Role of imaging in salivary gland disorders
Imaging modalities available to assess salivary glands include:
1. Plain film radiography
3. CT scan
5. Diagnostic ultrasound
6. Nuclear scintigraphy
These imaging modalities play an important role in evaluating a patient with pain, swelling, or other symptoms related to possible salivary gland disorders. Imaging helps in differentiating lesions of salivary glands from those of parapharyngeal space, masticator space, and mandible, submandibular and submental spaces. In addition to localizing the lesion, it also aids in determining the extent of disease, involvement of skull base, mandible, and neural spread in case of malignant lesions.
Plain film radiographs: were used to determine salivary gland abnormalities before the development of more sophisticated imaging techniques. It is the least sensitive of all the imaging modalities. It is least expensive and readily available. It will readily demonstrate calculi. It also helps in differentiating bony / cartilaginous lesions that may mimic salivary gland pathology.
Salivary glands should be imaged in multiple planes. Attempt should be made to isolate calcification / overlying calculi from bony mandible. Plain film imaging is useful in evaluating calculi, detecting calcification in hemangiomas, and evaluation of lymph nodes.
Plain film imaging of parotid region should be performed with the patient positioned with extended chin, open mouth posterioanterior view with cheek blown out. This method will help in imaging stenson’s duct. In viewing submandibular region an extended chin, open mouthed lateral view is obtained. The patient is instructed to depress the tongue with his finger.
It was first performed by Arcelin in 1913. He used bismuth as the contrast media. He demonstrated submandibular gland calculus using this imaging technique. Before the advent of modern imaging modalities, Sialography was the mainstay in diagnosing inflammatory and neoplastic lesions of salivary glands. Because of their greater sensitivity CT / MRI have replaced Sialography in the evaluation of salivary gland. Sialography will give some indication about whether a mass is intrinsic / extrinsic to the gland.
Digital subtraction Sialography is still a favoured imaging modality in detection of sialolithiasis within Stensen and Wharton’s ducts.
Sialography is contraindicated in cases of acute infections of salivary glands.
This test is performed by injecting oil / water soluble iodine containing contrast solution into the Stenson’s / Wharton’s duct. Water soluble diatrizoate meglumine and iodipimide meglumine are preferred because of their low viscosity and ease of injection. Water soluble agents also cause very little foreign body reaction. This imaging modality is very rarely used in sublingual imaging because the ducts are numerous and small. They open directly to the floor of the mouth. Sublingual Sialography is possible only in patients with anatomic variations like filling up of Bartholin’s duct from injection of Wharton’s duct during submandibular Sialography.
Since about 80% of salivary gland calculi are radio opaque, plain film study is a must before Sialography.
Stenson’s duct / Wharton’s duct is progressively dilated gently. Infiltration of local anesthetic agent may be necessary. Sialographic canula is connected to a contrast laden syringe and is inserted into the duct. After fluoroscopically verifying the position of the canula, the contrast material is slowly injected. Irregular contrast pooling and ductal obstruction are considered features of malignancy.
CT scan: This has revolutionized salivary gland imaging. It provides precise anatomical detail and is routinely used as a primary imaging modality in salivary gland lesions. It is very useful in evaluating salivary gland calculi and associated adnexal inflammation. It is inferior to Sialography in studying the ductal system.
Parotid gland CT density is highly variable. It tends to be intermediate between fat and muscle. The submandibular glands have less fat than the parotids. Their density is closer to that of muscle.
CT of parotid gland is obtained by using thin section slices in axial and rarely coronal images. This is usually performed after intravenous injection of contrast solution. Mild enhancement of parotid gland takes place after contrast injection. Images are usually filmed with conventional soft tissue windows. Bone windows are useful for evaluating calcification, calculi formation, or adjacent bone involvement.
MRI: Usually is complementary to CT because of its excellent soft tissue delineation capability. It is better than CT in differentiating parotid from extra parotid masses.
Ultrasonography: High resolution Ultrasonography (7.5 – 10 MHz) of parotid and submandibular salivary glands is actually a quick and non invasive method for assessing these glands. It helps in the assessment of vascularity of the gland, whether the lesion is solid or cystic. It also helps in accurately guiding the FNAC needle to perform biopsy from the suspected lesion.
It fails to demonstrate the parotid gland in its entirety because of intervening mandible. It also does not clearly demonstrate the intraglandular facial nerve branches.
Color Doppler studies of salivary glands indirectly identifies malignant lesions of salivary glands as these lesions boast high vascularity in comparison with their benign counterparts.
Radionuclide imaging: Is rarely used to evaluate salivary glands. Sodium pertechnetate (technetium 99m) is the commonly used element. This isotope is concentrated and excreted by salivary glands. Since most salivary gland tumors do not accumulate the radionuclide, a tumor mass will appear as a filling defect on this scan. Tumors like warthin’s readily take up pertechnetate resulting in formation of “Hot spots”.
Gallium 67 citrate is useful for studying inflammatory / neoplastic disease of salivary glands and adjacent areas. Since Gallium 67 is actively taken up by actively dividing cells, excessive gallium accumulation is seen in inflammatory and neoplastic processes like sarcoidosis / lymphoma etc.