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imaging of SALIVARY GLAND Sangam Kanekar, MD Assistant Professor Dept of Radiology and Neurology Penn State Milton S Hershey Medical Center Hershey, PA, USA INTRODUCTION There are three 3 major, paired salivary glands: the parotid, the submandibular, and the sublingual glands and multiple (600-1,000) minor salivary glands which line the mouth, palate, lip and rarely found in the nose. Embryology: The major salivary glands develop from the 6th-8th weeks of gestation from the buds from the epithelial lining of the mouth. The parotid develops first, and grow surrounding the VIIth nerve but it is the last to become encapsulated (after the lymphatics develop), resulting in entrapment of lymphatics in the parenchyma of the gland. A mesh of unique glandular stroma consisting of epithelial cells, myoepithelial cells, adipocytes, lymphatic channels, and lymph nodes. Note: The other major salivary glands do NOT have intraparenchymal lymph nodes. The minor salivary glands develop after the major salivary glands and arise from oral ectoderm and nasopharyngeal endoderm. PAROTID GLAND ANATOMY The paired parotid glands (para=around & otid=ear) are the largest of the major salivary glands located in the preauricular region. The deep cervical fascia continues superiorly to form the parotid fascia, which is split into superficial and deep layers to enclose the parotid gland. The thicker superficial fascia is extended superiorly from the masseter and sternocleidomastoid muscles to the zygomatic arch. The deep layer extends to the stylomandibular ligament, which separates the superficial and deep lobes of the parotid gland. Superficial layer of the deep cervical fascia Retromandibular vein PAROTID GLAND ANATOMY Stylomandibular canal deep lobe Superficial lobe deep lobe Gland is divided by the facial nerve into a superficial & deep lobe. The superficial lobe, is lateral to the facial nerve while deep lobe is medial and lies within the parapharyngeal space. Most benign neoplasm are found within the superficial lobe & can be treated with superficial parotidectomy. Tumors arising in the deep lobe of the parotid gland requires wider excision. The parotid gland is bounded superiorly by the zygomatic arch. Inferiorly, the tail of the parotid gland extends down and abuts the anteromedial margin of the sternocleidomastoid muscle. This tail of the parotid gland extends posteriorly over the superior border of the sternocleidomastoid muscle toward the mastoid tip. PAROTID GLAND ANATOMY buccinator muscle parotid duct Accessory parotid glands tumor Stensen’s duct (parotid duct) arises from the anterior border of the parotid and parallels the zygomatic arch. Duct runs superficial to the masseter muscle, then turns sharply medially to pierce the buccinator muscle at the level of the second maxillary molar where it opens onto the oral cavity. Accessory parotid glands are found in approximately 20% of the population and lie over the massester muscle. Histologically they are identical to normal parotid gland tissue. Accessory parotid tissue is susceptible to most salivary pathologies. Between 17% of all parotid neoplasms arise from accessory glands, 50% of which are malignant. SUBMANDIBULAR GLAND ANATOMY The Submandibular gland (SMG) is the second largest salivary gland and lies in the submandibular triangle formed by the anterior and posterior bellies of the digastric muscle and the inferior margin of the mandible. Besides SMG, lymph nodes, facial artery and vein, mylohyoid muscle, and the lingual, hypoglossal, and mylohyoid nerves also lie within the triangle. The middle layer of the deep cervical fascia encloses the submandibular gland. The Submandibular duct (Wharton’s duct) exits the medial surface of the gland and runs between the mylohyoid (lateral) and hyoglossus muscles and on to the genioglossus muscle. SUBLINGUAL GLAND ANATOMY SLG is almond shape and is the smallest of the major salivary glands. It lies just deep to the floor of mouth mucosa between the mandible and Genioglossus muscle and bounded inferiorly by the Mylohyoid muscle. Unlike the Parotid and SMG, the SLG has no true fascial capsule. Gland lacks a single dominant duct, instead, it is drained by approximately 10 small ducts (the Ducts of Rivinus), which either secrete directly into the floor of mouth, or empty into Bartholin’s duct that then continues into Wharton’s duct. SL & SM SPACES ANATOMY Fig SLS and SMS anatomy. Mylohyoid muscle (yellow arrow), Submandibular gland in submandibular space (green arrow), sublingual space (red arrow). SUBLINGUAL SPACE (SLS): is inferomedial to the myelohyoid muscle in the oral cavity. There is no true fascial lining of SLS. No fascia separates the posterior SLS from SMS as the result lesions of the SLS readily spread to the SMS. SUBMANDIBULAR SPACE (SMS) is located inferolateral to the mylohyoid muscle and superior to the hyoid bone. The superficial layer of deep cervical fascia split to encircle the SMS. SMS is a horseshoe shape space found between the hyoid bone below and the mylohyoid muscles sling above. Posteriorly, SMS runs into parapharyngeal space and posterior portion of SLS. HYPOPLASIA of GLAND absence of left SMG absence of left parotid gland Hypoplasia or absence of the major salivary glands with or without the absence of Stensen’s duct has been documented and may be partial or total. It is not usually associated with accessory or ectopic salivary tissue. Familial absence of these glands have also been documented. Lacrimal apparatus malformations (agenesis, aplasia) may be associated, this combination is known as lacrimo-auriclo-dentodigital (LADD). Total parotid agenesis may lead to reduction in salivary flow and may lead to dental caries and infection. Note: The submandibular, sublingual and minor salivary glands contribute most significantly to the total resting salivary flow. Therefore unilateral or partial parotid agenesis may not be noticed by the patient. IMAGING OF THE SALIVARY GLANDS CT, MR, and ultrasound are the imaging modalities of choice with respect to the salivary glands. General rule: CECT for inflammatory diseases and MR for the evaluation of tumors. Ultrasound is very useful for inflammatory or superficial disease. Sialograms (rarely performed today) used to be the mainstay of major salivary gland imaging, and they remain the most detailed way to image the ductal system. Healthy parotid gland is fatty with numerous thin strand-like structures interlacing through it. In children and some adults, a clinically normal parotid can have less fat content, making it difficult to distinguish healthy from diseased parotid. Normally, the facial nerve is NOT imaged. USG CT MR LESIONS NON-NEOPLASTIC Infective Sialolithiasis Acute Bacterial Viral Chronic TB Sarcoid NEOPLASTIC Trauma RT Autoimmune Sialosis Cystic lesions Hyperlipidemia Congenital Acquired Vascular Hemangiomas infantile hemangioma, rapidly involuting congenital hemangioma (RICH), non-involuting congenital hemangioma (NICH). Vascular malformations Venous Capillary (port-wine stains) Lymphatic (cystic hygromas) Arteriovenous Mixed Venous-lymphatic Venous venular INFECTION Bacterial infection Fig Bacterial sialadenitis. CECT, axial and coronal images show diffusely enlarged left SM gland with diffuse inflammation in the SM space and enhancement of the duct. Bacterial infection: Most of this infection ascend from the oral cavity related to the decrease in the salivary flow due to various reasons (trauma, surgery, radiation). Ascending infections are more common in parotid than SMG because: Stenson’s duct orifice is larger then Whartson’s, SMG secretions are mucinous and are bacteriostatic. Eosinophic mucus plug or food, seed particles are seen at the orifice. Most common organism is S.Aureus followed by S.viridans, H. influenzae and E. colli. INFECTION Bacterial infection Fig: CECT: axial & coronal images show large Fig: coronal and sagittal CT reformatted images abscess. show enlarged SMG with abscess in the lateral aspect of the gland. In severe infections liquefactions have to be excluded, especially in diabetic and HIV patients. SMG abscess are more common than parotid due to obstructive stone in SMG. S. Aureus are the commonest type of pathogen and are responsible 40% of abscess. In gray-scale sonography meticulous investigations are necessary in order to visualize moving debris in an abscess. CT may show enlarged gland with hypodense collection which may show peripheral enhancement. An abscess can be drained under US guidance INFECTION Viral infection Fig: Left viral parotitis: Left parotid is inflammed and enlarged and shows mild enhancement on post contrast. Fig: Bilateral viral parotitis: CECT shows bilateral enlargemnt of gland and moderate enhancement. Clinically glands were painful bilaterally. The most common cause of viral parotitis is mumps by RNA virus. It mainly involves the parotid gland and rarely SM or SL glands. The disease is characterized by acute painful swelling of the involved gland. Saliva is infective during the acute phase. Imaging findings are non-specific. The gland is usually enlarged and has a slight greater CT attenuation than normal. On MR gland tends to have a slight higher signal intensity then normal. HIV Sialopathy Fig: Lymphoepithelial cyst in HIV patient. INFECTION Fig: Uunilateral lymphoepithelial cyst in HIV patient. Parotid enlargement is seen in around 5% of HIV positive patients and most of these are due to BLEL. BLEL are frequently multiple; bilateral or unilateral; cystic, partial cystic, or solid. The cystic lesions are not entirely simple and contain internal echoes and septations. Occasionally, HIV causes a large solitary unilateral cyst with internal echoes. The lack of internal color Doppler flow and FNA confirm the cystic nature of these lesions. The CT and MR features of BELL are nonspecific. Solid BHEL cannot be radiologically differentiated from benign or malignant tumors. Partially cystic lesions mimic Warthin’s tumor.