MRI Physics 5 — Questions and Answers
Question 1: What is the primary source of acoustic noise in an MRI scanner?
- Rapid switching of gradient coils within the static magnetic field (Correct answer)
- The radiofrequency transmitter coil vibrating
- Cryogenic helium boil-off
- The rotating fan of the RF amplifier
Correct answer: Rapid switching of gradient coils within the static magnetic field
Lorentz forces act on gradient coil conductors carrying rapidly switched currents within B0, causing coil vibration and the characteristic loud knocking noise.
Question 2: Which type of MRI coil provides the best SNR for imaging a small, superficial structure?
- Small surface coil placed directly over the region of interest (Correct answer)
- Large volume (birdcage) coil surrounding the patient
- Phased-array coil designed for the spine
- Solenoid body coil at isocenter
Correct answer: Small surface coil placed directly over the region of interest
A small surface coil placed close to the target anatomy maximizes signal reception from that area while minimizing noise from surrounding regions.
Question 3: In MRI safety, what is the specific absorption rate (SAR)?
- The rate of RF energy absorbed per unit mass of tissue, measured in W/kg (Correct answer)
- The rate of gradient switching measured in T/m/s
- The static field strength absorbed by implants
- The rate of acoustic energy deposited in tissue
Correct answer: The rate of RF energy absorbed per unit mass of tissue, measured in W/kg
SAR measures RF power deposition in tissue; regulatory limits exist to prevent tissue heating, especially important at higher field strengths and with certain sequences.
Question 4: Which MRI pulse sequence is most commonly used to perform MR angiography without contrast injection?
- Time-of-flight (TOF) angiography (Correct answer)
- Inversion recovery with fat suppression
- Echo planar imaging (EPI)
- Spin echo with long TR/TE
Correct answer: Time-of-flight (TOF) angiography
TOF MRA relies on flow-related enhancement: unsaturated blood flowing into a saturated imaging slice produces high signal relative to stationary tissue.
Question 5: What is the relationship between magnetic field strength (B0) and the Larmor frequency?
- Larmor frequency is directly proportional to B0 (ω₀ = γ·B0) (Correct answer)
- Larmor frequency is inversely proportional to B0
- Larmor frequency is proportional to B0²
- Larmor frequency is independent of B0
Correct answer: Larmor frequency is directly proportional to B0 (ω₀ = γ·B0)
The Larmor equation states ω₀ = γ·B0, so doubling the field strength doubles the resonance frequency (e.g., 63 MHz at 1.5T, 128 MHz at 3T for hydrogen).
Question 6: Fast spin echo (FSE/TSE) sequences reduce scan time compared to conventional spin echo by:
- Acquiring multiple phase-encoding lines after each 90° excitation (echo train) (Correct answer)
- Using a shorter repetition time
- Reducing the number of slices acquired
- Increasing the voxel size to reduce k-space lines needed
Correct answer: Acquiring multiple phase-encoding lines after each 90° excitation (echo train)
FSE uses a train of 180° pulses after each excitation to fill multiple k-space lines per TR, reducing total scan time by a factor equal to the echo train length.
Question 7: Magnetization transfer contrast (MTC) in MRI works by:
- Saturating bound protons in macromolecules, which reduces free water signal through exchange (Correct answer)
- Applying a gradient to transfer magnetization between slices
- Using two different RF frequencies simultaneously to cancel fat signal
- Transferring longitudinal magnetization to neighboring voxels
Correct answer: Saturating bound protons in macromolecules, which reduces free water signal through exchange
Off-resonance RF pulses saturate the broad resonance of bound macromolecular protons; this saturation is transferred to free water protons via chemical exchange, reducing their signal.
MRI Physics 5
What is the primary source of acoustic noise in an MRI scanner?