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  • Print publication year: 2011
  • Online publication date: July 2011

10 - Magnetic resonance imaging

Summary

Regarding nuclear magnetic resonance imaging (MRI):

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

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

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

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

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

Which of the following are true about MRI signal formation in biological tissues?

A free induction decay (FID) signal can only occur after a 90° pulse

After a 90° pulse, the longitudinal magnetization recovers at the same rate as the transverse magnetization decays

The rate of the longitudinal magnetization recovery does not depend on the external magnetic field strength

Dephasing of the transverse magnetization is mainly due to spin–lattice relaxation

The FID is not usually considered in clinical MRI

In spin–echo (SE) imaging:

The first radiofrequency pulse in a spin–echo pulse sequence is a 180° pulse

Rephasing of the transverse magnetization is caused by the 180° pulse

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