Modeling the effect of in-plane magnetic field gradients on asymmetric spin-echo images with echo-planar imaging...

To describe the impact of macroscopic magnetic field gradients (MFGs) in the phase-encoding direction on MR images acquired with an asymmetric spin echo (ASE) sequence with echo-planar imaging (EPI) readout.

In EPI, the center of k-space is read out at a shifted time point in the presence of phase-encoding direction MFGs. The ASE signal equation was extended to account for a locally varying temporal offset ${\tau }_{\mathrm{eff}}$ between the spin echo and echo time due to MFGs. The impact on estimated quantitative blood oxygen level dependent (qBOLD) parameters was assessed using simulations. A $B_0$ map and ASE images with four different phase-encoding directions and two different parallel-imaging factors were acquired from 2 healthy volunteers. A robust linear regression was performed between the signal dependence on the phase-encoding direction and MFGs calculated based on the $B_0$ map to test the derived signal equation.

Simulated qBOLD parameters were substantially modified by the local ${\tau }_{\mathrm{eff}}$ . The volunteer images showed a logarithmic signal intensity ratio between images acquired with reversed phase-encoding directions that showed a linear dependence on both the calculated MFGs in the phase-encoding direction and the nominal temporal offset $\tau$ . The effect was strongly reduced for the images with the higher parallel-imaging factor.

The effects of phase-encoding-direction MFGs on volunteer ASE images is consistent with the proposed signal model and relevant for qBOLD measurements. This highlights the necessity to correct or mitigate in-plane MFGs in ASE EPI, such as using a high parallel-imaging factor.