Cervical dystonia is the most common form of focal dystonia in adults (50-82%). It manifests itself by a abnormal attitude of the head, intermittent or permanent, due to involuntary contraction of the cervical muscles which appears or is accentuated on the occasion of voluntary movement and maintenance posture. The distribution of dystonic muscles is specific to each patient explaining the diversity of patterns encountered. The therapeutic management of DC is essentially local and symptomatic. It is based on the realization of injections of neuro botulinum toxin (BoNT) targeting target (dystonic) muscles responsible for involuntary movements or posture abnormal. Identifying the muscles involved is a step prerequisite for therapeutic intervention.The obliquus capitis inferior (OCI) also known as Lower Oblique belongs to the group of suboccipital muscles.It is the only suboccipital muscle that does not attach to the skull. Its unilateral contraction causes ipsilateral rotation of C1 therefore of the head. The length of the transverse process of the atlas gives it considerable rotary efficiency. It is described as the cephalic rotation starter muscle. It would perform the 30 first degrees of rotation. The rotation of the whole column cervical would then be continued by the synergistic action of the muscle contralateral sternocleidomatoid and Spl. ipsilateral. The level of joint complex C1-C2 the amplitude of rotation corresponds to approximately 50% of the total rotation of the cervical spine. In order to better understand the part played by the OCI muscle in the disorganization of posture and cervical movements in the axial plane (plane of rotation) in the rotary DC, the investigators want biomechanically analyze its function in pathological situation. The physiology of this muscle is richly documented in healthy subjects. But does this knowledge apply in DC? Acquisition of imagery by the Cone Beam or CBCT system (Cone Beam Computed Tomography) before and 5 weeks after the injection of BoNT, will allow the analysis of the displacement of each vertebrate.