**1. Introduction**

Magnetic Resonance Imaging (MRI), as its name implies, is based on a magnetic resonance signal originating in the "spins" of hydrogen protons of a given patient's tissue undergoing magnetic resonance imaging under the action of a magnetic field [1].

Concerning the identification and characterization of tissues, the potential of MRI began to become apparent only in 1971, when it was realized that the magnetic relaxation properties of the nuclei differ among biological tissues. Furthermore, in the same tissue, this relaxation relied on the state of the vitality and integrity of tissues [2].

P. C. Lauterbur was the pioneer of imaging techniques for medical practice using MRI. In 1973, he described a method that produced a generation of a two-dimensional projection showing the density of the protons and the distribution of the relaxation times in a sample consisting of two water tubes. His studies were further improved by groups led by Hinshaw and Mansfield in England, Hutchinson in Scotland, Ernst in Switzerland, and Cho in Korea. Thus, alternative techniques have been developed to generate images that can assist both medical diagnoses and "in vivo" studies of biochemical reactions that occur at the cell level [1,3,4].

The most important factor for the formation of MRI is the "spin." In essence, the "spin" is a fundamental property of particles that make up the nucleus of the atom. Its concept was proposed by Samuel Abraham Goudsmit and George Eugene Uhlenbeck in 1925 [1].

Unlike the known images of Rx and CT, MRI does not use ionizing radiation but radiofre‐ quency pulses.

The phenomenon of Magnetic Resonance Imaging manifests itself in molecular, atomic, electronic, and nuclear levels. In the latter case, its nature is magnetic, and therefore it is called nuclear magnetic resonance (NMR). It arises from the fact that certain nuclei possess an intrinsic angular moment referred to as "spin" and an associated magnetic moment. In

medicine the term used is MRI. The term nuclear associated to it caused panic among patients, who believed the tests were harmful and painful to the tissues. In clinical trials, MRI is used to produce images of the body structures. This method has provided valuable assistance, since it is not invasive to biological tissues, and provides an excellent contrast between soft tissues [2,5,6].

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

Spin Echo Magnetic Resonance Imaging http://dx.doi.org/10.5772/53693 33

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

In MRI, the most important pulse sequence is the "spin-echo" and its parameters are the repetition time (TR) and echo time (TE). Another important additional sequence is the "inver‐ sion-recovery" sequence, which promotes fat suppression, highlighting areas of injury with an

Therefore, the keys to understanding MRI are physical principles, which include the magnetic properties of nuclei in biological tissues, the collective behavior of these biological tissues when excited by radio waves, and their relaxation properties, as well as the devices and techniques

The technical parameters used to run a MRI were pulse sequences in "spin-echo" (SE) and " inversion-recovery " (Short T1 inversion STIR) to obtain images in T1 relaxation time (before and after injection of gadolinium contrast), in T2 relaxation time, and precontrast proton density (PD); Repetition time (TR), echo time (TE), and inversion time (TI); Section Plans (coronal or axial); Field of view (FOV), matrix size, number of acquisitions (NAQ), and number of sections, thickness, and interval between slices, and increment (F1), besides other functions

The "spin-echo" pulse sequence [9,10,11] is used to obtain a signal by means of a 90º excitation pulse and a 180º inversion pulse, which were sent to the nuclei of hydrogen atoms of the tissues present in the region to be analyzed (Figure 2). These nuclei presented a rotating motion (precession), and when excited by a radio frequency coil (antenna), they start to rotate all at the same excitation frequency, resonating with each other. Once the stimulation is ceased, the

resonance signal, which will be captured by the antennas of the MR apparatus [2,9].

carbon, oxygen, and sodium [7,8,9].

**3. Spin–echo sequence**

additional parameter - the inversion time (TI) [8,9,10].

MR signal is captured in form of signal or echo (Figure 3).

used to differentiate the tissues [7,9,10,11].

to improve image quality [9,11].
