Introduction
MRI at 7 tesla is strongly affected by inhomogeneous B₁ fields causing signal variations throughout the brain, which can be mitigated with parallel transmit (pTx). For clinical application, either predefined pTx Universal Pulses (UPs) [1, 2] (1) or Fast Online Customized (FOCUS) pulses [3] (2) are desirable. Here we investigate the general usability of these two pulse types with one commercial and one self-built 8Tx/32Rx RF head coil [4]. Four non-selective pTx pulses were designed for each coil, consisting of UP/FOCUS excitation/inversion pulses for use in an MPRAGE sequence.

Methods
All measurements were performed on two 7T whole-body MR systems (Magnetom Terra, Siemens Healthcare, Erlangen, Germany) one using the self-built and one the commercial coil (Nova Medical, Wilmington, USA). From each system, previously acquired datasets (B₁⁺/B₀ maps) were used for optimizing pTx pulses.
The excitation pulses used an extended SPINS k-space trajectory [3, 5], defined by the parameters P_SPINS, had a target flip angle (FA) of 5° and a fixed duration of 1ms. The inversion pulses used a k_T point trajectory (7 k_T points, first one at the k-space centre) [6] and a maximum duration of 4.8ms. Parameters defining the trajectory P_kT consisted of the Cartesian coordinates of the nonzero k_T points and the respective subpulse durations. The trajectory parameters, combined with energy regularization weights λ, were defined as ‘Combined Optimization Values’ (COV) for offline optimization:

$$COV_{Exc} ∶= [P_{SPINS},λ] $$
$$COV_{Inv} ∶= [P_{kT},λ] $$

For each set of COV, individual pulse shape optimization and a Bloch simulation were performed for all datasets. The returned subject specific values for normalized root mean square error (NRMSE_p) of the FA and maximum local specific energy dose (SED_p) in [J/kg] were used for the cost function shown below [3]. SED_p was determined by summing SAR values over the pulse using the coil specific virtual observation point (VOP) models [7].

$$min⁡∑_{p=1}^{N_p} [w_{Hom} exp⁡(NRMSE_p (COV_{Exc/Inv}))+ w_{SED} exp⁡(SED_p(COV_{Exc/Inv}))] $$

This approach used empirically determined weights w_Hom for homogeneity and w_SED for SAR ([w_Hom, w_SED ]=[5,1] for excitation, [w_Hom, w_SED ]=[100,1] for inversion pulses).
After an optimized set of COV was found, corresponding universal pulse shapes were generated with B₁⁺/B₀ maps of all respective subjects using the Variable Exchange algorithm [8] for the excitation pulses (98 variable coefficients per channel, calculated RF shapes only) and a customized interior-point algorithm [9] for the inversion pulses (7 variable coefficients per channel, recalculated k_T point locations and subpulse durations). FOCUS pulses were calculated using these algorithms and corresponding COV/UPs in maximum 45s. The generated pulses for the self-built/commercial coil and for excitation/inversion are denoted accordingly: ‘UP/FOCUS_{inv/exc, sb/com}’.
When applying FOCUS pulses, B₁⁺ and B₀ mapping was performed during the sequence preparation phase. B₁⁺ mapping was done using a magnetization-prepared, saturation-recovery TurboFLASH sequence (T_Aq = 40s) [10]. B₀ mapping was performed with a sagittally oriented gradient echo sequence (TE1 = 2.39s, TE2 = 4.59s, TE3 = 7.09s; 4x4x6mm³; T_Aq = 12s). A mask was generated cutting off low intensity voxels. The pTx pulses were used in a 3D-MPRAGE sequence (TR = 3s, TI = 1.1s, TE = 2.28ms, FoV = 250x224x160mm³, 1mm³ iso, T_Aq =4min 54s). B₀ shimming was performed in all measurements.
At each site, nine MPRAGE scans were performed in a different healthy subject. Five MPRAGE images were acquired using the UP designed for the respective coil for excitation and either UP/FOCUS_{inv sb/com} or an standard adiabatic pulse for inversion. Four additional MPRAGE images were acquired using the other UP/FOCUS_{exc sb/com} pTx pulses or a CP pulse for excitation and the same adiabatic hyperbolic secant pulse for inversion.

Results and Discussion
Figure 1 shows NRMSE, energy and SED values of all excitation pulses evaluated on all datasets compared to CP mode excitation. UPs outperform the CP excitation for their respective coils, but may result in high NRMSE values for the other (nonbrain tissue included). FOCUS pulses generally reach lower NRMSE values. The self-built coil’s VOP model produces lower SED estimates than the other and here UP/FOCUS_sb have higher energy but lower SED than the CP pulse.
Figure 2 shows the same of all inversion pulses. The advantage of FOCUS compared to UPs is generally smaller than for excitation pulses, which could be related to less degrees of freedom (less variable RF coefficients per channel).
Figure 3 shows FA maps and MPRAGE images for all excitation pulses on both volunteers (same inversion pulse). UPs generally show better homogeneity than CP pulses, both FOCUS pulses generally further improve homogeneity.
Figure 4 shows FA maps and MPRAGE images for all inversion pulses (coil-specific UP excitation pulse). UP_{inv, sb} shows severe, UP_{inv, com} and FOCUS_{inv, sb} show minor imperfections on the commercial coil. On the self-built coil, minor imperfections are visible for UP/FOCUS_{inv, sb}.

Conclusion
For excitation, UPs provide better brain coverage than CP pulses, even when trained on a different coil with similar configuration yet different VOP models. Inversion pulses, however, may fail in that case. FOCUS pulses outperform both UPs also when trained on a different coil and thereby demonstrate a more general usability. FOCUS pTx inversion pulses may serve as a low-SAR alternative to adiabatic inversion pulses.

Acknowledgements

No acknowledgement found.