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Cellular pathology > MRI and MRS > Flashcards

MRI and MRS Flashcards

1
Q

What is CT best used for?

A

CT is best used for bony structures but can also be used for soft tissue

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2
Q

What is MRI best used for?

A

MRI is best used for soft tissue as signals is being picked up from protons in the fat and water

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3
Q

What is CT contrast due to?

A

CT contrast is due to tissue density dependent attenuation of x-rays

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4
Q

What does the Hounsfield number measure?

A

Hounsfield number measures how much X-ray has attenuated as it passes through any material

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5
Q

What is the first call for brain imaging and why?

A

Ct is the first call for brain imaging as its quick and cheap

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6
Q

What does CT show the difference between?

A

Clearly shows the difference between bone and tissue but anatomical detail is faint

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7
Q

What is image contrast?

A

Image contrast is the relative signal intensities between different tissue types and pathologies

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8
Q

What does MRI image contrast depend on?

A

Depends on physical propterties of the tissue such as water and fat content, cellular structure, cell density

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9
Q

What is T2 weighted MRI more sensitive to and how does CSF show up in it?

A

T2 weighted MRI is more sensitive to water

-CSF shows up bright

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10
Q

What is T1 weighted MRI more sensitive to and what does it show?

A
  • T1 weighted MRI is more sensitive to structural qualities

- Shows white and grey matter

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11
Q

What is nuclear magnetism?

A

-Nuclear magnetism is the positive charge of a spinning proton producing a magnetic moment

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12
Q

What is resonance?

A

Resonance in a magnetic field B0 is the magnetic moment of a proton which processes at the Larmor frequency

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13
Q

Steps involved in MRI

A
  • Patient lies in scanner with powerful magnet
  • All protons in their body align with the magnetic field but don’t generate a signal until a radiofrequency pulse is inserted which knocks. Out magnetisation to 90 degrees, generating a MRI signal
  • Magnetic field gradients are used to encode the signal in space so computer can generate an image
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14
Q

What do strong magnetic fields create?

A

Strong magnetic field creates magnetisation in all the tissue

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15
Q

What is magnetisation from in water and tissue?

A

Magnetisation is from protons in water and fat in tissue

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16
Q

What can magnetisation be manipulated by?

A

Magnetisation can be manipulated by radiofrequency pulses to produce an MRI signal to create an image

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17
Q

What does intensity in image depend on?

A

Intensity in image depends on water content, tissue structure, blood flow., perfusion, diffusion, paramagnetic

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18
Q

When is the strongest signal in MRI received?

A

Strongest signal is received when the first radiofrequency pulse is introduced and gets smaller with time

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19
Q

What does image density depend on and what does it provide?

A

Image signal intensity depends on T1 and T2 and provides contrast between tissue in a MRI image

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20
Q

What is a pulse sequence?

A

A pulse sequence is the time of putting in the radiofrequency pulse

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21
Q

What is echo signal?

A

Echo signal(TE) is the time between radiofrequency pulse and the MRI signal being picked up

22
Q

What is the repetition time(TR)?

A

Repetition time(TR) is the time between different radiofrequency pulses

23
Q

What is repeated several times to create a single image?

A

Echo signal is repeated several hundred times to create a single image

24
Q

What determines image contrast?

A

TE and TR determine image contrast

25
Q

What does T2 MRI use?

A

Uses 2 radiofrequency pulses and one knocks it to 90 degrees and one to 180 degrees

26
Q

What can T2 MRI detect?

A

Can detect loss of brain structure, tumours, oedema because these cause increased amount of free water and hence decreased number of fixed protons in cell membranes.

27
Q

How do MRI with longer T2 show up as?

A

Longer T2 show up as brighter

28
Q

What does T2 of tissue determine

A

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

29
Q

What is T2 very dependent on?

A

T2 very dependent on how mobile the water is in the tissue

30
Q

What does T2 decrease with?

A

Decreases with:

  • Oedema
  • Demyelination
31
Q

What is T2 reduced by?

A

T2 is reduced by the presence of paramagnetic ions like Fe

32
Q

What are the properties of contrast agents?

A

Properties:

- Paramagnetic 
- Chelated to reduce toxicity
33
Q

What does water in the vicinity of contrast agents experience and hence what happens to the T1 and T2 values?

A

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

34
Q

What are contrast agents used in and why?

A

Used in brain tumours as blood vessels created by the tumour are poor quality hence a large contrast of agent molecules can cross the poorly developed vascular wall

35
Q

Steps in T1 MRI

A
  • First radiofrequency pulse is put in and this causes magnetisation in the XY plane which generates a signal but no magnetisation in the z axis
  • Must then wait for magnetisation to come back along the z axis before putting in another radiofrequency pulse
36
Q

What does intensity of image depend on in T1 MRI?

A

Intensity of image depends on T1 relaxation time of tissue

37
Q

What do tissues with long T1 produce?

A

Tissues with long T1 produce a smaller signal than tissue with short T1

38
Q

What is T1 MRI useful for looking at?

A

Useful for looking at pathological changes that relate to neurodegenerative processes where there are changes in volume of the brain tissue, in particular, grey matter

39
Q

What matter is T1 lower in and why?

A

-T1 is lower in white matter than grey matter due to myelinated neurons

40
Q

What is T1 dependent on and what does it increase with?

A

-T1 is dependent on how mobile the water in the tissue is and increases slightly with oedema

41
Q

What ions is T1 very dependent on and what does it reduce?

A

-T1 is very dependent on the presence of paramagnetic ions which reduce T1

42
Q

What is used to determine the volume of the hippocampus?

A

-T1 images were used to determine the volume of the hippocampus

43
Q

What happens the longer someone is a taxi driver and what was proposed to explain this?

A
  • The longer the time spent as a taxi driver, the larger the volume of the hippocampus
    - Neurogenesis proposed to explan the need for increased capacity of spatial memory
44
Q

What can magnetic resonance spectroscopy specify and using what?

A

-Can specify a small region of the brain(15mm) and develop an MR spectrum of biochemicals using three selective pulses

45
Q

What does each peak in the spectrum of MRS refer to?

A

-Each peak on the spectrum refers to a different chemical

46
Q

What do chemicals have around them?

A

Electron clouds

47
Q

What does a stronger shielded nucleus have?

A

A stronger shielded nucleus has a lower resonant frequency

48
Q

Why does water have a higher frequency signal than fat as in lipids?

A
  • For instance water:
    • Electrons are closer to the oxygen than H
    • There’s weaker shielding around protons
    • Hence there’s a higher frequency signal than fat as in lipids the electrons are shared
49
Q

Why is MRS important when MRI looks normal?

A

-MRS is important when the MRI looks normal as the spectroscopy detects an imbalance in chemicals

50
Q

Safety issues with MRI

A

Safety issues:

  • No ferromagnetic objects in the exam room
  • Contraindications:
    • Pacemakers
    • Infusion pumps
    • 1st trimester pregnancy
    • Aneurysm clips
    • Metallic foreign bodies

Cellular pathology (12 decks)

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