**4.1. Tissue relaxation time T1**

Required for recovery of about 63% of the magnetization along the longitudinal direction after a 90º pulse are generally more anatomical, since the fat planes are hyperintense, perfectly delimiting muscle planes and vascular structures. When paramagnetic agents (contrast) are associated, they demonstrate the skin changes with much more specificity. It is used to evaluate the anatomic structures of the injured limb in MRI and SE sequences before and after contrast. The mechanism is based on the application of a 90º RF pulse that diverted the longitudinal magnetization towards the transverse plane. Subsequently, there is a recovery of this energy diverted to the initial longitudinal axis. In a more simplified way, T1 is the time required for the initial 63% recovery of the magnetization along the longitudinal axis after the application of 90-degree RF pulse (Figures 6 &7) [7,9,10].

Thus, the signal intensity (brightness) emitted by the tissues depends solely on its ability to recover the magnetization faster or slower after the application of a 90-degree RF pulse.

**Figure 7.** Relaxation time T1: recovery 63% of the magnetization along the longitudinal direction after a 90º pulse [3].

Spin Echo Magnetic Resonance Imaging http://dx.doi.org/10.5772/53693 39

Tissue relaxation time T2 is used throughout the SE sequence to detect lesions. At T2 time, there is a magnetization shift or loss. The tissues' capacity to lose magnetization faster or slower is what determines the signal strength. T2 time is the time required for the transverse magneti‐ zation to drop up to 37% of its initial value after the application of a 90-degree pulse (Figure

**4.2. Tissue relaxation time T2**

**Figure 8.** Schematic representation of T2 relaxation time.

8 & Figure 9) [7,9,10].

**Figure 6.** Schematic representation of T1 relaxation time.

Note that the relaxation time T1 begins in (A) before the 90º pulse when the magnetization M0 is in the axis. Just after the 90º pulse, the magnetization is zero and the transverse is maximum (B). A short time later, there is the recovery of the resulting longitudinal magneti‐ zation (C) representing the start of recovery T1 (D, E), and in (F) occurs the 63% recovery of the initial magnetization [16].

**Figure 7.** Relaxation time T1: recovery 63% of the magnetization along the longitudinal direction after a 90º pulse [3].

### **4.2. Tissue relaxation time T2**

**4.1. Tissue relaxation time T1**

of 90-degree RF pulse (Figures 6 &7) [7,9,10].

**Figure 6.** Schematic representation of T1 relaxation time.

the initial magnetization [16].

Required for recovery of about 63% of the magnetization along the longitudinal direction after a 90º pulse are generally more anatomical, since the fat planes are hyperintense, perfectly delimiting muscle planes and vascular structures. When paramagnetic agents (contrast) are associated, they demonstrate the skin changes with much more specificity. It is used to evaluate the anatomic structures of the injured limb in MRI and SE sequences before and after contrast. The mechanism is based on the application of a 90º RF pulse that diverted the longitudinal magnetization towards the transverse plane. Subsequently, there is a recovery of this energy diverted to the initial longitudinal axis. In a more simplified way, T1 is the time required for the initial 63% recovery of the magnetization along the longitudinal axis after the application

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

Thus, the signal intensity (brightness) emitted by the tissues depends solely on its ability to recover the magnetization faster or slower after the application of a 90-degree RF pulse.

Note that the relaxation time T1 begins in (A) before the 90º pulse when the magnetization M0 is in the axis. Just after the 90º pulse, the magnetization is zero and the transverse is maximum (B). A short time later, there is the recovery of the resulting longitudinal magneti‐ zation (C) representing the start of recovery T1 (D, E), and in (F) occurs the 63% recovery of Tissue relaxation time T2 is used throughout the SE sequence to detect lesions. At T2 time, there is a magnetization shift or loss. The tissues' capacity to lose magnetization faster or slower is what determines the signal strength. T2 time is the time required for the transverse magneti‐ zation to drop up to 37% of its initial value after the application of a 90-degree pulse (Figure 8 & Figure 9) [7,9,10].

**Figure 8.** Schematic representation of T2 relaxation time.

In (A) are representative protons of a tissue section. Soon after a 90-degree pulse, the protons are on the same transverse plane and in phase with each other. Their magnetic vectors all point in the same direction. (B) After a very short period of time, these protons are out of phase, and their magnetic vectors are pointing to different directions. This decreases the power of the transverse magnetization vector Mxy. (C) T2 is shown as the time interval required for the transverse magnetization drops to 37% of its original value [16].

**Figure 9.** T2 shown as the time interval required for the transverse magnetization drops to 37% of its original value [3].

**Figure 10.** Normal tissue in MRI in axial sections in the "spin echo" sequence taken from the lower limbs (calf) in T1 pre (A) and (B) post-contrast injection, T2 relaxation times (C) and inversion-recovery" sequence (D) used to promote sup‐

Spin Echo Magnetic Resonance Imaging http://dx.doi.org/10.5772/53693 41

In these images, the tissues present themselves with their normal callibre vascular structures and anatomic topography, as well as their musculature with preserved sign and normal morphological aspect. The images also present the bone structure of their cortical portions and

For images of the central nervous system, "Figure 11" illustrates the characteristics in normal tissue relaxation time T1 before and after contrasts, which are used to differentiate normal

(a) (b)

**Figure 11.** Image of a normal central nervous system (sagittal plane) on pre-contrast (A) and post-contrast (B) sequen‐

characteristic medullar signal, and preserved anatomical aspect [16].

pression or fat saturation [16].

tissue from the pathological ones [19,20].

ces spin-echo T1-weighted images.
