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Cellular Pathology > MRI & MRS > Flashcards

MRI & MRS Flashcards

1
Q

What is image contrast?

A

Ability to see differences in signal between different anatomical / pathological regions

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2
Q

What is colour cryo section?

A

Full multislice CT & MRI carried out and cadavers frozen and laser dissected to get prosection anatomical info to compare with scans

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3
Q

What do CT scans image?

A

CT contrast dependent on bony structures - can be altered to see brain and soft tissue

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4
Q

What do MRI show?

A

MRI images soft tissue and no bony structures: PD and T2 show fat and water tissues

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5
Q

What does the potential difference signal tell us?

A

PD signal proportional to amount of fat and water tissues present

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6
Q

What does T2 signal tell us?

A

T2 signal proportional to T2 relaxation time - tissue property: higher T2 the more fluid the tissue is

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7
Q

What creates the CT contrast?

A

CT contrast is due to tissue density dependent attenuation of x-rays (Hounsfield number)

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8
Q

Compare the contrast of CT to MRI

A

CT contrast a lot lower than MRI

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9
Q

How does hounsfield no. change depending on tissue type?

A

Hounsfield no. high in bone as x rays are very highly absorbed - shows up brighter

Fluid / water has lower attenuation (hounsfield No.) shows up darker

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10
Q

How will a haemorrhage look on CT?

A

Region of haemorrhage - blood breakdown products absorb more x rays∴shows up brighter on CT

Area of oedema; increased water content = attenuation of x rays is lower

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11
Q

What is MRI signal intensity dependent on?

A

The relative signal intensities between different tissue types and pathologies – depends on physical properties of the tissue such as water and fat content, cellular structure, cell density

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12
Q

What does MRI get signals from?

A

MRI - signals from water and fat molecules

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

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13
Q

Describe MRI T2 property

A

T2: fluid = bright, fatty tissues = dark → good for picking up pathological changes in brain ie. lesions, stroke, tumours etc.

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14
Q

What is the T1 tissue property of MRI?

A

T1: fluid = dark, deep contrast between rigid structures → good for showing anatomical changes relating to grey matter volume

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15
Q

Where is magnetic signal found fro MRI?

A

Magnetic signal comes from within us as we are biological magnets

We’re composed of 75% water (H₂O) - H+ has proton at its core and is spinning on its axis creating a magnetic moment

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16
Q

How does our magnetism create an MRI image?

A

All the protons in our body line up with the strong magnetic field of the scanner and are manipulated using radio frequency particles to generate an image

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17
Q

What creates a high contrast MRI image?

A

The more water = stronger signal = better image

Also depends on T1 and T2 parameters

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18
Q

What is nuclear magnetism?

A

The positive charge of a spinning proton produces a magnetic moment μ, dependent on radioactive isotopes

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19
Q

What causes resonance?

A

In a magnetic field Bo the magnetic moment of a proton precesses at the Larmor frequency νL

Frequency is proportional to the magnetic field

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20
Q

How is MR image formed?

A

MR Imaging is formed using a radiofrequency pulse to generate an MR signal from a slice of tissue

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21
Q

Outline how an MRI is formed

A
  1. Patient lies in bore of magnetic
    ( 30,000x uniform magnetic field )
  2. Imaging coil (loops of wire) around body part
  3. Insert radiofrequency current through imaging coil
  4. Sends in a radiofrequency variation in magnetic field into
    body part
  5. Signal picked up by imaging coil and sent to amplifier
    (radiofrequency receiver) → CPU → produces image
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22
Q

What angle does MRI take images?

A

Can image various slices at any orientation

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23
Q

How does a computer generate an image from MRI signals?

A

Magnetic field gradients are used to encode the signal in space so that the computer can generate an image

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24
Q

Give examples of types of MRI scanners

A

The following scanners contain a magnetic field 30,000x higher than the earths

  • Chair
  • Iv pole
  • Oxygen cylinder
  • Floor buffer
25
Q

What safety regulations need to be in check for MRI?

A

No ferromagnetic objects in the exam room
- Scissors, stethoscopes, wheel chairs, gas cylinders
- Hearing aids, watches, spectacles, (dentures – image
quality

26
Q

What contraindications need to be considered before doing an MRI?

A
  • Pacemakers
  • Infusion pumps
  • 1st trimester pregnancy
  • Aneurysm clips (refer to manufacturers specifications
  • Metallic foreign bodies (orbit x-ray, shrapnel)
27
Q

What is the MRI signal source?

A

Strong magnetic field from protons in water and fat in tissue creates magnetisation in all the tissue

28
Q

How is magnetisation turned into an MRI image

A

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

29
Q

What does image density depend upon?

A

The intensity in the image depends on water content, tissue structure, blood flow, perfusion, diffusion, paramagnetics etc

30
Q

Describe the regular magnetic field in all tissues

A

Magnetisation aligned with strong magnetic field in equilibrium (B0)

31
Q

What happens to the normal magnetisation when exposed to radiofrequency pulses?

A

A radiofrequency pulse is fired that interacts with magnetisation
Knocks it out of magnetic field (no longer points within axis of B0 (z axis)
- knocked sideways into x/y plane

32
Q

What happens to the signal once in the x/y plane?

A

One of 2 relaxation times can take place determining how strong the signal is:
T1 and T2

33
Q

What is T2 relaxation time?

A

T2 signal decay in xy: Magnetisation signal dies away with time (T2 relaxation)

34
Q

What is T1 relaxation time?

A

T1 signal recovery along z: Slow recovery of magnetisation along z axis (T1 relaxation)

35
Q

What factors determine the MR image formed?

A

These are MR parameters that vary with tissue type e.g. grey or white matter, & with disease → allow images to be made that demonstrate anatomy and pathology

36
Q

Describe the effect of T2 relaxation time on MR image?

A

Mxy decays according to T2 which affects how long the MR signal lasts - dependent on tissue structure (higher liquid = quick, more fat = slow)

37
Q

Outline the effects of T1 relaxation time on an MRI image

A

Mz recovers according to T1 which affects how much M there is available to be excited to give the next signal

38
Q

What causes the build up of an MRI image?

A

An MR image is built up from a series of signal acquisitions

Waves of radiofrequency signals pick up MRI signals from tissues several times to create an image

39
Q

What is meant by ‘TE’?

A

TE = echo time delay which can vary

- Signal picked up depends on T2 relaxation time

40
Q

How does echo time effect the quality of an MRI?

A

MR signal is strongest after a radiofrequency pulse and then decays with time = T2 relaxation time
Longer the echo time = weaker signal

41
Q

Give examples of brain structure TE

A

(TE) T2 relaxation times of brain structures:
White Matter : 90 ms
Grey Matter: 80 ms
CSF >1000 ms

42
Q

How does T2 effect signal intensity?

A

As T2 increases, signal intensity also increases

43
Q

How does TE differ within tissues?

A

Bound water decays v. quickly
Free water decays slower

ICF and ECF in cells is highly mobile

Membrane proteins also contain H+ within macromolecules or water in hydration complexes of cell membrane - have a shorter relaxation time

44
Q

How does tissue structure effect T2 signal intensity?

A

As tissue structure changes amount of bound and free water changes → alters signal intensity and contrast

45
Q

What does T2 relaxation time tell us?

A

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

46
Q

Under which circumstances does T2 signal increase?

A

T2 is very dependent on how mobile the water is in the tissue and increases with

  • Oedema, an increase in water content
  • Demyelination, a loss of brain tissue structure
47
Q

How is T2 reduced?

A

T2 is reduced by the presence of paramagnetic ions

  • Fe from blood breakdown products
  • Gd from contrast agents
48
Q

What is ‘TR’?

A

Repetition time (TR): how quickly signal returns to z axis

T1 dependent on TR

49
Q

Give examples of TR of brain structures

A

(TR) T1 relaxation times of brian structures:
> inverse of T2 image

White matter : 1000 ms
Grey matter : 1800 ms
CSF > 2500 ms

50
Q

How does repetition time effect an MRI?

A

When repetition time (TR) between pulses is shorter than T1, the magnetisation is reduced (“saturated”) = T1 weighted

51
Q

Describe how T1 effects signal intensity

A

The MR signal is then T1-weighted: Tissue with long T1 produces a smaller signal than tissue with short T1.

52
Q

Why is T1 shorter in white matter than grey?

A

T1 is lower in white matter than grey matter because of myelinated neurones

53
Q

What increases T1?

A

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

54
Q

What decreases T1?

A

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

  • Fe from blood breakdown products
  • Gd from contrast agents
55
Q

Give examples of contrast agents used in MRI

A

Paramagnetic (unpaired electrons) or superparamagnetic (ferrites) - Chelated to reduce toxicity

56
Q

Why do contrast agents reduce T1 / T2?

A

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

57
Q

Outline the basis of a dynamic contrast agent enhanced MRI

A
  1. Contrast agent injected intravenously into the tissue
  2. Diffuses out into cells decreasing T1
  3. T1 weighted image of T1 decreases : signal intensity increases
58
Q

How can we identify characteristics of multiple sclerosis lesions using MRI?

A

Active lesion with Gd contrast.
Number of lesions/lesion type can aid monitoring of disease progression or treatment response

MRS A 1H spectrum of biochemicals can be obtained from a localised region of the brain using three slice selective pulses.

Cellular Pathology (13 decks)

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