Enhanced susceptibility-weighted angiography (ESWAN) of cerebral arteries and veins at 1.5 Tesla

Simultaneous time-of-flight MR angiography and quantitative susceptibility mapping with key time-of-flight features

A technique for combined time-of-flight (TOF) MR angiography (MRA) and quantitative susceptibility mapping (QSM) was developed with key features of standard three-dimensional (3D) TOF acquisitions, including multiple overlapping thin slab acquisition (MOTSA), ramped RF excitation, and venous saturation. The developed triple-echo 3D TOF-QSM sequence enabled TOF-MRA, susceptibility-weighted imaging (SWI), QSM, and R2* mapping. The effects of ramped RF, resolution, flip angle, venous saturation, and MOTSA were studied on QSM. Six volunteers were scanned at 3 T with the developed sequence, conventional TOF-MRA, and conventional SWI. Quantitative comparison of susceptibility values on QSM and normalized arterial and venous vessel-to-background contrasts on TOF and SWI were performed. The ramped RF excitation created an inherent phase variation in the raw phase. A generic correction factor was computed to remove the phase variation to obtain QSM without artifacts from the TOF-QSM sequence. No statistically significant difference was observed between the developed and standard QSM sequence for susceptibility values. However, maintaining standard TOF features led to compromises in signal-to-noise ratio for QSM and SWI, arising from the use of MOTSA rather than one large 3D slab, higher TOF spatial resolution, increased TOF background suppression due to larger flip angles, and reduced venous signal from venous saturation. In terms of vessel contrast, veins showed higher normalized contrast on SWI derived from TOF-QSM than the standard SWI sequence. While fast flowing arteries had reduced contrast compared with standard TOF-MRA, no statistical difference was observed for slow flowing arteries. Arterial contrast differences largely arise from the longer TR used in TOF-QSM over standard TOF-MRA to accommodate additional later echoes for SWI. In conclusion, although the sequence has a longer TR and slightly lower arterial contrast, provided an adequate correction is made for ramped RF excitation effects on phase, QSM may be performed from a multiecho sequence that includes all key TOF features, thus enabling simultaneous TOF-MRA, SWI, QSM, and R2* map computation.

Simultaneous Variable-Slab Dual-Echo TOF MR Angiography and Susceptibility-Weighted Imaging

In this paper, we propose a new 3-D dual-echo method for simultaneous multislab time-of-flight MR angiography (TOF MRA) and single-slab susceptibility-weighted imaging (SWI). The previous echo-specific k-space reordering scheme for compatible dual-echo arteriovenography (CODEA) was advanced to applying excitation RF pulses for multiple thin slabs and a single thick slab to the first (TOF MRA) and second (SWI) echoes, respectively. Single-slab CODEA and multislab CODEA (fixed-slab CODEAs) were additionally acquired as comparison reference to the proposed variable-slab CODEA. Parallel imaging was also tested for feasibility of accelerating the proposed method. TOF MRA and SWI from the proposed variable-slab CODEA were visually and quantitatively comparable to multislab TOF MRA and single-slab SWI, respectively, separately acquired from the fixed-slab CODEAs. The parallel imaging reduced the scan time from 10.3 to 5.6 min. Furthermore, the proposed variable-slab approach improved the vessel continuities at slab boundaries of TOF MRA for CODEA as well as for the conventional single echo method. The proposed variable-slab CODEA provided multislab TOF MRA and single-slab SWI simultaneously in a clinically reasonable scan time of ~5 min with minimal impact on image qualities, while suppressing slab boundary artifacts in TOF MRA.