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Medical Imaging > MRI > Flashcards

MRI Flashcards

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USMLE® Step 1 flashcards
1
Q

Brief outline of how MRI works

A
  • A high magnetic field is applied to the tissue to be imaged.
  • The magnetic field causes hydrogen nuclei (protons) in the tissue to respond to radio waves.
  • The radio waves enter the tissue – these will be absorbed and then re-emitted.
  • The re-emitted radio waves provide the detailed image data, of soft tissues.
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2
Q

3 clinical applications of MRI

A
  • Blood vessels
  • Abnormal tissue
  • Breasts
  • Bones and joints
  • Organs in the pelvis, chest and abdomen (heart, liver, kidney, spleen)
  • Spinal injuries
  • Tendon and ligament tears
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3
Q

Which one of the following statements about MRI is not true?

A.MRI scanners do not use ionising radiation

B.MRI gives excellent soft tissue details

C.MRI records carbon nucleus

D.It is a time consuming process

A

C.MRI records carbon nucleus

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4
Q

2 benefits of MRI

A
  • No ionising radiation
  • Uses magnetic fields and radiofrequency pulses
  • Non-invasive
  • Excellent soft tissue detail
  • 2D slice through the patient
  • High-quality images
  • Anatomical and functional information
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5
Q

Why is hydrogen used is MRI

A
  • Has spin. Other nuclei like carbon and oxygen do not
  • Plentiful in body (density varies with tissue type)

Hydrogen + carbon = fat

Hydrogen + oxygen = water

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6
Q

Which of the following is most commonly used type of magnet in MRI machines?

A.Permanent magnet

B.Superconducting magnet

C.Resistive magnet

D.Temporary magnet

A

B.Superconducting magnet

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7
Q

Unit of measurement for magnetic field strength is TESLA (T)

Most common in clinical use is —– T

A

1.5

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8
Q

The coil windings of superconducting magnets are made of wires of a type 2 superconductor (mostly used is niobium-titanium)

These coils have ——- resistance when operated at temperatures near absolute zero (-273.15°C, -459°F, 0 K).

A

No resistance

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9
Q

What is commonly used as a coolant for MRI?

A

Liquid helium (4.2 K) is commonly used as a coolant

(sometimes in addition with a second cryogen liquid nitrogen as an intermediate thermal shield to reduce the boil-off rate of liquid helium)

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10
Q

Define an MRI quench

A

•Quench

Deliberate or accidental release of liquid helium removes magnetic field

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11
Q

What is the risk to human health of an MRI quench

A
  • Risk of frostbite
  • Suffocation (due to replacement of oxygen)
  • Trapping of personnel (due to increased ‘air’ pressure).
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12
Q

Which of the following components of a MRI machine allows it to choose exactly where in the body to acquire an image?

A.Gradient magnets

B.Radiofrequency generating coil

C.Radiofrequency receiving coil

D.Gadolinium based contrast

A

A.Gradient magnets

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13
Q

Purpose of gradient magnets in an MRI

A

Some smaller magnets whose strength varies with location (Gradient magnets)-allows spatial information to be collected

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14
Q

What is spatial localisation

A
  • The magnetic field varies with position
  • Energy difference between states dependent on magnetic field
  • Therefore energy difference between states dependent on position
  • Therefore the RF energy you collect will have an energy that is position dependent
  • This means the signal you now collect will contain a ‘code’ that tells you where it originated.
  • This is the key to constructing an image which is basically a ‘signal map’.
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15
Q

Is following statement true or false?

“In T1 weighted images fat appears dark and fluid appears bright”

A

False

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16
Q

What are images weighted according to?

A

•Images can be weighted according to the their relaxation time behaviours

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17
Q

What are the three basic MRI weightings?

A

•Basic weightings are T1W, T2W and proton density weighting (PDW)

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18
Q

Generally, how are different MRI weightings achieved?

A

•These weightings are achieved by manipulating two timing parameters called “time to repetition” (TR) and “time to echo” (TE)

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19
Q

Define: TR (time to repetition)

A

is the time in milliseconds between one radio wave excitation of the atomic nuclei and the next.

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20
Q

Define: TE (Time to echo)

A

The time in milliseconds between a radio wave excitation and the received signal (which is called an echo, not to be confused with echoes in ultrasound)

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21
Q

Short TR ——-T1 contrast

A

Short TR maximises T1 contrast

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22
Q

Long TR ——T1 contrast

A

long TR minimises T1 contrast

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23
Q

Short TE ——–T2 contrast

A

Short TE minimises T2 contrast

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24
Q

Long TE ——–T2 contrast

A

long TE maximises T2 contrast

25
Q

We use —– and ——- as reference tissues as these have the fastest and slowest relaxation respectively

A

•We use fat and water (body fluids) as reference tissues as these have the fastest and slowest relaxation respectively

26
Q
  • Body fluids
  • ——-signal on T2 weighted images (appear bright)
  • ——-signal on T1 weighted images (appear dark)
A
  • Body fluids
  • High signal on T2 weighted images (appear bright)
  • Low signal on T1 weighted images (appear dark)
27
Q

Fat

  • ——–signal on T2 weighted spin echo images (appear dark)
  • Intermediate – ——-signal on T2 weighted fast spin echo images*
  • ———signal on T1 weighted images (appear bright)
A
  • Fat
  • Low signal on T2 weighted spin echo images (appear dark)
  • Intermediate – high signal on T2 weighted fast spin echo images *
  • High signal on T1 weighted images (appear bright)
28
Q

What is the correct order of for the following scans (L-R)

  • T1W
  • T2W
  • PDW
A
29
Q

pros of MRI

A

No ionising radiation

(uses magnetic field and radiofrequencies)

Allows repeat studies

Non-invasive

Excellent soft tissue detail

30
Q

Cons of MRI

A

Expensive hardware

-and needs specialised building

Time consuming scans (cf CT secs v 10 x min)

Some participants cannot be scanned

  • size of patient
  • magnetic field: metal implants

Claustrophobic Issue

31
Q

How strong are clinical use MRIs

A
  • Most common in clinical use is 1.5T (research 3.0T)
32
Q

why is an MRI magnet always on

A

contains permanent ferromagnets (ion put current through)

Usually use superconducting magnets

  • Do not need a power source once current initially supplied
  • Sudden increase in strength closer you get to it
33
Q

why does MRI uses magnets

A
  • When the body is placed in a strong magnetic field, such as an MRI scanner, the protons’ axes all line up.
  • When additional energy (in the form of a radio wave) is added to the magnetic field, the magnetic vector is deflected.
  • When the radiofrequency source is switched off the magnetic vector returns to its resting state, and this causes a signal (also a radio wave) to be emitted.
  • It is this signal which is used to create the MR images.
34
Q

MRI generally uses Hydrogen- why?

A
  • Has spin
    • Other nuclei like carbon and oxygen do not
  • Plentiful in body (density varies with tissue type)
  • Spatial distribution of hydrogen

Hydrogen + carbon = fat

Hydrogen + oxygen = water

35
Q

Two orientations: parallel or antiparallel with introduction of the external magnetic field.

Which is this

A

Antiparallel

36
Q

Systems prefer —– energy to —– energy, so:

  • The parallel orientation (the “right” way) is a low-energy state
  • The antiparallel orientation (the “wrong” way) is a high-energy state
A

Systems prefer low energy to high energy

37
Q

Why aren’t all the protons the “right” way, in the parallel alignment?

A
  • Because heat provides energy to move protons to the high-energy (antiparallel) state-and energy gap is very small (depending on magnet strength)
  • That’s why the magnetic field must be strong – to overcome the effect of heat and produce a significant net alignment of the protons
38
Q

Purpose of the radiofrequency generator

A

If you put energy of the right value into the system can move protons from the low energy state to the high energy state

The energy difference between the two states is very small-corresponds to a radio wave energy level

39
Q

Once you stop putting energy into the system what happens?

A

The absorbed RF energy is retransmitted at the resonance frequency

40
Q

purpose of antanae

A
  • Needed to collect the signal that is transmitted
  • Within MRI, typically called ‘coils’
  • Shaped to fit the body part
41
Q

why is spatial localisation important in mri

A

you want to have signal arising from different locations have different qualities, that allow you to work out where it came from

42
Q

how do the gradient magnets allow for the identification of specific organs?

A
  • The magnetic field varies with position
  • Energy difference between states dependent on magnetic field
  • Therefore, energy difference between states dependent on position
  • The RF energy you collect will have an energy that is position dependent
  • This means the signal you now collect will contain a ‘code’ that tells you where it originated.
43
Q

what property of tissues allows for different types of MRI scan to be performed?

A

Different types of tissue have different characteristics in the rate they lose energy

44
Q

why does MRI take so long to complete

A
  • An MRI scan typically consists of a repeating sequence of energy in-signal collected
  • Depending on resolution this may occur a few hundred times.
  • Need lots of repetitions as apply slightly different magnetic field gradients with each repetition.
  • Higher the resolution more repetitions are required
45
Q

what is echotime in MRI

A
46
Q

what is Relaxation

A

the rate at which energy is lost

47
Q

what is image contrast due to in MRI

A

Image contrast is mainly due to changes in relaxation behaviour between tissue types

48
Q

What contrast material do contrast MRI’s usually use

A

Contrast MR typically uses a Gadolinium based agent-modifies the rate signal is lost from the system

49
Q

what does diffusion MRI measure

A

the movement of water molecules

50
Q

what does perfusion MRI measure

A

microcapillary delivery of blood to tissues

51
Q

what does the STIR sequence suppress

A

fat

52
Q

what does the FLAIR sequence suppress

A

fluid

53
Q

why do perfusion weighted scans show up brightly

A

grey matter has more H than the white matter, so in the PDW shows up brighter

54
Q

what pathology is this?

A

haemorrhage

brightest spots = blood

55
Q

What is the purpose of using IV contrast

A
  • Shows the boundaries of the tumour
  • Removed the signal from the reaction of surrounding tissue
56
Q

What property of MRI is this liver scan showing?

A

The dynamic ability to measure changes as and when they’re happening

57
Q

What type of MRI scan would this be

A

Diffusion Tensor Imaging (DTI)

Fibre Tracking:

Movement of water molecules along specific fibre pathways

58
Q

What is the purpose of FMRI

A
  • Measures brain activity by detecting changes associated with blood flow.
  • When an area of the brain is in use, blood flow to that region also increases.
  • Shows function of areas of the brain during movement patterns
59
Q

What is the purpose of MR spectroscopy

A
  • Quantitative technique
  • Utilises chemical shift (based on nuclear spin) different chemicals respond to different chemical fields at different parts of the scan
  • Spectral peaks indicate proportion of chemical present in voxel

Medical Imaging (13 decks)

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