MRI Knowledge 4 — Questions and Answers
Question 1: In a spin-echo sequence, what is the role of the 180° refocusing pulse?
- It excites protons from equilibrium into the transverse plane
- It reverses phase dispersion caused by static field inhomogeneities to form an echo (Correct answer)
- It suppresses fat signal by inverting its magnetization
- It encodes spatial information along the frequency direction
Correct answer: It reverses phase dispersion caused by static field inhomogeneities to form an echo
The 180° pulse flips the transverse magnetization so that faster and slower precessing spins rephase, forming a spin echo and eliminating fixed inhomogeneity effects.
Question 2: What does a high b-value in diffusion-weighted imaging (DWI) indicate?
- Shorter echo time and less diffusion weighting
- Stronger diffusion-sensitizing gradients and greater sensitivity to restricted diffusion (Correct answer)
- Lower magnetic field strength required
- Faster acquisition with less motion sensitivity
Correct answer: Stronger diffusion-sensitizing gradients and greater sensitivity to restricted diffusion
The b-value reflects the strength and duration of diffusion-sensitizing gradients; higher b-values increase diffusion contrast but also reduce SNR.
Question 3: Which MRI phenomenon occurs when protons at the boundary of fat and water tissues experience a frequency offset, causing a dark or bright band?
- Gibbs ringing
- Chemical shift artifact (Correct answer)
- Magnetic susceptibility artifact
- Wraparound (aliasing) artifact
Correct answer: Chemical shift artifact
Chemical shift artifact results from the ~3.5 ppm frequency difference between fat and water protons, causing spatial misregistration along the frequency-encode direction.
Question 4: What is the function of the shim coils in an MRI system?
- Generate the main static magnetic field
- Compensate for inhomogeneities in the B0 field to improve field uniformity (Correct answer)
- Produce the rapidly switching gradients for slice selection
- Amplify the RF signal received from the patient
Correct answer: Compensate for inhomogeneities in the B0 field to improve field uniformity
Shim coils generate small corrective magnetic fields that counteract spatial variations in B0, improving homogeneity and image quality.
Question 5: On a T1-weighted MRI, which substance normally appears hyperintense (bright)?
- Cerebrospinal fluid
- Cortical bone
- Subacute blood (methemoglobin) (Correct answer)
- Edema
Correct answer: Subacute blood (methemoglobin)
Methemoglobin in subacute hemorrhage shortens T1 relaxation time, producing a bright signal on T1-weighted images.
Question 6: What is the term for the MRI safety classification that indicates a device has been tested and found safe under specified conditions?
- MR Safe
- MR Conditional (Correct answer)
- MR Unsafe
- MR Compatible
Correct answer: MR Conditional
MR Conditional means the device poses no known hazards when used within the specific conditions (field strength, SAR, gradient limits) stated by the manufacturer.
Question 7: Which gradient is responsible for determining slice position in a 2D MRI acquisition?
- Frequency-encoding gradient
- Phase-encoding gradient
- Slice-selection gradient (Correct answer)
- Spoiler gradient
Correct answer: Slice-selection gradient
The slice-selection gradient is applied simultaneously with the RF pulse to excite only protons in the desired anatomical plane.
MRI Knowledge 4
In a spin-echo sequence, what is the role of the 180° refocusing pulse?