Spin-echo pulse sequence

The SE pulse sequence was devised in the early days of NMR days by Carr and Purcell (Fig.1.) and consists - in the preparation phase (see MR pulse sequence) - of a 90 radiofrequency pulse which flips the longitudinal magnetization Mz into the xy-plane, whereby the transverse magnetization Mxy starts to precess with the Larmor frequency (Bloch equations). This preparation phase is then followed in the acquisition phase by a train of refocusing 180 pulses which serve to generate repetitive signal echoes (hence the name spin-echo). The 180 pulses occur at times

1/2TE + iTE i = 0,1,...,n,

and the signal echoes at iTE. The echoes serve to rephase all the coherences in the xy-magnetization which are lost during the time between 90 and 180 pulse due to dephasing. This dephasing is due to static magnetic field inhomogeneities intrinsic (i.e. susceptibility changes at tissue interfaces) and extrinsic (i.e. field inhomogeneities of the main magnetic field) to the examined object. The signal peaks of the echoes fall onto a T2 decay curve, while at each echo the signals arise and decay with T2* (Fig. 1) with typical T2 relaxation times being of the order of 5-200 ms in the human body. The echo time TE may vary from echo to echo in the echo train (asymmetrical echoes). The recovery of the z-magnetization occurs with the T1 relaxation time and typically at a much slower rate than the T2-decay, because in general T1 > > T2 for living tissues and is in the range of 100-2 000 ms. The signal intensity measured is related to the square of the xy-magnetization, which in a SE pulse sequence is given by

M_xy = M_xy0(1-exp(-TR/T1)) exp(-TE/T2) 

where M_xy0 = M_z0 is proportional to the proton- or spin density, and corresponds to the z-magnetization present at zero time of the experiment when it is tilted into the xy-plane. TR is the pulse sequence repetition time (Fig. 1). This formula is directly derived from the Bloch equations. It shows the major problem with signal intensity interpretation in MR imaging: in general there is no intuitive approach to signal behaviour as signal intensity is a very complicated function of the contrast-determining tissue parameter, viz. proton density, T1 and T2, and the machine parameters TR and TE. For this reason, the terms T1 weighted image, T2 weighted image and proton density weighted image were introduced into clinical MR imaging. If the machine parameters are chosen so that TR &l Proton density weighted images are generated by choosing TR and proton density weighted image were introduced into clinical MR imaging. If the machine parameters are chosen so that TR &l Proton density weighted images are generated by choosing TR > > T1 (typically ³ 2 000 ms) and T2 < < TE (typically £ 30 ms), which when used in Eq. 1, shows that the two exponential terms are both close to one and therefore M is relatively independent of T1 and T2, thereby emphasizing M_xy0, which is proportional to the proton density. The T2 relaxation time of a tissue is readily found by fitting an exponential of the form exp (-iTE/T2) to the echoes obtained in a Carr Purcell CP sequence.

In standard SE MR imaging, each image line measured at each echo after the excitation 90 pulse is assigned to a different image, hence resulting in a multiecho pulse sequence. In the simplest form of SE imaging, the pulse sequence shown in Fig. 1 has to be repeated as many times as the image has lines; in the example, four images with different echo times would result. If the image lines from multiple echoes are used for the same image, this results in the RARE pulse sequence and if multiple image lines are obtained during a single echo, the imaging pulse sequence is of the spin echo echo planar imaging SE EPI or GRASE pulse sequence type. Since, in SE imaging TR > > TE, the machine would be idling most of the time, if a single slice would be acquired,. multiple slice imaging was introduced early on. SE pulse sequences can also be used in black blood angiography.

Spin-echo pulse sequence, Table 1

⁽¹ Rapid Acquisition with Relaxation Enhancement (RARE pulse sequence

⁽² Gradient and Spin Echo (GRASE pulse sequence, or Gradient and Spin Echo (GSE)

⁽³ Fast Spin Echo

⁽⁴ Fourier Acquired Steady State

⁽⁵ Half-Fourier Single Shot Turbo Spin-Echo

⁽⁶ Rapid Acquisition Spin Echo

⁽⁷ Dual Echo FSE

⁽⁸ Hybrid Echo-Planar Imaging

GvS

GvS