Neurodiagnostics, Pt. 2 Flashcards

1
Q

What is MRI based on? How does it work?

A

magnetic properties of living tissue, primarily spinning hydrogen atoms (protons)

  • patient is placed in a magnetic field to produce a net magentization
  • radiofrequency (RF) pulses are then administered to displace protons out of the z axis
  • as protons relax back to their original energy states, RF energy is released at variable intensities, recorded by a receiver coil, and converted into images
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2
Q

What is phase coherence?

A

the degree to which precessing (wobbling) nuclear spins are synchronous

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3
Q

What is the difference between T1 and T2 relaxation?

A

T1 (longitudinal) = time which determines the rate at which excited protons return to equilibrium, measurement of the time taken for spinning protons to realign with the external magnetic field = spin-lattice relaxation

T2 (transverse) = time which determines the rate at which excited protons reach equilibrium or go out of phase with each other, measurement of time taken for spinning protons to lose phase coherence among the nuclei spinning perpendicular to the main field = spin-spin relaxation

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4
Q

What is echo time (TE)? Repetition time (TR)?

A

time between the delivery of the RF pulse and the receipt of the echo signal by receiver coils —> time the tissue is being imaged to release energy for collection by MRI coils

amount of time between successive pulse sequences applied to the same slice —> time the tissue is being imaged to relax back toward the direction of the magnetic field before excited again

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5
Q

What is the utility in using different sequences in MRIs?

A
  • grey vs white matter
  • water-rich or not
  • fatty structure
  • free fluid vs tissue-bound fluid
  • breaks in BBB
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6
Q

What is the differenct between T1, T2, and intermediate-weighted images?

A

T1 = water dark, fat bright, white matter brighter than gray matter

T2 = water bright, fat gray, gray matter brighter than white matter

intermediate = fluid-attenuation where free water is suppressed (CSF, cysts) —> FLAIR most common

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7
Q

What are the 2 major advantages to using MRI?

A
  1. superior soft tissue detail
  2. better discernment of different tissue types
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8
Q

What makes up the magnetic vector (M) of patient tissue? What will it align with? How is its energy distinguished from stronger magnetic fields?

A

numerous spinning protons

strong magnetic fields inside the MRI gantry along the z axis

it must be separated from along the z axis

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9
Q

What is Larmor frequency?

A

the rate of precession (non-zero spin) of the magnetic moment of the proton around the external magnetic field

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10
Q

What is T1 relaxation?

A

spin-lattice / longitudinal - transfer of energy from the main magnetic vector of the patient back to the surrounding environment (magnetic field)

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11
Q

What is T2 relaxation? What causes it?

A

spin-spin / transverse - loss of energy between individual spinning protons within the main tissue magnetic vector —> energy released is less and less useful for signal generation and image formation the faster it is

inhomogeneities within the magnetic field caused by loss of coherence and poor signal formation

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12
Q

What is magnetic field inhomogeneity? What patient factors also cause this?

A

magnetic field within the MRI gantry is close to uniform, but not close enough to prevent loss of phase coherence

  • water content
  • hemorrhage
  • air-soft tissue interfaces
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13
Q

What are the 3 basic spin echo sequences?

A
  1. T1-weighted images
  2. T2-weighted images
  3. proton density-weighted images
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14
Q

What is the point of manipulating echo times and repetition times?

A

manipulated in basic spin echo sequences to emphasize certain tissues based on T1 and T2 relaxation times (that occur concomitantly)

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15
Q

What are 3 characteristics of T1-weighted images? What tissues provide the most signals?

A
  1. short echo time
  2. short repetition time
  3. 90 degree radiofrequency pulses

tissues with short T1 relaxation - fullest potential for energy transfer, release the most energy when short echo times are used —> fat > other soft tissues > water

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16
Q

How do tissues show up on T1-weighted images?

A
  • fat = bright (hyperintense)
  • water/CSF = very dark (hypointense)
  • parenchyma = intermediate, white matter brighter than grey matter
17
Q

What contrast is commonly used with T1-weighted images? What does it do?

A

gadolinium - paramagnetic compound administered IV

shortens T1 relaxation times in tissues where it localizes, making a very hyperintense signal —> inflammatory foci, tumors, etc.

18
Q

What are 3 characteristics of T2-weighted images? What tissues give the greatest signal?

A
  1. long echo time
  2. long repetition time
  3. following the 90 degree radiofrequency pulse, 180 degree refocusing pulse is applied to regain phase coherence

tissues with long T2 relaxation times, which are able to accept the rephasing refocusing pulse —> water

19
Q

How do tissues with long repetition times affect T1/T2 effects?

A

minimizes T1 effects

20
Q

How do tissues show up on T2-weighted images?

A
  • fat = bright, but not as bright as T1
  • water (CSF) = very bright (hyperintense)
  • parenchyma = imtermediate, gray matter brighter than white matter
21
Q

T1 vs T2-weighted images:

A

T1 = fat hyperintense, CSF hypointense, white matter brighter

T2 = fat bright, CSF hyperintense, gray matter brighter

22
Q

What are the 2 major characteristics of proton density-weighted images? What does this result in? What information does it give?

A

long relaxation time, short echo time = minimized T1 and T2 effects

based upon the amount of hydrogen protons available in a tissue, providing more tissue contrast (white vs gray matter, bone vs parenchyma)

23
Q

T1 vs T2 vs PD:

A

T1 = fat hyperintense, CSF hypointense, white matter brighter

T2 = fat bright, CSF hyperintense, gray matter brighter

PD = more tissue contrast

24
Q

What are inversion recovery (IR) sequences? How does it work?

A

T2-based, manipulates T1 relaxation times

  • based on suppressing signal from a specific tissue by applying additional 90 degree inversion pulse after 180 degree refocusing pulse
  • refocusing pulse pushed the magnetic vector of tissue to be suppressed on the z axis, so no signal is generated

(FLAIR and STIR)

25
Q

What is FLAIR?

A

Fluid Attenuation Inversion Recovery - free fluid protons (CSF) with longer T1 relaxation times compared to tissue-bound fluid protons (edema)

after the long inversion time, the 90 degree inversion pulse knocks the free fluid vector to the negative z axis, but tissue-bound fluid has already relaxed enough to produce a signal

26
Q

What image is produced by FLAIR?

A
  • free fluid (CSF) suppressed
  • tissue bound fluid (edema) is hyperintense
27
Q

FLAIR:

A
28
Q

How is short time (Tau) inversion recovery (STIR) used? How does it work?

A

suppress signals from fat (short T1 relaxation time)

  • 180 degree refocusing pulse is applied
  • short inversion time is allowed before the 90 degree inversion pulse
  • this results in the magnetic vector from fat constantly being displaced to the negative z axis, nullifying its signal
29
Q

What are gradient (T2*) images? What is it usually used for?

A

T2-weighted sequence without the refocusing pulse, allowing it to occur much more quickly than T2 relaxation

hemorrhage (signal void)

(very susceptible to inhomogeneities)

30
Q

What is FATSAT-chemical selective saturation?

A

tissue-specific pulse frequency is applied prior to the standard pulse, resulting in the saturated target tissue no contributing to image formation

31
Q

What are diffusion-weighted images based on? What images are produced?

A
  • diffusivity or random Brownian motion of water molecules
  • internal thermal energy of water molecules

cytotoxic edema impairs diffusion, increasing signal intensity of that area

32
Q

How does diffusion-weighted imaging work?

A

T2-weighted sequence with motion-probing gradients added to either side of 180 degree refocusing pulse

  • the degree of diffusion weighting applied is refered to as the b-value
  • two or more images with different b-values are obtained and an ADC map is created
33
Q

What are 4 disadvantages to using MRI?

A
  1. expense of equipment and maintenance
  2. longer time required compared to CT
  3. more expertise required
  4. limitation on area to be imaged
34
Q

What are 7 clinical uses for MRI?

A
  1. seizure disorders
  2. inflammatory brain disease
  3. ischemic/vascular brain and spinal disease
  4. trauma
  5. disk extrusions
  6. neoplasia
  7. post-op assessment of titanium plates