MRI allows characterizing the shape and position of speech articulators, but not tracking flesh points, since there are no physiological landmarks reliably associated with the highly deformable tissues of these articulators. This information is, however, interesting for studying the biomechanical properties of these organs as well as for modelling the relations between measurement modalities such as MRI and electromagnetic articulography. We have therefore attached fiducial markers made of MRI-visible polymers to a speaker's articulators, and recorded a corpus of MRI midsagittal images. We then determined and analyzed the articulators' contours and the markers' centre coordinates. We found that the "apparent sliding" of markers with respect to lips and tongue contours ranges between 0.6 and 1.5 cm. Specifically, the curvilinear distances between two tongue flesh points relative to the total length varied up to about ±20%, confirming a non longitudinal elasticity of the contours. However, we have also found that the markers and jaw coordinates can predict the articulators' contours with a variance explanation of about 85 %, and an RMS reconstruction error between 0.08 and 0.15 cm, compared with 74 to 95 % of variance and 0.07 to 0.14 cm of RMS error for the original articulatory models.