MRI Physics 2

Question 1

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

Accepted Answer

High natural abundance and large magnetic moment

Answer

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

Question 2

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

Accepted Answer

To tip net magnetization away from B0

Answer

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

Question 3

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

Accepted Answer

Magnetic field inhomogeneities in addition to spin-spin interactions

Answer

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

Question 4

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

Accepted Answer

A reversed readout gradient

Answer

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

Question 5

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

Accepted Answer

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

Answer

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

MRI - Magnetic Resonance Imaging Practice Test