A regular MRI is usually done lying down. If symptoms are worse upright, with movement, or with the head in certain positions, a scan done flat on a table may not recreate the conditions that make the problem visible.

The systematic review literature on CCI in Ehlers-Danlos syndrome specifically recommends dynamic and upright weight-bearing imaging because the standard supine MRI was developed for a different kind of instability.

CCI in the context of EDS is often ligamentous and position-dependent. The instability appears in the way the joints move under load. Supine imaging removes the load. 

When my standard supine MRI was done, it was read as unremarkable. The instability that the scan could not show was the same instability that defined the diagnosis once the imaging was done in the positions that provoked my symptoms.

The 2022 systematic review on craniocervical instability in Ehlers-Danlos syndrome (link) remains one of the core references behind the shift toward dynamic and upright imaging in this patient population.