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Cone beam computed sialography of sialoliths

Published Online:https://doi.org/10.1259/dmfr/90784441

Abstract

Cone beam CT is an emerging imaging modality used in maxillofacial imaging. This paper describes the use of cone beam sialography in two patients with salivary gland obstruction. In both cases, the obstruction was identified. The dose is comparable with conventional fluoroscopic techniques and has the advantage that the data may be viewed in a series of fine slices and in three dimensions, which may help in diagnosis and further management of the patient.

Case reports

Case 1

A 59-year-old fit and well female was referred to the dental radiology department by her general medical practitioner for a right submandibular sialogram. She complained of intermittent swelling of the right submandibular gland. Clinically, a small yellow nodule could be identified in the anterior right floor of the mouth consistent with a salivary calculus (Figure 1). Plain films confirmed a round 2 mm radiopacity in the floor of the mouth. A sialogram was carried out to identify any further obstructions within the duct. A hand injection technique was employed using Omnipaque 300 as the contrast medium. Once the gland was full, the patient was positioned in the Classic i-CAT (Imaging Sciences International, Hatfield, PA) cone beam CT (CBCT) unit and a 5 cm height, 20 s, 0.4 mm voxel scan was performed of the submandibular region. The data were examined using the Xoran Software supplied with the unit (Imaging Sciences International). Although there had been some reflux of contrast into the floor of mouth, the 2 mm calculus at the distal submandibular duct was easily identified, although this would have been hard to distinguish if conventional sialographic imaging had been employed (Figure 2). In addition there was some narrowing of the proximal third of the duct. The DICOM data were then exported and a three-dimensional (3D) reconstruction was made using Voxar 3D (Barco, Edinburgh, UK). This reconstruction confirmed the narrowing of the duct proximally (Figure 3). In addition, a horizontal right-angle bend of the duct at the distal third of the duct just distal to the genu was identified. No further calculi were identified in the duct system, so immediately after the investigation the calculus at the distal part of the duct was removed under local anaesthesia (Figure 4).

Case 2

A 54-year-old woman presented with a 1 year history of regular meal-time-related swelling of the right parotid gland and a prolonged history of occasional discomfort since her early twenties. Plain films showed no abnormality, but ultrasound suggested an obstruction of the main duct because dilatation of the hilar branches of the ductal system could be seen. A sialogram was performed using 1 ml of Urografin 370 contrast media, which was injected by hand. CBCT was used to image the filling phase of the investigation, using a Next Generation i-CAT (Imaging Sciences International) CBCT unit. A 10 cm height, 20 s, 0.4 mm voxel scan of the parotid region was performed. Images clearly demonstrated a point stricture at the entry to the hilum of the parotid gland, with marked proximal dilatation of the hilum and primary branches (Figure 5). A small filling defect was seen within a branch arising superiorly from the hilum, thought to represent a small calculus. This stone was, however, only demonstrated once the CBCT data were displayed in i-CAT VisionQ (Imaging Sciences International) in very fine para-sagittal slices, representing a significant advantage of the CBCT scan over conventional sialographic imaging (Figure 6). The ultrasound investigation was therefore repeated immediately and demonstrated a very small and mobile calculus with a 2.9 mm diameter that showed only the slightest of acoustic shadows, suggesting a very lightly calcified stone that had not been observed on initial ultrasound examination (Figure 7). Arrangements were made to perform a radiologically guided balloon dilatation of the duct stricture and Dormia basket extraction of the stone. This procedure is performed under local anaesthesia and fluoroscopic radiological guidance. A balloon catheter is inserted periductally into the region of the strictured parotid duct and the balloon dilated to stretch periductal fibrous tissue and relieve the stenosis. A Dormia basket may then be passed down the duct to capture and retrieve small calculi. Details of these procedures have been described previously.1

Discussion

Salivary gland obstruction has been diagnosed by a variety of methods including plain films, sialography, ultrasound, CT and MRI.27 Sialography is an excellent modality for demonstrating duct anatomy and the presence of stones and strictures.810 The added advantage of sialography is that an assessment of the suitability of the ductal system for interventional sialographic procedures can be made and that radiolucent stones may also be identified.1, 11

Conventional CT has been used for the detection of salivary calculi and, although more sensitive than plain films in the detection of salivary calcifications, would not be used routinely in the UK on a patient with suspected obstruction. Conventional CT sialography involved the introduction of contrast media into the salivary duct followed by immediate imaging while supine within the scanner. This became popular in the 1980s and was used mainly in the assessment of salivary masses,1215 although with the introduction of MRI its use has fallen into decline. In a case reported by Szolar16 et al, 3D reconstructions of CT data of the ductal system were shown to be superior to sialography in demonstrating duct strictures.

CBCT sialography has not been previously reported. Its use in these two cases explored the advantages that CBCT might be able to offer in sialographic imaging, above those normally available in traditional sialography, so that an informed assessment of benefit against risk might be made. In each of our cases the CBCT scan identified the main duct and the primary intraglandular ducts and the obstruction. A scan parameter using a voxel size of 0.4 mm was chosen; therefore, high contrast objects of 0.8 mm or greater should be detected. Decreasing the voxel size will enable smaller filling defects to be identified, but the dose will be increased proportionately. In the first case, the 2 mm calculus surrounded by contrast was easily identified in the scan.

Prior to the patient being scanned, contrast containing 300 mg of iodine per millilitre had been introduced into 22G and 27G sialography catheters. These were placed in a head phantom and radiographed on the CBCT unit to check that the radiopacity of the solution was suitable. The catheters were demonstrated well with no artefact. In the cases shown, this degree of radiopacity of the solution did cause a small amount of artefact around the main duct and a similar appearance was evident on the second CBCT scan of the parotid gland, which used a higher iodine concentration (370 mg). Perhaps for future patients a lower concentration of iodine within the contrast medium should be used for sialographic procedures (180 mg or 240 mg).

When CBCT is undertaken, the patient's position is secured in the unit using the chin-cup. However, this does not stabilize the soft tissues and so it is important that the patient be instructed not to swallow during the rotation. This could lead to a marked artefact in submandibular sialography that could render the scan undiagnostic.

A conventional submandibular sialogram in our clinical department comprises two occlusal views and four oblique lateral views. A conventional parotid sialogram would normally comprise four oblique lateral views, assuming that each was diagnostic and no repeat exposures were required. The exposure factors for the oblique lateral views are similar to those of a periapical view, so a conservative dose estimate for this procedure is between 16 µSv and 24 µSv. CBCT of the mandible using the Classic i-CAT with scan parameters similar to those used in our investigations is calculated as 96–134 µSv.17 A fluoroscopic sialographic examination with digital subtraction can be employed when conventional plain film sialography fails to image a complex ductal area adequately, and is used during interventional sialography. This would be expected to deliver between 34 µSv and 113 µSv (personal communication with Dr R Ngu). The CBCT sialogram therefore gives an exposure similar to fluoroscopic sialographic procedures, as performed in C-arm fluoroscopy X-ray units, but higher than an examination performed with plain or static images.

The advantage of CBCT sialography over conventional sialography is that 3D reconstruction can be performed and then viewed from any direction and in any slice thickness, and from which cross-sectional slices may be obtained in any direction. This may prove useful for demonstrating areas of complex anatomy, such as the route of the submandibular duct over the posterior free margin of the mylohyoid muscle, the paths of the ducts to the deep pole of the parotid gland or the route of the anterior parotid duct over the anterior margin of masseter muscle. It may also be useful for planning interventional procedures such as basket retrieval of stones and balloon dilation of strictures. In the first case presented here, the sharp and sigmoid-shaped bend in the posterior third of the submandibular duct would not be appreciated fully on an oblique lateral view. This “genu” region of the submandibular duct is a frequent site for the impaction of submandibular calculi and is often difficult to image adequately. In addition, by using the Xoran and i-CAT VisionQ software, it is possible to scroll through images of the main ducts to identify calculi that may otherwise be obscured by contrast, as was proved in the second case where a small radiolucent parotid stone was present and only revealed by fine slices through the superior branch of the parotid gland. This technique may also prove useful in trauma cases when the surgeon needs to know the location of a transected duct.

Indications and justification for the use of CBCT imaging in sialography are when plain film sialography is suspected to be or has been found to be inadequate in imaging the more complex cases of salivary duct obstruction, as in the use of fluoroscopic digital and digital subtraction sialography which delivers similar radiation doses. Selection criteria for CBCT are required and the role in sialography would need to reflect the fact that this method of imaging delivers a slightly higher radiation exposure and should therefore be reserved for more difficult cases.

Figure 1
Figure 1

Clinical photograph showing the calculus, visible in the right floor of mouth

Figure 2
Figure 2

Distal part of the submandibular duct showing the filling defect. (a) Axial view and (b) coronal view

Figure 3
Figure 3

Three-dimensional reconstructions showing the narrowing of the submandibular duct proximally and the horizontal right angle bend just distal to the genu. (a) Posterior view and (b) medial view

Figure 4
Figure 4

Photograph of the calculus immediately after retrieval

Figure 5
Figure 5

Parotid sialogram showing that the calculus is not visible in superior branch prior to fine slicing

Figure 6
Figure 6

Fine parasagittal slices through the duct demonstrating a calculus (arrow) in the dilated superior duct

Figure 7
Figure 7

Ultrasound scan of the left parotid gland showing the stone (arrow) within a dilated duct

References

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Volume 38, Issue 5July 2009
Pages: 247-306

© The British Institute of Radiology


History

  • ReceivedMarch 22,2008
  • RevisedMay 09,2008
  • AcceptedMay 16,2008
  • Published onlineJanuary 28,2014

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