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Fall 2022 > Magnetic Resonance Imaging Lecture 2 > Flashcards

Magnetic Resonance Imaging Lecture 2 Flashcards

1
Q

MRI uses ionizing radiation

TRUE/FALSE

A

False

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2
Q

MRI has become a vital tool for diagnosing:

A

Brain tumors and other disease of CNS.

Also for spotting soft-tissue injuries in muscles and ligaments

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3
Q

What is the fundamental property of matter in MRI?

A

Magnetism

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4
Q

In MRI, magnetism is generated by moving charges which are usually

A

Electrons

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5
Q

Atoms can have electron orbitals that are

A

paired (even # of electrons cancel magnetic field) or unpaired (magnetic field present)

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6
Q

Most materials do not show noticeable magnetic properties, except for

A

permanent magnets

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7
Q

The magnetic field exists as dipoles. There are two poles:

A

North pole: where magnetic field lines originate
South pole: where magnetic field lines return
Like poles repel, and opposite poles attract

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8
Q

Number of magnetic lines per unit area. It also decreases approx. with the square of the distance from source

A

Magnetic field strength (B)

also called magnetic flux density

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9
Q

The SI unit for magnetic field is

A

Tesla (T)

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10
Q

1T = 10,000 G (gauss)

Earth’s magnetic field=

A

0.00005T=0.05mT=0.5G

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11
Q

The magnetic field is induced by a

A

moving charge in a wire

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12
Q

What does the magnetic field direction depend on when it comes to the moving charge in the wire?

A

sign and direction

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13
Q

What is the right hand rule when it comes to the magnetic field?

A

fingers point in the magnetic field direction and thumb in the direction of a moving positive charge (opposite to the direction of electron movement)

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14
Q

Coiled current carrying wire increases the magnetic field strength inside the coil, with rapid falloff of field strength outside the coil
What does the amplitude of the current in the coil determine?

A

The overall magnetic field strength

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15
Q

Types of magnets in MRI

A

Air core magnet and solid core magnets

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16
Q

This magnet include wire-wrapped cylinders. It is also 1m in diameter and 2-3m long.
The magnetic field (B0) is generated by current flowing through the wire
B0 is horizontal (along z-axis)

A

Air core magnets

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17
Q

In air core magnets this is the peripheral magnetic field outside the magnet core.
It increases for larger bore diameters and higher field strengths.

A

Extensive fringe field

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18
Q

At what measurement are air core magnets superconducting?

A

=>1T

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19
Q

This magnet is constructed from permanent magnets or electromagnets.
B0 runs between magnet poles in the vertical direction.
Fringe fields are confined

A

Solid core magnets

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20
Q

Solid core magnets are commonly used in what type of MRI?

A

low-strength MRI

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21
Q

What kind of magnets are used in clinical MRI?

A

superconductive magnets with field strength of 1.5-3T

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22
Q

This is a magnetic property of materials. It is the extent to which a material becomes magnetized when placed in a magnetic field.

A

Susceptibility

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23
Q

What are the three types of susceptibility?

A

Diamagnetic, paramagnetic, ferromagnetic

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24
Q

This is a slightly negative susceptibility.

Oppose applied magnetic field: paired electrons the electron orbitals

A

Diamagnetic

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25
Q

Slightly positive susceptibility.

Enhance applied magnetic field: unpaired electrons in the electron orbitals

A

Paramagnetic

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26
Q

Supermagnetic-increase the applied magnetic field significantly
Can exhibit self-magnetism

A

Ferromagnetic

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27
Q

In an atom, these two have magnetic properties affected by spin and charge distributions intrinsic to _______ and _______/

A

Protons and neutrons

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28
Q

This part of the atom has a positive charge. The magnetic dipole produced by nuclear spin

A

Protons

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29
Q

This part of the atom is uncharged, but with subnuclear particles. The magnetic dipole produced in opposite direction of proton, and approximately same magnitude

A

Neutron

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30
Q

Magnetic moment of a nucleus that arises from the spin of the protons and neutrons
Represented by a

A

vector with magnitude and direction

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31
Q

With the nuclear magnetic moment, does the net magnetic moment exist with an odd or positive number of neutrons and/or protons?

A

Positive number

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32
Q

In the nuclear magnetic moment, the magnetic moment of many nuclei make up the MRI _____

A

signal

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33
Q

The protons in our bodies are typically oriented randomly and the magnetic moments cancel each other out ->

A

Net field=0

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34
Q

Putting a body in an MRI;
When placed in a strong field (the B0 field), the protons line up with (parallel) or against (antiparallel) the main field. In which direction are there slightly more protons?
Net magnetic moment in the direction of B0

A

Parallel direction

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35
Q

Putting a body in an MRI;
As the magnetic field strength increases, the energy separation of the low- and high- energy levels increases, and the number of protons in the low-energy state

A

Increases

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36
Q

Precession;
Bo=0
Spinning charge on proton generates magnetic dipole.

A

The spinning proton with magnetic moment u

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37
Q

Precession;

Bo is applied which imposes a torque on the magnetic moment u

A

The spin axis is tilted and the magnetic moment precesses about the axis of Bo

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38
Q

How can you figure out precession frequency?

A

Larmor equation

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39
Q

Larmor equation;

Gyromagnetic ratio is _______ for hydrogen protons

A

42.58 MHz/T

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40
Q

Net magnetic moment (Mo);
Component of that is Mo parallel to Bo
This is maximum at equilibrium

A

Longitudinal magnetization (Mz)

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41
Q

Net Magnetic Moment (Mo);
Component of that is Mo that is in the X-Y plane
This is zero at equilibrium

A

Transverse magnetization (Mxy)

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42
Q

What are the two parts of the net magnetic moment (Mo)?

A 
Longitudinal Magnetization (Mz)
Transverse magnetization (Mxy)
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43
Q

What are some frames of reference?

A

Laboratory frame and rotating frame

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44
Q

Magnetic field B0 is parallel to the z-axis

A

Frames of reference

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45
Q

This is one type of frame of reference.
Observer is detached from the system
M0 rotating about the z-axis at the Larmor frequency

A

Laboratory frame

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46
Q

This is one type of frame of reference
Observer is part of the system and rotating about the z-axis at the Larmor frequency
M0 rotating about the z-axis appears stationary

A

Rotating frame

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47
Q

Make small adjustments to make B0 uniform throughout the volume

A

Shim coils

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48
Q

Placed within the main bore of the magnet to transmit energy to the patient and receive returning signals

A

Radiofrequency (RF) coils

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49
Q

Placed within the main bore to produce linear variation of the magnetic field strength across the magnet volume

A

Gradient coils

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50
Q

Magnetic component of the RF excitation pulse (B1 field) matches the precessional frequency of the protons
Displaces the equilibrium magnetization if the frequency matches the Larmor frequency

A

Resonance

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51
Q

Result of exciting the tissue sample with RF radiation.

Causes displacement of the equilibrium magnetization by causing antiparallel and parallel spins to flip.

A

Excitation

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52
Q

RF signal is emitted by the rotating magnetic moment.
__ _______ quantifies the rate of the decay of the magnetization within the xy plane (Mxy).
At equilibrium Mxy=0
After a 90-degree RF pulse, the Mz converted to Mxy
Initially the spins are in phase with each other.
With time, the spins begin to dephase gradually and Mxy=0

A

T2 relaxation

Also called the transverse or spin-spin relaxation

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53
Q

In T2 relaxation, Mxy follows what kind of decay?

A

exponential

54
Q

In T2 relaxation; this is the transverse magnetization at time t

A

Mxy(t)

55
Q

At T2 relaxation; this is the initial transverse magnetization at t=0

A

Mo

56
Q

___ is the time it take for Mxy to decay to 1/e or 37% of its initial magnitude
At t=T2, Mxy=0.37Mo

A

T2

57
Q

When returning to equilibrium, this is ___ ________.
Mz begins to recover immediately after the RF pulse, simultaneous with transverse decay
___ ________ is the process by which Mz grows to its initial maximum value

A

T1 relaxation

also called longitudinal and spin-lattice relaxation

58
Q

This is the process by which Mz grows to its initial maximum value

A

T1 relaxation

59
Q

In T1 relaxation, Mz regrows

A

exponentially

60
Q

In T1 relaxation:
Mz(t)= longitudinal magnetization at time t
M0 = initial transverse magnetization at t = 0
T1 is the time needed for the recovery of 63% of Mz
At t = T1, = Mz= 0.63 M0

A

T1 relaxation

61
Q

___ is the time needed for the recovery of 63% of Mz

A

T1

62
Q

What are the basic acquisition parameters?

A

Time of Repetition (TR)
Time of Echo (TE)
Time of Inversion (TI)

63
Q

Because of this, there are differences in T1 and T2 relaxations times
WE WANT TO ACCENTUATE THESE DIFFERENCES IN T1 & T2 by changing how frequently we put in RF pulses (Trep) and how frequently we listen for a return signal (Techo)
This together is a

A

Pulse sequence

64
Q

Acquiring an MRI relies on the repetition of a sequence of events in order to sample the volume of interest and to build a complete dataset over time
___ is the time between the RF excitation pulses.

A

Time of repetition (TR)

65
Q

During the TR interval, T2 decay and T1 recovery occur in ______ tissue

A

the same

66
Q

Initial 90° RF pulse produces maximum Mxy
Signal decays exponentially
At time TE/2 following the initial 90° pulse, a 180° RF inversion pulse is applied.
induces rephasing of Mxy
At time ___, peak amplitude of the echo is reached due to rephasing of the spins.
A gradient polarity reversal can also induce echo formation (gradient echo).

A

Time of echo (TE)

67
Q

At time TE, peak amplitude of the echo is reached due to

A

rephasing of the spins

68
Q

Time between an initial inversion/excitation (180) RF pulse that produces maximum tissue saturation, and a 90 RF pulse.
During the __, Mz recovery occurs.
The 90 degree RF pulse converts the recovered Mz into Mxy, which is then measured with the formation of an echo at time TE as shown earlier

A

Time of inversion (TI)

69
Q

Basic Pulse Sequences

A 
Spin-echo (SE)
Inversion recovery (IR)
Gradient Echo (GE)
70
Q

90 RF pulse flips the spins onto the transverse plane, and spins begin to dephase with time
At TE/2, a 180 RF pulse inverts the spins and re-establishes phase coherence to produce an echo at TE
The signal from the echo is captured and recorded to produce the image

A

Spin Echo (SE)

71
Q

Contrast proportional to the difference in signal intensity between adjacent pixels in an image
Signal intensity variations in different tissues depend on TR and TE settings
(look at powerpoint for equation)

A

SE contrast weighting

72
Q 
Signal intensity differences due to differences of  T1 of the tissues
Short TR (400-600ms) and short TE (10-30ms)
A

SE sequence T1-weighting

73
Q

This maximizes the differences in M_z recovery during the return to equilibrium
400-600ms

A

Short TR

74
Q

This minimizes the T2 decay during signal acquisition

10-30ms

A

Short TE

75
Q

In T1-weighted MRI scan

Tissues with short T1=

A

higher signal intensity

76
Q

In T1-weighted MRI scan

Tissues with long T1=

A

Lower signal intensity

77
Q

In T1-weighted MRI scan

CSF is

A

dark

78
Q

In T1-weighted MRI scan

Fat is

A

bright

79
Q

In T1-weighted MRI scan

white matter is _____ than gray matter

A

brighter

80
Q 
Signal intensity differences due to differences in proton density
Long TR (1,500-3,000ms) and shirt TE (10-30ms)
A

SE sequence: proton density (PD)-weighting

81
Q

In SE sequence: PD-weighting
this minimizes T1 relaxation differences of tissues
1,500-3,000ms

A

Long TR

82
Q

In SE sequence: PD-weighting
this preserves PD differences without allowing significant T2 decay
10-20ms

A

Short TE

83
Q

In a PD-weighted MRI scan

CSF is

A

gray

84
Q

In a PD-weighted MRI scan

Fat is

A

bright

85
Q

In a PD-weighted MRI scan

gray matter is _____ than white matter

A

brighter

86
Q 
Signal intensity differences due to differences of  T2 of the tissues
Long TR (1,500-3,000 ms) and long TE (60-150 ms)
A

SE sequence T2-weighting

87
Q

In SE sequence T2-weighting
this minimizes T1 relaxation differences of tissues
1,500-3,000ms

A

Long TR

88
Q

In SE sequence T2-weighting
this emphasizes T2 decay differences of tissues
60-150ms

A

Short TE

89
Q

In T2-weighted MRI scan

Tissues with large T2 =

A

higher signal intensity

90
Q

In T2-weighted MRI scan

Tissues with low T2 =

A

lower signal intensity

91
Q

In T2-weighted MRI scan

CSF is

A

bright

92
Q

In T2-weighted MRI scan

Fat is

A

dark

93
Q

In T2-weighted MRI scan

Gray matter is _____ than white matter

A

brighter

94
Q

Null signal from certain tissues
Initial 180° inverting pulse flips Mz into the –z direction and starts to recover with a time dependent on the tissues T1
At TI, a 90° RF pulse is applied to remove the signal from a specific tissue
Since there is no Mz, there will be no Mxy to create a signal
In MRI we only collect data from Mxy
Fat and fluid have different T1, so Mz reaches 0 at different times
By selecting when to apply the 90° RF pulse, we can select which tissue to null

A

Inversion Recovery (IR)

95
Q

Look at powerpoint for this equation

IR signal intensity

A

S∝ρ_H [1−〖2e〗^((−TI)⁄T1) ] [1−e^(−(TR −TI)/T1) ] [e^(−TE/T2) ]

96
Q

Used to null fat signals
Uses shorter TI
Bounce point is when MZ = 0
90o RF pulse applied during the bounce point
If MZ = 0 at TI, maximum possible signal is 0
Selection of appropriate TI can suppress tissue signals depending on their T1 relaxation times

A

Short TAU inversion recovery (STIR)

97
Q

Used to null CSF signals
Uses longer TI
TI selected at the bounce point of CSF to allow null the signal from CSF and provide a better visualization of the surrounding anatomy

A

Fluid Attenuated Inversion Recovery (FLAIR)

98
Q

No 180 degree refocusing RF pulse, but use gradients to rephase the spins
Small flip angle < 90o (α) and short TR
(Shorter scan times)
Transverse magnetization decays at a rate denoted by T2*
(T2* results from inhomogeneities in the B0 field due to the use of gradients)

A

Gradient Echo (GE)

99
Q

This gradient is (produced by special coils within the magnet housing) applied to change the local magnetic fields, which changes the resonance frequencies across the patient. This leads to an accelerated dephasing of the spins.

A

Dephasing gradient

100
Q

This gradient is applied with the same magnitude but opposite polarity to the dephasing gradient to reverse the phase shift. This leads to the formation of a gradient echo.

A

Rephasing Gradient

101
Q

This is is important for creating MR images and for determining the location of the sample volumes

A

Spatial localization

102
Q

How is spatial localization achieved?

A

Achieved by superimposing linear magnetic field variations on the B0 field to generate corresponding position-depended variations in the precessional frequency of the protons
Simultaneous application of the RF pulse excited only the protons in resonance withing the frequency bandwidth of the RF pulse

103
Q

Three sets of gradient coils along the x, y, and z axis produce magnetic field variation based on the magnitude of the current applied in each coil
What is each one used as?

A

Use one to select slice, one as the phase encode, and one as the frequency encode
Net gradient= √(G_x^(2 )+G_y^(2 )+G_z^(2 ) )

104
Q

This gradient is applied along the z-axis and RF pulse turned on simultaneously
Determines the slab of tissue to be imaged
Localizes a proton to a slice by changing the frequency of the protons
Applied gradient and the bandwidth of the RF pulse determines the slice thickness

A

Slice select gradient

105
Q

The slice select gradient determines what?

A

The slab of tissue to be imaged

106
Q

Linearly increasing frequency encoding gradient applied along the x-axis to specify position within a slice
Localizes the protons in a slice along the x-axis by changing the frequency
Effective field at any point (x) along the x-axis is: B(x)=Bo+xGf
From Larmor equation, we can see the resonant frequency f(x) varies linearly with position x along the x-axis:
Each column has unique frequency

A

Frequency encoding gradient (also known as the readout gradient)

107
Q

Localizes the protons along the y-axis by changing the phase of the protons
Causes dephasing of the protons according to their position along the gradient
Once gradient is turned off, the protons go back to their original frequency, but have a different phase
Applied repeatedly, at different gradients, to each slice
Each row has a unique phase.

A

Phase encoding gradient

108
Q

This artifact is due to patient motion (breathing, cardiac, swallowing, involuntary muscle movement,…)
Seen strongly in the phase encode direction

A

Motion artifact

109
Q

This artifact is due to local distortions in magnetic field
Tissue interfaces (air-fat, air-water interfaces)
Metal (fillings, braces, implants,…)
Signal void and geometric distortion

A

Metal and susceptibility artifact

110
Q

This artifact Appears as a series of lines in the MR image parallel to abrupt and intense changes in the object at this location
Ex: CSF-spinal cord and skull-brain interface
Fine lines visible in an image are due to undersampling of high spatial frequencies because of incomplete digitization of the spin echo signal

A

Truncation artifacts

111
Q

This type of artifact Appears as an edge enhancement between regions containing predominantly water molecules and those containing predominantly fat molecules
Fat and water have different resonance frequencies
Occurs in frequency encode direction
One side there is signal enhancement where the fat and water signals are superimposed, while on the other there is signal nulling because fat has shifted away from where it is supposed to be

A

Chemical shift artifact

112
Q

Chemical shift artifact occur in

A

frequency encode direction

113
Q

motion artifact is seen strongly in the

A

Phase encode direction

114
Q

This artifact occurs when the field of view is too small for size of imaged object
Mismapping of anatomy outside the FOV to inside the FOV

A

Aliasing (wrap around) artifact

115
Q

For MRI safety, there are some things you must do before you enter because the magnet is ALWAYS ON:

A

Patient Safety Questionnaire
Posted Signs
Only MRI approved equipment can go into the room (crash carts, needles, catheters, stethoscopes, wheelchairs, oxygen tanks, etc.

116
Q

On slide 70 in powerpoint, there are the approved limits for human imaging

A

Magnet- device interference 4.0T
Gradients: nerve stimulation-<6T/s
deafness 105dBA over 1 hr or 200 pascals peak
RF coils- tissue heating <0.4W/kg (body) and <3.2W/kg (head)

117
Q

Do we use shielding in an MRI?

A

Yes

118
Q

For shielding in an MRI, we are worried about outside RF interacting with our RF pulses and interfering with our pulse sequences
What lines the whole room?
This basically creates a faraday cage of the scanner room

A

Copper paneling

119
Q

The magnet is cooled by liquid helium
this happens when the liquid cryogen boils off rapidly
this results in helium escaping very rapidly from the cryogen bath
coils cease to be superconducting
accompanied by a loud bang
might activate the STOP magnet, but it might not
it can be a partial ______ and the magnet is still active…proceed with caution

A

Quenching

120
Q

Encloses body part to be imaged
Increases signal by being close to body part
Does not detect noise from the rest of the body

A

RF coils

121
Q

What are the two types of RF coils?

A

Birdcage coils and Surface coils

122
Q

In MRI, we measure the magnetization that is _______ to the main B0 field

A

Perpendicular

123
Q

What is the time of repetition

A

Time between successive 90 degree RF pulse

124
Q

What is the correct order of a spin echo pulse sequence?

A

2 Mxy is max and Mo is 0
3 Spins begin to dephase and Mxy decays according to T2
1 90 degree RF pulse flips spins onto xy plane
4 At TE/2, a 180 degree RF pulse is applied to invert the spins and reestablish phase coherence
5 Spin echo is produced at TE

125
Q

What is the direction of the Bo field in a cylindrical air core scanner?

A

Parallel to the long axis of the cylinder

126
Q

Spatial localization in MRI depends on?

A

Varying magnetic field across the patient through the use of gradients

127
Q

Slice selection is performed by

A

Turning on the slice select gradiatient during Rf excitation

128
Q

FLAIR is used to null signal from ____ and STIR is used to null the signal from ____

A

CSF, fat

129
Q

Which is true about T1-weighted image

A

Shirt TR and short TE

130
Q

Which is true about T2 weighted image?

A

Long TR and long TE

131
Q

Which is true about PD weighted image

A

long TR and short TE

132
Q

Contrast in MRI relies on

A

Proton density, T1 and T2 relaxation time

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