Introduction

Although parallel transmission (pTx) has great utility for mitigating radiofrequency field inhomogeneity at ultra-high field, its use in routine scans has remained marginal due to a cumbersome calibration procedure that requires subject-specific field mapping and pulse design. Recently, a novel method aiming at providing calibration-free solutions by designing universal pulses (UPs) was proposed to skip the entire calibration procedure while still being able to counteract the RF inhomogeneity problem in an effective and safe way^1,2. The wide use of this new method however requires the pulses to be immune to inter-site variability, which is known to exist due to slight variations in tune-up across sites³. The purpose of this work was to examine how robust the non-selective UPs are against inter-site variability.

Methods

Universal pulse design aims at calculating RF solutions that are robust against inter-subject variability. Universal pulses were used at 7T for brain imaging at two sites: 1) the German Center for Neurodegenerative Diseases (DZNE, Bonn, Germany) and 2) the Center for Magnetic Resonance Research (CMRR) at the University of Minnesota. At both sites, brain images were acquired on a 7T Magnetom scanner (Siemens Healthcare, Erlangen, Germany) equipped with the SC72 gradient and using the commercially available Nova 8Tx-32Rx head coil (Nova Medical, Wilmington, MA, USA). At DZNE, a database of 10 adult subjects (5 men, 5 women, age=40±15 years) were first scanned to return their respective transmit B₁ and B₀ field maps. These maps were used to design non-selective UPs for T1w MPRAGE and T2w SPACE sequences. In particular, they were used to design UPs for inversion (MPRAGE), excitation (MPRAGE, SPACE) and refocusing (SPACE) pulses. These UPs were loaded by the modified pTx-enabled MPRAGE and SPACE sequences for data acquisition. All UPs were based on the k_T-points parametrization and were designed with simultaneous optimization of the k-space trajectory⁴ under explicit hardware and SAR constraints based on Virtual Observation Points (VOPs). The cost-function of the minimization algorithm was the flip angle (FA) Normalized Root Mean Square Error averaged over the 10 database subjects. The relevant imaging parameters for the MPRAGE protocol were: 1 mm isotropic resolution, TI=1.1s, TR=2.6s, ETL=160, iPAT=2, FA=5°, TA=4:28. For the SPACE protocol, relevant imaging parameters were: 0.8 mm isotropic resolution, TR=3s, ETL=117, iPAT=2, Partial Fourier 6/8, TA=7:48 with variable FA train⁵. After offline pulse design, the calculated UPs were blindly applied at DZNE to acquire brain images from new subjects (10 subjects for MPRAGE² and 5 subjects for SPACE³), without any subject-specific pTx calibration. The exact same protocols including the same UPs and same setups were then utilized at the CMRR to acquire brain images on 5 additional adult subjects (4 men, 1 woman, 18 < age < 62) to examine the robustness of the UPs against inter-site variability. For comparison, brain images were also acquired from the same subjects at each site using the Circularly-Polarized (CP) mode, with the average FA over a central axial slice reaching the nominal FA.

Results

At both sites, the use of UPs to acquire MPRAGE images led to largely improved tissue contrast across the entire brain, especially in such lower brain regions as temporal lobes and cerebellum (Figs. 1 and 2); this improvement was observed in all of the five subjects shown. More strikingly, the use of UPs to acquire SPACE images effectively restored signals in the lower brain regions including temporal lobes, occipital lobes and cerebellum, leading to substantially improved tissue contrast across the entire brain; again this improvement was seen in all of the 5 subjects shown. The peak 10g SAR values of the UPs, estimated with the VOPs, were 5.6 W/kg for MPRAGE and 7.9 W/kg for SPACE acquisitions.

Discussion and conclusion

Here we have shown that the nonselective UPs designed using a database of pTx field maps acquired at one site (DZNE) can be blindly applied not only at the same site but also at another site (CMRR) to collect high quality, high resolution whole brain structural T1w and T2w images at 7T. Despite the existing inter-site variability, our results show that the use of UPs can robustly and substantially improve the tissue contrast across the entire brain at both sites, without requiring the cumbersome pTx calibration procedure. To further validate the universality of these pulses, we welcome more sites to join us in testing these UPs so that the travelling pulses can continue to travel along their journey toward being widely accepted as a calibration-free, B₁⁺ mitigation solution that can benefit a large array of neuroimaging applications at ultrahigh field.

Acknowledgements

The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Program, Proof of Concept Grant Agreement n. 700812 and from NIH grants including U01 EB025144 and P41 EB015894.