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Cellular Pathology > MRI & MRS > Flashcards

MRI & MRS Flashcards

1
Q
  1. What is CT best used for?
A

Bony structures

Can be used for soft tissue though

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2
Q
  1. What is MRI best used for and why?
A

Soft Tissue and signal is being picked up by protons in fat, in water

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3
Q
  1. What are two types of MRI?
A
  • Proton Density (PD)

- T2- Weighted (T2)

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4
Q
  1. What is the contrast in CT images due to?
A

Tissue density density dependent attenuation of x-rays

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5
Q
  1. What does signal density depend on?
A

Hounsfield Number (how much of the x-rays are being attenuated)

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6
Q
  1. What does attenuated mean?
A

Attenuation is the reduction of the intensity of an x-ray beam as it traverses matter. The reduction may be caused by absorption or by deflection (scatter) of photons from the beam

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7
Q
  1. In terms of hounsfield number, what would be high and what would be low?
A 
High = Bone
Low = Tissue
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8
Q
  1. What is the range of hounsfield number?
A

-1000 (Air) to 1000 (Cortical Bone)

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9
Q
  1. What does the range -100 to +100 cover in hounsfield numbers?
A

Water

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10
Q
  1. MRI image contrast shows the relative signal intensities between diff tissue types and pathologies , what does this depend on?
A

Physical properties of the tissue such as water and fat content, cellular structure and cell density.

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11
Q
  1. Is MRI sensitive? If so, what to?
A

MRI contrast is very sensitive to changes in a large variety of the physical properties of tissue water and blood

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12
Q
  1. What does T1 relaxation show?
A

Paramagnetic blood breakdown products

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13
Q
  1. What does T2 relaxation show?
A

Tissue Fluidity - oedema, deoxyhaemoglobin

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14
Q
  1. What does water diffusion/movement (micro) ?
A

Cell membrane integrity, cell size

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15
Q
  1. What does flow (macro) show?
A

Blood flow

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16
Q
  1. What does perfusion show?
A

Blood flow, blood vessel density

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17
Q
  1. How does T2 weighted MRI look?
A

Well, its more fluid bright so CSF shows up bright and fatty tissue shows up light –> indicates pathological changes

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18
Q
  1. How does T1 weighted MRI look?
A

CSF shows up dark–> used for anatomical changes due to contrast

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19
Q
  1. What does anisotropy mean?
A

It means that something is directional - has different properties depending on the direction its in

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20
Q
  1. What do anisotropy maps show?
A

Show anistropic water diffusion ( diff in diff directions) in the brain

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21
Q
  1. What are the magnets within us?
A

Proton from H atom of water -> Magnetic moment (spins on its axis) –> protons line up with magnetic field of scanner—> can be manipulated by radio freq to produce signals which can be picked up.

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22
Q
  1. What does the positive charge of a spinning proton produce?
A

A magnetic moment (u )

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23
Q
  1. What magnetic field do the protons line up in?- how do they align?
A

B0

They don’t align perfectly along compass needle but instead they align with earths magnetic field.

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24
Q
  1. What do the protons do after they align?
A

They precess (rotate )

25
Q
  1. What is Lamor Frequency V( with a little capital L next to it)
A

When the freq of precession is proportional to the magnetic field

26
Q
  1. BRIEFLY describe process of MR imaging please?
A
  1. BO HAS MAGNETIC FIELD
  2. RF PUT THROUGH COILS
  3. SIGNALS –> AMPLIFIER–> COMPUTER
  4. MAGENTIC FIELD GRADIENTS -> ENCODE SIGNALS–> IMAGE
27
Q
  1. What is the Bo? What does it contain?
A

Bo is what the patient lies in
It has a strong and uniform magnetic field
it contains imaging coils that a radio-frequency current is put through

28
Q
  1. How do magnetic field gradients encode the signals sent to amplifiers, to create an image on the computer?
A

Spatial Variation in frequency allows us to see where the signal is coming from

29
Q
  1. What is one rule for safety in MRI?
A

No ferromagnetic objects in the examination room ( Scissors, stethoscopes, wheelchairs, gas cylinders, hearing aids,watches , spectacles )

30
Q

30 . What are some Contraindications (cant be done because it would be harmful for the patient) for MRI

A 
•	Pacemakers
•	Infusion pumps
•	1st trimester 
        pregnancy
•	Aneurysm clips
•	Metallic foreign 
        bodies (orbit x-ray, 
        shrapnel)
31
Q
  1. What does the strong magnetic field create in all tissues?
A

The strong magnetic field creates magnetisation (from protons in water/fat) in all the tissue which can be manipulated by radiofrequency pulses to produce an MRI signal to create an image.

32
Q
  1. What does the intensity of the image depend on?
A 
Water content
Tissue structure
Blood Flow
Perfusion
Diffusion
Paramagnetic
33
Q
  1. What is it called when the magnetisation from tissue is aligned with the magnetic field
A

Equilibrium (MO)

34
Q
  1. Radiofrequency interacts with and knocks out the magnetism from its previous alignment, into which plane?
A

The xy plane (Mxy)

35
Q
  1. Whats the difference between T2 and T1 relaxation?
A

T2 relaxation: When the magnetisation dies with time, its referred to as T2 signal decay in xy.

T1 relaxation: When the magnetisation recovers into alignment, it’s referred to as T1 signal recovery along the z plane.

36
Q
  1. Explain the process of MRI signals and contrast (T2) ?
A

o A radiofrequency pulse generates an MRI signal from the tissue, the MR image is built up from a series of signal acquisitions.
o Initially there is a delay in picking up a signal. The time for this (echo time) depends on T2 relaxation.
o Initially, the signal is intense but it decays with time in an exponential function.
o Longer echo time: weaker signal.

37
Q
  1. The following questions refer to T2 weighted image of a brain:
    - Why would be put in delays
    - What has a longer relaxation time, CFG or white/grey matter?
    - Do Lesions increase or decrease T2 ?
A

o We can put in delays so that the signal intensity is dependent on the tissue.
o Relaxation time for CSF is much longer (1000ms) than white or grey matter (80/90ms).
o Lesions increase T2 causing an increase in signal intensity.

38
Q
  1. Where does the signal come from in T2 relaxation in tissues?
A

o The signal comes from all the water protons in tissue (intra- and extracellular water and also cellular structures e.g. cell membranes).

39
Q
  1. What decays quicker Free water or bound water
A

Bound water–> Decays very quickly

Free water—> Decays very slowly

40
Q
  1. What will provide a stronger signal - a shorter or longer echo time?
A

Shorter echo time ( due to less decay)

41
Q
  1. If you had a graph measuring signal intensities at diff echo time, what would change the position of the curve on the graph?
A

Pathological Changes

42
Q
  1. What would expect to see on a T2w image of a high grade brain tumour?
A

more brighter because the signal from the normal brain decays more quickly than the CSF (outside).
The signal intensity is higher at any given echo time

43
Q
  1. Whats the difference between proton density and T2 weighted?
A

o Proton density (short echo time): signal is proportional to the tissue and water.
o T2 weighted (longer echo time): more brighter because the signal from the normal brain decays more quickly than the CSF (outside).

44
Q
  1. What does a quantitive T2 map show?
A

T2 relaxation time

45
Q
  1. Summarise T2 ?
A

o The T2 of tissue determines how quickly the MRI signal decays away after the radiofrequency pulse

46
Q
  1. T2 is very dependent on how mobile the water is in the tissue and increases with what two things?
A
  • Oedema, an increase in water content

* Demyelination, a loss of brain tissue structure

47
Q
  1. What is T2 reduced by?
A

presence of paramagnetic ions e.g. Fe from blood breakdown products and Gd from contrast agents.

48
Q
  1. How does MRI signals and contrast work for T1?
A

o An MR image is built up from a series of signal acquisitions from when the magnetisation returns to alignment to the Z plane.
o CSF magnetisation has a long T1 so the magnetisation takes much longer to come back.
o Brain tissue magnetisation has a shorter T1 so it comes back much more quickly.
o Image contrast: comparison of how quickly the magnetisation recovers at a certain point echo time.

49
Q
  1. How do we form T1w images?
A 
o	Enter a sequence of pulses with short echo time (no T1 recovery) and short repetition time. 
o	CSF (longest T1) shows up dark and fat (shortest T1) shows up bright). 
o	White matter has a shorter T1 as it’s more rigid and myelinated. 
o	Useful for detecting volumetric changes that occur with various diseases
50
Q
  1. Does grey matter increase or decrease with age?
A

Grey matter volume decreases with age.

51
Q
  1. What would you see in a T1w image of a dementia patient?
A

Larger dark space due to more CSF in the area as the hippocampus gets smaller and enlargement of the ventricles.

52
Q
  1. What is the taxi driver effect?
A

o 3D T1 images used to determine an increase volume of hippocampus.
o Taxi drivers have a larger hippocampal volume than normal - neurogenesis proposed to explain need for increased capacity of spatial memory.

53
Q
  1. Summarise T1 relaxation?
A

o Reduced repetition time - magnetisation producing the MRI signal is reduced (“saturated”)
o Tissue with long T1 produces a smaller signal than short T1.
o T1 is also dependent on how mobile the water is in the tissue and T1 increases slightly with oedema.

54
Q
  1. What are contrast Agents?
A

MRI contrast agents are contrast agents used to improve the visibility of internal body structures in magnetic resonance imaging (MRI)

55
Q
  1. Give an example of contrast agents used?
A

Paramagnetic (unpaired electrons) or superparamagnetic (ferrites) are chelated to reduce toxicity and then injected intravenously.

56
Q
  1. What happens after the injection of contrast Agents?
A

o Water in the vicinity of the contrast agent experiences strong fluctuating magnetic fields hence T1 and T2 are reduced.

57
Q
  1. What would you expect to see on a T1w image of glioma (tumour of glial cells)?
A

Tumour core distinguished as contrast enhancement in areas of BBB breakdown – areas of neo-angiogenesis stimulated by tumour growth

58
Q
  1. What are MRI characteritstics of MS?
A

o Active lesion with Gd contrast.
o Number of lesions/lesion type can aid monitoring of disease progression or treatment response.
-Inflammation and Demyelination makes tissue more fluid (T2)
-BBB leakage (T1)

Cellular Pathology (13 decks)

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