Advances in imaging in recent years have improved the sensitivity of imaging diagnosis of diseases that involve the major salivary glands. Imaging with Ultrasound (US), plain radiography and sialography, Magnetic Resonance Imaging (MRI), Computed Tomography (CT), Cone Beam Computed Tomography (CBCT) and nuclear scintigraphy/Positron Emission Tomography (PET) play part on diagnosis and planning of further management, operative or otherwise. This article reviews the Salivary gland diseases; methods used for their imaging and indications for these methods to the principal pathological processes.
It is responsibility of Dental Clinician to detect disorders of the salivary glands that may be presenting with intraoral and extraoral signs and symptoms. Imaging the salivary glands play important role in this task. In recent times the advancements in the imaging modalities have made it relatively feasible to make more accurate diagnosis, when findings of these methods are combined with clinical and histopathological findings.
Disease of salivary gland may arise due to inflammatory process, non-inflammatory process or Space occupying mass. Diseases of Salivary glands present with variety of signs and symptoms including Pain, Swelling, and altered salivation presenting as either xerostomia or Ptylism. Table 1 classifies the diseases of salivary glands. The cause of swelling that appears in major salivary gland lesion can be glandular or extra-glandular. Table 2 & Table 3 represent differential diagnosis of swellings that appear in Parotid and Submandibular salivary gland region .
Imaging of salivary gland is helpful in Detecting site, nature
and extension of lesion; evaluating the ductal morphology,
selecting site of Biopsy and treatment planning. It also helps in
differentiating between diffuse and focal Suppurative lesions.
Depending on pathology and patient factors Selection of
Imaging modality should be done. Starting point of any Salivary
gland pathology imaging is plain radiography. It demonstrates
Sialolith and involvement of adjacent osseous structures. If plain
radiography fails to provide any information next stage of imaging
is Sialography that provides morphologic information of ongoing
intra glandular process. However this condition uses contrast
medium which may be contraindicated in certain conditions and
in these conditions the imaging should be carried out by MRI.
CT and CBCT also provide information of presence and exact
location of Sialolith in relation to gland being imaged. US provide
information that can be helpful in differentiating the nature of
lesion associated with the salivary gland. Scintigraphy can be
used in the functional evaluation of the gland. Sialendoscopy is
comparatively recent advancement that helps with diagnostic as
well as interventional endoscopy of salivary glands.
Plain film imaging helps in demonstrating presence of
Sialolith and involvement of adjacent osseous structures caused
by lesions related (benign or malignant tumor of salivary gland)
or not related (inflammatory or neoplastic process of mandible)
to salivary glands .
Soft tissue radiograph of upper vestibule against maxillary
second molar region using intraoral Periapical film can help locate
Sialolith present in anterior Stenson duct of parotid. Occlusal
radiographs, i.e. Standard mandibular occlusal radiographs and
over the shoulder mandibular occlusal radiographs can be used
to diagnose Sialolith in anterior part of submandibular gland duct
and posterior part of submandibular gland respectively.
Extraoral radiographic methods to locate parotid duct include
panoramic radiograph and Postero-Anterior (PA) radiograph. In
order to avoid superimposition of osseous structure of the region
while taking PA radiograph, patient is asked to puff the cheek.
Methods to detect submandibular gland duct include Panoramic
and Lateral oblique radiograph.
Sialography is method in which contrast medium is injected
in salivary gland and following it the gland is imaged with plain
radiographs or CT/CBCT. Here plain radiograph is made in three
stages. First is scout view which provides baseline image. In second
stage radiograph is made after injecting the contrast medium. Last
stage is emptying phase in which radiograph is made to confirm
the empting of gland. It is contraindicated in patients with acute
inflammation, iodine allergy and in patients who are supposed to
undergo thyroid function test .
MRI is modality of choice for soft tissue imaging. It provides
superior soft tissue contrast than provided by any other imaging
modality. Its benefits include no ionizing radiation and soft tissue
contrast. Imaging method of choice for displaying presence of
salivary gland mass, its internal structure, its nature, its extension,
its perineural spread .
The major salivary glands appear hypodense than surrounding
muscles. Coronal section in contrast enhanced CT is best view in
visualizing of submandibular and sublingual glands. Imaging
with CT helps in detecting acute inflammatory process, abscess,
mucocele, cyst, Sialolith and benign and malignant neoplasms .
Cone Beam computed tomography can be used with or
without sialography to detect presence of Sialolith and to study
the ductal morphology of the lesion. CBCT is superior to CT in
the fact that it provides information of osseous tissue with lesser
radiation exposure and more speed. It, however, cannot provide
any information about the soft tissue .
Ultrasound is inexpensive, easily available, painless, non
invasive and nonionizing modality that differentiated solid tumor
from cystic mass. It is also helpful in detecting presence of Sialolith.
Color dopler USG also helps in detecting nature of lesion based on
its vascularity .
Scintigraphy takes the advantage of selective uptake of
radiopharmaceuticals by salivary gland tissue in various
pathological conditions. Injection of i.v. 99MTc- Pertechnate causes
concentration and excretion of it in glandular structures like
thyroid, salivary glands and mammary glands. Based on presence of increased, decreased or absence of uptake the condition is
diagnosed as inflammatory or neoplastic condition .
Latest edition in imaging modalities of Salivary glands, this
method uses Fiberoptic endoscopic principles in salivary gland. In
this method the sialendoscope is inserted in major salivary gland
duct opening to visualize live ductal and glandular condition. In
addition to diagnose the condition it also helps in intervention
in form of irrigation, medication delivery, lithotripsy and use of
basket to remove the smaller Sialoliths .
Imaging the salivary gland is important aspect of management
of salivary gland disorders. Selection of appropriate imaging
method and accurate interpretation of images will help in correct
diagnosis of condition.