POSTERIOR FOSSA - CISTERNA MAGNA

• Portion of subarachnoid space that bathes the posterior fossa with CSF.

• Seen as early as 10-14 post menstrual weeks.
• Echolucent structure posterior to the cerebellum.

• Connection with the IV ventricle through the median aperture can be visualized.

• Arches around cerebellum posteriorly.
• Deepens in midline due to invagination of the vermis.
• Measurement (evaluated in the same plane as the cerebellum).
• Measure from posterior aspect of cerebellar vermis to the inner table of the occiput.
• Normal = 5mm SD +/- 3mm (<10mm considered normal) (1-3).
• Constant during pregnancy (Pilu and co-workers) 0.69 ± 0.13cm = 2 SD)
• Linear echoes - may be 1, 2, or many echoes within it (thought to be due to arachnoid).

• Giant cisterna magna.

Normal Cisterna Magna	Giant Cisterna Magna – 13 mm

• Pitfall in measurement: Evaluation should be made on an axial image of the posterior fossa which includes the IV ventricle if possible, as angled semicoronal images may falsely enlarge the cisterna magna (2,3).

Large Cisterna Magna (10.2mm) Normal cerebellar hemispheres. Normal cerebellar vermis. Normal trans-cerebellar diameter.

REFERENCES

1. Filly RA, Cardoza JD, Goldstein RB et.al. Detection of fetal CNS anomalies: A practical level of effort for a routine sonogram. Radiology 1989;172:403-408.
2. Goldstein RB. Sonography of the fetal neural axis: A practical approach. Fetal Maternal Med Rev 1995;7:47-60.
3. Laing FC, Frates MC, Brown DL et.al. Sonography of the fetal posterior fossa: False appearance of mega-cisterna magna and Dandy-Walker variant. Radiology 1994;192:247-251.