Ranulas are benign mucus retention cysts of the FOM and submandibular regions that often develop in young people but can also be seen later in life. Because of the absence of true epithelial linings, they are not true cysts and develop as a result of blockage of the sublingual gland ducts (Rivinus duct). The underlying aetiology may be idiopathic or related to previous trauma or inflammation.^6,10

Ranulas are broadly classified into two categories: simple ranulas are confined to the SLS, bordered inferiorly by the mylohyoid muscles (Figure 6); while the plunging (diving) variety of ranula dissects into the inferior submandibular space through the communication posterior to the mylohyoid muscle or dissects through the mylohyoid muscles^9,10 (Figure 7).

Ranulas can present as an intraoral mass or as a cystic, fluctuant, submandibular mass of varying size. The role of imaging is to establish the anatomical location (sublingual or plunging variety), to demonstrate the benign cystic nature of the lesion, to differentiate it from other cystic lesions of the FOM (lymphangioma, epidermoid, dermoid cyst and branchial cleft cysts), to look for integrity of the mylohyoid muscles and above all, for pre-surgical assessment with the aim of complete excision of the sublingual gland.^7,10

Ultrasound is the initial imaging of choice, particularly in the paediatric age group. An uncomplicated ranula appears as a unilocular, anechoic cystic lesion, while complicated or chronic haemorrhagic ranulas demostrate varying degrees of debris and wall-thickening^9,12 (Figure 5). Ultrasound may have a limited role in large plunging ranulas or suspected ranulas with indeterminate features. In evaluating plunging ranulas, CT and MRI can better demonstrate the site of communication between the sublingual and submandibular spaces posterior to the mylohyoid, where the communicating narrowed cystic tract is seen directed anteriorly. This appearance is referred to as the ‘tail sign’, which is diagnostic of plunging ranula^10,13 (Figure 5e and Figure 7c).

The squamous epithelium lined epidermoid and dermoid cysts and cystic teratomas of the neck are rare developmental entities, mostly seen at the FOM as slow-growing midline cysts and constitute only 1.6% to 6.9% of head and neck cystic masses.^14,15,16 Ultrasound, CT and MRI can confirm the cystic nature of the lesion; however, MRI is the most sensitive modality for further characterisation and aids in differentiation from other cystic sublingual, submandibular and submental masses like ranula, cystic hygroma, lymphangioma and also, rarely, abscesses in cases of infected cysts.¹⁵

The ectoderm-lined dermoid cyst demonstrates the diagnostic features of an intracystic cluster of floating fatty corpuscles, known as the ‘sack of marbles’ sign, which on CT appears as a cluster of low attenuating densities and on MRI, demonstrates typical features of fat content.¹⁶

The simple, squamous epithelium lined epidermoid cyst typically appears as a midline FOM, simple, fluid-containing cyst on imaging and can attain a very large size because of slow growth^14,17 (Figure 8). Restriction of diffusion is the most characteristic feature of epidermoids on MRI (Figure 8d), differentiating it from the dermoid cyst.

The most common congenital lesions of the neck are thyroglossal duct cysts, which are rarely located in the FOM.¹⁸ The majority of these cysts are infrahyoid in location, with the suprahyoid site being less common, seen in only 20% of cases. A characteristic thyroglossal cyst appears as a painless, midline cystic mass in the paediatric population, the paramedian location is more common with the infrahyoid cysts. Ultrasound is the initial imaging of choice, which will show a simple, uncomplicated cyst as an anechoic, thin-walled cyst without internal solid components or vascularity. However, a more complex appearance on ultrasound would be seen in the presence of infection, haemorrhagic contents, or the rare coexistence of thyroid carcinoma within the cyst. Further imaging with CT or MR imaging would be essential to characterise more complex lesions, to demonstrate the extent of the deeper, larger lesions and for pre-surgical planning.

Acute infection of various neck spaces, including the FOM, is often associated with significant morbidity and mortality, particularly in elderly, diabetic and immunocompromised patients. The major underlying factors for acute FOM infections (cellulitis and abscesses) are related to salivary ductal calculus disease or odontogenic causes.⁸ The submandibular (Wharton) duct is a common site for intraductal calculi with the potential of causing post-obstructive suppurative adenitis and abscesses (Figure 9). Through the free anatomical communication posteriorly, cellulitis from an infected submandibular gland carries the risk of extension into the SLS and vice versa.^8,11

The FOM is also involved in other acute infections of the neck spaces, like Ludwig’s angina, which is a serious progressive acute bacterial infection, originating commonly from mandibular molar tooth infection. This disease entity is more common in diabetics and immunocompromised patients, and the infection can progress rapidly, causing airway compromise. Imaging demonstrates extensive cellulitis, predominantly involving the submandibular, submental and SLSs with a lesser likelihood of underlying abscess formation,^11,19 as demonstrated in Figure 10.

Ultrasound is helpful for the assessment of superficial abscesses in the submandibular region, nodal disease and duct calculi. In acute settings, contrast-enhanced CT neck (from the base of the skull to the superior mediastinum) is the imaging modality of choice for confirmation and localisation of the abscesses, which appear as rim-enhancing fluid collections (Figure 11), pre-surgical planning, assessment of the patency of the major neck vessels and evaluation of the bony mandible for osteomyelitis.^11,19,20 Recent studies have, however, also established the role of MRI in deep neck infection as a more accurate imaging modality because of its high soft tissue resolution.²⁰ The major limitation is the availability of MR imaging in emergency settings.

Squamous cell carcinoma is the most common malignancy of the oral cavity (more than 90%), most frequently involving the oral tongue and mucosal lip, followed by the floor of mouth (17% of oral cavity SCC). Other malignant conditions affecting the FOM are the minor salivary gland tumours (mucoepidermoid carcinoma, low-grade adenocarcinoma and adenoid cystic carcinoma), lymphoma and rarely, sarcomas.^1,2,21,22

Malignancies of the mandible and submandibular gland can also invade the FOM,^1,21 as demonstrated in Figure 12 (T4 SCC mandible invading the FOM) and Figure 13 (SCC of submandibular gland infiltrating the FOM). Malignancies of the FOM spread further by invading the muscles of the FOM, the mandible, the musculature of tongue, the submucosal spaces and extending along the neurovascular structures in the SLS.

CT and MRI are complementary imaging techniques to characterise and evaluate the extent of disease. MRI is an excellent technique and imaging of choice, with particular reference to the assessment of the depth of tumour invasion, which is a crucial prognostic marker for T-staging of oral cavity cancers.^23,24 MRI is also very sensitive for detecting early bone marrow infiltration and establishing perineural spread of disease, which is particularly a concern for adenoid cystic cancers.^22,23,25

According to the revised T-staging guidelines of the American Joint Committee on Cancer (AJCC) 2018,²⁶ depth of invasion and tumour size are the criteria for T1 to T3 staging, full-thickness osseous destruction of the mandible and maxilla for T4a staging and invasion of the masticator spaces and skull base, and encasement of the internal carotid arteries for T4b staging (Box 1).²⁶ Imaging of the superficial, smaller, mucosal T1 tumours poses diagnostic challenges where clinical inspection and biopsy are superior in assessment. However, MRI remains excellent in establishing spread to the tongue and FOM musculature, ductal obstruction and bone marrow invasion.

On imaging, SCC tumours typically appear as poorly defined, heterogeneously enhancing masses, most likely with cystic and necrotic components (Figure 14, Figure 15 and Figure 16). Figure 13 demonstrates a rare case of primary SCC of the submandibular gland with invasion of the muscles of the FOM and bony invasion of the mandible. Patients can present with features of recurrent sialadenitis secondary to submandibular duct obstruction at the FOM (Figure 16 and Figure 17). The most common condition mimicking malignancies of the FOM is infection of the oral cavity with associated cellulitis or abscess.^21,22,25

Lymphomas contribute 14% of head and neck malignancies and the vast majority are non-Hodgkin lymphomas (96%). Lymphoma of the oral cavity is generally uncommon and constitutes only 5% of oral malignancies. Primary non-Hodgkin lymphoma of the mandible is extremely rare, accounting for only 0.6% of extranodal lymphomas and 8% of mandibular malignancies.^27,28

At imaging, these rare FOM lymphomas present as characteristically bulky, homogeneous soft tissue masses with a varying degree of significant contrast enhancement and pronounced restriction of diffusion. The osseous abnormalities can appear as ill-defined lytic destruction or an infiltrative pattern with invasion and widening of the inferior alveolar nerve canal. Typical radiological findings are presented in Figure 18.