Normal masticator space and different grades of masticator space involvement in magnetic resonance images. (A) Axial T2-weighted magnetic resonance (MR) image (1800 ~ 3000 ms/90 ~ 150 ms, TR/TE) at the level of the nasopharynx shows the anatomic masticator space (circled in red). LP = lateral pterygoid muscle, M = masseter muscle, MP = medial pterygoid muscle, TP = temporalis muscle. (B) Grades of masticator space involvement. Grade 0: without MSI; Grade 1: with medial pterygoid muscle involvement but without lateral pterygoid muscle involvement or infratemporal fossa involvement; Grade 2: with medial and /or lateral pterygoid muscle but without infratemporal fossa involvement; Grade 3: with infratemporal fossa involvement. (C) T1-weighted axial contrast medium–enhanced MR image and (D) T2-weighted MR image show extensive tumor infiltration in the left masticator space. Medial and lateral pterygoid muscle involvement is marked with a red asterisk in each image. (E) T1-weighted axial MR image, (F) T1-weighted axial contrast medium–enhanced MR image, and (G) T2-weighted MR image show left infratemporal fossa involvement (red arrow).