An Objective Study of Anatomic Shifts in Intracranial Hypotension Using Four Anatomic Planes

Figure 1

Incisural and foramen magnum related anatomic planes of reference and their MRI equivalent reference lines. Technical note. A symbol (rather than a line) has been used in the figures, when a measured distance is close to or equal to zero.

(a) Anatomic reference planes and measured distanced intervals for supratentorial structures adjacent to the tentorial hiatus (incisura). The supratentorial reference planes are noted as follows: the plane of the incisura (approximated by the black line), the tuberculum-venous confluence line (approximated by the white line), and the long axis of the visual pathway (approximated by the gray reference line). The measured distances are shown by the mammillary body to incisura line distance (triangles measured interval), the splenium of the corpus callosum to incisura line distance (white circle), and the iter (point) to tuberculum-venous confluence line distance (black snowflake) demonstrated in a normal patient

(b) Anatomic reference planes and measured distanced intervals for additional supra- and infratentorial structures. This sagittal midline MRI image of the posterior fossa demonstrates additional measurements. The first of these is the measured interval (black line) between the undersurface of the optic chiasm and the plane of the diaphragm sella (white line). The interpeduncular fossa distances are also demonstrated with the mammillary body-pontine distance (black dotted line) representing the apex; the base (width of the floor of the tegmentum portion of the midbrain) is shown as well (white dotted line). The ratio is calculated from the apex to base relationship. The plane of the foramen magnum (gray line) is defined by the interconnection of the opisthion with the basion points of the skull. Since the decussation of the pyramids cannot be determined reliably on sagittal MRI, the distance between the obex and the foramen magnum line (black triangle line) was used as an alternative distance interval in order to assess caudal brain stem displacement. The distance from the caudal aspect of the cerebellar tonsil to the foramen magnum line is shown (white triangle line)