**2. MRI fundamental**

In nuclei in which the "spin" protons are not paired, there is a resultant magnetic field which can be represented by a dipole magnetic vector. The magnitude of this field is called nuclear magnetic moment, and its existence causes the nuclei to respond actively to external magnetic fields. The nuclear magnetic vector does not remain static in one direction, but has a preces‐ sional motion or rotation around its axis (Figure 1).

**Figure 1.** Schematic representation shows the spins in (A) the absence and (B) in the presence of an external magnetic field [3].

It is noted that in (A) without application of an external magnetic field, the protons are oriented in a random motion, while in (B) when placed in an external magnetic field B0, the protons are aligned in the same direction, or in an opposite direction to the magnetic field. The slight preponderance of the spins in the same direction of the field creates a small resulting magnetization vector named M0. This slight imbalance makes it possible to obtain images by RMI [3].

Two-thirds of the atoms that constitute the human body are hydrogen atoms, which contain only one proton in its nucleus. Therefore, they present a high-intensity magnetic vector, which increases their sensitivity to respond to external magnetic fields. In addition to hydrogen being the most abundant nucleus in biological tissues, its single proton results in more powerful magnetic moment than any other element. Due to these features, the hydrogen nucleus of biological tissues is the same one currently used to obtain the signal for the formation of images in MR procedures. However, other types of nuclei may be used to generate information on both the physiopathologic status and anatomy of tissues. Among other elements, we can cite carbon, oxygen, and sodium [7,8,9].

A radiofrequency pulse or excitation must be applied perpendicular to the main magnetic field in the frequency of precession or rotation of the hydrogen atoms (Larmor frequency) in order to obtain MR images. This radiofrequency pulse supplies energy to the resulting magnetization vector so that it is deflected to the transverse plane. Once the stimulation ceases, the magnetic vector returns to balance. This turning back to balance is measured and provides the generated resonance signal, which will be captured by the antennas of the MR apparatus [2,9].
