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MRI > Week 3 > Flashcards

Week 3 Flashcards

1
Q

What is the static magnetic field?

A

The constant external magnetic field, and the most important component for MRI

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2
Q

What is the magnetic moment?

A

The specific, magnetic property individual protons endure at the microscopic level

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3
Q

What is net magnetization?

A

The direction most protons point when inside a magnetic field

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4
Q

What is the net magnetization vector?

A

The sum of each hydrogen atoms magnetic moment while participating in net magnetization

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5
Q

What is B0?

A

Directly related to net magnetization, B0 specifically denotes the strength and orientation of the static magnetic field

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6
Q

What happens to low energy protons when introduced to a magnetic field?

A

They become parallel with the direction of B0. Known as “spin-up”

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7
Q

What happens to high energy protons when introduced to a magnetic field?

A

They become anti-parallel with B0 and point the opposite direction. Known as “spin-down“

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8
Q

Every hydrogen atom that aligns parallel with B0 constantly precesses in the same direction, but at varying rates. True or False?

A

False, static hydrogen atoms spin in the same direction at the same rate

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9
Q

What happens to the number of static hydrogen atoms, the precessional frequency, and image quality if the magnetic strength is increased?

A
  1. Increase number of static hydrogen atoms.
  2. Increase precessional frequency.
  3. Increase in image quality.
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10
Q

What are two other terms for precessional frequency?

A
  1. Resonant frequency
  2. Larmor frequency
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11
Q

What is precessional frequency?

A

The rate at which protons spin around B0

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12
Q

What is the gyromagnetic ratio and what equation is it used in?

A
  1. 42.6
  2. The numerical constant used to calculate the Larmor equation.
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13
Q

What is the gyromagnetic frequency of a hydrogen atom?

A

42.6 MHz/T (megahertz per Tesla)

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14
Q

What formula is used for calculating precessional frequency?

A

The Larmor equation
PF= (y/2n)B

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15
Q

In the equation PF= (y/2n)B, What does PF stand for and what is it measured in?

A

Precessional frequency; megahertz (MHz)

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16
Q

In the equation PF= (y/2n)B, what does the (y/2n) represent and what is its value?

A

The gyromagnetic ratio; in the context of MRI, it will always be 42.6.

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17
Q

In the equation PF= (y/2n)B, what does B represent and what is it measured in?

A

The magnetic field strength, measured in Tesla

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18
Q

What are the four main tissue types and their appearance in MRI?

A
  1. Fat (bright)
  2. Bone (grey)
  3. Muscle (darker grey)
  4. Tendon (dark)
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19
Q

How is tissue differentiation achieved in MRI?

A

By using radio-frequency excitation pulses and measuring the relaxation times of each tissue’s hydrogen atoms

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20
Q

What is the longitudinal direction?

A

The direction in which aligned protons are pointing while in the presence of the magnetic field.

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21
Q

What is the transverse direction?

A

The direction protons point after interacting with a radio frequency excitation pulse

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22
Q

What is relaxation time?

A

The time it takes a proton to return back to the longitudinal direction after being pushed into the transverse direction via an excitation pulse.

23
Q

What is relaxation time measured in?

A

Milliseconds (ms)

24
Q

What are the two types of relaxation times and when do they occur in relation to one another?

A
  1. T1
  2. T2
    *Simultaneously
25
Q

What condition must the excitation pulse meet to flip net magnetization from the longitudinal to the transverse direction?

A

The pulse must be equal to the precessional frequency of hydrogen atoms.

26
Q

What is a flip angle?

A

The degree in which the net magnetization tilts from longitudinal to the transverse direction after a RF pulse.

27
Q

What two factors does the flip angle rely upon?

A
  1. The RF pulse amplitude.
  2. The RF pulse duration.
28
Q

What is spatial localization?

A

The MRI scanner’s ability to excite a specific location within a patient’s body for imaging

29
Q

What is the unit of measurement for gradient field applications?

A

Millitesla per meter (mT/m)

30
Q

What are the gradients inside the scanner composed of and why?

A

Coiled wire to allow for the flow of electrical currents

31
Q

Where are the gradients located inside the scanner?

A

They are positioned around the patient inside the bore

32
Q

What are the three gradient types?

A
  1. Sagittal (X)
  2. Coronal (Y)
  3. Axial (Z)
33
Q

The gradients correlate to coordinates within the body and are always linear in nature. True or false?

A

True

34
Q

How do gradients achieve spatial localization?

A

By varying the magnetic field, both positively and negatively, using a radio frequency pulse.

35
Q

What are the two gradient variation formulas?

A
  1. B+ = B0 + [(G * D)/1000]
  2. B- = B0 - [(G * D)/1000]
36
Q

What does the gradient variation formula solve for?

A

How positively and negatively the gradient will vary the magnetic field with its RF pulse

37
Q

In the gradient variation formula, what does B0 refer to and what is it measured in?

A

The strength of the magnet; Teslas

38
Q

In the gradient variation formula, what does the G correlate to?

A

The strength of the gradient

39
Q

In the gradient variation formula, what does the D correlate to?

A

The distance from the center of the magnet

40
Q

Describe a Lumbar image obtained with the X gradient

A

The image would be taken on the sagittal plane, from left to right. A single image would appear like a lateral lumbar spine x-ray.

41
Q

Describe a lumbar image obtained with the Y gradient

A

The image would be on the coronal plane, anterior to posterior. The image would appear as an AP lumbar spine x-ray.

42
Q

Describe a lumbar image obtained on the Z gradient

A

The image would be on the axial plane, superior to inferior. This would be a cross-sectional view and unattainable on x-ray.

43
Q

What are the two components used in T1 and T2 imaging?

A
  1. Fat
  2. Water
44
Q

What is measured during T1 relaxation?

A

The time it takes both fat and water atoms to relax back to the longitudinal direction after the RF pulse.

45
Q

What’s another term for T1 relaxation?

A

Spin-lattice relaxation

46
Q

Where does spin-lattice relaxation get its name from?

A

As the fat and water protons are relaxing back to the longitudinal direction, they emit energy into their surroundings (a.k.a. the lattice)

47
Q

A certain percentage of all the protons affected by the RF pulse must be relaxed to longitude before any significant tissue differentiation occurs. What is this percentage known as and what must its value be?

A

T1 amplitude; 63%.

48
Q

How do the T1 relaxation times of fat differ from those of water?

A

Fat has a shorter T1 relaxation time than water

49
Q

What is T2 relaxation?

A

The amount of time it takes both fat and water hydrogen atoms to become dephased.

50
Q

What is dephasing?

A

The slow, incoherent spin of protons as they relax back to the longitudinal direction

51
Q

What is another term for T2 relaxation and why?

A

Spin-spin relaxation; Because it uses the spinning habits of atoms in two different components, fat and water.

52
Q

In T1 relaxation imaging; fat appears bright and water appears dark. How does T2 relaxation imaging compare? Why?

A

T2 imaging is the opposite, with fat appearing dark and water bright. This is because fat hydrogen atoms dephase significantly faster than water.

53
Q

What percentage of dephasing must be reached for both the fat and water atoms in order for there to be a significant degree of tissue differentiation? What is this percentage known as?

A

37%; the T2 amplitude

MRI (11 decks)

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