PHYSICS FRCR 1 > FRCR Physics MRI > Flashcards

FRCR Physics MRI Flashcards

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1
Q

In MRI: Is the Larmor frequency represents the rate of proton precession?

A

(TRUE) It describes the ‘spinning top’ motion of protons in a magnetic field.

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2
Q

In MRI: Is the Larmor frequency proportional to the field strength B0?

A

(TRUE) Frequency increases linearly with magnetic field strength (f = γB0).

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3
Q

In MRI: Does T1 relaxation time determine the Larmor frequency?

A

(FALSE) Larmor frequency is a function of the field and the nucleus; T1 is a measure of tissue recovery time.

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4
Q

In MRI: Is frequency inversely proportional to the gyromagnetic ratio?

A

(FALSE) It is directly proportional; different nuclei precess at different speeds in the same field.

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5
Q

In MRI: Does the Larmor frequency drop as the signal decays?

A

(FALSE) The signal strength (amplitude) decays, but the frequency of precession stays the same.

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6
Q

In MR imaging: Does the bulk magnetisation vector (Mo) represent a single proton?

A

(FALSE) Mo is the ‘Net Magnetisation Vector,’ which is the mathematical sum of millions of individual proton magnetic moments. Individual protons precess at an angle, but their net effect points along the Bo axis.

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7
Q

In MR imaging: Is the magnitude of Mo independent of the strength of Bo?

A

(FALSE) Mo is directly proportional to Bo. A stronger magnetic field aligns a higher percentage of protons in the ‘parallel’ state, creating a larger net magnetisation and higher Signal-to-Noise Ratio (SNR).

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8
Q

In MR imaging: Are the longitudinal and transverse magnetisation vectors equal in size after a 90 degree RF pulse?

A

(FALSE) A 90° pulse tips the entire longitudinal vector (Mz) into the transverse plane (Mxy). Immediately after the pulse, longitudinal magnetisation is ZERO and transverse magnetisation is at its MAXIMUM (Mo).

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9
Q

In MR imaging: Is the nuclear spin population inverted after a 180 degree RF pulse?

A

(TRUE) This is an ‘inversion pulse.’ It flips the net magnetisation vector 180 degrees, putting the majority of protons into the higher-energy ‘anti-parallel’ state (pointing against Bo).

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10
Q

In MR imaging: Does the rotation of the magnetisation vector in the x-y plane give rise to the MR signal?

A

(TRUE) Only precessing transverse magnetisation induces a current in the receiver coil (Faraday’s Law). Magnetisation along the longitudinal (z) axis does not rotate around the coil and cannot produce a signal.

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11
Q

During the spin echo pulse sequence: After the 90 degree RF pulse, magnetic field inhomogeneities cause the protons to lose phase coherence.

A

(TRUE) This is the cause of T2* decay. Because the magnetic field isn’t perfectly uniform, some protons precess slightly faster and some slower, causing them to ‘fan out’ and lose signal.

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12
Q

During the spin echo pulse sequence: The 180 degree RF pulse rephases the spins.

A

(TRUE) The 180° pulse acts like a ‘pancake flip.’ It reverses the positions of the fast and slow protons so that the fast ones are now behind the slow ones; they eventually catch up, creating a rephased ‘echo’.

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13
Q

During the spin echo pulse sequence: The spin echo signal appears at a time TE after the 180 degree pulse.

A

(FALSE) The echo appears at time TE (Echo Time). However, the 180° pulse is delivered at TE/2. Therefore, the echo appears at a time TE/2 after the 180° pulse, not a full TE after it.

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14
Q

During the spin echo pulse sequence: It can be used to generate either T1 or T2 weighted images.

A

(TRUE) By adjusting the TR (Repetition Time) and TE (Echo Time), you can emphasize different tissue properties. Short TR/Short TE = T1; Long TR/Long TE = T2.

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15
Q

During the spin echo pulse sequence: Multiple sequential 180 degree pulses can be applied after the 90 degree pulse to improve contrast in T1-weighted images.

A

(FALSE) Multiple 180° pulses are used in ‘Turbo’ or ‘Fast’ Spin Echo to create a train of echoes for T2 weighting (improving speed). They do not improve T1 contrast; T1 contrast is primarily controlled by the TR.

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16
Q

Gradient recalled echo pulse sequences: Frequently use flip angles < 90 degrees.

A

(TRUE) GRE sequences often use small flip angles (Alpha angles) to allow for faster imaging. Since not all longitudinal magnetization is tipped, recovery is faster, allowing for a very short TR.

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17
Q

Gradient recalled echo pulse sequences: Apply a 180 degree RF pulse to generate the MR signal.

A

(FALSE) GRE sequences do NOT use a 180° refocusing pulse. Instead, they use a gradient magnetic field reversal to rephase the spins and create the echo.

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18
Q

Gradient recalled echo pulse sequences: Can be used to generate T2-weighted images.

A

(FALSE) Because there is no 180° refocusing pulse, GRE sequences cannot compensate for field inhomogeneities. Therefore, they produce T2* (T2-star) weighted images, not pure T2.

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19
Q

Gradient recalled echo pulse sequences: Are often used for MR angiography.

A

(TRUE) GRE is excellent for MRA because it is very sensitive to flow. Moving blood enters the slice with ‘fresh’ spins that haven’t been saturated, making the vessels appear very bright (Inflow effect/TOF).

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20
Q

Gradient recalled echo pulse sequences: Are affected by main field inhomogeneities.

A

(TRUE) This is the defining characteristic of GRE. Without the 180° pulse to ‘cancel out’ fixed magnetic field errors, the signal decays quickly according to T2*.

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21
Q

In MR imaging: The frequency of electromagnetic radiation needed to excite nuclei is independent of the static magnetic field strength.

A

(FALSE) The resonance frequency (Larmor frequency)** is directly proportional to the magnetic field strength** (f = γB0). A 3T scanner requires double the frequency of a 1.5T scanner for excitation.

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22
Q

In MR imaging: The direction of the static magnetic field must be parallel to the long axis of the body.

A

(FALSE) While this is true for traditional closed-bore (superconducting) magnets, open MRI systems often use a vertical magnetic field that is perpendicular to the long axis of the body.

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23
Q

In MR imaging: A magnetic field gradient is used to define the slice to be imaged.

A

(TRUE) The slice-select gradient creates a linear variation in the magnetic field. Only the thin ‘slice’ where the Larmor frequency matches the RF pulse frequency will be excited.

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24
Q

In MR imaging: Gadolinium-based contrast agents shorten both T1 and T2 of tissues.

A

(TRUE) Gadolinium is paramagnetic and provides an additional pathway for protons to lose energy and dephase. However, at clinical doses, the T1-shortening effect is more dominant, making tissues appear bright on T1-weighted images.

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25
In MR imaging: Hydrogen nuclei in fat and water precess at the same frequency.
(FALSE) This is the basis of 'Chemical Shift.' The electron clouds around hydrogen in fat shield the nucleus differently than in water, causing fat protons to precess at a slightly lower frequency (approx. 3.5 ppm difference).
26
During the acquisition of an MR image: A slice select gradient is permanently applied along the major axis of the body.
(FALSE) The slice select gradient is only applied *during* the RF excitation pulse. If it were permanently applied, it would interfere with the subsequent phase and frequency encoding steps.
27
During the acquisition of an MR image: The slice thickness depends on the bandwidth of the RF pulse.
(TRUE) Slice thickness is determined by the ratio of the RF bandwidth to the gradient strength. A narrower RF bandwidth or a steeper gradient results in a thinner slice.
28
During the acquisition of an MR image: A frequency encode gradient is applied while the MR signal is measured.
(TRUE) Also known as the 'readout gradient,' it is turned on exactly when the signal (echo) is being collected. This causes protons at different positions to precess at different frequencies, allowing for spatial mapping.
29
During the acquisition of an MR image: The spins within each slice are sequentially dephased to provide spatial information.
(TRUE) This refers to 'Phase Encoding.' A gradient is applied briefly before readout to give protons at different positions a specific 'phase' (starting angle), which is essential for filling K-space.
30
During the acquisition of an MR image: The acquisition time is proportional to the matrix size.
(TRUE) Specifically, Scan Time = TR × Npy × Nex. Since Npy (the number of phase encoding steps) is a dimension of the matrix size, increasing the matrix resolution directly increases the scan time.
31
In MRI, SNR can be increased by: Increasing the slice thickness.
(TRUE) Thicker slices mean larger voxels, which contain more protons and therefore produce a stronger signal. MRI SNR relationships SNR increases with: ✔ Slice thickness ✔ Voxel size ✔ Field strength (B₀) ✔ Number of signal averages (NEX) ✔ FOV SNR decreases with: ✖ Smaller voxel ✖ Higher receiver bandwidth ✖ Long TE
32
In MRI, SNR can be increased by: Increasing the echo time (TE).
(FALSE) Longer TE allows for more transverse decay (T2/T2*), which reduces the available signal and lowers SNR.
33
In MRI, SNR can be increased by: Increasing the bandwidth of the receiver.
(FALSE) A wider bandwidth captures more frequencies, which includes more random electronic noise, thereby decreasing the SNR.
34
In MRI, SNR can be increased by: Increasing the field of view (FOV).
(TRUE) For a fixed matrix, a larger FOV creates larger voxels. Larger voxels contain more signal-producing protons.
35
In MRI, SNR can be increased by: Increasing the number of phase encoding steps.
(FALSE) Increasing the matrix size (phase steps) while keeping FOV constant makes voxels smaller, which significantly reduces the SNR.
36
Regarding MR scanners: Do they have three sets of gradient coils?
(TRUE) There are three sets (X, Y, Z) used to provide spatial encoding by linearly varying the magnetic field.
37
Regarding MR scanners: Is the main magnet surrounded by cryogenic liquids?
(TRUE) Liquid helium is used to maintain the coils at superconducting temperatures (~4 K) so the current flows without resistance.
38
Regarding MR scanners: Can the RF coil be used to detect the MR signal?
(TRUE) RF coils serve as the antenna that detects the oscillating magnetic field from the patient (Faraday’s Law).
39
Regarding MR scanners: Must they be quenched at the start of each day?
(FALSE) A quench is an emergency or accidental loss of superconductivity. Magnets are kept 'at field' 24/7 to maintain stability and save costs.
40
The MR signal: Is it produced following any RF pulse?
(FALSE) Signal is only produced if the RF pulse creates transverse (x-y) magnetization. A pure 180° inversion pulse produces no immediate signal.
41
The MR signal: Does it increase with main field strength (Bo)?
(TRUE) Higher field strengths create a larger net magnetization vector (Mo), providing more available signal.
42
The MR signal: Does it decrease in size over time?
(TRUE) This is known as Free Induction Decay (FID). The signal disappears as protons lose phase coherence (T2/T2* decay).
43
The MR signal: Is it created by spinning hydrogen nuclei?
(TRUE) Protons possess 'spin' and 'charge,' giving them a magnetic moment that allows them to interact with the scanner.
44
The MR signal: Does it consist of a voltage induced in a coil of wire?
(TRUE) According to Faraday's Law, the precessing transverse magnetic field induces an electrical voltage in the receiver coil.
45
Regarding MRI relaxation: Is T1 typically 20-200ms for most tissues?
(FALSE) T1 is much longer, typically 200-2000ms. T2 times are more likely to fall in the 20-200ms range.
46
Regarding MRI relaxation: Does T1 decrease as magnetic field strength increases?
(FALSE) T1 INCREASES with higher field strength because the higher Larmor frequency makes energy transfer to the lattice less efficient.
47
Regarding MRI relaxation: Is T1 always longer than T2 for any given tissue?
(TRUE) Transverse dephasing (T2) is always a faster process than longitudinal energy recovery (T1).
48
Regarding MRI relaxation: Is T2 relaxation a measure of signal decay speed?
(TRUE) T2 describes the decay of transverse magnetization; as protons lose phase, the signal induced in the coil disappears.
49
Regarding MRI relaxation: Does T1 influence the TR (Repetition Time) of a sequence?
(TRUE) TR is chosen to allow for specific amounts of T1 recovery. Short TR emphasizes T1 differences; Long TR minimizes them.
50
Spatial resolution in MRI: Does decreasing slice thickness improve it?
(TRUE) Reducing slice thickness decreases voxel volume in the z-axis, minimizing partial volume averaging and improving detail.
51
Spatial resolution in MRI: Does increasing the matrix size improve it?
(TRUE) A larger matrix (e.g., 512 instead of 256) divides the FOV into more, smaller pixels, enhancing spatial detail.
52
Spatial resolution in MRI: Does decreasing the Field of View (FOV) improve it?
(TRUE) For a fixed matrix, a smaller FOV results in smaller individual pixels, which increases spatial resolution.
53
Spatial resolution in MRI: Does increasing the magnetic field (Bo) improve it?
**(FALSE) Field strength improves Signal-to-Noise Ratio (SNR), not the geometric resolution itself.**
54
Spatial resolution in MRI: Does increasing the Echo Time (TE) improve it?
(FALSE) TE determines T2 contrast and affects SNR, but it has no impact on the pixel or voxel dimensions.
55
The chemical shift artifact: Does it occur at fat-water interfaces?
(TRUE) It is caused by the 3.5 ppm difference in precessional frequency between protons in fat and protons in water.
56
The chemical shift artifact: Does it get worse at higher field strengths (3T vs 1.5T)?
(TRUE) The frequency difference in Hz increases linearly with B0, leading to a larger spatial displacement of the fat signal.
57
The chemical shift artifact: Can it be minimized by a STIR sequence?
**(TRUE) Fat suppression (STIR or Fat-Sat) removes the fat signal entirely, thereby eliminating the fat-water shift.**
58
The chemical shift artifact: Is it seen in the frequency encode or phase encode axis?
**(TRUE) Frequency encode axis**. The scanner misplaces the fat signal because it uses frequency to determine spatial position.
59
The chemical shift artifact: Is it caused by poor magnet shim (inhomogeneities)?
(FALSE) It is caused by 'molecular shielding' (the chemical environment of the protons), not by imperfections in the main magnetic field.
60
Regarding MRI bioeffects: Do static fields of 5T interfere with brain function?
(FALSE) There is no evidence of brain dysfunction at 5T. The safety threshold for clinical imaging is generally 8 Tesla for adults.
61
Regarding MRI safety: Is every metal object a projectile hazard?
(FALSE) Only ferromagnetic materials (iron, nickel, cobalt) are subject to the 'missile effect.' Non-ferrous metals like titanium are safe from torque but can still heat up.
62
Regarding MRI safety: What causes peripheral nerve stimulation?
(TRUE) Rapidly changing magnetic field gradients (dB/dt) induce electric currents in the body, which can trigger nerve depolarization and muscle twitches.
63
Regarding MRI safety: What does SAR measure?
(TRUE) Specific Absorption Rate (W/kg) measures the rate at which RF energy is absorbed by tissue, which manifests as heat.
64
Regarding MRI safety: Can metal implants be affected by both Bo and RF pulses?
(TRUE) Bo causes mechanical torque (twisting) on ferromagnetic items, while RF pulses can cause electrical induction and heating in conductive materials.
65
Regarding MR relaxation: Does T1 involve energy transfer to surrounding atoms?
(TRUE) Known as 'Spin-Lattice' relaxation, protons dissipate absorbed energy into the surrounding molecular framework to recover longitudinal alignment.
66
Regarding MR relaxation: What percentage of M0 is recovered after 1 T1?
(TRUE) T1 is defined as the time taken for longitudinal magnetization to recover to 63% of its original maximum value.
67
Regarding MR relaxation: Does T1 increase at higher field strengths?
(TRUE) As the Larmor frequency rises with Bo, the efficiency of energy transfer to the lattice decreases, prolonging T1 recovery times.
68
Regarding MR relaxation: Is T2 caused by main field inhomogeneities?
(FALSE) T2 is caused by intrinsic 'Spin-Spin' interactions. T2* (T2-star) is the decay caused by both T2 and external magnetic field inhomogeneities.
69
Regarding MR relaxation: Is T2 typically 300-800 ms for soft tissues?
(FALSE) Soft tissue T2 is much shorter, typically 30-150 ms. T1 values are longer, usually in the 300-2000 ms range.
70
In clinical MRI: Do all hydrogen protons precess at the same frequency in the same field?
(FALSE) Protons in fat and water precess at slightly different frequencies due to chemical shift (molecular shielding).
71
In clinical MRI: Does a short TI null the signal from CSF?
(FALSE) A short TI nulls Fat (STIR). A long TI is required to null CSF (FLAIR).
72
In clinical MRI: Why does fat look bright on T2-weighted Fast Spin Echo (FSE)?
(TRUE) The rapid chain of 180 degree pulses in FSE reduces the 'J-coupling' interaction in fat, keeping it hyper-intense.
73
In clinical MRI: Why is blood dark on Spin-Echo sequences?
(TRUE) This is the 'flow void.' Blood moves out of the slice between the 90 degree and 180 degree pulses, so it cannot produce an echo.
74
In clinical MRI: How is the echo refocused in a GRE sequence?
(TRUE) Instead of an RF pulse, a gradient reversal is applied along the frequency encode (readout) axis to rephase the spins.
75
SNR in MRI: Why is SNR higher in T1-weighted than T2-weighted images?
(TRUE) **T1-weighted images use a short TE, whereas T2-weighted images require a long TE, allowing more signal decay to occur before readout.**
76
SNR in MRI: Does increasing receiver bandwidth increase SNR?
(FALSE) Increasing bandwidth allows more noise into the system. SNR is inversely proportional to the square root of the bandwidth.
77
SNR in MRI: What happens to SNR if you change matrix from 128 to 256?
(FALSE) For a fixed FOV, doubling the matrix in both dimensions reduces voxel volume to 1/4, which reduces SNR by a factor of 4.
78
SNR in MRI: Is SNR inversely proportional to TE?
(FALSE) Signal decreases as TE increases, but the relationship is an exponential decay (e^-TE/T2), not a simple inverse.
79
SNR in MRI: Is SNR proportional to proton density?
(TRUE) More protons per unit volume (higher proton density) result in a larger net magnetization vector and a stronger signal.
80
MRI Artifacts: Do B0 inhomogeneities cause ghosting along the phase axis?
(FALSE) Ghosting is caused by motion. B0 inhomogeneities cause geometric distortion and signal loss (susceptibility).
81
MRI Artifacts: Are GRE sequences more prone to motion than SE?
(FALSE) GRE is faster and often used for breath-holding, making it less prone to motion than slow conventional SE sequences.
82
MRI Artifacts: Why is metal artifact worse on GRE than Spin Echo?
(TRUE) GRE lacks the 180 degree refocusing pulse found in Spin Echo, making it unable to correct for the dephasing caused by metal (susceptibility).
83
MRI Artifacts: Does a shorter TR help reduce motion artifacts?
(TRUE) A shorter TR reduces total scan time, minimizing the window for patient movement and enabling breath-holding.
84
MRI Artifacts: How does increasing FOV affect wraparound (aliasing)?
(TRUE) It prevents the anatomy from extending outside the sampled area, stopping the signal from 'folding' back into the image.
85
MRI Safety: Does the Faraday cage contain the fringe magnetic field?
(FALSE) The Faraday cage blocks external RF interference. The magnetic fringe field is contained by 'active' or 'passive' magnetic shielding.
86
MRI Safety: What is the significance of the 0.5 mT (5 Gauss) line?
(TRUE) This is the safety limit for the general public and patients with pacemakers; access is restricted beyond this point.
87
MRI Safety: Do diamagnetic materials pose a projectile risk?
(FALSE) Diamagnetic materials are weakly repulsive. Only ferromagnetic materials (e.g., steel, iron) present a projectile/missile hazard.
88
MRI Safety: Can gradient switching cause cardiac arrhythmias?
(TRUE) Rapidly changing magnetic fields (dB/dt) induce currents; if they exceed the cardiac threshold, they can theoretically trigger arrhythmias.
89
MRI Safety: Is Gadolinium contrast safe in the 2nd/3rd trimester?
(TRUE/FALSE - STATEMENT WAS TRUE) It is avoided because it is associated with an increased risk of stillbirth and neonatal inflammatory conditions.
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