Fornix

Fornix

The fornix is one of the most important white matter pathways involved in memory function. As the principal output tract of the hippocampus, it forms a key component of the limbic system and plays a central role in memory consolidation, learning, and the transfer of information within the brain’s memory networks. Through its connections between the hippocampi, mammillary bodies, and other limbic structures, the fornix is essential for the formation and retrieval of memories. Injury to the fornix has been associated with memory impairment and other cognitive deficits. Because of its critical role in cognitive functioning, there is increasing interest in understanding the relationship between radiation dose to the fornix and treatment-related neurocognitive outcomes. Accurate delineation is therefore important for both clinical practice and research.

Co-registration with a T1-weighted MRI is recommended to facilitate accurate identification of the structure. The fornix is a paired, C-shaped white matter tract consisting of two symmetrical fibre bundles that connect the hippocampi to the mammillary bodies. It follows an arching course through the central part of the brain and can be divided into several recognizable segments along its trajectory.

Key anatomical landmarks include:

• Posterior origin: the hippocampi.
• Initial course: the fibres travel superiorly and medially along the medial aspect of the temporal horns of the lateral ventricles.
• Body of the fornix: the fibre bundles arch anteriorly beneath the corpus callosum.
• Midline portion: the left and right bundles join beneath the septum pellucidum.
• Anterior columns: the fibres diverge again and descend anteriorly, passing posterior to the anterior commissure and anterior to the interventricular foramina (foramina of Monro).
• Inferior termination: the mammillary bodies.

For contouring purposes, the structure should be followed from the hippocampal region to the mammillary bodies. The contouring endpoint is the superior border of the mammillary bodies.

The accompanying video demonstrates a step-by-step approach for identifying and contouring the fornix using anatomical landmarks on MRI. This educational resource is intended to support consistent, accurate, and reproducible delineation for clinical and research applications.

Please site as follows:

Hofstede, D., Di Perri, D., Roelofs, E., Postma, A., Troost, E., & Eekers, D. EPTN International Neurological Contouring Atlas (INCA) instruction videos. CancerData, 2022. doi:10.17195/candat.2022.02.1