The acquisition of dynamic articulatory data is crucial to improve our understanding of speech production. Ultrasound (US) imaging presents the interest of offering a good temporal resolution without any health hazard and at a reasonable price. However, it cannot be used alone because there is no reference coordinate system and no spatial calibration. We describe a multimodal acquisition system which uses electromagnetography sensors to locate the US probe, and the method used to calibrate the US modality. We experimented this system to investigate the most appropriate acquisition protocol for Magnetic Resonance Imaging. Three strategies were explored for one speaker: (i) stopping phonation just before acquisition, (ii) reiterated acquisitions, and (iii) silent acquisition. The measurements of the minimal tongue to palate distance show that silent acquisition generally offers the minimal articulatory drift while guaranteeing a small tongue to palate distance, i.e. a clear articulation.