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K. Orhan (ed.)Ultrasonography in Dentomaxillofacial Diagnosticshttps://doi.org/10.1007/978-3-030-62179-7_9
9. Sonographic Anatomy and Pathology Salivary Glands
UltrasonographySalivary glandsOral pathologyElastography
9.1.1 General Sonographic Anatomy
As superficial anatomical structures, the salivary glands are easily accessible to ultrasound. Ultrasound has several advantages that make it the examination of choice in the study of their pathology.
Main advantages are the easy accessibility and the low cost of the method, the safety of technique since radiation is avoided, and the excellent image analysis in real-time with the newer high-frequency transducers used since they provide the details of the superficial structures required for the examination [1, 2].
Although the application of ultrasound is generally accepted in various medical specialties, the value of the method in assessing and examining salivary glands has emerged over the last two decades with the partial abolition of sialography and its replacement by the use of ultrasound as a noninvasive technique that provides real-time information on their function.
The first papers on the application of ultrasonography on salivary glands pathology goes back to the 70s [3, 4].
Regarding the glands, the superior lobe of the parotid is easily assessed with the use of ultrasonography [2, 3]. However, the deeper lobe is not as easy to access due to the intervention of the ramus of the mandible, which blocks the sound waves.
9.1.2 The Parotid Gland
Anatomically the parotid is the largest of the salivary glands and weighs about 25 gm. It is located in the retromandibular area, with skeletal borders the posterior border of the ramus anteriorly, the mastoid process posteriorly, and the temporomandibular joint and external acoustic meatus superiorly. It has the shape of a three-sided pyramid with its tip inside and base outside with three surfaces. In 20% of patients, a nodule of the accessory parotid gland can be identified on the masseter muscle [4–6].
The superior surface includes branches of the facial nerve and superficial intraparotid lymph nodes and more posteriorly the temporomandibular joint.
The anterior surface is bordered by the ramus of the mandible and section of the masseter muscle.
The posterior surface is related to the mastoid process, the styloid process and its muscles, the sternocleidomastoid muscle, and part of the internal carotid artery.
The parotid gland is assessed in both transverse and sagittal image scans except for the retromandibular section of the gland. The normal parotid gland is homogenous in echostructure and demonstrates medium echogenicity. The borders are clearly demonstrated .
Upon ultrasonographic imaging, the tissue layers visible with the probe are the following from the outer surface towards the deeper layers: facial skin, subcutaneous fat, and parotid gland.
Its borders are as follows:
The anterior border separates the upper from the anterior surface and is the border that includes the parotid duct, the distal branches of the facial nerve and the vessels that accompany it.
The posterior border separates the upper from the posterior inner surface and sits on sternocleidomastoid muscle.
The inner border separates the anterior upper surface from the posterior inner surface and is related to the distal part of the pharynx.
Imaging of the parotid should be obtained with a high-frequency transducer due to the superficial structure of the gland. Usually, linear probes of 9–20 MHz are used. The entire gland should be evaluated in two perpendicular planes. The retromandibular vein, which lies directly deep to the facial nerve and lateral to the external carotid artery, is an excellent landmark to separate the superficial and deep lobes.
In the parotid gland, we see branches of the external carotid and the retromandibular vein.
9.1.3 The Submandibular Gland
The submandibular gland is bordered by the mandible laterally and the mylohyoid muscle superiorly and medially. A small portion of the gland may pass posterior to the mylohyoid muscle to lie within the sublingual area. Note should be taken on the anterior portion of the gland since the area contains lymph nodes [5–7].
Upon US examination of the submandibular gland, the ultrasound probe is placed sideways at the floor of the mouth at each side, parallel to the inferior border of the mandible. Regarding the anatomy of the region the following structures should be noted [6, 7]: The gland itself, the crypts and acini and the main duct.
With the applications of modern ultrasonographic techniques available, many authors claim that ultrasound can help identify inflammatory lesions and differentiate between benign and malignant lesions.
To help differentiate the types of lesions the variations in the shape and borders of the lesions as well as the reflected sound echostructure is studied [3, 5, 6].
Benign lesions are usually tumors with well-defined borders and in majority of cases of homogenous density.
Unlike benign tumors, malignant lesions are depicted with ill-defined borders and an inhomogeneous sound absorption pattern. There have been reported rare cases in the first stages of malignancy with low-grade differentiation where the lesions have well-defined borders.
9.2 Inflammatory Disease
9.2.2 Other Inflammatory Conditions
Other inflammatory conditions with similar ultrasonographic features are autoimmune diseases and recurrent sialolithiasis [7–11]. Sjögren’s syndrome is an autoimmune disorder that results in inflammation and destruction of the exocrine glands, primarily the lacrimal and salivary glands. To avoid cumulative radiation to the patient at follow-ups it is preferable to monitor them with ultrasound and not with CT scan.