Respiration induced B1+ changes and their impact on universal and tailored 3D kT-point parallel transmission pulses for 7T cardiac imaging

Abstract

Purpose: Human heart imaging at ultra-high fields is highly challenging because of respiratory motion-induced artefacts and spatially heterogeneous $B_1^{+}$ profiles. This work demonstrates that respiration resolved 3D $B_1^{+}$ -maps can be used with a dedicated tailored and universal parallel transmission (pTx) pulse design to compensate respiration related $B_1^{+}$ changes in subjects performing shallow and deep breathing (SB/DB).

Methods: Three-dimensional (3D) $B_1^{+}$ -maps of the thorax were acquired in 31 subjects under SB and in 15 subjects under SB and DB. Different universal and tailored non-selective pTx pulses were designed from non-respiration resolved (NRR) and respiration resolved (RR) reconstructions of the SB/DB $B_1^{+}$ -maps. The performance of all pulses was tested with RR-SB/DB $B_1^{+}$ -maps. Respiration-robust tailored and universal pulses were applied in vivo in 5 subjects at 7T in 3D gradient-echo free-breathing scans.

Results: All optimized pTx pulses performed well for SB. For DB, however, only the universal and the tailored respiration-robust pulses achieved homogeneous flip angles (FAs) in all subjects and across all respiration states, whereas the tailored respiration-specific pulses resulted in a higher FA variation. The respiration-robust universal pulse resulted in an average coefficient of variation in the FA maps of 12.6% compared to 8.2% achieved by tailored respiration-robust pulses. In vivo measurements at 7T demonstrate the benefits of using respiration-robust pulses for DB.

Conclusion: Universal and tailored respiration-robust pTx pulses based on RR $B_1^{+}$ -maps are highly preferred to achieve 3D heart FA homogenization at 7T when subjects perform DB, whereas universal and tailored pulses based on NRR $B_1^{+}$ -maps are sufficient when subjects perform SB.

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