Excitation
The hydrogen proton is the most common atom in our body and can be found in substances including water (body consists for > 70% of water) and fat. Hydrogen protons are electrically charged (H+) and may be regarded as little magnets with a north pole & south pole. This makes hydrogen protons susceptible to external magnetic fields.
Each proton rotates 360° around its own axis (like a spinning top) and has a positive and negative pole. In order to understand MRI technology, you need to be aware that each proton spins with a certain speed, the so-called Larmor frequency (fig. 2). Because of its spin, the proton continually changes ‘phase’ (each phase is a snapshot, as it were). The relevance of this phase of the proton will be explained later in this module.
Figure 2. Larmor frequentie en fase (‘snapshot’)
When hydrogen protons enter a strong external magnetic field (e.g. of the MRI scanner) most hydrogen protons will align themselves parallel to the strong external magnetic field. Most hydrogen protons will be aligned in the same direction.
The sum of direction & force of the parallel protons is represented on paper as a vector; the net magnetization. This is expressed with an arrow in the literature. When a patient is placed in the MRI scanner, most protons will align themselves parallel to the magnetic field of the MRI device. In this resting phase, the net magnetization (vector) will always point towards the patient’s head. The Z axis represents the MRI scanner’s magnetic field. This is also termed B0 (fig. 3).
Figure 3. Protons align themselves parallel to the MRI device’s magnetic field; the Z axis, also termed B0).
Despite the parallel alignment of protons in their resting phase, they will nevertheless have a different spin movement. In other words, the hydrogen protons do not spin synchronously, this is also termed ‘out-of-phase’. Hydrogen protons may be triggered by radiofrequent pulses with a specific frequency. When the frequency of the hydrogen proton (= Larmor frequency) matches the transmitted radiofrequent wave, resonance will occur. Energy is then transferred (as the opera singer breaking a glass). This is termed excitation. Excitation will cause all hydrogen protons to spontaneously spin simultaneously, the are spinning in-phase. The transmitted radiofrequent pulse will not only cause the protons to spin in-phase, but will also rotate their magnetization in a plane at a right angle to the Z axis; the XY axis (= the transversal plane). The X, Y and Z axes can be used to visualize the net magnetization vector (fig. 4).
Figure 4. Excitation. A 90° rotation of the net magnetization to the transversal plane. The red arrow indicates the direction of the net magnetization. RF pulse = radiofrequent pulse.
In rotation to the XY axis, the net magnetization of the protons changes from longitudinal magnetization (= Z axis) to transversal magnetization (= XY axis). The degree of rotation to the XY axis depends on the strength and duration of the transmitted radiofrequent pulse. This may vary from 1° to 180° and is also termed the ‘flip angle’.
In summary
By giving radiofrequent pulses, protons will spin in-phase and ‘flip’ from the Z axis (= longitudinal plane) to the XY axis (= transversal plane). This process is termed excitation. When the protons are flipped to the XY axis, longitudinal magnetization will decrease and transversal magnetization will increase.
Eventually, the induced magnetic signal changes are registered by receiver coils and then processed into the MRI image (this will not be discussed further in this module).
Important: Signals can only be received and processed if:
- The protons are in the transversal plane (= XY axis). Explanation: minor signal changes in the Z axis will be drowned out by the strong magnetic field of the MRI device (= Z axis/B0). In other words, you can only measure signals in the transversal plane. In other words, you can only measure signals in the transversal plane.
- Protons must be in-phase. Explanation: when protons are not in-phase, the sum of all microscopic transversal magnetizations together will be negligibly small (protons ‘neutralize each other’ if they are not aligned).
