Whole brain inhomogeneous magnetization transfer (ihMT) imaging: Sensitivity enhancement within a steady-state gradient echo sequence

Samira Mchinda (1) , Gopal Varma (2, 3) , Valentin H. Prevost (1) , Arnaud Le Troter (1) , Stanislas Rapacchi (1) , Maxime Guye (1) , Jean Pelletier (1, 4) , Jean-Philippe Ranjeva (1) , David C. Alsop (2, 3) , Guillaume Duhamel (1) , Olivier M. Girard (1)

1 CRMBM - Centre de résonance magnétique biologique et médicale
2 HMS - Harvard Medical School [Boston]
3 BIDMC - Beth Israel Deaconess Medical Center [Boston]
4 Neurologie, maladies neuro-musculaires [Hôpital de la Timone - APHM]

PURPOSE: To implement, characterize, and optimize an interleaved inhomogeneous magnetization transfer (ihMT) gradient echo sequence allowing for whole-brain imaging within a clinically compatible scan time. THEORY AND METHODS: A general framework for ihMT modelling was developed based on the Provotorov theory of radiofrequency saturation, which accounts for the dipolar order underpinning the ihMT effect. Experimental studies and numerical simulations were performed to characterize and optimize the ihMT-gradient echo dependency with sequence timings, saturation power, and offset frequency. The protocol was optimized in terms of maximum signal intensity and the reproducibility assessed for a nominal resolution of 1.5 mm isotropic. All experiments were performed on healthy volunteers at 1.5T. RESULTS: An important mechanism driving signal optimization and leading to strong ihMT signal enhancement that relies on the dynamics of radiofrequency energy deposition has been identified. By taking advantage of the delay allowed for readout between ihMT pulse bursts, it was possible to boost the ihMT signal by almost 2-fold compared to previous implementation. Reproducibility of the optimal protocol was very good, with an intra-individual error \textless 2%. CONCLUSION: The proposed sensitivity-boosted and time-efficient steady-state ihMT-gradient echo sequence, implemented and optimized at 1.5T, allowed robust high-resolution 3D ihMT imaging of the whole brain within a clinically compatible scan time.

Mots clés: magnetization transfer model; crmbm; ihMT; myelin; Provotorov theory of radiofrequency saturation; dipolar order; snc; Inhomogeneous magnetization transfer; dual frequency RF saturation

Domaines: Imagerie; Ingénierie biomédicale

Samira Mchinda, Gopal Varma, Valentin H. Prevost, Arnaud Le Troter, Stanislas Rapacchi, et al.. Whole brain inhomogeneous magnetization transfer (ihMT) imaging: Sensitivity enhancement within a steady-state gradient echo sequence. Magnetic Resonance in Medicine, 2017, ⟨10.1002/mrm.26907⟩. ⟨hal-01657938⟩