI vs fMRI

A

MRI looks at anatomy, while fMRI looks at the brain function

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2
Q

Magnetic fields

A

Magnets produce these magnetic fields
- attraction and repulsion of materials
- MRI machine can be dangerous if used incorrectly, due to the strong magnetic pull
- influence on nuclei -> nuclear magnetic resonance imaging

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3
Q

Nuclear magnetic resonance imaging NMR

A

Part of nuclei align with direction of the magnetic field
=/= radioactivity
- can cause nausea
- but no consequent short- or long-term illness

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4
Q

Hydrogen in fMRI

A
  • human body is 55-78% water
  • water molecules contain hydrogen atoms
  • hydrogen atoms only contain one single proton
  • atoms with uneven number of protons act as dipoles (even cancel eachother out and are not magentic)
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5
Q

Precess

A

Nuclei with odd number of protons/neutrons spin (precess)
- Larmor frequency
- depends upon magnetic field strength of the magnet

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6
Q

What causes Nuclear magnetic Resonance Imaging

A

If additional magnetic field oscillates with the Larmor frequency, nuclei absorb energy from the field

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7
Q

Static magnetic field

A

Atoms align with direction magnetic field

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8
Q

Oscillating magnetic field

A

If you add another magnetic field = radio frequency pulse (RF)
-> atoms spin/recess and get in phase (they will synchronise)
-> atoms flip and take direction of oscillating field
-> increase in energy state

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9
Q

When you remove RF pulse

A
  • dephasing of atoms (spinning will be random
  • realigning to static field (flip back) and this emits energy which you can see in the scan!
    –> small signals over all the re-aligning nuclei integrate (stronger signal, when less dephasing)
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10
Q

Gradient of field strenght
- where does the signal come from?

A

Gradients are additional magnetic field over space: 3 different ones
–> they rely on the fact that the Larmor frequency depends on the field strenght
- if you add magnetic fields the Larmor will become larger

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11
Q

If magnetic field strength differs across space

A
  • nuclei in different locations have a different Larmor frequency
  • RP fulse only affets the nuclei with matching Larmor frequency
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12
Q

Three orthogonal gradients of field strength

A
  • slice sletion gradient (at time of RF pulse Z)
  • phase selection gradient (dephasing after RF pulse Y)
  • frequency encoding gradient (at time of read out to signal X)
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13
Q

Slice selection gradient

A
  • Applied during RF pulse
  • RF pulse only affects nuclei that experience a total field strength with matching Larmor frequency
  • Slice: volume of excited nuclei
  • one slice per RF pulse for 2D, for full 3D there are as many RF pulses as there are slices
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14
Q

Interleaved slice acquisition

A

To minimize cumulative effects due to cross-slice excitation
- first all the odd slices, then the even slices for better results

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15
Q

Phase encoding gradient

A
  • 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 differenes in phase persist
  • all nuclei at a certain position in the gradient have same phase, that phase is informative about position
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16
Q

Frequency encoding gradient

A
  • applied during data acquisition
  • = the read-out direction
  • all nuclei at a certain position in gradient have same resonane frequency, thus frequency at read-out is informative about position
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17
Q

Summary of the gradients

A
  • z-gradient (slice) cause slice selection
  • y-gradient (phase) shows different phases
  • x-gradient (frequency) shows the different frequencies
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18
Q

Pulse sequence

A

Succession of RF pulses and gradient changes
- pulse sequences differ in a number of ways

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19
Q

Pulse sequenes differ in a number of ways

A
  • what happend prior to the RF pulse
  • form and amplitude of the RF pulse
  • direction and the amplitude of the gradients
  • occurrence of one or multiple so-called gradient reversals
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20
Q

Voxels

A

Unit of space
- the shorter the time in which an image has to be taken, the lower the number of slices that can be imaged
–> the number of voxels per row/column in the slice relates back to number of steps of phase/frequency encoding gradient

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21
Q

How do these physical principles give rise to an image with anatomical structure?

A

Emitted signal decays over time and signal intensity depends upon sevel factors:
- density of H protons
- T1-recovery
- T2-decay
Factors are different in different tissues, resulting in signal contrast
Pulse sequence and parameter choice determine which factor has most weight

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22
Q

T1-recovery

A

Recovery of longitudinal orientation = spin-lattice relaxation
- realign with static magnetic field

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23
Q

T2-decay

A

Loss of transverse magnetization due to the loss in phase coherence = spon-spin interctions.
- Immediately after application of 90* RF pulse, transverse magnetization is maximized. It then begins to dephase due to natural interaction at anatomic or molecular levels. The signals from these dephasing protons begin to cancel out => MR signal decreases

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24
Q

T1-recovery time

Study These Flashcards
A

= time it takes the longitudinal magnetization to grow back to 63% if its final value (all flipped back is 100%)
= spin-lattice relaxation time

25
T1-weighted image
Images only the T1 recovery time - Faster T1-recovery means less dephasing and therefor it will show up brighter on the image - because this differs for different tissues, you can clearly see the differences between different tissues and you get a nice image
26
T2-decay time
= time that it takes the transverse magnetization to decrease to 37% of its starting value = spin-spin relaxtion time --> basically the opposite of T1
27
T2-weighted image
It is basically the same at T1, but the opposite.
28
Recap T1 and T2 time
T1-time: time it takes to relax back in alignment with B0 T2-time: time it takes to dephase -> two processes, which happen simultaneously and are both tissue-dependent - using various pulse sequences that orchestrate the gradient coils, we can measure location specific activations - YT video slide 25
29
3 components MRI scanner
Magnet (static field) Gradient coild (varying magentic field) Radio frequency coil (RF pulse) --> magnet and gradient coils create varying magnetic fields. Radio frequency coils transmits and measures radio frequency waves
30
Magnet
Electrons flow along a wire - Faraday's principle
31
Faraday's principle
Electric current in a loop of wire generates transient magnetic fied perpendicular to the loop of wire - left hand principle
32
Magnetic field strength
Proportional to number of loops - increase in current also increases field strength Higher field strength: - higher signal, higher resolution and more contrast - more expensive and also more artefacts
33
CT scans vs. MRI
Shows a difference in contrast - CT scan mainly in clincial context (inflammation, infection, TBI, stroke, tumor, ...)
34
3 structural imaging methods
- structural T1-weighted MRI - diffusion weighted imagin DWI - magnetic resonance spectoscopy MRS
35
Image artefacts
Things that can disrupt the outcome of the image - due to fixed materials, like braces - due to removable items, like hair pins - due to movement
36
Finding anatomical abnormalities
Goal of study (clinical diagnostics) or incidental
37
Routine pulse sequences
Robust to acquisition problems - but e.g. high-field scanners
38
Voxel based morphometry VBM
Morphometry = quantity specific properties of brain anatomy - compare regional volumes of tissue and produce map of statistially significant differences among populations of subjects Problems: normalization problems and tot brain volume differences
39
Relevance of brain structure for behavior
- reveal neuroanatomical abnormalities with devestating effects upon behavior - normal brain structure: relation to various behavioral variables
40
Connectivity
'Nothing defines the function of a neuron better than its connections' - neurons gets input and gives output to and from other neurons as well
41
Pattern of action potentials
From wehre and to where are the action potentials - Paths = axons - in humans use DWI (non-invasive connectivity imaging)
42
Diffusion weighted imaging
Image with large bunderls of 1000s of axons - axons that start and end in each other's vicinity,s tay together --> white-matter pathways or tracts
43
How to DWI
Pulse seqeunce is adapted to be sensitive for diffusion of molecules => DWI - molecules move from parts with higher to parts with lower concentration - Cell walls and myelin impede such motion --> anisotropy in diffusion
44
Types of diffusion
Isotropic diffusion: can go whereever, equal in all directions (restrictend an unrestricted) Anisotropic diffusion: restricted so can only go in certain directions
45
Diffusion tensor imaging DTI
Quantify amount of diffusion in each possible direction in 3D space - diffusion described by tensor
46
DTI indices
- mean diffusion MD - fractional anisotrpy FA - axial and radial diffusivity AD and RD --> differential sensitivity to diffusion-related phenomena
47
Mean diffusion
Overall amount of diffusion
48
Axial and radial diffusivity (AD and RD)
Amount of diffusion in certain direction: main axis or other directions
49
Fractional anisotrpy (FA)
From zero (isotropic diffusion) to 1 (only diffusion along main axis)
50
Tractography
Trace a line following main diffusion direction - projection: connect cortical with subcortical - commissural: connect hemispheres - association: connect distal areas
50
Orientation map
Slide 44
51
DWI relevance for behavior
- can look for disconnections (influences behavior) - FA affected in various mental disorders (schizophrenia, depression, autism etc.)
52
Magnetic Resonance Spectroscopy MRS
Quantify concetration and spatial distribution of specific molecules in brain - spectroscopy - spectrum
53
Spectroscopy
Method in which a signal is decomposed into its freqeuncy components
54
Spectrum
Shows strength/amplitude of each frequency component - abnormal ratios between the peaks prove metabolic dysfunction
55
Underlying principle of MRS
Similar to MRI: radio-frequency waves affect spin of nuclei in magnetic field MRI: spin of H in water molecules <-> MRS reflects resonance of other molecules
56
Intended vs unintended variations in frequency
Signal generated by atom is affected by its local chemical environment => each molecule has different resonant freqeuncy = chemical shift - use variation in frequency to measure checmical composition of brain tissue
57
MR spectrum
Reflects particle resonance (ppm; parts per million) of metabolites that are associated with specific neurottransmitters or other substances in the brain tissue - reference: Larmor frequency of tetramethylsilane

Brain Imaging Methods (9 decks)

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