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Deep part of masseter muscle

The deep part of the masseter muscle is a distinct anatomical subdivision of the masseter, one of the primary muscles of mastication. It lies beneath the superficial part and is essential for mandibular elevation and stabilization during chewing. This part of the muscle originates from the posterior one-third of the zygomatic arch and inserts onto the upper part of the lateral ramus of the mandible. The deep masseter is particularly relevant in imaging due to its role in various jaw pathologies and TMJ disorders.

Synonyms

  • Deep masseter muscle

  • Pars profunda musculi masseteris

  • Internal part of the masseter

MRI Appearance

1. Proton Density (PD)

  • The deep part of the masseter appears as a well-defined, homogenous structure.

  • Isointense signal compared to adjacent muscles of mastication.

  • Muscle fibers can occasionally be distinguished, especially in high-resolution PD images.

2. Short Tau Inversion Recovery (STIR)

  • Normally shows uniformly low to intermediate signal intensity, similar to other skeletal muscle.

  • Pathological processes (e.g., edema, inflammation) will cause the deep masseter to appear hyperintense (bright signal).

  • STIR is useful for detecting subtle muscle injuries or early myositis involving the deep masseter.

3. T1-weighted Imaging

  • The deep part of the masseter demonstrates low to intermediate signal intensity.

  • The margins are typically well-circumscribed, allowing differentiation from adjacent fat and the superficial masseter part.

  • T1 is optimal for assessing muscle bulk, atrophy, fatty infiltration, or mass lesions involving the deep

MRI images

Deep part of masseter muscle