**3.2. Fast spin–echo sequence**

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

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 respiratory artifacts are sometimes problematic in cases where the respiratory compensation techniques are not compatible with the programs fast spin-echo, which is counterbalanced to some extent by the fact that shorter examination times in fast spin-echo sequence enable the production of images with fewer respiratory artifacts in [9,10,11,13,14,15].

There are two differences in contrast between the pulse sequence of the conventional spinecho and fast spin-echo, both of which are due to the 180º pulse repeated at short intervals following the sequence of echoes. First, the adipose tissue remains clear on T2-weighted images due to multiple RF pulses that reduce the effects of spin-spin interactions in this tissue. However, the fat saturation techniques may be used to compensate for this. Second, the 180º repeated pulses may increase the magnetization transfer, so that the muscles appear darker on the fast spin-echo images than on the conventional spin-echo images. Additionally, multiple 180º pulses reduce the effects of magnetic susceptibility, which may be detrimental when looking for small haemorrhages [10].

The advantages of fast spin sequence are that metal implant artifacts are significantly reduced in rapid sequences.

In fast spin-echo T1-weighted images, effective TE and TR are short; on T2-weighted effective TE and TR are long TR; on proton density weighting/T2-weighted images, effective TE is short and effective TR is long [10,11,13,15].

mined by the pixel area and the thickness of the section. Thus, the size of the matrix is determined by the number of pixels of the anatomy covered during the selection of the tissue section to be analyzed. This size is indicated by two values. The first one corresponds to the number of frequencies sampled and the second to the number of phase codings performed

**Figure 5.** Illustration of the resonance image inversion-recovery pulse sequence. A 180º pulse inversion is applied fol‐

are also shown [16].

Spin Echo Magnetic Resonance Imaging http://dx.doi.org/10.5772/53693 37

lowed by a 90º recovery pulse, as well as a 180º repolarization pulse. TR, TE and TI

Frequency codification is the reading of a signal along the longest axis of the anatomy. The phase codification is the reading of a signal along the short axis of the anatomy. Thus, a matrix size of 256 x 192 indicates that 256 encoding frequencies and 192 encoding phases are per‐

The number of acquisitions (NAQ) represents the number of times that data are acquired

The number, thickness and intervals of the sections are defined according to the type of lesion. Other functions are used to improve image quality. Its use allows viewing only the sections

The images primarily reflect the distribution of free hydrogen nucleus and the way it responds to an external magnetic field. Thus, this response determines different relaxation times known as T1 and T2. The pathological processes cause relaxation time to change in relation to the tissues of the nervous and musculoskeletal system, and the signal intensity is reflected [7,9,16].

[7,10,13].

selected [10,11].

**4. Tissue parameters**

formed during a sequence [9,10].

within/into the same pulse sequence [10,11].

The advantages are: Greatly reduced examination times, better image quality, and more information on T2-weighted images. We can use high-resolution matrices and multiple numbers of excitations (NEX). However, some effects of increased flow and movement are incompatible with some options of image acquisition, such as fat tissue bright on T2-weighted images, blurred images can occur because data were collected at different TE time, decreased magnetic susceptibility effect, because multiple 180º pulses produce excellent returning phase, so that one must not use it in case of suspected bleeding [4, 9,10,13,14,15].

The "inversion-recovery" sequence is used to promote suppression or fat saturation, high‐ lighting areas of injury. The process was the reverse of the "spin-echo" sequence. There was an inversion followed by a recovery by applying 180º inversion pulses, which inverted the spins of the fatty tissue region examined by 180º, followed by 90º recovery pulse. Subsequently, a 180º repolarizing pulse was applied to produce a spin-echo. In this sequence, the repetition time (TR) is the time between each 180º pulse. The inversion time (TI) is the length of time the fat (spins) took to recover from this complete inversion (Figure 5).

This process allowed the fat to become dark or hypointense, differing itself from the lesions. This happened because the inversion of its spins caused a total loss of energy/magnetization. Consequently, there is no sign for it [10].

The field of view (FOV) determines the size of the anatomy covered during the selection of the tissue section to be analyzed either in a coronal or axial plane.The forming unit of a digital image is the pixel. The brightness of each pixel represents the power of the MR signal produced by a volumetric imaging of the patient or volumetric pixel or Volumetric Picture Element (voxel). The voxel is a volume element representing the tissue inside the patient. It is deter‐

**Figure 5.** Illustration of the resonance image inversion-recovery pulse sequence. A 180º pulse inversion is applied fol‐ lowed by a 90º recovery pulse, as well as a 180º repolarization pulse. TR, TE and TI are also shown [16].

mined by the pixel area and the thickness of the section. Thus, the size of the matrix is determined by the number of pixels of the anatomy covered during the selection of the tissue section to be analyzed. This size is indicated by two values. The first one corresponds to the number of frequencies sampled and the second to the number of phase codings performed [7,10,13].

Frequency codification is the reading of a signal along the longest axis of the anatomy. The phase codification is the reading of a signal along the short axis of the anatomy. Thus, a matrix size of 256 x 192 indicates that 256 encoding frequencies and 192 encoding phases are per‐ formed during a sequence [9,10].

The number of acquisitions (NAQ) represents the number of times that data are acquired within/into the same pulse sequence [10,11].

The number, thickness and intervals of the sections are defined according to the type of lesion. Other functions are used to improve image quality. Its use allows viewing only the sections selected [10,11].
