When the radiofrequent pulse is switched off, the protons will return to their original resting phase; the XY axis reverts back to the Z axis. This is termed relaxation. Two separate processes take place during relaxation: longitudinal relaxation (T1 relaxation) and transversal relaxation (= T2 relaxation). Again, these two processes are independent and should be regarded as two separate processes.
In T1 relaxation, protons will return to their original position and the energy received from the radiofrequent pulse is transferred to their surroundings. T1 relaxation describes what happens in the Z axis (fig. 5).
Figure 5. When the radiofrequent (RF) pulse is switched off, T1 relaxation occurs; longitudinal magnetization increases.
The T1 relaxation time is defined as the time needed to achieve 63% of the original longitudinal magnetization (in Z axis) (fig. 5). Each tissue has its own T1 relaxation time and curve (fig. 6).
Figure 6. T1 relaxation times of fat and water.
Fat has a short relaxation time as compared to water because it can more easily transfer its received energy to its surroundings.
Simultaneously, something changes in the transversal plane; the protons spinning synchronously (= in-phase) will no longer spin synchronously once the radiofrequent wave has been switched off (= out-of-phase); this process is termed dephasing. Dephasing occurs because the magnetic field of the MRI scanner is no longer 100% homogeneous. The protons will be affected by irregularities in the magnetic field and no longer spin synchronously. Protons may be regarded as little magnets and thereby accelerate the dephasing process. Explanation: consider in-phase protons as a group of soldiers marching synchronously. When the leader (‘radiofrequent pulse’) stops giving commands to the soldiers (‘radiofrequent pulse is switched off’), the soldiers (‘protons’) will no longer march synchronously. Only one soldier (‘proton’) in the rank needs to trip to set off a rapid chain reaction of soldiers (‘protons’) no longer marching synchronously. Dephasing is an undesired phenomenon, seeing the protons must be in-phase for the receiving coils to receive the signal.
T2 relaxation describes what happens in the XY axis. To avoid confusion: net magnetization consists of both a longitudinal component (Z axis) and a transversal component (XY axis). Together they constitute the net magnetization vector. Figure 7 illustrates the transversal component when the protons are in-phase.
Figure 7. When transmitting a radiofrequent (RF) pulse, the protons in the transversal plane (XY axis) will be in-phase.
Dephasing in T2 relaxation is rapid, much quicker than in T1 relaxation. When the radiofrequent pulse is switched off, transversal magnetization will be lost (fig. 8/9).
Figure 8. Dephasing occurs when the radiofrequent (RF) pulse is switched off.
T2 relaxation time is defined as the time needed to dephase up to 37% of the original value. Each tissue has its own T2 relaxation time and curve (fig. 9). In comparison with water, fat has a short T2 relaxation time and will therefore dephase quicker.
Figure 9. T2 relaxation times of fat and water.
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