Fast magnetization transfer saturation imaging of the brain using MP2RAGE T1 mapping

Abstract

Purpose: Magnetization transfer saturation (MT_sat) mapping is commonly used to examine the macromolecular content of brain tissue. This study compared variable flip angle (VFA) T₁ mapping against compressed-sensing MP2RAGE (csMP2RAGE) T₁ mapping for accelerating MT_sat imaging.

Methods: VFA, MP2RAGE, and csMP2RAGE were compared against inversion-recovery T₁ in an aqueous phantom at 3 T. The same 1-mm VFA, MP2RAGE, and csMP2RAGE protocols were acquired in 4 healthy subjects to compare T₁ and MT_sat. Bloch-McConnell simulations were used to investigate differences between the phantom and in vivo T₁ results. Ten healthy controls were imaged twice with the csMP2RAGE MT_sat protocol to quantify repeatability.

Results: The MP2RAGE and csMP2RAGE protocols were 13.7% and 32.4% faster than the VFA protocol, respectively. At these scan times, all approaches provided strong repeatability and accurate T₁ times (< 5% difference) in the phantom, but T₁ accuracy was more impacted by T₂ for VFA than for MP2RAGE. In vivo, VFA estimated longer T₁ times than MP2RAGE and csMP2RAGE. Simulations suggest that the differences in the T₁ measured using VFA, MP2RAGE, and inversion recovery could be explained by the magnetization-transfer effects. In the test-retest experiment, we found that the csMP2RAGE has a minimum detectable change of 2.3% for T₁ mapping and 7.8% for MT_sat imaging.

Conclusions: We demonstrated that MP2RAGE can be used in place of VFA T₁ mapping in an MT_sat protocol. Furthermore, a shorter scan time and high repeatability can be achieved using the csMP2RAGE sequence.

Keywords: MRI; T1; brain; macromolecules; magnetization transfer; myelin.