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STP 2024 > MRI > Flashcards

MRI Flashcards

1
Q

What is spin?

A

A fundamental property of nature like electrical charge or mass.

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2
Q

What multiples does spin come in?

A

1/2

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3
Q

Can spin be positive or negative? (Y/N)

A

Yes

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4
Q

Do protons have spin? (Y/N)

A

Yes

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5
Q

Do electrons have spin (Y/N)

A

Yes

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6
Q

Do neutrons have spin? (Y/N)

A

Yes

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7
Q

How much spin do unpaired electrons, protons and neutrons possess?

A

1/2

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8
Q

What is the total electronic spin of a deuterium atom?

A

1/2

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9
Q

What is the total nuclear spin of a deuterium atom?

A

1

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10
Q

How is the observable manifestation of spin eliminated?

A

By two or more particles with spin pairing up

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11
Q

Where is unpaired nuclear spin important?

A

Nuclear magnetic resonance

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12
Q

Can a particle with a net spin absorb a photon? (Y/N)

A

Yes

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13
Q

What does the gyromagnetic ratio depend upon?

A

Frequency, ν and γ

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14
Q

What is the relationship between v and γ?

A

v = γB

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15
Q

What is γ for hydrogen?

A

42.58 MHz/T

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16
Q

What holds nuclei together?

A

Nuclear forces

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17
Q

Do protons and neutrons have approximately the same mass?

A

Yes

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18
Q

What term refers to neutrons and protons?

A

Nucleons

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19
Q

What is the shell model of an atom?

A

It supposes that electrons orbiting a nucleus fill orbital shells.

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20
Q

What are the limits of each orbital shell?

A

2, 8, 20, 28, 50, 82 and 126

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21
Q

Can spins cancel out by pairing up within an atom? (Y/N)

A

Yes

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22
Q

Do the majority of elements have an isotope with a non zero nuclear spin?

A

Yes

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23
Q

What makes an isotope useable in NMR?

A

It must be in a natural abundance such that it can be detected.

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24
Q

What are the nuclei of interest in NMR?

A

Hydrogen, Deuterium, Phosphorous-31, Sodium-23, Nitrogen-14, Carbon-13 and Fluorine-19 (and many more!)

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25
Q

What is the starting point for considering energy levels?

A

A proton with spin

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26
Q

What property of an atom is used in NMR?

A

Spin

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27
Q

How can spin be considered in NMR?

A

As a magnetic moment vector, behaving like a tiny magnet with a north and south pole

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28
Q

What happens to a proton placed in an external magnetic field?

A

The spin vector aligns itself with the external field (like a magnet would)

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29
Q

What is the low energy configuration/state of a particle in an external magnetic field?

A

Where the poles are aligned N-S-N-S

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30
Q

What is the high energy configuration/state of a particle in an external magnetic field?

A

N-N-S-S

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31
Q

What is a magnetic monopole?

A

A hypothetical object with only one magnetic pole.

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32
Q

Why can a magnetic monopole never exist physically?

A

Without poles, flux lines cannot be imagined (It requires a closed loop which is not physically possible)

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33
Q

What are rectangular poles?

A

Poles with rectangular geometry

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34
Q

What is an energy state transition?

A

An interaction whereby the energy state of an atom is changed by the absorption of a photon.

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35
Q

How can a particle transition to a higher energy state?

A

It must absorb a photon which exactly matches the energy difference.

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36
Q

What is meant by the energy difference between two states?

A

The amount of energy needed to cause a transition between two energy levels/states

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37
Q

How is the energy of a photon related to its frequency?

A

E =h v where h is Planck’s const

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38
Q

What is the value of h?

A

6.626x10^-34 J s

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39
Q

What is the Lamour frequency?

A

The associated frequency that causes transitions (resonance frequency) which are both represented by v

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40
Q

What do energy level diagrams represent in NMR?

A

The energy of two spin states

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41
Q

What is the photon energy needed to cause a transition?

A

v = γB and E = hv

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42
Q

What is the relationship between E, h, γ and B?

A

E = h γ B

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43
Q

What condition is required for a transition to occur?

A

The energy of the photon must match the energy difference between the two spin states. Absorption can then occur

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44
Q

What range are the photon energies in NMR in?

A

Radio frequency (RF) range

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45
Q

What can v vary between in NMR?

A

60 and 1000MHz for hydrogen nuclei

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46
Q

What can v vary between in clinical NMR?

A

15-125 MHz for hydrogen imaging

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47
Q

What is the simplest NMR experiment?

A

The continuous wave experiment

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48
Q

How is the continuous wave experiment conducted?

A

A constant frequency is continuously on probing the energy levels while the magnetic field is varied.

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49
Q

What is the blue line representing in the diagram below?

A

The energy of the aforementioned frequency

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50
Q

What is the 2nd method for the CW experiment?

A

A constant magnetic field is applied and frequency is varied.

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51
Q

What is the magnitude of the constant magnetic field in the CW experiment?

A

Constant

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52
Q

How many possible orientations are there for a group of spins in a magnetic field?

A

2

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53
Q

At room temperature, what is the ratio of lower energy level spins to upper energy level spins?

A

The lower energy level, N+, slightly outnumbers that of the upper level

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54
Q

What equation dictates Boltzmann statistics?

A

N-/N+ = e^(-E/kT)

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55
Q

What does E represent in Boltzmann statistics?

A

The energy difference between the two possible energy states.

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56
Q

What is k in Boltzmann statistics?

A

Boltzmann’s constant = 1.3805x10^-23 J/Kelvin

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57
Q

What is T in Boltzmann statistics?

A

Temperature

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58
Q

What happens when the temperature of a system decreases?

A

The ratio of N-/N+ decreases too

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59
Q

What happens when the temperature of a system increases?

A

The aforementioned ratio approaches 1

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60
Q

What generates the signal that is picked up in NMR spectroscopy?

A

The difference between the energy absorbed by the spins which makes a transition from low to high energy states

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61
Q

Why is the energy between two energy states important in NMR?

A

It dictates the magnitude of the signal

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62
Q

What is the signal proportional to in NMR?

A

The population difference between the states

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63
Q

Is NMR spectroscopy particularly sensitive? (Y/N)

A

Yes

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64
Q

What makes NMR spectroscopy so sensitive?

A

It is capable of detecting very small population differences

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65
Q

What gives NMR its sensitivity?

A

The resonance (exchange of energy) at a specific frequency between the spins and spectrometer

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66
Q

What two other factors influence MRI signal?

A

Natural abundance and biological abundance

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67
Q

What does the natural abundance of an isotope mean in NMR?

A

The fraction of nuclei have a given number of protons and neutrons i.e. atomic weight

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68
Q

What is the natural abundance of Hydrogen?

A

99.985%

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69
Q

What are the relative natural abundances of nuclei studied in NMR? (Expressed as %)

//<Difficult>|<Non-examinable></Non-examinable></Difficult>

A

Hydrogen: 99.985; Deuterium: 0.015; Carbon-13: 1.11; Nitrogen-14: 99.63; Nitrogen-15: 0.37; Sodium-23: 100; Phosphorus-31: 100; Potassium-39: 93.1; Calcium-43: 0.145

70
Q

What is the biological abundance of nuclei studies in NMR? (Expressed as a fraction)

//<Difficult>|<non-examinable></non-examinable></Difficult>

A

Hydrogen: 0.63; Sodium: 0.00041; Phosphorus: 0.0024; Carbon: 0.094; Oxygen: 0.26; Calcium: 0.022; Nitrogen: 0.015

71
Q

Is it difficult to describe NMR on a microscopic scale? (Y/N)

A

Yes

72
Q

What picture of NMR is most helpful to look at?

A

The macroscopic picture

73
Q

What element is the first to be defined in NMR models?

A

The spin packet

74
Q

What is a spin packet?

A

A group of spins experiencing the same magnetic field strength

75
Q

What is this diagram represnting?

//{Not complete}

A

The spins in each grid section

76
Q

What can be used to describe the spin vector in a spin packet?

A

The magnetic field due to the spins in each spin packet can be represented by a magnetization vector

77
Q

What is the size of each vector proportional to?

A

N+ - N-

78
Q

What is the vector sum of the magnetization vectors from all of the spin packets?

A

The net magnetization

79
Q

How do you describe pulsed NMR?

A

In terms of net magnetization

80
Q

What is the conventional NMR coordinate system?

A

Cartesian: X Y Z

81
Q

Where are the external magnetic field and the net magnetization vector at equilibrium in the coordinate system?

A

Both along the Z axis

82
Q

At equilibrium, where does the net magnetization vector lie?

A

Along the direction of the applied magnetic field B0

83
Q

What is the equilibrium magnetization?

A

M0

84
Q

What is the longitudinal magnetization?

A

Mz

85
Q

Is there any transverse magnetization?

A

No

86
Q

It is possible to change the net magnetization?

Consider change in energy

A

By exposing the nuclear spin system to an energy of a frequency equal to the energy difference between spin states

87
Q

What happens if sufficient energy is put into the system?

A

The spin will be saturated making Mz = 0

88
Q

What is the time constant in NMR?

A

It describes how Mz returns to equilibrium value

89
Q

What equation governs the behaviour as a function of time?

A

Mz = M0(1 - e^{-t/T1})

90
Q

What is T1 in NMR?

A

The time taken to reduce the difference between the longitudinal magnetization Mz

91
Q

By what factor does the equilibrium value reduce by?

A

e

92
Q

What happens if the net magnetization is placed along the -Z axis?

A

It will gradually return to equilibrium position along the +Z axis at a rate governed by T1

93
Q

What equation governs the behaviour of Mz as a function of time t after its displacement?

A

Mz = M0(1 - 2e^{-t/t1})

94
Q

What does T1 reduce by in most cases?

A

The spin lattice relaxation time (T1) is the time taken to return to equilibrium after displacement

95
Q

By how much does the equilibrium value decrease by?

A

e

96
Q

What is precession?

A

Rotation about the Z axis

97
Q

What causes precession?

A

The placement of the net magnetization is placed in the XY plane. It rotates about the Z axis at a frequency equal to that of the photon which would cause a transition between the two energy levels

98
Q

What is the Lamour frequency?

A

The frequency about which procession occurs

99
Q

What happens when the net magnetization starts to de-phase?

A

Each spin packet experiences a slightly different magnetic field strength and rotates at its own Lamour frequency

100
Q

What produces a greater phase difference?

A

A longer elapsed time

101
Q

What is seen when the net magnetization vector is initially along the +Y direction?

A

The vector is the overlap of several thinner vectors from the individual spin packets

102
Q

What time constant describes the return to equilibrium of transverse magnetization?

A

Mxy (the spin-spin relaxation time, T2)

103
Q

What equation determines the spin-spin relaxation time?

A

Mxy = Mxy0 e^-{-t/T2}

104
Q

How do T1 and T2 compare?

A

T2 is always less than T1

105
Q

When does the net magnetization go to zero in the XY plane?

A

When the longitudinal magnetization grows in until we have M0 along Z

106
Q

Does transverse magnetization behave the same as other forms of magnetization? (Y/N)

A

Yes

107
Q

What does the transverse component rotate about?

A

The direction of applied magnetization. It also de-phases

108
Q

In brief, what is the spin-spin relaxation time?

A

The time to reduce the transverse magnetization by a factor of e

109
Q

When do the magnetization vectors fill the XY plane?

A

They can do this before growing back up along the Z axis. Both processes occur simultaneous with the only restriction being that T2 is less than or equal to T1

110
Q

What two factors contribute to the decay of transverse magnetization?

A

Molecular interactions (leads to a pure T2 molecular effect) and variations in B0 (leads to a pure inhomogeneous T2 effect)

111
Q

What is the combination of the two factors that influence decay in NMR?

T2* ?

A

The combination of these two factors result in the decay of transverse magnetization. The combined time constant is called T2 star and is given the symbol T2*

112
Q

What is the relationship between the T2 from molecular processes and that from inhomogeneities in the magnetic field?

A

1/T2* = 1/T2 + 1/T2(inhomo)

113
Q

What is a frame of reference?

A

A frame of reference is a set of coordinates that can be used to determine positions and velocities of objects in that frame; different frames of reference move relative to one another

114
Q

What defines a rotating frame of reference?

Consider rotation frequency

A

It rotates about the Z axis at the Lamour frequency

115
Q

How do we distinguish the rotating coordinate system of reference from the laboratory system?

A

X’ and Y’

116
Q

Why does the lab frame appear stationary?

A

The magnetization vector rotates about the Z axis

117
Q

Why does the rotating frame appear stationary?

A

The relaxation of Mz magnetization to its equilibrium value looks the same as it did in the lab frame

118
Q

What velocity does the transverse magnetization vector rotate about?

A

The same velocity

119
Q

The magnetization vector is faster than the rotating frame clockwise. True or false?

A

True

120
Q

A magnetization vector traveling slower than the rotating frame rotates in which direction?

A

Counter-clockwise about the Z axis

121
Q

In a sample, spin packets are seen traveling slower and faster than the frame. False or True?

A

True

122
Q

What is the consequence of faster and slower rotating magnetizations?

A

The mean frequency of the sample is equal to the rotating frame, dephasing Mx’y’

123
Q

What is the effect of a coil of wire with a current passing through on the X plane?

A

It produces a magnetic field along the X axis

124
Q

What is the effect of an alternating current through a coil on the X axis?

A

A magnetic field that alternates direction

125
Q

What is observed when a coil with an AC of frequency equal to the rotation?

A

The magnetic field will be constant along the X-axis the same as a DC in the lab frame

126
Q

How many vectors describe the B1 magnetic field?

A

2

127
Q

What are the names of the Three Stooges?

A

Moe, Larry, Curly, Shemp and Joe (the mythical 5th stooge)

128
Q

How do the vectors behave in the lab frame of reference?

A

One rotates clockwise at v0 and the other counter clockwise at -v0

129
Q

How does the net B1 magnetic field appear?

A

As an oscillating vector along the X axis

130
Q

How does the B1 vector component appear in the lab frame of reference?

A

It will appear stationary despite rotating clockwise

131
Q

How does the B1 vector component that rotates counter clockwise travel?

A

About the X axis at -2v0

132
Q

What does the component that rotates about Z at -2v0 appear as?

A

It spins so far away from the resonance frequency that it can be ignored

133
Q

How does B1 field appear appear in the rotating frame of reference?

A

It appears stationary

134
Q

How does a moving coil appear in the rotating frame of reference?

A

It appears to rotate at the Lamour frequency

135
Q

In NMR, how does the coil’s field appear?

A

As a coil with an AC at the Lamour frequency

136
Q

What is seen when the magnetic field has an AC?

A

It generates a pulsed B1 magnetic field along the X’ axis

137
Q

How do the spins respond to a B1 magnetic field pulse along the X’ axis?

A

In such a way that the net magnetization vector rotates about the direction of the applied B1 field

138
Q

What does the rotation angle of the B1 field depend on?

A

The length of time the field is on, tau, and the field’s magnitude B1

139
Q

What assumption can be made about T1 and T2 in most cases? (Consider Tau)

A

Tau is assumed to be much smaller than T1 and T2

140
Q

What happens to the magnetization vector when a 90 degree pulse is applied?

A

If a 90 degree pulse is applied, it rotates the magnetization vector clockwise by 90 degrees about the X’ axis.

141
Q

The pulse to rotates the equilibrium magnetization down the Y’ axis. True of False, False or true?

A

True

142
Q

What relationship defines the rotation angle?

A

theta = 2 pi γ τ B1

143
Q

In the lab frame, the equilibrium magnetization spirals down around the Z axis to which plane?

A

The XY plane

144
Q

The rotating frame of reference helpful to describe the behaviour of magnetization in response to a pulsed magnetic field. True or false?

A

True

145
Q

A 180 degree pulse will rotate the magnetization vector in response to a pulsed magnetic field. True or false?

A

True

146
Q

What governs the behaviour of the orientation of the rotation of the net magnetization?

A

The rotation equation

147
Q

What happens to a net magnetization vector between X’ Y’ and X’ and -Y’ after the application of a 180 degree pulse of B1 along the X’ axis?

A

It ends up between X’ and -Y’

148
Q

What is the rotation matrix?

A

It is a matrix used to predict the result of a rotation

149
Q

What is theta in the rotation matrix?

A

The rotation angle about the X’ axis

150
Q

What do X’ Y’ and X describe in the rotation matrix?

A

The initial location of the vector

151
Q

What do X’’ Y’’ and Z’’ represent?

A

The location of the vector after the rotation

152
Q

Motion in a solution which result in time varying magnetic fields cause what phenomenon?

A

Spin relaxation

153
Q

What do time varying fields cause at the Lamour frequency?

A

A change in Mz

154
Q

What is seen when observing hydrogen?

A

Rotation about the external field B0 and a magnetic field from the hydrogen.

155
Q

What is the form of the field experienced by hydrogen?

A

Sinusoidal

156
Q

Is there a distribution of rotation frequencies in a sample of molecules?

A

Yes

157
Q

What will affect T1 in this case?

A

Frequencies around the Lamour frequency

158
Q

What is the Lamour frequency proportional to?

A

B0 and T1

159
Q

How does the Lamour frequency vary with magnetic field strength?

A

It is proportional to magnetic field strength

160
Q

What is T1 inversely proportional to?

A

The density of molecular motions at the Lamour frequency

161
Q

What does the rotation frequency distribution depend on?

A

The temperature and viscosity of the solution

162
Q

Does T1 vary as a function of T?

A

Yes

163
Q

At the Lamour frequency v0, is T1 greater at 280K or 340K?

A

340K

164
Q

Does temperature variation in the human body affect T1?

A

No

165
Q

Viscosity varies between different tissue types, true or false? What does it influence?

A

True and this influences T1

166
Q

Fluctuating fields which perturb the energy levels of the spin states cause the transverse magnetization to de-phase. Vrai ou faux?

A

Vrai

167
Q

What is the number of molecular motions less than and equal to the Lamour frequency inversely proportional to?

A

T2

168
Q

In general, do relaxation times get longer or shorter as B0 increases?

A

They get longer as there are fewer relaxation causing frequency components in the random motion of molecules

169
Q

Bloch equations are a set of coupled differential equations which can be used to describe the behaviour of what?

A

A magnetization vector under any conditions

170
Q

When properly integrated, the Bloch equations will yield the X’ Y’ and Z components of magnetization as a function of what?

A

Time

171
Q

What are the forms of the Bloch equations?

A

dMx’/dt = (ω0 - ω)My’ - Mx’/T2

dMy’/dt = -(ω0 - ω)Mx’ + 2pi γ B1 Mz - My’/T2

dMz/dt = -2 pi γ B1 My’ - (Mz - Mz0)/T1

STP 2024 (4 decks)

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