Shaped saturation with inherent radiofrequency-power-efficient trajectory design in parallel transmission

Abstract

Purpose: A target-pattern-driven (TD) trajectory design is introduced in combination with parallel transmit (pTX) radiofrequency (RF) pulses to provide localized suppression of unwanted signals. The design incorporates target-pattern and B1+ information to adjust denser sampling and coverage in k-space regions where the main pattern information lies. Based on this approach, two-dimensional RF spiral saturation pulses sensitive to RF power limits were applied in vivo for the first time.

Theory and methods: The TD method was compared with two state-of-the-art spiral design methods. Simulations at different spatial fidelities, acceleration factors and anatomical regions were carried out for an eight-channel pTX 3 Tesla (T) coil. Human in vivo experiments were performed on a two-channel pTX 3T scanner saturating shaped patterns in the brain, heart, and thoracic spine.

Results: Using the TD trajectory, RF pulse power can be substantially reduced by up to 34% compared with other trajectory designs with the same spatial accuracy. Local and global specific absorption rates are decreased in most cases.

Conclusion: The TD trajectory design uses available a priori information to enhance RF power efficiency and spatial response of the RF pulses. Shaped saturation pulses show improved spatial accuracy and saturation performance. Thus, RF pulses can be designed more efficiently and can be further accelerated.

Keywords: RF pulse optimization; k-space trajectory, target-pattern design; parallel transmit; shaped saturation; variable-density sampling.

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