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Identify this location in MRI brain
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This is an axial MRI of the posterior fossa (cerebellum/brainstem level). The crosshair cursor with the measurement annotation is placed at a small structure in the midline, just anterior to the fourth ventricle. Based on the anatomy visible, that appears to be the fastigial nucleus region or, more likely given its midline position at the roof/floor of the fourth ventricle, the vermis of the cerebellum at the level of the nodulus/uvula.
Let me look more carefully and consult the textbook for confirmation.
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The textbook reference (Grainger & Allison's Diagnostic Radiology) includes an axial T2 MRI at the cerebellar level that matches this image well. Let me fetch the relevant figure to compare.
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This reference image confirms the anatomy clearly. Now I have everything I need for a definitive answer.
Location Identified: Fastigial Recess / Roof of the Fourth Ventricle (Fastigium)
Looking at the axial MRI in the image, the crosshair cursor with the measurement (~0.56 mm / 1.00 mm) is placed midline, at the apex (roof) of the fourth ventricle - this is the fastigium.
Anatomy at this level:
| Structure | Location in image |
|---|---|
| Cerebellar hemispheres | Large bilateral foliated structures (bright on this sequence) |
| Cerebellar vermis | Midline posterior structure |
| Fourth ventricle | Dark (CSF) midline cavity - the tent-shaped space |
| Fastigium | The pointed roof/apex of the 4th ventricle where the crosshair sits |
| Dentate nuclei | Deep within each cerebellar hemisphere (visible as areas of different signal) |
| Pons/medulla | Anterior to the 4th ventricle |
More precisely, the cursor is at the fastigium - defined as:
The pointed, tent-like roof of the fourth ventricle formed by the superior medullary velum (SMV) anteriorly and the nodulus/uvula of the cerebellar vermis posteriorly. It is the most dorsal/superior point of the fourth ventricle lumen.
The image appears to be a T2-weighted or susceptibility-weighted (SWI/GRE) axial sequence given the bright cerebellar cortex signal and dark CSF in the ventricle. The sequence settings (W:612, L:283) are consistent with a GRE or FLAIR-type window.
Comparing with the textbook axial T2 reference (Grainger & Allison's Diagnostic Radiology, Fig. 53.16B - shown above), the plane matches the level of the pons and cerebellar peduncles, with the arrows in the reference image pointing to the dentate nuclei (showing iron deposition as low signal on T2).
Summary:
The marked location is the fastigium - the roof/apex of the fourth ventricle, at the midline junction between the superior medullary velum and the cerebellar vermis (nodulus). This is an important anatomical landmark in posterior fossa imaging, relevant in conditions such as fastigial epidermoid cysts, medulloblastoma extension, and Dandy-Walker malformations.
- Grainger & Allison's Diagnostic Radiology, Chapter 53, Fig. 53.16
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