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Medical Radiation Sciences 2 > MRI > Flashcards

MRI Flashcards

1
Q

T/F

When electric current flows through a wire, a magnetic field is induced around the wire

A

True

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2
Q

Which way is the magnetic field generated in relation to the flow of current?

A

Magnetic field is generated in a direction perpendicular to the flow of current

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3
Q

How can we limit the resistance to the flow of current that is normally generated?

A

Resistance to the flow of current can be reduced if the metal conductor is cooled substantially

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4
Q

Why is having low resistance in a magnetic machine so imporant?

A

Low resistance allows the use of high electric currents to produce high strength magnetic fields

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5
Q

What is the type of magnet (magnet coils) ussually used in MRI machines?

A

Super conducting metal-alloy magnet

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6
Q

What temperature are superconducting magnets within an MRI machine cooled down to? What substance cools it?

A

Cooled to approx. 4K or -269oC, using expensive cryogenic helium

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7
Q

What is the symbol for magnetic feild?

A

B0

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8
Q

What is the strength of the magnetic field is measured in?

A

Units of T (Tesla)

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9
Q

What is the typical strength of the magnetic feild in MRI machines?

A

1.5T and 3T

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10
Q

How is the current of a magnet induced?

A

By injecting electric current into the coil of wire, which allows us to remove the power supply.

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11
Q

T/F

The MRI machine can maintain current for years.

A

True

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12
Q

How often does liquid helium levels in the magnet need to be filled?

A

Once per month to once every few years, depending on the magnet design

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13
Q

What gradient is the arrow showing?

A

The Z-gradient

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14
Q

What are 1 and 2 representing?

A
  1. Flow of current
  2. Magnetic feild
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15
Q

What is the arrow showing?

A

Longitudinal Magnetization (B0)

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16
Q

Give 3 examples of magnetic resonance active atoms?

A
  1. H
  2. N
  3. C
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17
Q

What creates a strong net spin?

A

Single proton + NO neutron

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18
Q

What are the most abundent atom in the body?

A

Hydrogen

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19
Q

What do all clinical images in MR come from?

A

Hydrogen atoms from water, fat and carbohydrates

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20
Q

T/F

Hydrogen has a positive electrical charge

A

True

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21
Q

T/F

Protons have thier own magnetic feild. Why or why not?

A

True; The positive charge of a proton is moving, and a moving electrical charge is a current, and an electrical current generates a magnetic charge.

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22
Q

What is the magnetic field of each proton called?

A

A magnetic dipole moment (MDM)/magnetic moment

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23
Q

T/F

Magnetic moments are usually in line and straight oriented.

A

False; Magnetic moments are usually randomly oriented

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24
Q

What happens when an external magnetic field is applied to protons with a magnetic moment?

A

They align either with (parallel) or against (antiparallel) the external field

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25
Q

What is the preferred state of alignment when protons with a magnetic moment are introduced to an external magnetic feild?

A

Alignment parallel to the external magnetic feild

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26
Q

What is net magnitization

A

The difference in the number of protons aligning parallel and antiparallel to B0

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27
Q

What does net magnetization depend on?

A

Depends on the strength of B0

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28
Q

What is the source of the MR image?

A

Net magnetization

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29
Q

T/F

As an estimate, for about 10 million protons aligning antiparallel to B0, 10,000007 protons align parallel to B0

A

True

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30
Q

What is the symbol for net magnetization?

A

M

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31
Q

What is the longitudinal magnetization denoted by?

A

M0

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32
Q

What is longitudinal magnetization?

A

The resulting net magnetization in the direction of the z-axis

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33
Q

T/F

M0 parallels the external magnetic field (B0)

A

True

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34
Q

What is the X axis?

A

left – right direction

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35
Q

What is the Y axis?

A

Anterior – posterior direction

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36
Q

What is precession?

A

When a spinning proton is placed in magnetic feild, the force from the magnetic field interacts with the spinning proton and tries to get it to “wabble and fall”

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37
Q

What is the frequency of precession?

A

The frequency of precession is the number of revolutions per second

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38
Q

What is precessional frequency denoted as?

A

ω

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39
Q

What is the unit for ω?

A

ω unit is MHz

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40
Q

How do you find the proton precessional frequency?

A

Larmor equation

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41
Q

What is the Gyromagnetic Constant?

A

It is a characteristic of each type of nuclei

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42
Q

What is the gyromagnetic constant for hydrogen?

A

For hydrogen protons, the gyromagnetic constant is equal to 42.6 MHz/T

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43
Q

For a typical MR system, with a magnetic field strength of 1.5T, what is ω?

A

ω = 64 MHz (64 million times/sec)

Use equation

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44
Q

T/F

The relationship between ω and B0 is linear.

A

True

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45
Q

Situation example:

Once B0 (magnetic feild) is applied, there still no signal. What could have caused this?

A

-The net magnetization is very small
-The magnetic force of the patient cannot be measured as it is in the same direction as the external field

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46
Q

Net magnetization has to be at an angle to B0 (the magnetic feild), what is this called?

A

Transverse Magnetization

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47
Q

T/F

Transverse magnetization is required to generate a signal

A

True

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48
Q

How did we get the magnetization to go from longitudinal to transverse?

A

Through the transference of energy from the radiofrequency pulse to the protons

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49
Q

When does transverse magnetization only occur with radiofrequency?

A

When the RF pulse has the same frequency as the precessional frequency of the protons

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50
Q

What is the purpose of the radiofrequency pulse?

A

The purpose of the RF pulse is to disturb the protons so that they fall out of alignment with B0

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51
Q

What is Resonance?

A

Resonance describes the phenomenon of increased amplitude of signal that occurs when the frequency of a RF pulse is equal (or close) to the precessional frequency of protons.

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52
Q

What is the RF energy is transmitted by?

A

RF energy is transmitted by an RF transmit coil (body coil, head coil, knee coil).

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53
Q

What is it called when the RF is transmitted for a period of time? How do we denote this?

A

RF pulse (B1)

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54
Q

T/F

For resonance to occur and for efficient transfer of energy from the RF coils to the proton to occur, RF pulse must be at the precessional frequency of the proton.

A

True

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55
Q

What would the RF energy for a 1.5 T magnet be?

A

64MHz

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56
Q

What happens to the net magnetization as energy is absorbed from the radiofrequency pulse? What is this phonomanon called?

A

-As energy is absorbed from the RF pulse, the net magnetization rotates away from the longitudinal direction
-Called flip angle

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57
Q

What does the flip angle depend on?

A

Depends on the strength and duration of the RF pulse

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58
Q

If the RF pulse rotates the net magnetization into the transverse plane, what is it termed?

A

Termed a 90 degree RF pulse.

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59
Q

T/F

The strength and/or duration of the RF pulse can be controlled to rotate the net magnetization to any angle

A

True

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60
Q

Does transverse magnetization occur?

A

No, the f of th RF is not the same as the precessional f of the proton

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61
Q

Does transverse magnetization occur?

A

Yes, f of RF and precessional f is the same

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62
Q

T/F

As the protons move away from B0, there is an overall gain of M0

A

False; As the protons move away from B0, there is an overall loss of M0

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63
Q

What does Mxy describe?

A

Transverse magnetization occurs, in which protons precess “in phase” in a transverse plane

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64
Q

T/F

When transmitting an RF pulse, the protons in the transversal plane end up in phase with each other

A

True

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65
Q

How do we measure the amount of transverse magnetization?

A

Through a reciever coil

66
Q

T/F

Transverse magnetization cannot induce a current in a loop of wire

A

False; Transverse magnetization can induce a current in a loop of wire

67
Q

What happens after the transverse magnetization is recieved by the reciever coil?

A

Induced electric current is digitized and recorded in the computer of the MR system for reconstruction as an MR image

68
Q

What happens to the protons as soon as we stop the radiofrequency waves?

A

As soon as the RF pulse is switched off the protons start to fall out of phase with each other and return to a lower energy state

69
Q

What are the two ways in which relaxation occurs?

A
  1. Longitudinal magnetization starts to return to its original value – (T1) relaxation
  2. Transverse magnetization begins to disappear – transverse (T2) relaxation
70
Q

T/F

When the transverse magnetism is completely in phase, our measured MR signal is at a max

71
Q

What happens to the MR signal when the transverse magnetization begins to dephase?

A

Measured signal strength begins to decrease until the magnetization is completely dephased, at which time the measured MR signal is zero

72
Q

What are two other terms for dephasing?

A

Spin-spin interaction or transverse decay

73
Q

When does dephasing occur?

A

Dephasing occur due to magnetic interaction between spins (protons)

74
Q

T/F

T2 is a parameter that is characteristic of specific tissue

75
Q

T/F

T2 is the rate of dephasing for the protons associated with that tissue

76
Q

What is the rate of decay in spin to spin interactions called?

A

Free induction decay

77
Q

T/F

Free induction decay is linear.

A

False; Free induction decay is exponential

78
Q

What is this graph showing?

A

The free induction decay curve

79
Q

What is T2?

A

T2 is the time that it takes for the transverse magnetization to decay to 37% of its value (looses 63% of its maximum signal)

80
Q

T/F

T2 depends on its local magnetic feild.

81
Q

T/F

Bones and lungs have a long T2 relaxation.

A

False; have a short T2 relaxation

82
Q

Is this short or long T2 relazation?

A

Short T2 relaxation

83
Q

T/F

CSF and water has a long T2 relaxation.

84
Q

Is this a short T2 relaxation or a long T2 relaxation?

A

Long T2 relaxation

85
Q

What type of T2 relaxation is being shown here?

A

Medium T2 relaxation

86
Q

T/F

Different tissues have different values of T2 and dephase at different rates

87
Q

What colour will Low intensity signal show up as?

A

Dark gray/black

88
Q

What colour will High intensity signal show up as?

89
Q

What colour will Intermediate intensity show up as?

90
Q

What is the duration of T2?

Bone/Calcium/Metal

91
Q

What is the duration of T2?

Fat

92
Q

What is the duration of T2?

Water

A

Very long T2

93
Q

What is the duration of T2?

White matter

94
Q

What is the duration of T2?

CSF

95
Q

What is the duration of T2?

Grey matter

A

Intermediate T2

96
Q

T/F

If we were to create an image at a time when the transverse magnetization curves were widely separated, then we would have high contrast between the tissues in our image

97
Q

What is this describing?

The time in which the technologist acquires the image

A

-Echo time (TE)
-Repetittion time (TR)

98
Q

Is this a T1 or T2 weighted image?

A

T2 weighted image

99
Q

T/F

After a 90 degree RF pulse, the longitudinal magnetization is 100%

A

False; After a 90 degree RF pulse, the longitudinal magnetization is ZERO

100
Q

What is another term for T1 relaxation?

A

Longitudinal relaxation

101
Q

What is the environment to which the longitudinal relaxation energy that is released is called? What is the interaction called?

A

Enviroment: The Lattice
Interaction: Spin-lattice interaction

102
Q

T/F

T1 is a parameter that is characteristic to specific tissue

103
Q

What happens to induce T1 relaxation?

A

Nuclei that are in transverse position are jostled by surrounding lattice and give up thier energy which then caues them to return to thier longitudinal position.

104
Q

T/F

Both the relaxation of T1 and T2 are exponential.

105
Q

What is T1 relaxation?

A

The time it takes for the longitudinal magnetization (Mz) to recover 63% of its maximum value.

106
Q

What does T1 depend on?

A
  1. The surrounding Lattice
  2. Surrounding molecules
  3. Strength of the main magnetic field
107
Q

T/F

On a T1 curve, magnetization increases with time

108
Q

What is the duration of T1?

Fat and protien

109
Q

What is the duration of T1?

Water

110
Q

What is the duration of T1?

Bone/calcium/metal

A

Very long T1

111
Q

What is the duration of T1?

White matter

A

Very Short T1

112
Q

What is the duration of T1?

CSF

113
Q

What is the duration of T1?

Gray matter

A

Intermediate T1

114
Q

T/F

If an image is obtained at a time when the relaxation curves are widely separated, T2-weighted contrast will be maximized

A

False; If an image is obtained at a time when the relaxation curves are widely separated, T1-weighted contrast will be maximized

115
Q

What will tissue with short T1 appear as in the image?

116
Q

Will tissues with a shorter T1 have a larger or smaller Mz value?

A

Large Mz value

117
Q

What colour will shorter T2 values appear as in the image?

118
Q

T/F

Tissues with a shorter T2 have a smaller signal.

119
Q

What type of magnetization is T2 relaxation focused towards?

A

Transverse magnetization (Mxy)

120
Q

What type of magnetization is T1 relaxation focused towards?

A

Longitudinal relaxation (Mz)

121
Q

T/F

The T1 and T2 processes occur simultaneously and independently

122
Q

How do we choose the repetittion time of the radiofrequency pulse?

A

On whether we want to see charecteristics from T1 or T2

123
Q

T/F

TR and TE influence the magnetic resonance signal.

124
Q

How can we influence the tissue weighting of the image?

A

By choosing values of the TE and TR

125
Q

What is the repetition time (TR)?

A

The amount of time betweeen sucessive pulse sequences

126
Q

What is the echo time (ET)?

A

The time between the delivery of hte RF pulse and the receipt of the echo signal

127
Q

What dictates the amount of agnetization each tissue begins its T2 decay with, following a 90 degree RF pulse?

A

The Repetition time

128
Q

With a short TR followed by very short TE, what property will contrast result from?

A

T1 Properties of tissue

129
Q

In a T1 weighted image, what is the duration of the TR for CSF?

A

Very long TR

130
Q

In a T1 weighted image, what is the duration of the TR for white matter?

A

Has an interrmediate TR

131
Q

In a T1 weighted image, what is the duration of the TR for grey matter?

132
Q

T/F

Very long TRs minimize T1 effects since all tissues have had time to recover between excitations.

133
Q

What properties are shown with a long TR and average TE time?

A

T2 properties

134
Q

In a T2 weighted image, what is the duration of the TE for CSF?

135
Q

In a T2 weighted image, what is the duration of the TE for white matter?

136
Q

In a T2 weighted image, what is the duration of the TE for grey matter?

A

Intermediatte TE

137
Q

T/F

CSF is dark on T1-weighted imaging and bright on T2-weighted imaging.

138
Q

How do we tell the difference between T1 and T2 images?

A

By looking at the CSF

139
Q

What are PD-weighted images?

A

PD-weighted images are related to the number of nuclei in the area being imaged (number of hydrogen protons

140
Q

When do PD weighted images result?

A

PD weighted images result when the contribution of both T1 and T2 contrast is minimized

141
Q

How do we minimize the effects of T1 and T2 to form a PD image?

A

Very long TR and very short TE

142
Q

How long is the TR in proton dense images? Why?

A

2000+ms to minimize T1 differences

143
Q

How long is the TE in proton dense images? Why?

A

20ms to minimize T2 differnces

144
Q

T/F

High PD tissues appear dark.

A

False; High PD tissues appear bright

145
Q

Whaty type of image is this?

A

A Proton Density Image

146
Q

What is the function of the RF coils?

A

Send out the pulse

147
Q

What is the function of gradient coils?

A

Gives us slight variation in magnetic feild strength

148
Q

Where are gradient coils located?

A

Lie between the main magnet and RF coils

149
Q

Are the gradient coils supercooled?

A

No, Not supercooled and operate close to room temperature

150
Q

T/F

The strength of the field changes position depending on the coil used

151
Q

What is this describing?

When switched on the magnetic field generated by the gradient coils is combined with B0 to generate MR signals that are spatially localized to the anatomy of interest.

A

Spatial encoding

152
Q

What are the 2 main purposes of the RF coils?

A
  1. Transmit RF energy to the tissue of interest
  2. Receive the induced RF signal back from the tissue of interest

(can be independent or both functions)

153
Q

What is the purpose of the Faraday Cage?

A

Faraday Cage used to block unwanted external radio waves

154
Q

What is RF noise caused by?

A

Caused due to RF interference from electronics in or near scanner room

155
Q

What type of artifact appears from RF noise?

A

Zipper artifact

156
Q

What is being shown here?

A

Zipper artifact

157
Q

What are the main concerns with the gradient feilds?

A

Excessive noise and potential auditory damage

158
Q

What is the main safety concern assosiated with the RF pulse?

A

RF pulses may lead to local tissue heating through the dissipation of energy.

159
Q

What type of contrast is used for MRI imaging? What type of magnet is this?

A

-Gadolinium
-Paramagnetic material

160
Q

What are the adverse reactions of Gadolinium?

A
  1. Fetal loss, slowed development
  2. Renal impairment
161
Q

Is it alright for a mother to breastfeed her child following recieving Gadollinum contrast?

A

Yes, no precautions

162
Q

How is Gadolinium introduced into the body?

A

It is injected

Medical Radiation Sciences 2 (13 decks)

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