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Assessment of the sensitivity of inhomogeneous Magnetization Transfer (ihMT) brain imaging with B1+ variations at 3T

Lucas Soustelle^1,2, Samira Mchinda^1,2, Thomas Troalen³, Maxime Guye^1,2, Jean-Philippe Ranjeva^1,2, Gopal Varma⁴, David C Alsop⁴, Guillaume Duhamel^1,2, and Olivier M Girard^1,2
¹Aix-Marseille Univ, CNRS, CRMBM, Marseille, France, ²APHM, Hôpital Universitaire Timone, CEMEREM, Marseille, France, ³Siemens Healthcare SAS, Saint-Denis, France, ⁴Division of MR Research, Radiology, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA, United States

As for many other MRI techniques, inhomogeneous Magnetization Transfer necessitates special attention to local B₁⁺ variations. In this work, a detailed analysis of the ihMT ratio sensitivity to B₁⁺ is performed on human brain at 3T using various concentration schemes for its optimization.

Figure 2: Left side (a): illustrative 3D ihMTR images (2mm isotropic resolution) obtained under strong RF energy concentration conditions (TR = 345 ms, Configuration #2). Right side: WM averaged ihMTR measured at nominal V_ref (b), WM averaged metrics calculated using the two reference voltage settings: δ_ihMTR (ihMTR relative variation; c), δ_{ihMTR-[-10%;10%]}(voxel fraction within 10% error margin; d) and δ_ihMTR-RMS (RMS relative error; e).

Figure 1: Simulations of the ihMT ratio in WM as a function of RF power B_1,RMS for distributed (short TR), intermediate and concentrated (long TR) RF power deposition. The dashed vertical line indicates the nominal B_1,RMS used in the current 3T study. a: absolute ihMTR values show that concentrating RF energy allows increasing ihMTR sensitivity; b: normalized ihMTR values provide insight into the corresponding relative sensitivity to B₁⁺ variations (approximated at first order by the derivative of the ihMTR vs. B_1,RMS characteristic curves at nominal B_1,RMS).