Calibration-free pTx of the human heart at 7T via 3D universal pulses

Abstract

Purpose: MRI at ultra-high fields in the human body is highly challenging and requires lengthy calibration times to compensate for spatially heterogeneous $B_1^{+}$ profiles. This study investigates the feasibility of using pre-computed universal pulses for calibration-free homogeneous 3D flip angle distribution in the human heart at 7T.

Methods: Twenty-two channel-wise 3D $B_1^{+}$ data sets were acquired under free-breathing in 19 subjects to generate a library for an offline universal pulse (UP) design (group 1: 12 males [M] and 7 females [F], 21-66 years, 19.8-28.3 kg/m² ). Three of these subjects (2M/1F, 21-33 years, 20.8-23.6 kg/m² ) were re-scanned on different days. A 4kT-points UP optimized for the 22 channel-wise 3D $B_1^{+}$ data sets in group 1 (UP22-4kT) is proposed and applied at 7T in 9 new and unseen subjects (group 2: 4M/5F, 25-56 years, 19.5-35.3 kg/m² ). Multiple tailored and universal static and dynamic parallel-transmit (pTx) pulses were designed and evaluated for different permutations of the $B_1^{+}$ data sets in group 1 and 2.

Results: The proposed UP22-4kT provides low $B_1^{+}$ variation in all subjects, seen and unseen, without severe signal drops. Experimental data at 7T acquired with UP22-4kT shows comparable image quality as data acquired with tailored-4kT pulses and demonstrates successful calibration-free pTx of the human heart.

Conclusion: UP22-4kT allows for calibration-free homogeneous flip angle distributions across the human heart at 7T. Large inter-subject variations because of sex, age, and body mass index are well tolerated. The proposed universal pulse removes the need for lengthy (10-15 min) calibration scans and therefore has the potential to bring body imaging at 7T closer to the clinical application.

Keywords: 7 Tesla; body MRI; heart; parallel transmission; universal pulse.