Simultaneous Optimization of MP2RAGE T1 -weighted (UNI) and FLuid And White matter Suppression (FLAWS) brain images at 7T using Extended Phase Graph (EPG) Simulations

Abstract

Purpose: The MP2RAGE sequence is typically optimized for either T₁ -weighted uniform image (UNI) or gray matter-dominant fluid and white matter suppression (FLAWS) contrast images. Here, the purpose was to optimize an MP2RAGE protocol at 7 Tesla to provide UNI and FLAWS images simultaneously in a clinically applicable acquisition time at <0.7 mm isotropic resolution.

Methods: Using the extended phase graph formalism, the signal evolution of the MP2RAGE sequence was simulated incorporating T₂ relaxation, diffusion, RF spoiling, and B₁ ⁺ variability. Flip angles and TI were optimized at different TRs (TR_MP2RAGE ) to produce an optimal contrast-to-noise ratio for UNI and FLAWS images. Simulation results were validated by comparison to MP2RAGE brain scans of 5 healthy subjects, and a final protocol at TR_MP2RAGE = 4000 ms was applied in 19 subjects aged 8-62 years with and without epilepsy.

Results: FLAWS contrast images could be obtained while maintaining >85% of the optimal UNI contrast-to-noise ratio. Using TI₁ /TI₂ /TR_MP2RAGE of 650/2280/4000 ms, 6/8 partial Fourier in the inner phase-encoding direction, and GRAPPA factor = 4 in the other, images with 0.65 mm isotropic resolution were produced in <7.5 min. The contrast-to-noise ratio was around 20% smaller at TR_MP2RAGE = 4000 ms compared to that at TR_MP2RAGE = 5000 ms; however, the 20% shorter duration makes TR_MP2RAGE = 4000 ms a good candidate for clinical applications example, pediatrics.

Conclusion: FLAWS and UNI images could be obtained in a single scan with 0.65 mm isotropic resolution, providing a set of high-contrast images and full brain coverage in a clinically applicable scan time. Images with excellent anatomical detail were demonstrated over a wide age range using the optimized parameter set.

Keywords: 7T; FLAWS; MP2RAGE; MRI; ultrahigh field.