Computed tomography and magnetic resonance imaging of lesions at masticator space
We aim to review the normal anatomy and imaging appearance of masticator space lesions. Because the masticator space is not amenable to direct examination, cross-sectional imaging with computed tomography and magnetic resonance imaging play an important role in diagnosis and characterization of lesions occurring there. Masticator space lesions can be classified on the basis of their origin into the following categories: inflammatory lesions, benign tumors, malignant tumors, vascular lesions, and developmental lesions. A diverse spectrum of malignant tumors and benign lesions are seen extending from the adjacent spaces. In addition, one should also be familiar with pseudolesions as well as post-treatment changes in the masticator space that can be mistaken for pathologic conditions.
KeywordsMasticator Infection Benign Malignant MR imaging CT scan
Classification of masticator space lesions
Granulomatosis with polyangiitis
Schwannoma and neurofibroma*
Fibrosing inflammatory pseudotumor
Primitive neuroectodermal tumor (PNET)
Malignant peripheral nerve sheath tumors
Displaced 3rd molar teeth
Dermoid and epidermoid cysts
Cephalocele and meningocele
Accessory parotid tissue*
Lateral pterygoid muscle changes in TMJ disease
Lesions extending into masticator space
Oral cavity carcinoma*
Minor salivary gland tumor*
Maxillary sinus malignancy
Pigmented villonodular synovitis
Recurrence versus post-radiation*
Bisphosphonate-related osteonecrosis of the jaw*
We aim to review the normal anatomy and imaging appearance of masticator space lesions and pseudolesions.
The masticator space contains the four muscles of mastication (masseter, temporalis, medial and lateral pterygoids) . This space also contains the ramus and the posterior body of the mandible, the mandibular division of the trigeminal nerve (V3) , and the inferior alveolar vein and artery and branches of the internal maxillary artery [3, 4, 5, 6] (Fig. 1).
The temporal fossa, or space, is the superior extent of the masticator space superior to the zygomatic arch. Radiologists may refer to this area as the suprazygomatic masticator space. The infratemporal fossa is the cranial extension of the masticator space adjacent to the skull base being the portion between the pterygopalatine fossa medially and the zygomatic arch laterally. The submasseteric space is a potential space located between the masseter muscle and the ramus of the mandible. There is no fascial division between these subspaces, and pathological processes can spread continuously from one to the other [1, 2, 3, 4, 5].
Methods of examination
Magnetic resonance (MR) imaging
The standard MR imaging examination of the masticator space should include the application of a T2-and T1-weighted fast spin-echo (SE) sequence, an axial short inversion time inversion-recovery (STIR) sequence, and fat-saturated T1-weighted imaging with a gadolinium-based contrast material. The image planes are mainly axial or coronal and may be sagittal. Section thickness should be 3–4 mm [1, 2, 3, 4]. Diffusion MR imaging has been used to differentiate malignant tumors of the masticator space from benign lesions  and infection , as well as for differentiation of recurrent tumors from post-treatment changes . MR spectroscopy has a role in differentiation of chronic infection from malignancy, but of limited value . Dynamic contrast-enhanced MR imaging and dynamic susceptibility-weighted contrast-enhanced MR imaging also show promise for characterizing space soft tissue tumors of the masticator space [3, 11].
Computed tomography (CT)
Multidetector CT of the masticator space is performed in the axial plane beginning from the skull base down to the base of the neck after intravenous administration of contrast material. Coronal and sagittal images may be reconstructed from the axial sections [3, 5]. PET/CT using fluoro-deoxy-glucose as a tracer (FDG-PET) is sensitive for detection of distant metastatic foci and to differentiate recurrent tumors from post-radiation changes . Recently, CT perfusion  and dual energy CT  have been used for characterization of head and neck masses including masticator space lesions.
MR imaging is the first-line study for the evaluation of suspected masticator space mass lesions, especially with a strong suspicion that the lesion is neoplastic. CT is the method of choice in patients suspected to suffer from inflammatory disease or in patients with contraindications for MR imaging [1, 2, 3].
Differentiating malignancy from inflammatory lesions
Routine contrast CT and MR imaging are commonly used for differentiation of malignancy from inflammatory lesions in the masticator space. Abscesses usually appear as well defined marginally enhanced lesions; on the other hand, malignancy shows an inhomogenous pattern of contrast enhancement with ill-defined margins [1, 2, 3, 4, 5]. However in some cases, this differentiation is difficult with routine CT and MR imaging. Diffusion MR imaging and MR spectroscopy helps in this differentiation. Malignancy reveals restricted diffusion with low ADC value compared to inflammatory lesions at diffusion [7, 8, 9]. There is a significant difference in the choline signals and choline/creatine between malignant tumors and inflammatory lesions of the masticator space .
Differentiating malignancy from benign lesions
Although differentiating benign lesions from malignant tumors is often difficult by routine imaging alone, certain characteristic imaging features such as calcification and MR signal intensity can be helpful in narrowing the list of differential diagnoses [1, 2, 3, 4]. Advanced MR imaging such as diffusion MR imaging  and dynamic susceptibility-contrast MR imaging  may aid in this differentiation. Benign tumors show higher ADC value compared to malignant tumors; however, some overlap has been reported. Malignant tumors tend to show earlier and faster uptake of contrast material than benign tumors at dynamic contrast study [1, 2, 3, 4, 5].
Differentiating primary tumors from tumors extending into the masticator space
The epicenter of tumors originating from the masticator space located within the masticator muscles or the mandible and located anterior to the fatty parapharyngeal space. The epicenter of small tumors can be identified; however, the origin of the large aggressive tumors may be difficult to determine with imaging [1, 2, 3, 4].
Role of imaging
In inflammatory lesions of the masticator space, it is crucial to determine the source of infection, extent of the lesion, to describe abscesses, and to detect potential osteomyelitis of the mandible and skull base in order to plan appropriate treatment. CT is the modality of choice, as it demonstrates the source of infection and may be helpful in depicting cortical erosion in osteomyelitis. However, MR imaging more accurately delineates inflammation involving the soft tissues [1, 2, 3, 4].
Origin of inflammatory lesions
Inflammatory lesions of the masticator space commonly arise from odontogenic infections (80 %), although other sources such as malignant otitis externa, maxillary sinus fracture, and parotid infection are possible etiologies as well [3, 15].
Phlegmon versus abscess
Of the masticator muscles involved in inflammatory lesions, the masseter (76 %) and medial pterygoid (63 %) muscles are most often affected. The temporalis (26 %) and lateral pterygoid (21 %) muscles were less frequently involved .
Extension of inflammatory lesions
Masticator space infection may extend into the parapharyngeal space, prevertebral space, temporal fossa (Fig. 3), and the orbit. Infection in the masticator space might easily spread to the base of the skull [1, 2, 3, 4].
Atypical (mycobacterial and fungal) infection
Fungal pathogens such as aspergillus and mucormycosis can gain access directly from an infected paranasal sinus . Tuberculous disease is usually due to hematogenous spread . The CT and MR imaging of these atypical infections are nonspecific and indistinguishable from those of other causes of masticator space infection [1, 2, 3, 4]. Actinomycosis is an important infection that causes mass formation and is difficult to distinguish from malignant tumors .
Rarely, parasitic diseases have been reported in the masticator space. Hydatid cysts appear as well defined simple cysts with high signal intensity on T2-weighted images. Also, cysticercosis has been reported in the masseter muscle as having high signal intensity on T2-weighted images with marginal enhancement [20, 21].
Granulomatosis with polyangiitis (Wegener’s granulomatosis) is a rare necrotizing vasculitis disorder characterized by a triad of necrotizing granuloma in the upper and lower respiratory tract, necrotizing vasculitis and glomerulonephritis. Granulomatosis with polyangiitis may spread to the masticator space directly or perineurally along the mandibular nerve. On MR imaging, the granulomatous lesions show hypointense signal on both T2-weighted and T1-weighted sequences. Contrast enhancement is usually observed .
Nerve sheath tumors (schwannoma and neurofibroma)
Hemangioma is commonly seen in children and tends to afflict females. It presents shortly after birth, grows rapidly, peaks at 1–2 years, and then usually shows slow spontaneous regression. Hemangiomas tend to have a low signal on T1-weighted images and a bright signal on T2-weighted images. Flow voids of blood vessels within the mass and intense contrast enhancement are fairly characteristic .
Fibrosing inflammatory pseudotumor
Fibrosing inflammatory pseudotumor is a nonspecific inflammatory process of uncertain cause that has been most frequently reported in the orbit and rarely involves the masticator space. MR findings of pseudotumors are ill-defined margins with hypointensity on T2-weighted images, relatively weak enhancement, and good response to steroid therapy [3, 22].
Osteosarcomas of the masticator space may arise from the mandible or from extraskeletal soft tissue of the masticator space. Osteosarcomas of the mandible affect patients in the third decade and one or two decades later than other lesions. On CT, the majority of osteosarcomas have matrix mineralization and calcifications of the osteoid or osteoid-like substance within the tumor, and some tumors show a sunburst effect caused by radiating mineralized tumor spiculae. On MR imaging, osteosarcoma is of low-to-intermediate signal intensity on T1-weighted images and is of high signal intensity on T2-weighted images. Calcifications and new bone formations appear as signal void regions within the lesion [1, 27].
Chondrosarcomas are malignant cartilaginous tumors that typically manifest in the fourth and fifth decades of life. On CT scan, chondrosarcoma shows a soft tissue mass with characteristic multiple stippled and amorphous areas of calcifications that may be associated with mandibular bone destruction and an inhomogeneous pattern of contrast enhancement. The signal intensity of the chondroid matrix is lower than bone matrix on T1-weighted images. There are hyperintense areas (chondroid tissue) and hypointense areas (calcified regions) on T2-weighted images. The tumor may show characteristic curvilinear septal and peripheral enhancement of fibrovascular tissue and non-ossified cartilage, a pattern that has been described as ‘‘ring-and-arc’’ [3, 27].
Primitive neuroectodermal tumor (PNET) and extraskeletal Ewing sarcoma
Primitive neuroectodermal tumor (PNET) and extraskeletal Ewing sarcoma are similar malignant soft tissue sarcomas which are likely neuroectodermal in origin. Males are affected more commonly than females. These disorders predominantly affect young adolescents. They have a nonspecific appearance and show low-to-intermediate attenuation on CT without evidence of calcification. MR imaging reveals nonspecific features with low-to-intermediate signal intensity on T1-weighted imaging and high signal intensity on T2-weighted imaging. Tumor margins may be relatively well-defined with a pseudocapsule, or appear infiltrative. MR imaging frequently shows definable high flow vascular channels within the mass, often more peripherally [1, 2, 3, 4].
Malignant peripheral nerve sheath tumors (MPNST)
Lymphoma is rarely reported in the masticator space. MR imaging of lymphoma involving the pterygoid muscle demonstrates signal intensity similar to other soft tissue tumors. Infiltrative lesions of lymphoma show a homogeneous low signal on T1 and high signal on T2-weighted images with intense enhancement. The associated nodal disease, extranodal lymphatic disease (Waldeyer’s ring), or multiple other extranodal extralymphatic sites (e.g. sinus, nose, orbit) are involved simultaneously, which suggests the diagnosis of lymphoma .
Soft-tissue plasmacytomas of the head and neck have a predilection for the nasal cavity or nasopharynx, but have been reported in the masticator space. At CT, they appear as a soft tissue mass. At MR imaging, plasmocytoma has low signal intensity on T1-weighted MR images, high signal intensity on T2-weighted images, and homogeneous marked enhancement on postcontrast T1-weighted images. Plasmacytomas may show intratumoral flow voids. They exhibit restricted diffusion due to their high cellularity [1, 2, 3, 4].
Metastases to the masticator space are most commonly seen in patients aged 50–70 years. Metastasis is most often located in the mandibular molar region, because of the abundant blood supplies. The CT and MR imaging appearance of metastasis to the masticator space is non-specific. The lesion may show intense homogeneous, heterogeneous, or marginal contrast enhancement. It may be associated with cortical destruction and invasion of bone marrow of the mandible, with mouth-eaten appearance [1, 2, 3, 4].
Condylar fracture and intramuscular hematoma
Fractures of the mandibular condyle may be displaced into the masticator space and can be associated with hematoma of the masticator muscles. Localization of the degree of dislocation of the condylar fracture has significant therapeutic implications. CT scans can be helpful in the assessment of dislocation of fractured bones and associated intramuscular hematoma .
Myositis ossificans is a benign, self-limited, proliferative mesenchymal response to soft tissue injury, resulting in formation of mature bone in the periphery of muscles of mastication by 6–8 weeks following trauma. CT demonstrates a well-defined geometric hypodense mass with peripheral calcification in the earlier phases, when mature, dense calcification can be seen. Familial forms have been reported .
Displaced third molar teeth
Displacement of maxillary third molars into the infratemporal fossa is usually associated with an incorrect extraction technique. These teeth usually displace through the periosteum into the infratemporal fossa just adjacent to the lateral pterygoid plate and inferior to the lateral pterygoid muscle. CT can provide an exact anatomical location .
Dermoid and epidermoid
Meningocele and cephalocele
Meningoceles and cephaloceles are rare lesions representing protrusions of arachnoid or dura mater through skull defects into the masticator space. They may be considered as congenital, spontaneous, or secondary. At imaging, cephaloceles are smoothly marginated and have the same signal intensity characteristics as CSF with all MR imaging sequences .
Denervation atrophy of masticator muscles
Denervation atrophy of the masticator muscles occurs as a result of perineural tumor infiltration along the mandibular division of the trigeminal nerve. In the acute stage, increased volume, increased signal intensity at T2-weighted images, and increased gadolinium enhancement is present on MR imaging and may mimic a mass lesion on the affected side. In the chronic stage, fatty replacement of the muscle fibers and volume loss become evident on the affected side, and the normal side can be mistaken for a mass lesion by an unaware radiologist .
Masseteric hypertrophy is a relatively rare benign condition characterized by enlargement of the masseter muscles. This is bilateral in approximately half of patients, and most cases exhibit habitual teeth grinding. In unilateral cases with the preservation of soft tissue planes, associated pterygoid and temporalis muscle enlargement is useful to improve diagnostic confidence [1, 2, 3, 4].
Accessory Parotid tissue
Accessory parotid tissue lies superficial to the masseter muscle along Stensen’s duct in approximately 20 % of patients. Asymmetry of the accessory parotid tissue may mimic a masseteric mass on palpation. This is easily distinguished on MRI, which reveals tissue identical in signal to the parotid gland [1, 2, 3].
Rabdomyolysis of muscles
Rhabdomyolysis is defined as a disintegration of skeletal muscles that causes myoglobinuria, raised creatine kinase, and renal failure in severe cases. CT scans initially show areas of low attenuation within affected muscles, which may become hyperattenuated in later stages, with or without subsequent atrophy. MR imaging shows hyperintensity on T2-weighted images with avid contrast enhancement and may demonstrate T1 hyperintensity later in the course of the disease in some patients .
Lateral pterygoid muscle changes in temporomandibular joint disease
In temporomandibular joint dysfunction syndrome, increased signal intensity on T2-weighted images can be identified in the superior head of the lateral pterygoid muscle. This change may reflect increased fluid signal intensity related to muscular edema or fatty change, or both, secondary to atrophy .
Lesions extending into the masticator space
The frequency of masticator space involvement in nasopharyngeal carcinoma is 19.7 %. Nasopharyngeal carcinoma demonstrates intermediate to high signal intensity on T2-weighted images, low signal intensity on T1-weighted images, and moderate enhancement on contrast-enhanced images. MR imaging provides excellent visualization of the soft tissue planes of the nasopharynx and is superior to CT for detecting perineural spread of tumor. Lymphadenopathy is present in up to 90 % of patients, with the retropharyngeal chain often being the first involved nodal site  (Fig. 13).
Oral cavity carcinoma
Squamous cell carcinomas originating from the retromolar trigone, tonsillar region, gingival and buccal space can also infiltrate the masticator space. The tumor may spread along the anterior portion of the mandibular ramus with bone destruction, or extend directly into the masticator space through the fat space between the medial pterygoid muscle and the ramus with or without bone destruction. Also, these tumors may spread perineurally through the mandibular deviation of the trigeminal menial nerve. The tumor demonstrates intermediate to high signal intensity on T2-weighted images, low signal intensity on T1-weighted images, and moderate enhancement on contrast-enhanced images [2, 3, 4].
Tumors in the superficial lobe of the parotid gland may spread along the neurovascular bundle into the masticator space via holes in the parotid fascia where the external carotid artery and nerves perforate the fascia and enter the masticator space. Also, a tumor in the superficial portion of the gland depresses the masseter muscle anteriorly. A tumor in the deep portion spreads through the stylomandibular tunnel into the parapharyngeal space and thereafter presses the medial pterygoid muscle [3, 4, 5, 6].
Minor salivary gland tumors
Tumors of minor salivary glands in the parapharyngeal space can often displace/infiltrate the masticator space. Of all the benign tumors, pleomorphic adenoma is the commonest. Pleomorphic adenoma can arise from displaced or aberrant salivary gland tissue within a lymph node in the parapharyngeal space. Pleomorphic adenomas usually show the following MR appearance: hyperintense signal intensity on T2-weighted sequences, related to their myxoid component, and often pronounced enhancement. Presence of an intact fat plane helps in distinguishing benign tumors from malignant ones [1, 2, 3, 4].
Ameloblastoma is the most common odontogenic tumor that may extend into the masticator space. Typical imaging findings are unilocular or multilocular (soap foam or honeycomb) lesions, often accompanied by absorption of the apices of adjacent teeth (40 %). At CT, cystic areas of low attenuation and enhancing solid areas of isoattenuation are seen. MR imaging of ameloblastoma presents several characteristic findings: multilocularity, mixed solid and cystic components, irregularly thickened walls, papillary projections, and marked enhancement of the walls and septa .
Extension of nasal and paranasal sinus malignancy posteriorly by direct extension or perineural spread may result in neoplastic invasion of the masticator space .
Pigmented villonodular synovitis
Pigmented villonodular synovitis frequently arises in the temporomandibular joint that may extend into the masticator space. This lesion has a predilection to hemorrhage, resulting in hemosiderin deposition. As a result, the tumor characteristically appears as an aggressive mass that partially encases the mandibular condyle and demonstrates low signal intensity on both T1- and T2-weighted MR images .
Synovial chondromatosis of the temporomandibular joint is characterized by the development of cartilaginous bodies in the synovial membrane and joint space. At CT, the lesion appears as a mass of water density with calcified loose bodies and bone erosion. MRI may misdiagnose loose bodies but detects joint swelling and accurately defines the boundaries of the lesion. MRI is particularly useful to intracranial extension and internal derangement [1, 2, 3, 4].
Extracranial meningiomas are classified into 4 subgroups: direct extension from primary intracranial meningioma, extracranial growth from cranial nerve sheath arachnoid cells, extracranial extension from embryonic arachnoid rests without connection to the skull base or cranial nerves, and distant metastasis from intracranial tumors. The typical MR imaging features of meningiomas are isointense or hypointense on T1-WI and isointense or hypointense on T2-weighted images and exhibit marked homogeneous contrast enhancement. On T2-weighted images, fibroblastic and transitional meningiomas are isointense to hypointense, but angiomatous and meningothelial meningiomas are usually hyperintense .
Response to therapy and tumor recurrence
Osteoradionecrosis of the mandible
Osteoradionecrosis of the mandible may occur after irradiation. Imaging findings include areas of osteolysis and mixed sclerosis within the irradiation portal. Fragmentation and sloughing of necrotic bone may also be found. The adjacent masticator muscles show abnormal hyper intensity on T2-weighted images, intense diffuse enhancement, and mass-like thickening that may mimic tumor recurrence or osteomyelitis. CT in these patients reveals cortical disruption, trabecular disorganization, fragmentation, and possible pathological fractures .
Radiation-induced sarcomas arise 5–10 years in the high-dose field zone after irradiation. Radiation-induced sarcomas have varied histologies, including osteosarcoma, malignant fibrous histiocytoma, chondrosarcoma, and malignant nerve sheath tumors. The presence of a heterogeneous tumor, or rapidly growing large destructive mass that displays different signal intensity from the primary tumor, within the radiation field that occurs after a sufficient latency period should suggest the possibility of a radiation-induced sarcoma. The presence of calcification or ossification points strongly to a diagnosis of radiation-induced sarcoma [42, 54].
Bisphosphonate-related osteonecrosis of the jaw
Bisphosphonate-related osteonecrosis of the jaw is characterized by nonhealing exposed bones of the jaw in patients who have undergone bisphosphonate treatment for bone metastasis and osteoporosis. CT scan shows osteolysis, sclerotic lesions, periosteal reaction, narrowing of the marrow space, and fractures. The lesion typically shows decreased signal intensity on T1-weighted images. T2-weighted images revealed intermediate or slightly increased signal intensity in early disease and increased or decreased signal intensity in later stages of disease. Contrast material-enhanced variable .
In conclusion, imaging plays an essential role in the assessment of the masticator space. Consideration of the various inflammatory, neoplastic or vascular lesions, lesions extending into the space, and recognition of pseudomasses allows an appropriate radiological differential diagnosis to be formulated. Accurate delineation of the extent of pathology in the masticator space is pivotal for directing drainage procedures for infective lesions, surgical intervention, or radiotherapy planning for neoplastic lesions.
Conflict of interest
The author declares that he has no conflict of interest.
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