Individually optimized dynamic parallel transmit pulses for 3D high-resolution SPACE imaging at 7T

Although clinical 7T MRI offers various advantages compared to lower field strengths, achieving spatially uniform flip angle distributions remains a challenge. Sampling Perfection with Application optimized Contrast using different flip angle Evolution (SPACE) sequences employing a long train of refocusing pulses with varying flip angles pose a particular challenge in that regard. In this study, we investigate scalable dynamic parallel transmission (pTx) pulses to achieve homogeneous 3D high-resolution SPACE brain imaging at 7T.

Non-parametrized and scalable dynamic pTx pulses were designed for excitation, refocusing and inversion in SPACE sequences by using fast online customization (FOCUS). First, a database of B₀ and multi-channel $B_1^{+}$ maps were used for optimizing universal pulses and parameters for flip angle homogeneity under strict specific absorption rate (SAR) constraints. During each new examination, B₀ and $B_1^{+}$ maps were acquired as additional calibration step and pTx pulses were tailored to the subject. For scalability, a symmetry condition was enforced. T₁, T₂, fluid-attenuated inversion recovery (FLAIR) and double inversion recovery (DIR) SPACE images were acquired in five healthy subjects at 7T using the proposed FOCUS pulses and conventional circularly polarized (CP) pulses for comparison.

Improved SNR and better image homogeneity were observed in every image acquired with FOCUS pulses in comparison to CP. Quantitative analysis showed a significant reduction in the coefficient of variation (COV) of image intensities in the cerebellum, a region notably affected by $B_1^{+}$ inhomogeneities across all contrasts. FLAIR images, for example, exhibited a 46% COV reduction.

Individually optimized dynamic pTx pulses for 3D high-resolution SPACE imaging delivered clinically acceptable image homogeneity, enabling the application of widely used clinical contrasts at 7T.