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  • Title: Calibration-free pTx of the human heart at 7T via 3D universal pulses.
    Author: Aigner CS, Dietrich S, Schaeffter T, Schmitter S.
    Journal: Magn Reson Med; 2022 Jan; 87(1):70-84. PubMed ID: 34399002.
    Abstract:
    PURPOSE: MRI at ultra-high fields in the human body is highly challenging and requires lengthy calibration times to compensate for spatially heterogeneous B1+ 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 B1+ 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 B1+ 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 B1+ data sets in group 1 and 2. RESULTS: The proposed UP22-4kT provides low B1+ 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.
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