9.1 Introduction

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 [5].

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.

The parotid duct or Stensen’s duct may be seen in ultrasonographic examinations as a linear and very thin hypoechoic structure that travel towards the upper mandible from the anterior border of the gland and with a slight low course angulation in the masseter muscle [6]. The end section of the duct corresponds to the second molar and can be visualized with a theoretical line from the inferior part of the tragus to the midsection between the nose and upper lip. (Fig. 9.1).

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.

During ultrasonographic examinations in transverse sections, the gland is depicted with triangular shape. Its crypts are depicted as slightly sub-echoic areas, while the main duct is partially seen inside the glandular parenchyma. (Fig. 9.2).

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

9.2.1 Parotitis

Parotitis which as the name implies refers to glandular inflammation is the most common cause of parotid swelling in developed countries. The most frequent symptom includes noncontinuous pain and fever as well as unilateral or bilateral swelling of the glandular area. The inflammation is limited only to the area of the parotid gland, without involvement of the submandibular or sublingual glands [7]. It is of unknown etiology in a number of cases and the differential diagnosis include mumps or the suppurative parotitis, which is easily excluded from the clinical symptoms (in the case of mumps the glands are involved bilaterally and the skin lesions are prior to the glandular swelling, while in suppurative parotitis, there is unusual secretion from the glandular duct). The majority of cases involve children while there is episode reduction as the children grow, and they cease near puberty or in late adolescence [7, 8]. The male sex is more frequently affected. For the cases of parotitis in the past, sialography was the prime modality for glandular imaging by showing punctate or globular areas of sialectasis. Ultrasound is now the favored imaging approach. Most sonograms of parotitis show the characteristic enlarged parotid glands with multiple round, hypoechoic areas measuring 2–4 mm in diameter, likely representing peripheral sialectasis and lymphocytic infiltration [9–11]. The vascularity of the glands may increase secondary to the inflammation process. (Fig. 9.3, 9.4, 9.5 and 9.6).

9.2.2 Other Inflammatory Conditions

Chronic sialadenitis may affect all the major glands and is caused by inflammation that is not treated and rests, leading to alterations of the acini and secretory function of the glands. It is attributed to bacterial or nonbacterial inflammations. Clinically, the patients present with swelling and pain. Causes usually include granulomatous conditions such as actinomycosis and histoplasmosis. When of granulomatous etiology, they may appear with less inflammatory image features at ultrasound, and demonstrate a hypoechoic mass with poorly refined margins. Bacterial gland infection is more common, usually presenting at childhood with more common ages from 2–4 years of age. In many cases, abscesses are formed and treatment is required (Figs. 9.7 and 9.8). With the use of color Doppler, abscesses present peripheral blood flow or none at all, due to necrotic debris.

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.

9.2.3 Inflammation from Drainage Failure Due to Calcification (Salivary Stone or Sialolithiasis)

The most common reason for inflammation of a single salivary gland is in 80% of cases the presence of a sialolith [7, 12, 13]. Salivary stones form usually within the duct of the glands with more frequent the Wharton’s duct of the submandibular gland. They are hard stone-like structures, that form within the gland. They are made of mineral stones, they are slow in development, and they may block the ductal system of the gland. When this happens, the drainage of the gland is blocked, the patient demonstrates pain and swelling and inflammatory features of the glandular system. With the use of ultrasound, the stone may be readily visible, but in many cases the acoustic shadow of the calculi may be all that is visualized. The use of ultrasound is crucial since it visualizes the glandular parenchyma easily [14, 15] (Figs. 9.9 and 9.10).

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