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SEM 4:Cellular Pathology > MRI and MRS > Flashcards

MRI and MRS Flashcards

1
Q

What is an MRI used for?

A

It images soft tissues therefore the denser material is fat. It works by getting signals from the water and the fat of the soft tissues.

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2
Q

What is proton density?

A

The signal picked up from the water and the fat that is proportional to the amount of water and fat in the tissue.

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3
Q

How is the contrast on the image determined?

A

By the T2 relaxation time. The higher the T2, the more fluid in the tissue e.g. CSF

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4
Q

What is the T2 relaxation time?

A

The time constant which determines the rate at which excited protons reach equilibrium or go out of phase with each other.

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5
Q

What does the detail from T2 weighted image reflect?

A

It reflects the real structural structure.

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6
Q

What is the structure of the grey and white matter?

A

Grey matter has a higher blood volume therefore looks more pink whereas the white matter is more white.

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7
Q

What is a CT?

A

It is a method used to distinguish the difference between dark and grey matter but not much detail. CT contrast is due to tissue-density dependent attenuation of x-rays.

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8
Q

What does the signal intensity of a CT image depend on?

A

It depends on the hounsfield number. This is the measure of how much the x-ray is attenuated e.g. the bone will have a higher attenuation as it absorbs more X-rays than fluid. The crontrast is much lower than you get in MRI. The attenuation is less in darker regions.

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9
Q

What are the advantages of a CT?

A
  • Good at finding the traumatic brain injury
  • Quick
  • Cheap
  • Usually used in medical field
  • Anyone can have a CT whereas MRI signals are more intense so need to screen the patient beforehand to see if they have metal or not in them
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10
Q

What does relative signal intensities in MRI dependent on?

A
  • Depends on the fat and water content
  • cellular structure
  • cell density
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11
Q

Where do MRI signals come from?

A

Come from the protons in water and fat molecules

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12
Q

Describe a T2 weighted image

A

In a T2 weighted image, the fluid shows up more bright whereas the fat shows up more dark and the jelly structure is in-between. The more fluid = the brighter the signal.

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13
Q

Describe a T1 weighted image

A

It is used within fatty tissues in the body such as subcutaneous fat and the bone marrow.

  • The CSF is dark as it contains no fat.
  • There is a good contrast between fluid and soft tissue.
  • It is good for picking up the anatomical changes that relate to the changes in volume of the grey matter
  • Pathological change –normal ageing or even Alzheimer’s can be shown.
  • Changes in molecular biology will cause changes in the MRI scan
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14
Q

What is a perfusion map? What can it be used for?

A

It is a quantitative diffusion map in red that highlights live blood vessels. A melanoma will show up on the map. It is relevant in stroke as can show change in blood volume.

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15
Q

What produces the signal in an MRI?

A
  • Water molecules that are moving between the extra and the intravascular spaces through diffusion so this can be measured.
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16
Q

What does the diffusion map show?

A
  • It shows the ventricles where the CSF is free - water is fully free to diffuse therefore shows up as orange and red.
  • Whereas, in the jelly like structures the diffusion is hindered by the structures - the water molecules can’t move as much therefore the colour is darker.
  • Use MRI to see how free the water is to diffuse therefore, showing the rigidity of the structure - detect strokes.
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17
Q

What are routine maps?

A

Used to detect acute stroke

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18
Q

What does diffusion anisotrophy show?

A

It shows cellular structure of white matter. The long myelinated structures send signals through structures in the brain. The water can only diffuse along the axis of the external structures.

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19
Q

Where does water diffuse in the brain?

A

It can diffuse freely along the structures but not on the side.

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20
Q

What is anisotropy? How is a diffusion map drawn?

A

It is the difference in the directionality of the diffusion. Then, it can measure the diffusion in different directions in the brain and a map can be draw. The bright areas is where the diffusion is restricted to moving only along the axonal structures therefore, it is white matter. If there is a low single intensity, it can diffuse in any direction so doesn’t show up as brightly on the anisotropy.

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21
Q

How are MRI signals produced?

A

MRI signals come from the magnets within us. The sagittal view of a brain is formed from the bright signal from CSF. The signal of the fatty tissue of the scalp is dark as the the bone marrow of the fatty tissue in the skull.

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22
Q

Describe a signal is created from an MRI - through magnets in the water of our body

A
  1. The signal comes from water. The water has a hydrogen atom that has a proton at its core.
  2. The proton is spinning on its axis and creates a magnetic moment. All the protons in the body have the bar magnets at their core.
  3. When you take an MRI, all the protons will line up with the magnetic firing of the scanner.
  4. The MRI scanner has a magnetic field. All the bar magnets in the body line along the field in the scanner to the form a signal. It can be manipulated to generate the signal.
  5. It is manipulated by using radio frequency pulses put in it. This generates a signal that we pick up and put together to form the image.
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23
Q

What is signal intensity of an MRI dependent on?

A

It is dependent on the amount of water, the T1 and T2. As the properties of the tissue, it enables us to make the image sensitive enough to see all the different anatomical structures.

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24
Q

What is nuclear magnetism?

A

The positive charge of a spinning proton produces a magnetic moment u.

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25
Q

What is magnetic resonance imaging or nuclear magnetic imaging?

A

The signal arises from the proton in the nucleus. Nuclear refers to ionizing radiation and as MRI does not use ionising radiation so the term nuclear was dropped.

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26
Q

What is resonance?

A
  • In a magnetic field Bo the magnetic moment of a proton processes at the Larmor frequency VL.
  • The proton is magnetic so aligns (process).
  • Bo, the magnetic field is within the magnet (water). They process instead of aligning completely with the magnetic field, they process and misalin.
  • The frequency of the procession is proportional to the magnetic field.
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27
Q

How can the magnetic field be manipulated?

A

If you use a radio frequency pulse that is exactly the same as th Lamour equation, then you can manipulate the magnetic field. Using a radio frequency at 64, then you can knock out the alignment and generate a sign`al that you can pick up.

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28
Q

Why are MR imaging used and formed?

A
  • It is formed using a radiofrequency pulse to generate an MR pulse from a slice of tissue.
  • The magnetic field gradients are used to encode the signal in space so that the computer can generate an image.
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29
Q

Describe the structure of the MRI and how does it work

A
  1. Loops of wire formed with imaging coil is wrapped around the head and the body.
  2. The radiofrequency current passes through the loops of wire and send a radiofrequency variation in the field that we are imaging.
  3. Individual slices at any orientation at any part of the body. Then signals are taken from these slices and picked up by the coil.
  4. This is then amplified by the rf receiver and put into the computer and forms an MR image.
30
Q

What happens when there is oedema?

A

There is an increase in T2.

Increase in T2 as there is oedema

31
Q

What is magnetic field plating?

A

The frequency that comes back is the same as B0. This can put on spatially variation in magnetic field. It goes from one part of the magnet to the other. The field changes through the frequency that is received by the rg receiver is also changed dependent on where it is in space.

32
Q

How to work out where objects are in space?

A

Can measure the frequency of the signal and work out where in space it came from.

33
Q

What is the contrast and frequency of a signal?

A

Contrast - the intensity of a signal

Frequency - where the signal comes from

34
Q

How are vibrations generated?

A
  1. The magnetic field gradients are switched on and off.
  2. Then, make the scanner vibrate and makes noises of low and frequency
  3. This causes vibrations
35
Q

What are the parts of an MRI?

A
  • Chair
  • IV pole
  • Floor buffer
  • Oxygen cylinder
36
Q

Why does the MRI need to be very safe?

A
  • The magnetic field is 30x stronger than the Earth’s magnet.
  • The force of the magnet is super super strong
  • Should not go in with anything that is ferromagnetic
37
Q

What are ferromagnetic objects not to be used around MRIs?

A
  • Scissors, stethoscopes, wheel chairs, gas cylinders

- Hearing aids, watches, spectacles (dentures - image quality)

38
Q

What are contraindications of MRI?

A
  • Pacemakers, Infusion pumps, 1st trimester pregnancy, aneurysm clips (refer to manufacturers specifications) and Metallic foreign bodies (orbit x-ray, shrapnel)
39
Q

What happens to magnetic objects around the MRI machine?

A

They can get sucked into the MRI machine and can therefore cause damage. Everybody gets checked before they enter the magnet room; the patient and anyone helping with the scan.

40
Q

How can contraindications affect MRI scan?

A

They can still be affected by the MRI scan even though not all metals are magnetic. When moving them through a magnetic field, there is still force on them. This could cause internal haemorrhage.

41
Q

Why is pregnancy and MRI to be done with caution?

A

It can cause some DNA changes as nothing too severe however not advised.

42
Q

How does the MRI receive a signal source?

A
  • The strong magnetic field creates magnetisation in all the tissue.
  • The magnetisation is from the protons in water and fat in the tissue.
  • The magnetisation can be manipulated by radiofrequency pulses to produce an MRI signal to create an image.
  • The intensity in the image depends on water content, tissue structure, blood flow, perfusion, diffusion, paramagnetic etc.
43
Q

How can a tumour be identified in an MRI?

A

A T1 weighted image checked against a contrast agent to show where the tumour is. There is a magnetic agent that changes the T1 relaxation property of the tissue.

44
Q

What does the intensity of a signal from an MRI depend on?

A

It is dependent on many factors that are specific to the pathology of the tissue that we are observing.

45
Q

What are the two relaxation times that determine how strong the MRI signal is?

A

T1 and T2 relaxation times. These are MR parameters that vary with tissue type e.g. grey or white matter, and with disease. Hence, they allow images to be made that demonstrate anatomy and pathology.

46
Q

What affects how long the MR signal lasts?

A

Mxy decays according to T2 which affects how long the MR signal lasts.

47
Q

What is the signal recovery of Mz dependent on?

A

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

48
Q

Describe in detail how the MRI and relaxation times work in detail

A
  1. Magnets align to the magnetic field. There is equilibrium. The protons are all aligned with the magnetic field.
  2. Then, a radio frequency pulse is given which knocks the magnetic field. The protons are no longer aligned with B0 and are sideqays in the X/Y plane.
  3. The signal/magnetisation that has been knocked over dies away time. This gives the T2 signal decay or signal relaxation. How quickly it decays depends on the tissue structure. More liquid is a slower process and the fat is a quicker process. The image intensity depends on the T2 relaxation time. When T2 change due to pathological changes or tissue types can create an image with the contrast picture.
  4. Magnetisation is reduced to 0. There is a slow recovery of the magnetisation to then reach equilibrium and match the B0 magnetic field. T1 recovery is along the Z axis. This depends on tissue structure: Liquid is a slower process and fat is a rapid process.
49
Q

What is the difference between T1 and T2 signals?

A
  • Both T1 and T2 are independent and can give different contrast images
  • The signal is from the water and fat therefore, the signal arises from chemicals that are dependent on the tissue structure.
  • The changes in T1/T2 reflect the changes in the tissue structure related to pathology
50
Q

How is an MR image developed?

A
  • Put a radiofrequency pulse in and pick up a MR signal
    • > usually about 100ms
    • > have 2 different angles, can put in the delay so that there is a signal intensity that is dependent on the T2
  • Do this many times to pick up an image
  • Have a delay - echo time (TE) can vary
51
Q

What is the relationship between echo time (TE) and T2 signal time?

A

The longer the echo time, the weaker the signal dependent on the T2 signal time

52
Q

What is the normal brain T2 relaxation time?

A

Usually greater than 1000ms

- If there are lesions, the T2 increases and increased signal intensity in the T2 weighted image.

53
Q

When is the echo time acheived?

A
  • The bound water decays quickly and the free water decays really slowly. The average of all of these in somewhere in-between the both. Then, need to wait a short amount and this is a strong signal from everything. Then, wait after the echo and there is an average between the two as some things would’be decayed.
54
Q

How is contrast seen through water changes?

A
  • As tissue structures changes and the amount of bound or unbound water changes, this is how contrast is seen. The average is more in between the two decay times.
55
Q

What is bound and unbound water?

A

Bound water is those within cellular structures and membrane which have a shorter relaxation time and decay quickly.

Unbound water decays more slowly and is between the intracellular and extracellular water and highly mobile.

56
Q

What is the consequence of a shorter echo time?

A
  • The proton density is proportional to how much fat and water is present. Within lesion, it shows up brighter as there is increased water content within the lesions where there is inflammation and loss of the immobile tissue structures.
57
Q

How do pathological changes appear on a T2 map?

A
  • Show up brighter due to the lesion having more of a fat content so more protons that occur in the tissue.
  • There will be different appearances between shape and heterogeneity
  • Need to have some type of idea on where the lesion arises from.
58
Q

Summarise T2 relaxation time

A
  • The T2 of tissue determines how quickly the MRI signal decays away after the radiofrequency pulse
  • T2 is very dependent on how mobile the water is in the tissue and increases with oedema and demyelination.
  • T2 is reduced by the presence of paramagnetic ions: Fe from blood breakdown products and Gd from contrast agents.
59
Q

What is the T1 time for CSF and Brain Tissue?

A

CSF - has a large T1 time - the magnetization takes a long time to return to the Z axis. The brain tissue has a shorter T1 and the magnetization returns faster. Within 10 seconds, both the brain tissue and CSF would have returned to Z axis.

60
Q

What is the T1 time for white and grey matter?

A

White matter has a shorter T1 than grey matter because it is a more rigid structure. This is because white matter has a more myelinated structures which causes a reduced T1, full of fat which interacts with water to give a reduced T1. Those with a longer T1 shows up in the dark.

61
Q

What happens to grey matter with age?

A

It decreases with age - reduced by 1% a year.

62
Q

What happens to a patient with Alzheimer’s?

A
  • In the hippocampal region, there is an increased dark space.
  • This means that there is more CSF fluid in that region. There is an increase in the sulci as well.
  • A disease that causers a rapid decrease in the hippocampal volume due to pathological changes and change s in the frontal volume.
  • Changes in reduction of hippocampus is clearer to see.
63
Q

What is the Taxi driver effect?

A

Found that taxi drivers had an increased volume of the hippocampus in taxi drivers and healthy controls. The longer a person is a taxi driver, the bigger the volumetric change in the hippocampal region. The theory was that taxi drivers needed to learn maps which meant a part of the hippocampal region increased = increased in grey matter.

64
Q

Summarise T1 relaxation time

A
  • When the repetition time (TR) between pulses is much shorter than T1, the magnetisation that can produce the MRI signal is reduced (“saturated”).
  • The MR signal is then T1-weighted
  • Tissue with long T1 produces a smaller signal than tissue with short T1.
  • T1 is lower in white matter than grey matter because of myelinated neurones
  • T1 is also dependent on how motile the water is in the tissue and T1 increases slightly with oedema
  • T1 is very dependent on the presence of paramagnetic ions which reduce T1. Fe from blood breakdown products and Gd from contrast agents.
65
Q

What are contrast agents?

A

Paramagnetic (unpaired electrons) or superparamagnetic (ferrites) which are chelated to reduce toxicity

66
Q

What happens when water and contrast agents are in the same vicinity?

A

There are strong fluctuating magnetic fields hence T1 and T2 are reduced.

67
Q

What is an example of a paramagnetic contrast agent?

A

Gadolinium (Gd)

  • Used by being connected to this large molecule
  • Toxic on its own
  • Can use it intravenously and when it gets close to a molecule - the T1/T2 is reduced.
  • If water gets close to the agent, then the T1/T2 is reduced and there is a change in image contrast.
68
Q

What are the basis of dynamic contrast enhanced MRI?

A
  • Contrast enhanced is injected intravenously
  • If the contrast agent can get out into the tissue, then T1 goes down.
  • The T1 in the weighted image goes down and the signal intensity goes up.
69
Q

How is a tumour highlighted?

A
  • In a normal brain, the contrast agent can’t get out of the vein. The blood brain barrier is very tight and the contrast agent stays in the intravascular region.
  • In the tumour, the blood brain barrier is broken. The agent can get out into the tumour tissue and therefore reduce the T1 weighted image. It causes large increase in signal intensity.
70
Q

When is a T2 weighted image used and a T1 weighted image used in a tumour?

A

T2w image is used to find the region to put the radiotherapy dose.
T1w image with the agent would be used to find the area to remove the tumour surgically.

71
Q

What are the different type of lesions?

A
  • Those that enhance and those that don’t
  • It can use the 2 different image techniques to measure the type and number of lesions.
  • Use it to monitor the disease over time

SEM 4:Cellular Pathology (13 decks)

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