Rapid B field mapping at 3 T using the 180° signal null method with extended flip angle.

PURPOSE

To extend the null signal method (NSM) for B mapping to 3 T magnetic resonance imaging (MRI).

BACKGROUND

The NSM operates in the steady state regime and exploits the linearity of the spoiled gradient recalled echo (SPGR) signal around the 180° flip angle (FA). Using linear regression, B maps are derived from three SPGR images acquired at different FAs with a short repetition time. While the conventional NSM allows accurate mapping of B for moderate B variation, we observed that this method fails for the larger B variations typical of high-field MRI.

METHODS

We analyzed the effect of the FA range of the acquired SPGR images on B determination using the NSM for 3 T MRI through extensive numerical and in vivo analyses. B maps derived from the extended angle-range NSM (EA-NSM) were calculated and compared to those derived from the conventional, more restricted angle range, NSM, and to those derived from the reference, but much more time-consuming, double angle method (DAM). Furthermore, we investigated the compatibility of EA-NSM B mapping and the half-scan and SENSE reconstruction methods for accelerating acquisition time.

RESULTS

Our results show that the use of the conventional FA range leads to substantial inaccuracies in B determination. Both numerical and in vivo analyses demonstrate that expanding the FA range of the acquired SPGR images substantially improves the accuracy of B maps. Furthermore, B maps derived from EA-NSM were demonstrated to be quantitatively comparable to those derived from the lengthy DAM protocol. We also found that B maps derived from SPGR images using the EA-NSM and imaging acceleration methods were comparable to those derived from images acquired without acceleration. Finally, the use of half scanning combined with SENSE reconstruction permits whole-brain B mapping in ~1 min.

CONCLUSIONS

The EA-NSM permits accurate, fast, and practical B mapping in a 3 T clinical setting.