Nuclear Medicine
CLINICAL DECISION SUPPORT
99mTc-labelled pertechnetate-anion [TcO4]-[]- is actively taken by the epithelial cells of the salivary glands and secreted into the glandular ducts and mouth along with saliva. The anions pertechnetate [TcO4]- and iodine I- probably share the same mechanism for salivary gland uptake and secretion, and Na+/I- symporters are probably the uptake mechanism. It has been shown that only ductal cells in salivary gland have Na+/I- symporters. After i.v. injection, maximum uptake uncorrected for background is after about an ½ hour.
Evaluate response after surgical intervention for the obstructed duct; following other surgical interventions (e.g. symptoms following subtotal parotidectomy, transposition of the duct orifice or surgical denervation to prevent drooling).
All three major salivary glands (parotid, submandibular, and sublingual) can be examined, but typically only the parotids and submandibular glands are included.
The examination is most useful for patients with severe xerostomia, as mild xerostomia is often subjective. Multiple time point salivary gland scintigraphy may be useful to monitor the dysfunction, when it could influence patient’s management.
The method is a functional test and cannot be used for differential diagnosis of aetiology of xerostomia, except for differentiating between parenchymal function loss and outflow problems.
For adults, the recommended administered activity is 37 MBq.
No recommendations are given for paediatric nuclear medicine.
The effective dose per administered activity is [3]: Na[99mTc]TcO4: 13 µSv/MBq
The effective dose for the suggested activity is 0.5 mSv.
Caveat
“Effective Dose” is a protection quantity that provides a dose value related to the probability of health detriment to an adult reference person due to stochastic effects from exposure to low doses of ionizing radiation. It should not be used to quantify the radiation risk for a single individual associated with a particular nuclear medicine examination. It is used to characterize a certain examination in comparison to alternatives, but it should be emphasized that if the actual risk to a certain patient population is to be assessed, it is mandatory to apply risk factors (per mSv) that are appropriate for the gender, the age distribution and the disease state of that population."
Interpretation is based on uptake and secretion after gustatory stimulation with, for example, citric acid. Normal uptake is uniform and symmetric with prompt washout after administration of citric acid. Evaluation can be performed visually (qualitative assessment), however, quantitative assessment is recommended. For visual assessment the salivary gland uptake is compared with background uptake and uptake in the thyroid gland and mouth. Several qualitative scores have been suggested [96,97].
Bilateral decreased uptake and blunted response to administration of citric acid are observed after radiation damage due to radioiodine treatment, in connective tissue disease (in particular Sjögren´s syndrome), after chemotherapy, in elderly persons, or in case of diseases, such as sarcoidosis, amyloidosis, diabetes, primary biliary cirrhosis, chronic pancreatitis and cystic fibrosis. Increased uptake and abnormal secretion of 99mTc pertechnetate is sometimes seen in conditions associated with sialadenosis (e.g. diabetes mellitus, liver cirrhosis and anorexia nervosa). Increased uptake is the result of reduced salivary secretion.
-In acute sialoadenitis intense activity is seen in the glands during the flow (perfusion) phase. The eventual accumulation and excretion are decreased due to swelling of the glands.
Scintigraphy is not useful in demonstrating salivary gland tumours, with the exception of adenolymphomas or Warthin’s tumours. These two tumours demonstrate increased pertechnetate uptake as well as retention (after stimulation) as compared to surrounding salivary gland tissue.
Reference values based on 50 voluntary healthy persons (59):
Percent uptake and uptake rate in each of the parotid glands can be calculated
Ejection fraction for the parotid glands is approximately > 55% and for the submandibular glands >45 % [96–98].
Ratio between each parotid gland and thyroid gland uptake is 0.7-0.9, and the ratio between each submandibular gland and the thyroid gland is 0.6-0.8.
Normal values will depend on the imaging procedure and the population.
The uptake ratio between the salivary glands and the thyroid gland will depend on thyroid function and should not be used in hypo- or hyperthyroid patients. It will also differ between patients living in iodine deficient and iodine supple areas.
Movement of the head during image acquisition may affect the semi- and quantitative analyses.
No EANMMI or SNMMI recommendations exist, and the method must be validated in each individual institution for a given population. A standard γ camera with a general all purpose or a high-resolution collimator should be used. In adults 185-370 MBq Na[99mTc]TcO4 is injected intravenously. Dynamic imaging beginning immediately after injection for about 30 seconds is commonly used for vascular flow information. Ten to 30 min after injection, a second dynamic imaging for about 20 min with orally administered citric acid after about 10 min is performed. Citric acid solution can be administrated through a tubing into the patient´s mouth. An even distribution of the citric acid in the mouth is important for symmetric stimulation. A standardized citric acid solution (e.g. 3 mL lemon juice) is probably better than lemon. The patient should be instructed to hold the lemon juice in the mouth for a standardized time, e.g. 30 seconds, before swallowing. Additional static images can be included, as well. Quantification or semi quantification of uptake and washout after stimulation is recommended.
Other tracers. 123I-, 131I-, 67Ga- and 111In-labelled white blood cells (WBC) have been used to study salivary glands, but these will not be discussed further.
Further information can be found in the published literature [99–101].