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MedSci 317 > Module A - Anatomical and functional brain imaging > Flashcards

Module A - Anatomical and functional brain imaging Flashcards

1
Q

What are the four main techniques of brain imaging?

A
  1. Radiography and X-ray angiography
  2. CT (computerised tomography)
  3. PET (positron emission tomography)
  4. MRI (magnetic resonance imaging)
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2
Q

Which imaging techniques can be combined to to show functional and anatomical information?

A

CT and PET

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3
Q

Which imaging techniques can be combined to reduce the radiation dose of functional and anatomical imaging?

A

PET and MRI

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4
Q

Describe the physical basis of radiography and X-ray angiography:

A

Absorption of X-rays by tissues (high energy visible light, short wavelength)

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5
Q

Describe the procedure of radiography and X-ray angiography:

A

Broad beam X-rays passes through the head onto X-ray film
Results in projection of 2D image of the 3D object
Intensity is in proportion to absorption of X-rays
For angiograms, contrast agent is injected intravascularly (contrast agent has high attenuation coefficient)

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6
Q

What is the attenuation coefficient?

A 
A summation of all attenuations along the part the X-ray had to travel of tissue
Thicker objects (higher density) have a high attenuation coefficient
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7
Q

Describe the uses of radiography and X-ray angiography:

A

Skull fractures, bone abnormalities (bone tumours) absorb X-rays as they are quite dense
Symmetry of structures can be observed (pineal gland, Ca2+ displaced to one side in abnormalities)

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8
Q

Describe the advantages of radiography and X-ray angiography:

A

Very good spatial resolution (0.05mm)
Good depication of the skull (fractures, bone tumours)
Suburb spatial resolution

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9
Q

Describe the disadvantages of radiography and X-ray angiography:

A

Projection images only (contrast not very good for soft tissue)
Poor intrinsic tissue contrast
Use of ionising radiation (interact with free-radicals to cause cancer)
Some risk associated with injection of contrast

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10
Q

Describe the physical basis of computerised tomography:

A

Absorption of X-rays by tissues (slices)

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11
Q

Describe the procedure of computerised tomography:

A

Very narrow X-ray beams are projected through the head onto detectors (array)The source is rotated about the head to acquire many views (projections)
2D image of the slice is reconstructed using filtered backprojection
An adjacent slice is imaged, building up a 3D image
Filtered back projection clears up the blurry image
There can be artifacts due to the limited number of projections

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12
Q

Describe the uses of computerised tomography:

A 
Emergency situations (checking for internal damage/bleeding)
Tumours, stroke, anatomical defects (haemorrhage, absess,, white/grey matter differences)
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13
Q

Describe the advantages of computerised tomography:

A

Tomographic
Can resolves grey and white matter, blood, CSF
High resolution (1mm)

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14
Q

Describe the disadvantages of computerised tomography:

A

Use of ionising radiation
Some risk associated with injection of contrast
Anatomical images only (not functional images)

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15
Q

Describe the physical basis of positron emission tomography:

A

Radioactive isotopes of certain elements (e.g. 11C, 13N, 15O, 18F) can be substituted/injected into biologically important compounds (glucose)
Decay of atoms emits positrons
Positrons collide with electrons, producing two gamma rays 180 degrees apart (matter meeting antimatter)
Detection of gamma rays reveals positron of labelled molecules
Like filtered back transmission

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16
Q

Describe the procedure of positron emission tomography:

A

Isotope labelled compounds are injected or inhaled
e.g. 18F labelled 2-deoxyglycose taken up by neurons, phosphorylated by hexokinase, cannot be further metabolised, builds up in cells
Gamma rays detected by array of crystal photomultipliers
Images of activity are reconstructed using tomographic techniques

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17
Q

Describe the uses of positron emission tomography:

A

Glucose imaging of metabolism
Blood flow imaging using labelled water (15O) (blood flow is a good indicator of local brain function through unknown mechanism)
Image subtraction can highlight area associated with specific tasks

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18
Q

Describe advantages of positron emission tomography:

A

Brain function can be imaged
Also distribution of receptors (e.g. dopamine)
Blood flow using labelled water

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19
Q

Describe the disadvantages of positron emission tomography:

A 
Anatomical images not obtained (only functional)
Poor resolution (2-8mm) (physical limitation of positron relocation until it annihilates)
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20
Q

What is the radiation dose?

A

Effective dose (tissue dependent)
Measured in millisieverts (mSv)
Measures probability of cancer/genetic damage
1Sv=5% probability of getting cancer

21
Q

Describe the radiation doses of imaging techniques:

A

Plain-film x-ray 0.05mSv
CT 5-20mSv
PET 5-7mSv
PET/CT 25mSv

22
Q

Describe the physical basis of magnetic resonance imaging:

A

Some nuclei (H, C, P, Na) have spin and small magnetic field
Nuclei become aligned in magnetic field
Radiofrequency energy can be used to excite the nuclei from their ground state
While the nuclei are relaxing back to their ground state they emit radiowaves
Radiowaves are detected by a coil of wire placed around the patient

23
Q

What is NMR sensitivity relative to?

A

The size of the magnetic moment (signal size given equal number of nuclei at equal size field)
Measurement of sensitivity is a combination of NMR sensitivity and relative abundance in the body (H very abundant and magnetic field very strong)

24
Q

Describe the frequency of precision:

A

w=yB0
Typical clinical scanner B0=1.5T
Earths magnetic field ~0.00005T
y=42.47MHz for proteinsl, w is about 65MHz and 1.5T

25
Q

What are the two distinct relaxation processes?

A

Spin-lattice (T1)- longitudinal relaxation ~800ms

Spin-spin (T2)- transverse relaxation ~30ms

26
Q

In terms of spin echo, what gives contrast to the image?

A

Altering TR and TR gives different contrasts to the image

27
Q

Shorter TR and TE gives more ________ weighting

A

T1

28
Q

Longer TR and TE gives more ________ weighting

A

T2

29
Q 
Time constants depend on the chemical environment of the nucleus:
Solids
Water
Tissue
White matter and grey matter
Tumours
Fat
A 
Very short T1 and T2 (us)
Very long T1 and T2 (seconds)
T2 shorter than T1
White matter shorter T1 than grey matter
Long T1
Shorter T1 than water
30
Q

Describe the appearance of CSF (and WM) in T1 and T2 weighted images:

A

T1 weighted has dark CSF and bright WM

T2 weighted has bright CSF

31
Q

Which times of T1 weighting apply in each of these tissues:
Grey matter
White matter
Cerebrospinal fluid

A

950ms
600ms
4500ms

32
Q

Which times of T2 weighting apply in each of these tissues:
Grey matter
White matter
Cerebrospinal fluid

A

100ms
80ms
2200ms

33
Q

Describe the uses of magnetic resonance imaging:

A

Proton decay imaging
T1/T2 weighting (T1 good white/grey matter constrast, T2 bright CSF)
Angiograms
Diffusion weighted imaging for early stroke (signal is dependent on diffusion coefficients)
BOLD
In vivo spectroscopy

34
Q

Describe the advantages of magnetic resonance imaging:

A

Signal can be dependent on tissue type (very good contrast between grey and white matter, tumours) due to the individual environment of nuclei
Ability to change contrast with a wide variety of imaging protocols
Sensitivity to motion (angiograms, diffusion)
Sensitivity to blood oxygenation (functional imaging)
Good resolution (<1mm)
Non-invasive (no ionising radiation)

35
Q

Describe the disadvantages of magnetic resonance imaging:

A

Motion, artifacts can destroy BOLD signal
Contraindications (pacemarkers, implanted devices, aneurism clips
Claustrophobia due to small bore (slow)

36
Q

Describe diffusion weighted imaging (MRI):

A

Diffusion is random molecular motion
MRI can be sensitive to diffusion through signal loss
Diffusion weighted imaging gives relative signal loss in areas of high diffusion
Useful in stroke

37
Q

Describe the effects of a low diffusion coefficient (DWI MRI):

A

Spins don’t move very far
Experience small differences in magnetic field environment
Little dephasing
Little signal loss

38
Q

Describe the effects of a high diffusion coefficient (DWI MRI):

A

Spins explore a larger region of space
Experience greater difference in magnetic fields
More dephasing
Significant signal attenuation

39
Q

Describe the uses of diffusion tensor imaging in MRI:

A

Can measure the direction of diffusion and amount of anisotropy
Useful in white matter tract injury

40
Q

What is anisotropic diffusion and how does it arise?

A

Diffusion is not necessarily isotropic

Signal attenuation changes for different diffusion gradient directions

41
Q

Describe white matter tracking (diffusion tensor imaging):

A

Cell boundaries restrict diffusion across the fibres (observed in white matter fibres and muscle cells)
Principle direction of diffusion can be used to track fibre bundles

42
Q

Describe the result of activation of neurons requiring energy in fMRI:

A

Increased demand for oxygen and other nutrients

Increased cerebral blood flow

43
Q

Describe the properties of diamagnetic Hb-O2:

A

Similar magnetic properties (isomagnetic) compared with surrounding tissues
Lower magnetisation
No magnetic field inhomogenities

44
Q

Describe the properties of paramagnetic Hb:

A

Without O2, paramagnetic compared with surrounding tissues
High magnetisation
Field inhomogeneities
Enhanced T2 decay

45
Q

What type effect is BOLD?

A

A T2 effect

46
Q

Describe the changes of BOLD from resting state to activated state:

A 
ACTIVE neurons
INCREASE regional O2 demand
INCREASE cerebral blood flow
INCREASE OxyHb and DECREASE DeoxyHb
DECREASE magnetic susceptibility
INCREASE T2*
INCREASE signal
47
Q

Describe the practical considerations of BOLD fMRI:

A

Not a direct measure of brain activity (input not output)
Good spatial resolution but poor temporal resolution
No anatomical information - can be overlaid with anatomical scans
Typical repeated multiple times (fMRI paradigms (pattern of tasks), e.g. 30 sec active, 30 sec inactive)

48
Q

Describe the parameters of spatial and temporal resolution of BOLD fMRI:

A

Blood flow response ~ slow 5 second delay
Needs to be compared to rest state
Can combine with other modalities like EEG, MEG

MedSci 317 (7 decks)

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