Controlling T2 blurring in 3D RARE arterial spin labeling acquisition through optimal combination of variable flip angles and k-space filtering.

PURPOSE: To improve the SNR efficiency and reduce the T2 blurring of 3D rapid acquisition with relaxation enhancement stack-of-spiral arterial spin labeling imaging by using variable refocusing flip angles and k-space filtering.
METHODS: An algorithm for determining the optimal combination of variable flip angles and filtering correction is proposed. The flip angles are designed using extended phase graph physical simulations in an analytical and global optimization framework, with an optional constraint on deposited power. Optimal designs for correcting to Hann and Fermi window functions were compared with conventional constant amplitude or variable flip angle only designs on 6 volunteers.
RESULTS: With the Fermi window correction, the proposed optimal designs provided 39.8 and 27.3% higher SNR (P < .05) than conventional constant amplitude and variable flip angle designs. Even when power deposition was limited to 50% of the constant amplitude design, the proposed method outperformed the SNR (P < .05) of these 2 conventional approaches by 32.5 and 20.7%. The sharpness and the contrast between gray and white matter were improved with the k-space filtering correction for all of the flip angle designs. The improvements were moderate for the Hann window correction.
CONCLUSION: This work demonstrates that variable flip angles can be derived as the output of an optimization problem. The combined design of variable flip angle and k-space filtering provided superior SNR to designs primarily emphasizing either approach singly.