Theoretical analysis and optimization of ultrashort echo time (UTE) imaging contrast with off-resonance saturation

Abstract

PurposeOff-resonance saturation (ORS) is a tool which can be used in ultrashort echo time (UTE) magnetic resonance imaging to selectively reduce short T2 signals. When these ORS prepared UTE images are subtracted from a non-suppressed UTE acquisition, the short T2 signals are highlighted. The aim of this paper is to develop a theoretical ORS model and optimize short T2 contrast. TheoryUsing a theoretical model the sequence parameters such as saturation flip angle and off-resonance frequency were optimized to maximize short T2 contrast. Bloch simulations were performed to demonstrate the accuracy of the theoretical model. MethodsVolunteer imaging was performed on the knee using different saturation flip angles and off-resonance frequencies using a Fermi RF pulse with a 3D UTE Cones acquisition. ResultsThe off-resonance saturation method showed good long T2 suppression, and highlighted short T2 signals such as the patella tendon. The theoretical signal curves generally agreed with simulated and experimentally measured signals. ConclusionOff-resonance saturation 3D UTE imaging can be used to effectively suppress long T2 signals and highlight short T2 signals. Theoretical modeling can be used to optimize sequence parameters to maximize long T2 suppression and short T2 contrast. Experimental results confirmed the theoretical predictions.