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Obex

The Obex (also known as Gierke) is an important anatomical landmark located at the caudal end of the fourth ventricle, marking the transition between the fourth ventricle and the central canal of the medulla oblongata. It is a small, pointed structure found on the dorsal surface of the medulla, just above the foramen magnum. Clinically and radiologically, identification of the obex is significant as it serves as a reference point in various pathological conditions, such as syringomyelia, Chiari malformations, and tumors involving the lower brainstem.

Synonyms:

  • Gierke

  • Caudal tip of the fourth ventricle

  • Lower end of the fourth ventricle

  • Dorsal medullary point

MRI Appearance:

  • T1-weighted Imaging:

    • The obex appears as a subtle, low-signal (isointense to slightly hypointense) point at the dorsal surface of the medulla, best appreciated in sagittal and axial planes.

    • It is often identified as the junctional region where the CSF signal of the fourth ventricle ends and the medullary tissue begins.

  • T2-weighted Imaging:

    • The obex is delineated as the point where the high signal intensity of the fourth ventricular CSF tapers into the medulla.

    • Appears as a bright CSF-fluid interface adjacent to the hypointense brainstem tissue.

    • Provides clear contrast between CSF (hyperintense) and medulla (intermediate to hypointense).

  • FLAIR (Fluid-Attenuated Inversion Recovery):

    • The CSF in the fourth ventricle is suppressed (dark) on FLAIR, making the obex less conspicuous but still recognizable as the lower tip of the ventricle.

    • The obex appears as a transition point at the dorsal medulla where the dark (suppressed) CSF signal ends.

CT Appearance:

  • The obex is generally not well visualized on non-contrast CT due to limited soft tissue contrast.

  • On high-resolution or thin-slice CT, it may be inferred as the anatomical point at the lower end of the fourth ventricle, seen as a small notch or point dorsal to the medulla.

  • Useful as a reference point in evaluating posterior fossa lesions, hemorrhage, or hydrocephalus affecting the fourth ventricle.

MRI images

Obex (Gierke) mri 3t axial image

MRI images

Obex (Gierke) mri 3t sagittal image