Balanced SSFP‐like steady‐state imaging using small‐tip fast recovery with a spectral prewinding pulse

Abstract

Purpose Small-tip fast recovery (STFR) imaging has been proposed recently as a potential alternative to balanced steady-state free precession (bSSFP). STFR relies on a tailored “tip-up” RF pulse to achieve comparable signal level as bSSFP, but with reduced banding artifacts and transient oscillations, and is compatible with magnetization-preparation pulses. Previous STFR implementations used 2D or 3D pulses spatially tailored to the accumulated phase calculated from a B0 field map, making the steady-state STFR signal contain some T2* weighting. Here we propose to replace the spatially tailored pulse with a recently introduced spectrally selective “pre-winding” pulse that is precomputed to a target frequency range. The proposed “spectral-STFR” sequence produces T2/T1-weighted images similar to bSSFP, but with reduced banding and potentially other benefits. Theory and Methods We investigated the steady-state signal properties of spectral-STFR using simulations, and phantom and human volunteer experiments. Results Our simulation and experimental results showed that the spectral-STFR sequence has similar signal level and tissue contrast as bSSFP, but has a wider passband and more consistent banding profiles across different tissues (e.g., less hyperintense signal at band edges for low flip angles). Care is needed in designing the spectral RF pulse to ensure that the small tip angle approximation holds during RF transmission. Conclusion Spectral-STFR has similar tissue contrast as bSSFP but a wider passband and more consistent CSF/brain tissue contrast across the passband. The spectral-STFR sequence is a potential alternative to bSSFP in some applications. Compared to a spatially tailored STFR sequence, spectral-STFR can be pre-computed, is easier to implement in practice, and potentially has more uniform image contrast and minimal T2* weighting.

Design of spectral‐spatial phase prewinding pulses and their use in small‐tip fast recovery steady‐state imaging

“…In previous experiments, we have noticed this relationship as an empirical off-resonance bandwidth limit. Therefore, we use bandwidth 1 T free [3] as a conservative upper limit for prewinding. Equation [3] suggests that the widest bandwidth (range of f values used in Eq.…”

“…Therefore, we use bandwidth 1 T free [3] as a conservative upper limit for prewinding. Equation [3] suggests that the widest bandwidth (range of f values used in Eq. [2]) for refocusing is obtained with a minimal free precession time.…”

“…To overcome the bandwidth limitations of the purely spectral prewinding design in Ref. 3, we propose a spectral-spatial RF pulse design approach that spatially adapts the spectral properties. The addition of spatial information allows the RF pulse to encode spatially varying off-resonance across the object.…”

“…Recently, spectrally selective phase prewinding pulses were incorporated into the small-tip fast recovery (STFR) steady-state sequence (3). The STFR sequence employs both a traditional "tip-down" pulse and then a "tip-up" pulse that quickly returns the magnetization to the longitudinal axis after free precession (4).…”

Factorized sensitivity estimation for artifact suppression in phase‐cycled bSSFP MRI

“…However, its elevated sensitivity to magnetic field inhomogeneity causes signal voids in acquired images known as banding artifacts. This undesirable sensitivity can be alleviated by pulse sequence designs that modify off‐resonance profiles, advanced field shimming procedures, or multi‐acquisition phase‐cycled bSSFP imaging . Multiple‐acquisition bSSFP has remained a common choice since it demonstrates improved reliability against field inhomogeneity and it can additionally offer fat‐water separation …”