Figure 2.46. Axial PET of the sublingual gland demonstrating the intense uptake seen in the sublingual glands bilaterally medial to the mandible (photopenic linear regions).
The SLG can be seen by CT and MRI and is similar in appearance to the SMG, although smaller (Sumi et al. 1999a). FDG uptake is less well defined since it is small and closely approximated to adjacent skeletal muscle, but the uptake is moderate.
Occasionally, accessory salivary tissue is found in the SMS along the anterior aspect (anterior to the normal submandibular gland). This is caused by herniation of sublingual gland through defects in the mylohyoid muscle, called a mylohyoid boutonniere, which typically occurs between the anterior and posterior parts of the mylohyoid muscle. The accessory gland may be accompanied by sublingual branches of the facial artery and vein. Although the accessory tissue may mimic a tumor, this should be readily identified as normal since the accessory tissue has the same characteristics on CT and MRI as normal sublingual or submandibular gland (White et al. 2001; Hopp et al. 2004).
MINOR SALIVARY GLANDS
The minor salivary glands are unevenly distributed throughout the upper aerodigestive tract and are submucosal in location. They are more concentrated in the oral mucosa where they inhabit the mucosa of the hard and soft palate, buccal mucosa, floor of mouth, as well as the mucosa of the lips, gingiva, and tongue. They are also found in the pharynx (nasal and oral), sinonasal spaces, larynx, trachea, and bronchi. Functionally, they are either mucinous (predominantly in the palatal mucosa) or mixed seromucinous glands. The serous minor salivary glands are found only on the tongue at the circumvallate papilla. The minor salivary glands do not have large defined ducts but do contain multiple small excretory ducts. MRI of minor salivary glands has been achieved with high‐resolution surface coils of the upper and lower lips. Patients with Sjogren disease had smaller gland area relative to normal, best demonstrated in the upper lip (Sumi et al. 2007).
Pathology of the Salivary Glands
Pathologic states of the salivary glands include tumors (epithelial and non‐epithelial), infections and inflammation, autoimmune diseases, vascular lesion, and non‐salivary tumors.
Of all salivary gland tumors, most (80%) are found in the parotid gland. The submandibular gland contains approximately 10% with the remainder in the sublingual and minor salivary glands. Of all parotid gland tumors, 80% are benign and 20% malignant. About 50% of submandibular gland tumors are benign and most of sublingual gland tumors are malignant. About 50% of minor salivary gland tumors are benign. The smaller the gland, the more likely that a mass within it is malignant. The pleomorphic adenoma and papillary cystadenoma lymphomatosum (Warthin tumor) account for most of benign salivary tumors, with the former being the more common at about 80% of benign and latter less common at about 15% of benign masses. Most of the malignant salivary gland tumors are represented by mucoepidermoid and adenoid cystic carcinomas.
Malignancies of the parotid gland may result in metastatic involvement of intraparotid and adjacent level II, and III jugular chain lymph nodes. The SMG drains primarily into adjacent level IB lymph nodes and then into the jugular chain and deep cervical nodes. The SLG drains into both level IA and IB nodes and then subsequently into the jugular chain and deep cervical nodes.
VASCULAR LESIONS
Lymphangioma (Cystic Hygroma)
The cystic hygroma is included in this discussion because of its transpatial location and that it might mimic other cystic masses. It is typically multilocular and has an epicenter in the posterior triangle, but may be found in the submandibular space and less commonly in the sublingual space. The imaging characteristics are those of cysts and follow fluid density on CT and signal intensity on MRI, although do typically demonstrate internal architecture from septation with varying thickness. CT typically demonstrates isodensity to simple fluid or slight hyperdensity if infected or contains products of hemorrhage (Koeller et al. 1999; Makariou et al. 2003) (Figure 2.47). US demonstrates anechoic spaces consistent with simple fluid with septa of variable thickness. Like cystic (and few solid lesions) lesions, there is increased thru transmission. Infection and hemorrhage cause variable degrees of echogenicity and thicker septations (Koeller et al. 1999; Makariou et al. 2003). MRI, however, can be variable on both T1 and T2 sequences based on the fluid characteristics. With simple fluid, T1 and T2 are isointense to simple fluid (CSF) but with infection, or hemorrhage products, the increased protein concentration as well as cellular debris and iron from hemoglobin can result in varying degrees of T1 hyperintensity and variable hypo‐ or hyperintensity on T2 (Figure 2.48). Any of these modalities may demonstrate fluid‐fluid or fluid‐debris layers. Both CT and MRI will demonstrate enhancement in the setting of infection (Macdonald et al. 2003). These lesions are more common in the pediatric age group, although small lesions may persist into adulthood. When found in the submandibular or sublingual space, they may be mistaken for a ranula (especially giant or plunging ranulae) and less likely hemangioma or thyroglossal duct cyst if midline (Kurabayashi et al. 2000; Macdonald et al. 2003). Although dermoids are in the differential diagnosis, they are usually identified by their imaging characteristics secondary to their contents of fat and dermal elements. Epidermoid cysts may be more difficult to differentiate from cystic hygromas and ranulae because of similar imaging characteristics (Koeller et al. 1999). Because the lymphangiomas have a vasculolymphatic origin, they may be associated with venous anomalies and rarely saccular venous aneurysms (Makariou et al. 2003). Vascular flow signals may be seen with Doppler US. The venous anomalies or aneurysms may be difficult to differentiate from other vascular malformations; however, their association with typical findings of lymphangiomas may assist in diagnosis.
Figure 2.47. Axial contrast‐enhanced CT of the neck at the level of the submandibular glands demonstrating a low‐density structure on the right of approximately fluid density (compare to the CSF in the spinal canal), which is intermediate in density relative to the muscles and subcutaneous fat. A large lymphangioma associated with the right submandibular gland was diagnosed.
Figure 2.48. Coronal STIR MRI of the face of a different patient with a very large lymphangioma with large septations. Note the lymphangioma fluid is brighter than the CSF and there is fat suppression of the subcutaneous fat.
Hemangioma
Hemangiomas are typically found in the pediatric age group. The majority are of the cavernous type and less likely the capillary type. They are best demonstrated by MRI and show marked enhancement. They are also very bright on T2 MRI. Foci of signal void may be vascular channels or phleboliths (Figures 2.49–2.51). They are typically slow flow lesions and may not be angiographically evident. US can vary from hypoechoic