**3.1. Conventional spin–echo sequence**

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

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

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

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

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 identified.

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