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PHYSL 372 > MRI > Flashcards

MRI Flashcards

1
Q

What are the goals of imaging?

A
  1. assessment of human anatomy, physiology, and function in-vivo, NON-INVASIVELY
  2. diagnostic/prognostic information (e.g. location/type of injury, stroke, etc.)
  3. improve our understanding of: normal physiology, mechanisms of disease, treatment strategies
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2
Q

List 3 brain imaging techniques.

A
  • computed tomography (x-ray CAT scan)
  • positron emission tomography (PET)
  • magnetic resonance imaging (MRI)
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3
Q

What kind of electromagnetic waves does MRI use? Is the wavelength larger or smaller than x ray?

A

radio; larger than x-ray

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4
Q

What are the pros of MRI?

A
  • contrast mechanisms
  • no ionizing radiation
  • soft tissue contrast (unlike CT)
  • any slice orientation
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5
Q

What are the cons of MRI?

A
  • scan time
  • contra-indications (i.e. not everyone can have MRI scan because it is a large magnet)
  • patient monitoring
  • availability
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6
Q

What basic concept in chemistry did MRI evolve from?

A

NMR

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7
Q

What does NMR measure?

A

intrinsic molecules in the body

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8
Q

What does NMR stand for?

A
  • Nuclear: looking at nuclei of atoms within molecules
  • Magnetic: sample placed in strong magnetic field
  • Resonance: deals with spinning frequency of nuclear spins
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9
Q

TRUE or FALSE: atomic number is the number of protons+ neutrons; mass number is the number of protons

A

FALSE:
- atomic number = # protons
- mass number = # protons + neutrons

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10
Q

A nucleus yields a magnetic moment unless the number of each nucleon type is _____________.

A

even

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11
Q

What exactly is imaged in MRI? Based on this, what does MRI detect?

A

protons of the hydrogen atom in water; detect change in magnetic properties of the proton

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12
Q

TRUE or FALSE: water has constant properties in various tissues, which makes it useful in MRI.

A

FALSE: water has DIFFERENT properties in various tissues, which makes it useful in MRI

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13
Q

Describe how MRI gives us tissue contrast.

A
  • water has different properties in various tissues
  • MRI takes advantage of differences in: concentration, magnetic properties, diffusion, flow, blood oxygen, etc.
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14
Q

When is blood flow considered paramagnetic? diamagnetic?

A
  • paramagnetic = deoxygenated blood
  • diamagnetic = oxygenated blood
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15
Q

What does the signal intensity of each voxel represent?

A

macroscopic average of the water properties in each compartment within the tissue (i.e. activity of H2O molecules)

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16
Q

TRUE or FALSE: one water molecule fits into one voxel

A

FALSE: many water molecules can fit into 1 voxel

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17
Q

How big is a single voxel?

A

1mm x 1mm

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18
Q

How thick are slices in MRI scans?

A

~ 1-5 mm thick

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19
Q

What does the radiofrequency (RF) coil do in MRI?

A

excites and detects the H signal from the tissue

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20
Q

What are the main hardware components of an MRI scanner?

A
  • magnet
  • RF coil
  • gradient coil
  • computer console
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21
Q

How is a static magnetic field (Bo) created in MRI? What does it do?

A
  • large magnet polarizes the sample by creating a net nuclear magnetization along the axis of the applied static magnetic field
  • nuclear magnetic moments align themselves along Bo (like how a compass needle lines up towards north pole)
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22
Q

What are MRI scanners characterized by?

A

strength of their magnetic field

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23
Q

How strong is the magnetic field in a:
- normal clinical scanner
- our strongest scanner
- Earth’s magnetic field

A
  • normal clinical scanner = 1.5 Tesla
  • our strongest scanner = 4.7 Tesla
  • Earth’s magnetic field = 0.00005 Tesla
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24
Q

What is the function of longitudinal magnetization (Mo) in MRI?

A

prepare the signal for transmission, by aligning the protons

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25
Q

TRUE or FALSE: signal-to-noise in image is inversely proportional to magnetic field strength

A

FALSE: signal-to-noise in image GOES UP LINEARLY with field strength (i.e. increase Bo (Tesla) = increase resolution and speed of MRI)

26
Q

What are some disadvantages of using a strong magnetic field in MRI?

A
  • distortions
  • cost
  • power
  • acoustic noise
27
Q

What do RF pulses do in MRI? Why do we need radiofrequency pulses (B1) in MRI? (i.e. how does it affect magnetization?)

A
  • need RF pulses to excite the magnetic field (Bo) that we created because we cannot detect magnetization along the z-axis (longitudinal magnetization)
  • i.e. change LONGITUDINAL magnetization (which cannot be detected) to TRANSVERSE magnetization
28
Q

What are the letters that symbolize static magnetic field, RF pulses, longitudinal magnetic field, transverse magnetization, gradient coil?

A
  • static magnetic field = Bo
  • RF pulses = B1
  • longitudinal magnetic field = Mo or Mz
  • transverse magnetization = Mxy
  • gradient coil = G
29
Q

Why do we need to convert Mz to Mxy in MRI with B1 rf pulses?

A
  • Mz cannot be detected
  • Mxy is measurable because it induces a current in an appropriately tuned RF coil
30
Q

Why is the Larmor frequency important in MRI? What is another name for the Larmor frequency?

A
  • it is the rate of spin of the proton magnetic moment around the magnetic field
  • also called the precessional frquency
31
Q

What are 2 types of brain RF coils?

A
  • standard “birdcages”
  • phased array
32
Q

How is the MR signal that is detected by the RF coil converted into a reconstructed image of the brain?

A

the FOURIER TRANSFORM (FT) is able to pull apart individual frequencies from a complex signal to form a reconstructed image of the brain

33
Q

What do gradient coils (G) create in MRI? What are they used for?

A
  • creates inhomogeneous linearly-varying magnetic field that adds or subtracts a small amount from the main static magnetic field
  • used for SPATIAL LOCALIZATION of the signal from the nuclear spins
34
Q

TRUE or FALSE: magnetic field and precessional frequency vary as a LINEAR function of position

A

TRUE

35
Q

What are the benefits of stronger gradients in MRI?

A
  • faster acquisition
  • can yield more signal and contrast potential
  • more signal means higher spatial resolution
36
Q

Describe the gradient-echo MRI pulse sequence.

A
  1. magnetize the sample (Bo)
  2. excitation of magnetization (B1)
  3. gradients - spatial localization of the signal (G)
  4. acquire the data - precession of magnetization (computer console)
37
Q

Order, strength, and timing of RF and G pulses - all of these parts affect __________________________.

A

final image signal and contrast

38
Q

TRUE or FALSE: different MRI pulse sequences are used for different body parts

A

TRUE

39
Q

What are the main classes of MRI sequences? What is the difference?

A

GRADIENT-ECHO:
- faster, sensitive to magnetic field inhomogeneities

SPIN-ECHO:
- refocuses field inhomogeneities
- bread and butter sequence used for proton-density, T1-, and T2-weighting

40
Q

Describe the spin-echo pulse sequence.

A

arrange different gradient pulses to create different images

41
Q

What are the key acquisition parameters of MRI?

A
  • slice thickness
  • plane of acquisition
  • number of slices
  • field of view (FOV)
  • matrix (in-plane resolution)
  • repetition time (RT)
  • echo time (TE)
  • flip angle
  • number of averages (signal-to-noise)
42
Q

TRUE or FALSE: more time is needed to make brain slice images thicker in MRI

A

FALSE: more time needed to make then THINNER

43
Q

Can stroke often be detected using the typical spatial resolution in MRI?

A

no, should use high spatial resolution

44
Q

What does 3D MPRAGE stand for? What does it do?

A
  • 3D Magnetization Prepared Rapid Acquisition with Gradient Echo
  • excite entire 3D brain volume with first RF pulse + alter delay before acquisition to maximize gray/white contrast
45
Q

What can contrast in MRI be particularly useful for?

A

detecting acute stroke

46
Q

what are some water properties that can be imaged using MRI?

A
  • proton density
  • T1
  • T2
  • FLAIR
  • diffusion
  • perfusion
  • flow
  • blood oxygenation
47
Q

What is one of the most important sources of tissue contrast in MRI?

A

differences in nuclear relaxation

48
Q

Explain nuclear relaxation in terms of Mxy and Mz.

A

transverse magnetization (Mxy) DECAYS away and the longitudinal magnetization (Mz) RECOVERS over time

49
Q

What IV contrast agent is used for MRI? How does it affect T1? Provide an example of what we would be able to see with Gd.

A
  • Gadolinium (Gd) - DTPA
  • shortens T1 (i.e. gets through leaky parts of BBB)
  • e.g. Gd allow us to visualize metastases
50
Q

What does FLAIR stand for? What does it do? What kind of image can you get with FLAIR?

A
  • Fluid Attenuated Inversion Recovery
  • uses difference in T1 between CSF and brain tissue to NULL SIGNAL FROM CSF
  • get T 2-weighted image without confounding bright signal from CSF
51
Q

What can DTI be used to image?

A

properties in brain connectivity (e.g. white matter tracts)

52
Q

What does cerebral perfusion in MRI allow us to measure/image?

A

blood flow in the brain

53
Q

TRUE or FALSE: in MRI, you can see increased blood flow in individuals with long covid

A

FALSE: DECREASED blood flow

54
Q

What can MRA be used to image?

A

vasculature in the brain

55
Q

What can fMRI be used to image? How does it work?

A
  • image task-specific brain activation
  • signal intensities change because of differing levels of oxygen
56
Q

What does MR spectroscopy measure? Provide some examples.

A
  • other molecules in tissue (i.e. not water)
  • e.g. phosphorus spectroscopy to detect ATP
  • e.g. carbon spectroscopy to measure glucose
57
Q

TRUE or FALSE: MR spectroscopy can be used to measure temperature

A

TRUE

58
Q

TRUE or FALSE: MRI can only be used to image the brain

A

FALSE: can also image spinal cord, and other body parts

59
Q

What are some MRI safety issues?

A
  • projectiles
  • metal implants
  • tissue heating (SAR)
  • acoustic noise
  • peripheral nerve stimulation
  • nausea/dizzy (high magnetic fields)
60
Q

How are MRI safety issues addressed?

A

pre-screening and secure scanner environment

PHYSL 372 (21 decks)

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