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body of corpus callosum vs fornix on mi brain coronal section

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Body of Corpus Callosum vs Fornix - Coronal MRI Section

On a coronal MRI through the brain at the level of the diencephalon, these two structures are close to each other in the midline but occupy distinct positions relative to the lateral ventricle. The key distinction is their vertical relationship:

Positional Relationship

StructurePosition on Coronal Section
Body of Corpus CallosumSuperior - forms the roof of the body of the lateral ventricle; appears as a broad, horizontal white matter band arching across the midline at the top of the ventricle
Body of FornixInferior - hangs below the corpus callosum, just beneath the septum pellucidum, appearing as a small rounded/oval white matter bundle at the midline

How to Tell Them Apart on Coronal MRI

  1. Corpus callosum body - broad, thick, flat band of white matter that forms the ceiling of the lateral ventricle. Appears low signal (dark) on T2, high signal on T1. Wider and more prominent.
  2. Body of fornix - a smaller, more discrete bundle that runs just inferior to the corpus callosum and superior to the third ventricle. The two columns of the fornix hang side by side, separated by the thin septum pellucidum above and the third ventricle below. The fornix is suspended from the undersurface of the corpus callosum by the septum pellucidum.

Memory Tip

"Callosum is the Ceiling, Fornix hangs below" - on coronal sections, corpus callosum (#1) is the roof, septum pellucidum (#2) descends from it, and the body of the fornix (#3) is attached inferiorly.

Coronal Brain Section (Gray's Anatomy for Students, eFig. 9.22)

The image below is a labeled coronal section through the diencephalon. Key labels:
  • 1 = Corpus callosum (the broad superior band)
  • 2 = Septum pellucidum (thin midline sheet below it)
  • 3 = Body of fornix (the small bundle just below the septum pellucidum)
  • 5 = Body of lateral ventricle (the cavity between them and the thalamus)
  • 6 = Thalamus
Coronal section of brain through diencephalon - labeled anatomy showing corpus callosum (1), septum pellucidum (2), body of fornix (3), lateral ventricle (5), and thalamus (6)
Gray's Anatomy for Students, eFig. 9.22 - Coronal Section Through the Diencephalon

MRI Coronal + Sagittal Reference (Grainger & Allison Radiology, Fig. 53.7)

The image below shows a coronal T2 MRI (A) and a sagittal T2 MRI (B). In the sagittal view, the corpus callosum body is labeled B, and the fornix is labeled F - note how the fornix runs inferior and parallel to the corpus callosum. The massa intermedia (MI) of the thalamus is also shown within the third ventricle.
Coronal and sagittal T2 MRI showing corpus callosum body B, fornix F, genu G, rostrum R, splenium S, and massa intermedia MI
Grainger & Allison's Diagnostic Radiology, Fig. 53.7 - The body (B), fornix (F), and massa intermedia (MI) are labeled on the sagittal T2 image

Summary for Exam

On a coronal MRI:
  • The body of corpus callosum = superior midline structure, the broad white roof of the lateral ventricles
  • The body of fornix = inferior midline structure, a small rounded bundle just below the corpus callosum and septum pellucidum, superior to the third ventricle
  • The septum pellucidum lies between them, separating left from right lateral ventricle
  • The massa intermedia (interthalamic adhesion) is found within the third ventricle, just below the fornix at this level
  • Gray's Anatomy for Students, eFig. 9.22
  • Grainger & Allison's Diagnostic Radiology, Fig. 53.7
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