MRI Physics 5

Question 1

What is the primary source of acoustic noise in an MRI scanner?

Accepted Answer

Rapid switching of gradient coils within the static magnetic field

Answer

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?

Accepted Answer

Small surface coil placed directly over the region of interest

Answer

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)?

Accepted Answer

The rate of RF energy absorbed per unit mass of tissue, measured in W/kg

Answer

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?

Accepted Answer

Time-of-flight (TOF) angiography

Answer

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?

Accepted Answer

Larmor frequency is directly proportional to B0 (ω₀ = γ·B0)

Answer

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).

MRI - Magnetic Resonance Imaging Practice Test

MRI - Magnetic Resonance Imaging Practice Test

MRI Physics 5