**3. Spin–echo sequence**

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

32 Imaging and Radioanalytical Techniques in Interdisciplinary Research - Fundamentals and Cutting Edge Applications

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‐

(a) (b)

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

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

between soft tissues [2,5,6].

**2. MRI fundamental**

field [3].

obtain images by RMI [3].

sional motion or rotation around its axis (Figure 1).

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

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

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 to improve image quality [9,11].

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

individual nuclei around the X-axis to rotate 180 degrees (5), rephasing (6) and regenerating

Spin Echo Magnetic Resonance Imaging http://dx.doi.org/10.5772/53693 35

The 90º pulse plus the 180º pulse produced an echo, which is repeated several times during the study in the analyzed region. This echo is referred to as the repetition time (TR). The echo time (TE) is the duration between the middle of a 90º pulse and the middle of an echo

The sequences of pulses in conventional spin-echo can be used in almost all tests. T1-weighted images are useful to demonstrate anatomy, but they can also demonstrate diseases when associated with contrast enhancement. T2-weighted images also demonstrated diseases. Tissues affected by diseases appear edematous and/or vascularized. They have higher water content and therefore, a strong signal on T2-weighted images. Thus, they can be easily

Usually, in conventional spin-echo sequence a short TR a short TE will give a T1-weighted image, a long TR and short TE (first echo) will give a proton density image, and a long TR and

The fast spin-echo sequence is a spin-echo sequence, but with the time of the exam dramatically shorter than the conventional spin-echo. To understand how rapid the fast spin-echo sequence is, we should review how data is obtained in the conventional spin-echo. A 90º excitation pulse is followed by a 180º rephasing pulse. Only one encoding phase step is applied by TR in each

Generally, the contrast observed in fast spin-echo images is similar to that of the conventional spin-echo images. Therefore, these sequences are useful in many clinical applications. In the central nervous system, pelvis, and musculoskeletal regions, the fast spin-echo sequence has practically substituted the conventional spin-echo. In the chest and abdomen, however, the

the signal, referred to as spin-echo (7).

**Figure 4.** SE pulse of 90º and applied time (TE/2) of pulse RF of 180º [3].

long TE (second echo) will give a T2-weighted image [10].

section and just one K-space line is completed by TR [10,12,13].

**3.1. Conventional spin–echo sequence**

**3.2. Fast spin–echo sequence**

(Figure 4).

identified.

**Figure 2.** Radiofrequency pulse: 90º excitation pulse and a 180º inversion pulse, the pulse can be any value [3].

**Figure 3.** Illustration of the "spin-echo" (SE) imaging sequence [9,10].

When a pulse of 90º (π/2) is applied, the magnetization M initially in its equilibrium condition along the Z-axis (1) undergoes a 90º-displacement towards the y-direction (2). The tissues show a distribution of frequency of precession (3). There is a loss of coherence of the initial state (4). This loss can be reversed by applying a 180-degree pulse (π), which causes the spins of individual nuclei around the X-axis to rotate 180 degrees (5), rephasing (6) and regenerating the signal, referred to as spin-echo (7).

The 90º pulse plus the 180º pulse produced an echo, which is repeated several times during the study in the analyzed region. This echo is referred to as the repetition time (TR). The echo time (TE) is the duration between the middle of a 90º pulse and the middle of an echo (Figure 4).

**Figure 4.** SE pulse of 90º and applied time (TE/2) of pulse RF of 180º [3].
