Variable Flip Angle Optimization for Fetal Brain Imaging With Reduced Specific Absorption Rate

Half-Fourier Acquisition with Single-Shot Turbo Spin Echo (HASTE) scans are routinely used for fetal brain imaging, but they have a high specific absorption rate (SAR) and can be inefficient due to SAR limits. Here, we have designed an optimized variable flip angle (VFA) pattern for fetal HASTE neuroimaging to reduce both the SAR and repetition time (TR) of the HASTE sequence while maintaining similar image quality to standard fetal HASTE imaging with a constant flip angle (CFA). The VFA pattern was optimized by minimizing the difference in expected signal between the VFA and CFA scans while constraining the SAR of the VFA scan to no more than 65% of the SAR of the CFA scan and reducing the TR by 29%. The expected signal was calculated using an extended phase graph formalism, and simulations were used to predict the performance of the two scans using different image quality metrics. The proposed VFA and standard CFA scans were tested in phantoms and fetuses at 3 T. SAR and acquisition times were recorded, and image quality was rated by three radiologists. The VFA scan showed similar signal-to-noise ratio and contrast-to-noise ratio values but slightly lower signal and relative contrast values than the CFA scan in phantom studies. In vivo, the VFA scan yielded significantly reduced SAR, measurement times, and total scan times. There was no significant difference in overall image quality ratings between the VFA and CFA scans. An optimized VFA scan can provide 65% of the SAR and 71% of the acquisition time of a CFA scan while being diagnostically equivalent. Lower SAR reduces heating, eliminates SAR pauses, and allows accelerated scans by reducing the TR. The time saved by faster HASTE acquisitions increases patient comfort and may be used to repeat scans with excessive fetal motion or to perform advanced sequences.