Standardized Universal Pulse: a fast RF calibration approach to improve flip angle accuracy in parallel transmission

Purpose: In parallel transmission (pTX), subject-tailored RF pulses allow achieving excellent
flip angle (FA) accuracy but often require computationally extensive online optimisations,
precise characterisation of the the static field (ΔB0) and the transmit RF field (B+1 )
distributions. This costs time and requires expertise from the MR user. Universal Pulses
(UP) have been proposed to reduce this burden, yet, with a penalty in FA accuracy. This
study introduces the concept of standardised universal pulses (SUP), where pulses are designed
offline and adjusted to the subject through a fast online calibration scan.
Methods: A SUP is designed offline using a so-called standardised database, wherein each
B+1 map has been normalised to a reference transmit RF field distribution. When scanning
a new subject, a 3-slice B+1 acquisition (scan time < 10 s) is performed and used to adjust
the SUP to the subject through a linear transform. SUP performance was assessed at 7T
with simulations by computing the FA-normalised root mean square error (FA-NRMSE)
and the FA profile stability as measured by the average and coefficient of variation of the
FA across 15 control subjects, along with in vivo experiments using an MP2RAGE sequence
implementing the SUP variant for the FLASH readout.
Results: Adjusted SUP improved the FA-NRMSE (8.8 % for UP versus 7.1 % for adjusted
SUP). Experimentally, in vivo, this translated in an improved signal homogeneity and more
accurate T1 quantification using MP2RAGE.
Conclusion The proposed SUP approach improves excitation accuracy (FA-NRMSE) while
preserving the same offline pulse design principle as offered by UPs.