Fig 4.2: The net magnetization when magnetic field is applied
When a radio frequency pulse is given, the net magnetization move from its equilibrium position so it no longer points along the magnetic field, it will subsequently precess around the field and the net magnetization shift to y-direction as shown in Fig 4.3.
The radio frequency is applied at the above mentioned frequency. Radio waves are magnetic fields that change direction in time. The powerful stationary field pushes the magnetization so that it precesses. Likewise the radio waves push the magnetization around the radio wave field, but since the radio wave field is many thousand times weaker than the static field, the pushes normally amount to nothing. Because of this, we will exploit a resonance phenomenon: By affecting a system rhythmically at an appropriate frequency (the systems resonance frequency), a large effect can be achieved even if the force is relatively weak. A well-known example: Pushing a child sitting on a swing. If we push in synchrony with the swing rhythm, we can achieve considerable effect through a series of rather weak pushes. If, on the other hand, we push against the rhythm (too often or too rarely) we achieve very little, even after many pushes.
Fig 4.3: change in magnetization when RF pulse is applied
With radio waves at an appropriate frequency (a resonant radio wave field), we can slowly rotate the magnetization away from equilibrium. “Slowly” here means about one millisecond for a 90 degree turn, which is a relatively long time as the magnetization precesses 63 million turns per second at 1.5 tesla (the magnetization rotates 63 thousand full turns in the time it takes to carry out a 90 degree turn, i.e., quite a lot faster). Eventually it will return to equilibrium (relaxation), but it takes a relatively long time on
this timescale (e.g. 100 ms). Meanwhile, radio waves at this frequency are emitted from the body. We measure and analyze those.
Notice: The position of the nuclei in the body does not change - only their axis of rotation.
The strength of the radio waves that are emitted from the body depends on the size of the net magnetization and on the orientation. The greater the oscillations of the net magnetization, the more powerful the emitted radio waves will be. The signal strength is proportional to the component of the magnetization, that is perpendicular to the magnetic field (the transversal magnetization), while the parallel component does not contribute (known as the longitudinal magnetization).
If the net magnetization points along the magnetic field (as in equilibrium, to give an example) no measurable radio waves are emitted, even if the nuclei do precess individually. This is because the radio wave signals from the individual nuclei are not in phase, meaning that they do not oscillate in synchrony perpendicular to the field. The contributions thereby cancel in accordance with the net magnetization being stationary along the B0-field (there is no transversal magnetization).
The interactions happening at near-collisions between nuclei give rise to the magnetization constantly approaching the equilibrium size. This is called relaxation. The speed at which relaxation occurs depends on the protons interactions with their neighbors, which in turn depends on the firmness of the substance (the consistency). It is the difference in consistency and the presence of large molecules that limit the waters free movement, which causes most of the contrast we see in MR images.
The relaxation occurs on two different time scales: The magnetization perpendicular to the magnetic field (the transversal magnetization) often decreases relatively rapidly, while it can take considerably longer to recover the magnetization along the field (the longitudinal magnetization) as shown in Fig 4.4.
The transversal magnetization (Mxy) decreases exponentially on a timescale T2 (e.g. around 100 ms for brain tissue. Several seconds for pure water).
The longitudinal magnetization (Mz) approaches equilibrium M0 on a timescale T1 (for example approximately 1 s for brain tissue. Several seconds for pure water).
The relaxation times depend on the mobility of the molecules and the strength of the magnetic field.