Gradient Echo Versus Spin Echo

Abstract

Spin Echo pulse sequences follow similar contrast behavior to that described in the previous section; however, there are some key differences between Gradient Echo and Spin Echo pulse sequences (Fig. 11.1). In the case of spin echo, a 180° refocusing pulse is applied at a time equal to half the echo time. This reverses the effect of the field inhomogeneities and the amplitude of the transverse magnetization at the echo times dependent on the pure T2 relaxation time of the tissue. As the T2 values for tissues are longer than the T2* values, the echo times chosen to achieve T2 weighting with Spin Echo are also longer than the echo times required to achieve T2* weighting with gradient echo sequences. The spin echo pulse sequence uses an excitation pulse that is normally 90°, so at each excitation pulse all of the z magnetization is transferred into the transverse plane. This combined with the refocusing pulse gives the largest possible signal provided the magnetization is allowed to recover sufficiently between repetitions.