Fast spin echo sequences with very long echo trains: Design of variable refocusing flip angle schedules and generation of clinical T₂ contrast

Abstract

Reducing and continuously varying the flip angle of the refocusing RF pulses in a rapid acquisition with relaxation enhancement (RARE; fast/turbo spin echo) sequence is a useful means of addressing high RF power deposition and modulation transfer function (MTF) distortion due to relaxation. This work presents a streamlined technique to generate a sequence of refocusing flip angles on a per-prescription basis that produces relatively high SNR and limits blurring in a wide range of materials encountered in vivo. Since the "effective TE" (traditionally defined as the time at which the center of k-space is sampled) no longer corresponds to the expected amount of spin-echo T 2 contrast due to the mixing of stimulated and spin echoes, a "contrast-equivalent" TE is defined and experimentally demonstrated that allows annotation of a more accurate effective TE that matches the contrast produced by 180°refocusing. Furthermore, contrast is shown to be manipulable by the addition of magnetization preparation pulse sequence segments, such as

Comparison of 3D Cube FLAIR with 2D FLAIR for Multiple Sclerosis Imaging at 3 Tesla

“…This results in slower signal decay as compared to conventional FSE sequences with constant flip angles of 180°. In this way many more echoes can be used to generate the T2 image without causing extensive blurring and artifacts [13]. Moreover, the specific absorption rate (SAR) of the Cube sequence is rather low as the flip angles are mostly much less than 180°.…”

Time‐efficient fast spin echo imaging at 4.7 T with low refocusing angles

“…Following previously published methodology (11,19,20), the amplitude of the transverse magnetization, M, at the n th echo can be described by:…”

“…The magnetization and coherence terms may be computed via the extended phase graph (EPG) algorithm (9,20); the relaxation term may then be determined by reorganizing Eq. [1].…”

“…HASTE images were acquired from a healthy 22-yearold in the sagittal and axial planes with a novel refocusing scheme based on the extended echo-train acquisition (20). The original method modulates refocusing angles to provide a near constant signal level in the first half of the echo train; flip angles are ramped to a higher value (115°in Ref.…”

“…The original method modulates refocusing angles to provide a near constant signal level in the first half of the echo train; flip angles are ramped to a higher value (115°in Ref. 20) in the later half of the train, during which time the signal decays gradually. In this scheme, RF power is concentrated in the second half of the echo train, where the angles are relatively high.…”