Question

Regarding nuclear magnetic resonance imaging (MRI):

a. In an external magnetic field, more hydrogen nuclei align with their magnetic moments parallel to the external field than antiparallel

b. Protons subject to a strong static external magnetic field start to precess in phase

c. The frequency of precession (Larmor frequency) of protons in a static magnetic field of 1.5 T equals 42.6 MHz

d. At 1.5 T, the precessional frequency of hydrogen nuclei in *** is 220 Hz lower than that of hydrogen nuclei in water

e. Apart from hydrogen, other nuclei that can be polarized in an external magnetic field include carbon 12C and oxygen 16O

Answer 1

a. True. Slightly more hydrogen nuclei align parallel (‘spin up’) than antiparallel (‘spin down’), giving rise to the net longitudinal magnetization MZ. The difference between protons with parallel and antiparallel alignment is only 3 per million in a 1 T static magnetic field. This can be increased using special techniques (hyper polarization).

b. False. Protons align with the external field, but precess with different phases, cancelling each other’* transverse magnetization. Therefore, the resulting net transverse magnetization (MXY) is zero. Their phases are synchronized (‘phase coherence’) by an external 90° radio frequency pulse whose frequency is equal to the Larmor frequency.

c. False. This is the precessional frequency in a 1 T magnetic field. At 1.5 T, it would be proportionally higher, i.e. 63.9 MHz.

d. True. This is called chemical shift and is utilized in some *** saturation techniques and opposed-phase imaging.

e. False. Nuclear magnetic resonance is a property of nuclei with odd numbers of protons and/or neutrons (e.g. 13C). Nuclei with even numbers of protons and neutrons have a zero net magnetic moment and cannot be polarized.