Synopsis

Keywords: Image acquisition: Fast imaging, Image acquisition: Sequences, Physics & Engineering: Physics

Trains of spin echoes have become essential parts of most imaging studies. They improve speed and signal-to-noise ratio and make possible 3D spin echo imaging in a feasible time. As more RF refocusing pulses are used, surprisingly complex effects arise that add new options and features for spin echo train imaging. We will review these surprising properties of echo trains and how they can be used to improve and optimize imaging.

We will review seven surprising topics of spin echo physics:
#1: The spin echo: An RF pulse can realign magnetization dephased by nonuniform fields. The spin echo, which is now often taken for granted in MRI, was originally a subject of surprise and amazement¹. Signal rapidly decayed in early NMR systems because of poor magnetic field uniformity. Spin echoes proved that the apparently irreversible decay of signal was actually reversible. Today, MRI magnets are uniform enough to allow for gradient echoes, but spin echoes and their ability to remove macroscopic magnetic field nonuniformity effects make them essential for long TE imaging.
#2: The stimulated echo: Two pulses can combine to store magnetization along the z direction and then realign. Introductory MRI physics courses invariably describe spin echoes as the effect of a single 180° RF pulse. But lower flip angle pulses create a mixture of effects^1,2. In addition to creating weaker spin echoes, reduced flip angles store dephased magnetization along the z direction where it decays with T1, not T2. A second RF pulse can then excite and refocus the stored magnetization. This echo, referred to as a stimulated echo, is largest when the two pulses are 90°.
#3: Echo trains and stability: A train of equally spaced RF pulses will create a stable series of echoes, but only if the phase of the refocusing pulses are 90° relative to the excitation pulse. Carr and Purcell³ introduced the idea of a train of echoes to observed T2 decay. They used 180° pulses, but it was found that very small deviations from perfect 180°s caused unstable echo amplitudes. Later Meiboom and Gill⁴ showed that a particular phase relationship between the RF pulses of the excitation pulse and the refocusing pulses stabilized the echo train. This CPMG echo train technique is used in almost all spin echo train imaging.
#4: The pseudo-steady state: Spin echoes are inherently decaying with T2 yet, surprisingly, concepts of steady state imaging from gradient echo imaging can be applied to spin echo trains with echo spacing short compared to T2. Neglecting T1 and T2 decay, one can show that a steady state exists for any flip angle, as long as the CPMG phase condition is met⁵. Slow changes of the flip angle during the train will slowly change the echo amplitude. This pseudo-steady state can help to understand and design flip angle trains for MRI. Slow variation of flip angles can be used to control contrast and blurring effects in long echo train T2 imaging^6,7,8.
#5: Low flip angle refocusing trains: Because a single RF pulse will refocus only sin²(flip/2) of the excited magnetization, the penalty for reduced flip angles is severe. For spin echo trains, however, the pseudo-steady state echo amplitude⁵ is approximately sin^1/2(flip/2). A 30° train, for example, will refocus almost 50% of the excited magnetization as long as the magnetization is transitioned into the steady state with a few larger flip angles at the outset, or potentially other preparations⁹. Because the magnetization decays more slowly with low flip angles, due to a mixture of spin (T2) and stimulated (T1) echoes, longer echo trains can be used and a SNR comparable to a 180° flip angle train can be achieved.
#6: Even echo rephasing: Motion and flow are much more attenuated in low flip angle than high flip angle trains. Motion of magnetization during a spin echo train introduces gradient induced phases shifts that violate the assumptions of the CPMG echo train sequence. Surprisingly, this is not a major issue when refocusing pulses are near 180°. The phase shift from the first echo is reversed by the second echo, a phenomenon known as even echo rephasing¹⁰. However, when flip angles are reduced, attenuation of motion and flow are very apparent¹¹. Low flip angle trains cannot reliably image flow, but can be useful if flowing spins are not of interest or in applications like black blood MRI¹² or vessel wall imaging¹³.
#7: Is True-FISP spin or gradient echo?: The motion induced phase effects in spin echo train imaging can be overcome if gradients are balanced and fully refocused during an echo spacing. For times short compared to T1, such balanced sequences can act like motion insensitive spin echo trains¹⁴. At longer times, recovered magnetization adds to the signal and the sequence is better described as balanced steady state free precession. Balanced SSFP retains spin echo like qualities including reduced sensitivity to magnetic field nonuniformity¹⁵.

Acknowledgements

No acknowledgement found.