The Diagnostic Accuracy of Contrast-Enhanced CT of the Neck for the Investigation of Sialolithiasis

Discussion

Sialolithiasis is the most common benign disorder of the salivary glands.¹ Eighty percent to 90% of calculi are found in the submandibular system, due to the more viscous, alkaline saliva and upward drainage of the duct promoting stasis.^13,14 Parotid and sublingual calculi account for only 5%–10% and 0%–5% of cases, respectively.¹⁵ In our study, 70% of the total calculi (35/50) were located in the submandibular system; 38% (19/50), in the submandibular gland and 32% (16/50), in the submandibular duct. This figure is slightly less than rates published in the literature and may be because we only imaged symptomatic patients; those with small calculi may be asymptomatic or those with distal ductal calculi may be clinically managed without imaging. Thirty percent (15/50) of the total calculi were in the parotid system; 24% (12/50), in the parotid gland and 6% (3/50), in the parotid duct. Postmortem studies have indicated that the prevalence of salivary calculi in the population is 1.2%.^10,16,17 The peak incidence occurs from 30 to 60 years of age.⁵ The mean patient age in our study was 50 years, with a range of 25–85 years and a slight female preponderance of 57% (Table), correlating with that in previously published reports.¹⁸ Clinically symptomatic calculi are less frequent; an incidence of 27–59 cases per million population per year was reported in a review of 15 health regions in England between 1991 and 1995, though a higher incidence rate than previously reported was demonstrated in a nationwide population-based study by Schrøder et al,¹⁸ of 14.10 per 100,000 person-years.^4,19,20 The prevalence of salivary calculi in our study was 34%. There was no significant difference in stone prevalence between patient groups referred by different specialties using the χ2 test of proportions (P = .44).

Imaging techniques available for investigation of salivary calculi include conventional radiography, digital subtraction sialography, sonography, CT, and MR sialography. Conventional radiography is dependent on stone composition, and only about 20% of calculi are radiopaque.²¹ The sensitivity of digital subtraction sialography ranges from 96% to 100%, and the specificity, from 88% to 91%.²² It is contraindicated in the acute setting due to the potential exacerbation of symptoms associated with infection.⁹ Sonography has been reported to have a sensitivity ranging from 59.1% to 93.7%, and a specificity, from 86.7% to 100%.^22,23 The sensitivity and specificity of MR imaging sialography have been reported to be 91% and 94%–97%, respectively.¹⁹

CT of the neck with and without contrast is given a rating of 5 by the American College of Radiology, defined as “may be appropriate,” in selected cases if a sialolith is suspected.¹¹ The disadvantage of NCCTN is the lack of detail about potential complications of an obstructing calculus or an alternative diagnosis, such as a salivary tumor.¹² CECTN, while useful for delineation of glandular inflammation or abscess, carries with it the risks associated with intravenous contrast such as allergy, anaphylaxis, extravasation, acute kidney injury, and so forth. Performance of both techniques results in an additional radiation dose to the patient. In our study, a mean reduction in effective dose of 1.6 mSv or a mean relative dose reduction of 42% per case would have been achieved if NCCTN had been omitted from the protocol (using a standard neck conversion factor of 0.0059, assuming use of a head CT dose phantom of 16 cm).

Contrary to previous reports that postulated that contrast may obscure visualization of a calculus,^9,11 there were no false-positives in our study; calculi can be confidently diagnosed on CECTN. This is the first study, to our knowledge, to directly compare CECTN with NCCTN, used as the criterion standard, for the investigation of suspected salivary calculi.

We report a sensitivity of 96% (95% CI, 86.3%–99.5%) and a specificity of 100% (95% CI, 94.1%–100%) for CECTN in the detection of salivary calculi. In this study, CECTN had an accuracy of 98% for the evaluation of salivary calculi. The diagnoses of the 66% of patients who did not have calculi included acute or chronic sialoadenitis (secondary to infective, granulomatous, or autoimmune etiology), ductal strictures, or cervical lymphadenopathy. We identified 3 masses: Fine-needle aspiration was performed in 2 cases, both benign, and the third case was lost to follow-up.

This study raises the question of the utility of the NCCTN before the CECTN if both studies are being performed, given the accuracy of CECTN in the diagnosis of salivary calculi. The introduction of recent technologic advances such as dual-energy CT may obviate performing both CT examinations. Dual-energy CT acquires images at 2 different energy levels simultaneously and uses the attenuation differences at the different energy levels for deriving additional information. Virtual unenhanced images can be generated from each dataset of a postcontrast CT by extracting the attenuation component attributed to the presence of iodine, thereby removing the need for a separate noncontrast CT and, hence, reducing the dose.^24,25

There were no false-positives in our study; all calculi correctly identified on the CECTN were identified on the NCCTN. During this review, we encountered other calcifications in the region of the salivary glands that could be erroneously labeled sialoliths. Entities that radiologists should be aware of that could potentially hamper CT interpretation include calcified lymph nodes (Fig 2A, -B); calcification of the stylohyoid ligament, which can be bilateral (Fig 2C); tonsilloliths (Fig 2D); and phleboliths. Potential false-negatives may be caused by dental amalgam causing metal streak artifacts in the floor of the mouth obscuring a calculus, particularly in the region of the anterior submandibular duct (Fig 3). This is a common location of symptomatic stones that can be easily overlooked if one is not cautious. Radiologists should maximize the basic tools at their disposal, namely manual adjustment of all 3 aforementioned standard window settings. Metal streak artifacts are common and caused by multiple mechanisms (beam-hardening, scatter, noise, undersampling, motion, and conebeam or windmill effects).²⁶ Manufacturers offer an array of techniques to reduce metal streak artifacts, such as filtered back-projection, linear interpolation, selective Algebraic Reconstruction Technique, and the Metal Deletion Technique.²⁷ These techniques were not routinely available at our institution during the study period.

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Fig 2.

Potential false-positives. A and B, Calcified lymph nodes. C, Bilateral calcification of the stylohyoid ligaments (arrows). D, Calcification of the left stylohyoid ligament (arrow) with an adjacent tonsillolith (asterisk) in the left palatine tonsil.

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Fig 3.

Potential false-negative. A, Axial contrast-enhanced CT. Significant streak artifacts in the floor of the mouth due to dental amalgam. B, Single section inferiorly with windows adjusted manually, revealing an obstructing calculus (arrow) in the right anterior submandibular duct. One should be cautious when analyzing the floor of the mouth because this is a classic location for obstructing submandibular duct calculi and can easily be missed.

There were 2 false-negatives in our study as previously mentioned; both were asymptomatic intraglandular calculi within the parotid gland.

There are several limitations to this study. It was retrospective and based on radiologic findings only. There was no surgical or pathologic correlation. The clinical outcome of the patients included is not known; therefore, we did not establish which calculi were clinically relevant.