Physics Flashcards

Which property of hydrogen makes it the most commonly used nucleus in clinical MRI?

Answer: High natural abundance and large magnetic moment

Hydrogen (¹H) is abundant in the body and has a large gyromagnetic ratio, producing strong MRI signals.

What is the purpose of the radiofrequency (RF) pulse in an MRI sequence?

Answer: To tip net magnetization away from B0

The RF pulse at the Larmor frequency tips the net magnetization vector away from the longitudinal axis to induce signal.

T2* relaxation differs from T2 relaxation primarily because T2* includes:

Answer: Magnetic field inhomogeneities in addition to spin-spin interactions

T2* accounts for both intrinsic spin-spin relaxation (T2) and additional dephasing caused by local magnetic field inhomogeneities.

In a gradient echo sequence, what replaces the 180° refocusing pulse used in spin echo?

Answer: A reversed readout gradient

Gradient echo sequences use a reversed readout gradient to rephase spins, making them sensitive to T2* rather than T2.

What determines the slice thickness in a standard 2D MRI acquisition?

Answer: The bandwidth of the RF pulse and the slope of the slice-select gradient

Slice thickness = RF bandwidth / (gyromagnetic ratio × gradient strength); a wider RF bandwidth or shallower gradient yields a thicker slice.

Which artifact arises when signal from outside the FOV wraps around to the opposite side of the image?

Answer: Aliasing (wrap-around) artifact

Aliasing occurs in the phase-encode direction when anatomy outside the FOV is undersampled, causing it to appear on the opposite side.

The signal-to-noise ratio (SNR) in MRI is proportional to which of the following?

Answer: Voxel volume × √(NEX) × √(scan time factors)

SNR increases with larger voxel volume, more signal averages (NEX), and longer acquisition times that allow more k-space sampling.