Knowledge Flashcards
7 cards from real MRI practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Knowledge flashcards as text
In a spin-echo sequence, what is the role of the 180° refocusing pulse?
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.
What does a high b-value in diffusion-weighted imaging (DWI) indicate?
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.
Which MRI phenomenon occurs when protons at the boundary of fat and water tissues experience a frequency offset, causing a dark or bright band?
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.
What is the function of the shim coils in an MRI system?
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.
On a T1-weighted MRI, which substance normally appears hyperintense (bright)?
Answer: Subacute blood (methemoglobin)
Methemoglobin in subacute hemorrhage shortens T1 relaxation time, producing a bright signal on T1-weighted images.
What is the term for the MRI safety classification that indicates a device has been tested and found safe under specified conditions?
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.
Which gradient is responsible for determining slice position in a 2D MRI acquisition?
Answer: Slice-selection gradient
The slice-selection gradient is applied simultaneously with the RF pulse to excite only protons in the desired anatomical plane.