“Tectal beaking” is MRI feature of:
Arnold Chiari I malformation
Chiari I malformation (cerebellar ectopia and seen in adulthood.
Definition: - Descent of otherwise normal cerebellar hemispheres below the foramen magnum, usually involving tonsils.
Features: Peg like, pointed tonsils displaced into upper cervical canal
 0–10 years à 6 mm at least
10–30 years à 5 mm
 30–40 years à 4 mm
Associated anomalies: Syringomyelia (30–60%), hydrocephalus (20–25%), basilar invaginations (25–50%), Klippel-Feil syndrome (5–10%) and atlanto-occipital assimilation (1–5%)
Chiari II malformation (childhood).
Definition: Descent of dysplastic cerebellar hemisphere below the foramen magnum, usually involving inferior vermis, which is everted instead being inverted, causing 4th ventricle to reduce to a coronal cleft and medulla oblongata to elongate and kink.
Features:
Dandy Walker malformation
Arnold-Chiari malformation
Aqueductal stenosis
Third ventricular tumor