3D T-weighted imaging at 7T using dynamic k-points on single-transmit MRI systems.

Objectives: For turbo spin echo (TSE) sequences to be useful at ultra-high field, they should ideally employ an RF pulse train compensated for the B inhomogeneity. Previously, it was shown that a single k-point pulse designed in the small tip-angle regime can replace all the pulses of the sequence (static k-points). This work demonstrates that the B dependence of T-weighted imaging can be further mitigated by designing a specific k-point pulse for each pulse of a 3D TSE sequence (dynamic k-points) even on single-channel transmit systems Materials and methods: By combining the spatially resolved extended phase graph formalism (which calculates the echo signals throughout the sequence) with a gradient descent algorithm, dynamic k-points were optimized such that the difference between the simulated signal and a target was minimized at each echo. Dynamic k-points were inserted into the TSE sequence to acquire in vivo images at 7T. Results: The improvement provided by the dynamic k-points over the static k-point design and conventional hard pulses was demonstrated via simulations. Images acquired with dynamic k-points showed systematic improvement of signal and contrast at 7T over regular TSE-especially in cerebellar and temporal lobe regions without the need of parallel transmission. Conclusion: Designing dynamic k-points for a 3D TSE sequence allows the acquisition of T-weighted brain images on a single-transmit system at ultra-high field with reduced dropout and only mild residual effects due to the B inhomogeneity. [ABSTRACT FROM AUTHOR]