Basic Pulse Sequences in Magnetic Resonance Imaging

Abstract

Magnetic resonance images are obtained by a combination of different radiofrequency pulses and gradient waveforms applied to the subject inside a magnetic field. There are multiple pulse sequences used in clinical and preclinical studies adjusted to whatever physician or researches want to analyze, from basic anatomic images to accurate diagnostic techniques as diffusion, perfusion, or functional imaging. In this chapter, we present the most used radiofrequency pulse combinations of the two groups of sequences available in magnetic resonance imaging: spin-echo and gradient-echo sequences.

Magnetic Resonance Imaging of TMJ

“…This number of the echoes is called echo train length (also known as the turbo factor). Instead of the one line in conventional spin-echo sequence, multiple lines are used to fill the k-space more rapidly in FSE sequences, and the scan time is decreased [2,20]. Echo train length or turbo factor has an important role in image weighting.…”

“…Inhomogeneity of the magnet results in partially saturated missing or incomplete fat [4]. A short frequency 90° fat sat pulse is applied; the fat protons magnetization deflected into the transverse plane before sequence imaging, and they produce no signal [20].…”

“…Frequency encoding is accomplished by applying gradients during data acquisition which are used to dephase (negative polarity) and rephase (positive polarity) transverse magnetization to create one or multiple echo signal [4]. In this sequence variable RF pulses (between 10° and 90°) also known as flip angle or tip angle are applied to create the transverse magnetization [20] (Fig. 10.11).…”

“…Since no 180° RF pulse is used, the main problem is the magnet inhomogeneities effect in this sequence. Depending on this technical characteristics, T2* dephasing with much shorter time than T2 occur in GRE [5,20]. The most important advantage of the GRE sequence is short imaging time.…”

“…10.11 Gradient echo pulse sequence formation diagram T2* contrast affected by TE and T1 contrast is related with the flip angle and TR in this sequence. Larger flip angle and short TR introduces higher degree of T1 contrast, while lower flip angle and short TE accentuate T2* contrast [2,3,20]. Proton density contrast dominates when lower flip angle, longer TR, and shorter TE are used [2].…”