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Topic

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Cerebellar tonsil (H IX)

The cerebellar tonsil is a rounded lobule of the inferior surface of the cerebellum, belonging to the posterior lobe. Positioned on either side of the medulla oblongata, the paired tonsils sit adjacent to the foramen magnum and are connected to the uvula of the vermis via the tonsillar peduncle. They form part of the roof of the vallecula cerebelli, a groove between the tonsils where the medulla lies.

The tonsils are clinically significant because their position makes them prone to downward herniation through the foramen magnum, particularly in Chiari malformation or raised intracranial pressure, potentially compressing the brainstem and upper cervical spinal cord.

Function

  • Contributes to coordination of voluntary motor activity as part of the cerebellar cortex

  • Participates in balance and posture regulation

  • Interacts with vermis and cerebellar hemispheres in fine motor control

  • Acts as an anatomical landmark in posterior fossa neuroimaging

  • Clinical role in pathology: tonsillar herniation (Chiari malformation) is a key diagnostic entity

Synonyms

  • Tonsil of cerebellum

  • Cerebellar tonsil lobule

  • Hindbrain tonsil

  • H IX (Terminologia Anatomica designation)

MRI Appearance

T1-weighted images:

  • The cerebellar tonsils appear as intermediate to low signal intensity gray matter, isointense to the rest of the cerebellar cortex

  • Surrounded by hypointense CSF in the cisterna magna, providing clear contrast

  • Herniation appears as inferior displacement of the tonsils below the foramen magnum

T1 Post-Contrast (Gadolinium-enhanced):

  • Normal tonsils do not enhance significantly, except for vascular structures

  • Pathology such as tumors, inflammation, or vascular malformations may demonstrate enhancement

T2-weighted images:

  • Tonsils show intermediate to slightly hyperintense signal, consistent with cerebellar cortex

  • CSF in the surrounding cisterns is bright hyperintense, creating sharp delineation

  • Useful for identifying edema, malformations, or downward displacement

STIR (Short Tau Inversion Recovery):

  • Tonsils appear intermediate signal

  • Edema, inflammation, or gliosis within or around the tonsils appears hyperintense, aiding in detection of pathological change

  • Effective for assessing acute Chiari-related cord or brainstem edema

CT Appearance:

  • The cerebellar tonsils appear as soft tissue density lobules within the posterior fossa, adjacent to the foramen magnum

  • Surrounded by hypodense CSF of the cisterna magna

  • CT clearly demonstrates posterior fossa bony anatomy and can detect tonsillar herniation by showing crowding at the foramen magnum

  • Less sensitive for intrinsic tonsil pathology compared to MRI, but valuable in trauma, hemorrhage, or bone anomaly assessment

MRI images

Cerebellar tonsil (H IX) of Cerebellum mri sag image -img-00000-00000

MRI images

Cerebellar tonsil (H IX)