Flow-suppressed 2D spin-echo imaging with high tolerance to B1 inhomogeneity using hyperbolic secant pulses

Abstract

Purpose: To demonstrate flow-suppressed two-dimensional (2D) spin-echo and spin-echo diffusion echo-planar imaging (EPI) sequences using hyperbolic secant (HS) pulses for both π/2 excitation and π refocusing.

Theory and methods: A theoretical framework to derive phase dispersion of moving spins under π/2 excitation and π refocusing using HS pulses was described. Numerical simulations were performed to verify the validity of the theoretical analysis. All experiments were performed on a 3T clinical scanner. Phantom and human-brain imaging was performed using 2D spin-echo sequence, and liver imaging was performed using 2D spin-echo diffusion EPI. The signal-to-noise ratio and residual blood flow signal of the proposed sequences were compared with those of conventional spin-echo sequences using sinc pulses.

Results: Results from human brain and liver images demonstrated that the proposed method substantially reduced blood flow artifacts. In the brain, venous blood flow was suppressed more effectively with the proposed method than with conventional spin-echo sequence using presaturation. In the liver, as compared with spin-echo sequence using sinc pulses, the proposed method showed noticeable attenuation of bright blood signals at low b-values, whereas the overall tissue signal in peripheral regions was higher. The signal-to-noise ratio was enhanced by 10% to 30%, indicating improved B₁ tolerance due to the adiabatic π refocusing HS pulse.

Conclusion: Flow suppression and partial B₁ insensitivity were achieved by replacing sinc pulses with HS pulses in conventional 2D spin-echo imaging and spin-echo diffusion EPI sequences. This approach may be particularly useful in various applications requiring reduced vascular signal contamination, such as liver and brain imaging.

Keywords: B1 insensitivity; DWI; flow suppression; hyperbolic secant pulse; spin‐echo MRI.