PLAIN RADIOGRAPHY AND IMAGING OF THE PARANASAL

PLAIN RADIOGRAPHY AND IMAGING OF THE PARANASAL SINUSES

Meher Ursekar
Consultant, Departments of CT and MRI, Bombay Hospital and Medical Research Centre, Mumbai 400020.

Plain radiographs such as Water’s view and the Caldwell view, were once the mainstay of diagnosis of sinus disease, but have now been replaced by high-resolution CT (HRCT) for the evaluation of sinus inflammatory disease. Plain radiographs do not allow adequate evaluation of the ostiomeatal complex or of the sphenoid and ethmoid sinuses, because of overlapping structures. High resolution CT scanning provides excellent bone detail and accurate soft tissue mapping. CT is the modality of choice for imaging inflammatory disease of the sinuses and the ostiomeatal complex. MRI best assesses intracranial complications of inflammatory diseases. For sinonasal tumours, both CT and MRI provide information that is complementary. CT provides bone detail and anatomic landmarks at the skull base, while MRI has the potential to differentiate tumour from adjacent inflammation. Soft tissues are more clearly imaged by MRI and most tissues can be more accurately differentiated as being infected, neoplastic or haemorrhagic, than by CT scans. High resolution CT is used routinely before endoscopic surgery to evaluate the extent of the inflammatory disease and to assess important anatomic landmarks and their variations. Normal appearance and pathology in the PNS is discussed as seen on plain radiography, CT scan and MRI.

Radiographic imaging of the paranasal sinuses (PNS) is achieved through plain radiography and cross sectional techniques such as computed tomography (CT) and magnetic resonance imaging (MRI). All the paranasal sinuses originate as evaginations from the nasal fossae. The frontal sinuses are the only sinuses absent at birth, and they start to develop after the second year of life. The maxillary sinus is the first to form and a rudimentary sinus can always be seen at birth.

PLAIN RADIOGRAPHY

Plain radiographs were once the mainstay to diagnose sinus disease and have now been replaced by high-resolution CT (HRCT) for the evaluation of sinus inflammatory disease. Plain radiographs do not allow adequate evaluation of the ostiomeatal complex or of the sphenoid and ethmoid sinuses, because of overlapping structures.

Technique: Although numerous radiographic views are available for the plain film evaluation of the PNS, there are only four commonly employed views - Caldwell, Water, lateral and base. The routine radiographic work-up must include a cross-table or erect lateral film obtained with a horizontal X-ray beam, which reveals the presence of fluid in the sinuses by the resulting air-fluid level. If on the other hand, the X-ray beam is overhead and the patient prone or supine with the head turned to one side, the fluid will layer along the dependant wall of the sinus, and no air-fluid level will be demonstrated.

The frontal sinuses are evaluated best in the Caldwell and Water’s projections. A properly positioned Caldwell view will show the petrous pyramids in the lower third of the orbits and is the best projection for examining the frontal and ethmoid sinuses in the frontal projection (Fig. 1). Lateral and base (submentovertex) views are used to evaluate the anterior and posterior walls of the frontal sinuses. Unfortunately, both views will permit visualization of only that portion of the sinus wall which is perpendicular to the film plane (parallel to the x-ray beam). Thus, on lateral views only limited portions, like the midsagittal anterior and midsagittal posterior frontal sinus tables are visualized.

fig1: Plain radiograph - Caldwell view: the frontal and ethmoid (arrow) are well demonstrated on this view. There is haziness of the left orbit and maxillary sinus area due to a soft tissue mass overlying these structures (arrowhead)

The frontal sinuses are usually asymmetric in size and the intersinus septum is usually displaced to one or the other side. Despite this, in the normal circumstance the lower insertion of the septum is typically, in the midline. If the lower margin is extremely displaced, an expansile process such as mucocoele should be suspected. Large sinuses have scalloped margins with septations within the sinus. A smooth, ovoid shape and loss of scalloping are indirect evidence of expansile sinus lesions. Finally, downward flattening of the superomedial orbital rim is also suggestive of mucocoele.

The density of the normal frontal sinuses should be similar to that of the superior orbital fissures on the Caldwell view. A hypoplastic sinus has little air compared to the amount of bone, and appears "clouded" even when it is disease free. The mucoperiosteal boundary of the frontal sinus separating it from the frontal bone, normally appears as a "white line". In chronic infection the line becomes sclerotic and is no longer sharp.

The ethmoid sinuses are evaluated best on the Caldwell view (Figure 1). The many ethmoid air cells are superimposed on each other in all radiographic views. Therefore, when only a few ethmoid air cells are opacified while the others are normally pneumatised, the diseased cells may be masked, giving the appearances of a completely normal ethmoid sinus. In these situations, CT may be required .The density of air within the sinus should be compared with the density of air in the inferior turbinates on a Caldwell view. On Water’s view only the most anterior ethmoid cells can be visualized, while the middle and posterior cells are overlapped with the nasal fossae. The ethmoid sinuses may extend laterally into the orbital plate of the frontal bones (supraorbital ethmoid cells). On Caldwell views, the supraorbital cells produce symmetric, curvilinear collections of air just above the orbital margins. As a rule, they appear symmetric, distinguishing them from the frontal sinuses.

The maxillary sinuses. These nearly symmetric sinuses are best visualized on the Water’s view (Fig. 2). A hypoplastic maxillary sinus shows a greater downward slope of the lateral margin of its roof than the normal sinus. This can be mistaken for a blowout fracture of the orbit. Careful attention should be paid to the lateral wall, which is thicker in the case of the hypoplastic sinus. Acquired causes of a small maxillary antrum include fibrous dysplasia, Paget’s, brown tumours and giant cell tumour. In these cases the air space looks asymmetric, and the sinus wall, abnormally textured. Lateral views are used to evaluate the anterior and posterior walls. The posterior wall forms the anterior boundary of the pterygopalatine fossa. The base view also exhibits the posterior wall as a curved structure. The medial wall is best seen on the Caldwell view. On this view, only the inferomedial wall is consistently seen. The important area of the ostiomeatal unit is not well visualized. The anterior antral wall is not well seen on any view. The inferior extension of the sinus and its relationship to the teeth and hard palate is seen well on lateral views. Unerupted molar teeth create a mound like elevation of the floor, which can be mistaken for a fluid level. On a Caldwell view, the foramen rotundum, which contains the maxillary division of the trigeminal nerve, is projected through the superomedial portion of the antrum. The superior orbital fissure can easily be recognized, as well. Normal overlapping soft tissue shadows like the upper lip, or moustache are projected over the sinus floor in Water’s view and can be mistaken for pathology like mucous retention cysts or polyps. Subcutaneous cellulitis with swelling of the cheek produce clouding of the antral air space on Water’s view and can mimic true clouding due to sinus disease.

Fig 2: Plain radiograph -Water view: The maxillary sinuses are well demonstrated .The right sinus is hazy due to sinusitis.

The sphenoid sinuses are deeply located within the face and are difficult to evaluate by routine films. The two sinuses are often asymmetric. The sinuses can show recesses or extensions into the greater wings of the sphenoid, lesser wings and pterygoid processes. Lateral, base and open mouth Water’s views are best for evaluating the sphenoids. The lateral view shows the relationship of the sinus with the sella, the floor of which forms the sphenoid sinus roof.

IMAGING TECHNIQUES

Plain radiographs of even the highest quality, will underestimate the presence and extent of soft tissue disease and bone erosion. When detailed sinus evaluation is required, and especially prior to functional endoscopic sinus surgery (FESS), some form of tomography is required. Before the advent of CT, this was achieved through multidirectional tomography, which provided fine bone detail and gross assessment of soft tissue masses.

High resolution CT scanning provides excellent bone detail and accurate soft tissue mapping. Image degradation from dental fillings remains a problem, but can be partially circumvented by angling the beam. Axial and direct coronal views are obtained with the head in neutral position and hyperextension, respectively. Although it is possible to reconstruct the axial images into coronal and sagittal planes, these reconstructed images lack the fine detail of scans obtained directly. CT is the modality of choice for imaging inflammatory disease of the sinuses and the ostiomeatal complex. Intracranial complications of inflammatory diseases are best assessed by MRI. For sinonasal tumours, both CT and MRI provide information that is complementary. CT provides bone detail and anatomic landmarks at the skull base, while MRI has the potential to differentiate tumour from adjacent inflammation.

Soft tissues are better imaged by MRI and most tissues can be more accurately differentiated as being infected, or neoplastic or haemorrhagic, than by CT scans. The sinuses can be evaluated in axial, coronal, sagittal and oblique planes without changing the position of the patient. The disadvantage is that the examination times are longer than with CT, and 10% of patients reject the examination on account of claustrophobia. MRI also does not depict bone in detail and in fact, bone can be identified only by an absence of MR signal. Air, dystrophic calcification and scar also produce no signal and can be confused. MRI cannot be performed on patients with pacemakers and ferromagnetic vascular clips.

NORMAL CT ANATOMY

FESS is widely used in the treatment of chronic or recurrent sinusitis. The goal of endoscopic surgery is to re-establish mucociliary clearance of the sinuses by limited resections of inflammatory tissue or anatomic obstruction along normal drainage channels of the sinuses. High resolution CT is used routinely before endoscopic surgery to evaluate the extent of the inflammatory disease and to assess important anatomic landmarks and their variations. The extent of surgery is dictated by the extent of disease as determined preoperatively by the CT scan. CT scanning is also useful in the evaluation of postoperative complications after FESS. When a CSF leak is suspected, the CT scan is performed after opacification of the cisterns with contrast material (CT cisternography), to identify the exact location of the leak.

Fig 3: Coronal high resolution Ct scan. Normal ostiomeatal unit on the left side. The uncinate process (long row), maxillary infundibulum (arrowhead) and middle meatus (short arrow) are marked. On theright side, there is encroachment upon the OMU by the bulla ethmoidalis (arrow).

The maxillary, frontal and anterior ethmoidal sinuses drain into the middle meatus. The anterolateral nasal wall structures that make up the drainage system for these sinuses are designated as the ostiomeatal unit (OMU). The important components of the OMU can be seen on coronal CT sections (Fig. 3). There are four important anatomic landmarks that are crossed during FESS. These are the uncinate process, the ethmoidal bulla, basal lamella of the middle turbinate, and the anterior wall of the sphenoid sinus. All of these can be identified on CT images. The posterior ostiomeatal complex includes drainage of the sphenoid and posterior ethmoid sinuses into the superior meatus via the sphenoethmoid recess. The sphenoid sinus ostium is best seen on axial CT sections, as it is situated anterosuperiorly.

ANATOMIC VARIANTS

There is great variation in the anatomy of the sinonasal passages. Some anatomic variations are seen more frequently in patients with chronic sinonasal inflammation. Anatomic variants identifiable on CT images include nasal septal deviation, septal spurs, paradoxical curvature of the middle turbinate towards the lateral sinus wall, balloon like pneumatisation of the middle turbinate (Fig. 4), Haller cells, large ethmoidal bulla (Fig. 3), and abnormal deviation of the uncinate process. Normal variations of anatomy can result in narrowing of the infundibulum or the maxillary ostium (Fig. 3). There are some "dangerous" nor mal variations that should be recognised before FESS, to avoid risk of injury. These include the presence of Onodi cells that pneumatise the medial wall of the optic canal, putting the nerve at risk during posterior ethmoidectomy. A pneumatised anterior clinoid process can also put the nerve at risk. A low placed fovea ethmoidalis may be at risk for injury during surgery, with the consequences of post operative CSF rhinorrhoea.

Fig 4: The right middle turbinate (arrow) is pneumatised (concha bullosa) and encroaches on the maxillary infundibulum ( arrowhead). and ostium are obstructed. The maxillary infundibulum and ostium are obstructed by soft tissue. One of the right anterior ethmoid air cells is opacified.

INFLAMMATORY LESIONS

The imaging findings in sinusitis are nonspecific and must be correlated with the clinical history and examination findings. Air-fluid levels in a sinus suggests acute sinusitis. Fluid levels can be seen after antral lavage and in the setting of trauma.

Mucosal thickening upto 2-3 mm in the nasal cavity and ethmoid sinuses is seen normally. In children under 3 years, sinuses are opaque due to mucosal redundancy. Recurrent bouts of acute sinusitis lead to chronic disease with mucosal hypertrophy, polypoidal thickening, areas of atrophy and fibrosis. Long-standing inflammation can lead to reactive bony sclerosis, which is differentiated at imaging, from osteomyelitis by the lack of rarefaction and sequestra.

Sinonasal secretions contain predominantly water, and therefore appear with low signal intensity on T1-weighted and high signal intensity on T2-weighted MR images (Fig. 5a and 5b). In chronically obstructed sinuses however, T1- and T2 relaxation times are very variable and depend on the amount of protein within the secretions. An increase in proteins upto 25%, leads to progressive T1 shortening, or a high signal on the T1-weighted images. When the secretions contain more than 25% protein, they become extremely viscous and behave like solids, appearing dark on T1 and T2-weighted images, thereby mimicking a normal, aerated sinus. This is a major pitfall of MRI. In addition, fungal infections may produce a dark signal on the T2-weighted images, which is partly due to high protein content, and the paramagnetic effect of fungal hyphae.

Mucous and serous retention cysts are seen as smooth, dome-shaped structures, often at the sinus floor. A solitary polyp and a retention cyst are indistinguishable. The imaging appearances of sinonasal polyposis are typical. The nasal fossae are expanded and filled with polypoid soft tissue masses. Bone erosion, thinning of the ethmoid trabeculae and centrally high density masses surrounded by a peripheral, low density rim are other signs (Figs. 6a and 6b).

Fig: 5A

Fig:5B

T1 - weighted axial MR image ( Fig. 5A).shows inflammatory secretions within the ethmoid and sphenoid sinuses (arrow) that are isointense (or similar in intensity ) to the temporalis muscle (arrowhead). the secretions are very bright (or hyper intense ) on the axial T2- weighted image( Fig. 5B).

Antrochoanal polyp is a solitary polyp which fills the maxillary antrum, completely opacifying it and expanding its walls and the ostium. The polyp extends into the nasal cavity through the widened ostium (Fig. 7).

Fig 6a : Fig 6b:

Coronal CT (Fig. 6A) shows multiple ethmoidal polypi obstructing the ostia of both maxillary sinuses. The trubeculae are thinned out (short arrow). The polyps typically appear hyperdense centrally (long arrow), with a peripheral hypodense rim. The T2- weighted MR image (Fig. 6B)shows hyperintense polypys obstructing the sphenoethmoid recesses (arrow).

fig 7 Coronal CT.An antrochoanal polyp is seen filling the right maxillary sinus (arrow) and extending medially into the nasal cavity

Allergic fungal sinusitis results from colonization of non invasive fungi like aspergillus in immunocompetent individuals. Findings are similar to sinonasal polyposis. In the immunocompromised or debilitated host, invasive forms are more common. Infection is associated with rapid destruction of bony sinus walls, extension into the orbital apex and cavernous sinuses, and into the anterior cheek (Fig. 8a and 8b). Imaging features may mimic that of carcinoma.

Mucocoeles occur when the sinus ostium is obstructed due to inflammatory scarring, trauma or tumour. CT shows a non-enhancing, low attenuation mass expanding the sinus walls (Fig. 9). Their MR signal intensity is highly variable.

COMPLICATED SINUSITIS

Orbital spread of infection from sinusitis may take on many forms. The infection may be limited to the preseptal space or extend into the postseptal space, in which case, more aggressive management is needed. Postseptal cellulitis may involve the extraconal region, retrobulbar region or both.

Intracranial complications are uncommon and include meningitis, epidural abscess, subdural abscess, cerebral abscess and cavernous sinus thrombosis. In general, MRI is superior to CT in the assessment of these complications (Fig. 10).

NEOPLASTIC LESIONS

A large variety of tumours arise in the nose and PNS. These range from benign to malignant, and epithelial to non-epithelial lesions. More common lesions and those with typical imaging features will be described.

Inverted papillomas are almost always unilateral and arise from the lateral nasal cavity wall, near the middle turbinate. They can be locally invasive and are frequently multicentric, and tend to recur after excision

Fig 8A : Fig 8B :

Non contrast coronal (Fig. 8A ) and axial (Fig. 8B ) CT images in a patient with aggresive fungal sinusitis. The hyperdense soft tissue is seen to breach the cribriform plate (arrow in A) and the medial wall of the left orbit (arrow in B)

Carcinomas arise most frequently within the maxillary sinuses. Squamous cell carcinoma accounts for 80% of all PNS malignancies. Other neoplasms include lymphoma, melanoma, plasmacytoma, malignant fibrous histiocytoma and minor salivary gland lesions. Accurate mapping of tumour extent and proper staging necessitate some form of imaging study. Sinonasal tumours may cause obstructive sinusitis by blocking the sinus ostium. Differentiating tumour from inflammatory changes is important and this is achieved best by MRI. MRI is also useful in detecting early intracranial spread (Fig. 11). Apart from these advantages, both CT and MRI play complementary roles in defining the tumour. CT depicts osseous involvement more clearly than MRI. The MR imaging signal intensities of tumours are usually not histologically specific.

Fig 9: A Soft tissue mass is seen filling the left maxillary sinus and expanding its walls (arrow) on this coronal CT image.

Fig10 Complicated sphenoid sinusitis. The intracranial complications are well shown on Gadolinium enhanced, T1-weighted MR images. The meninges along the floor of the temporal fossae are thickened and exhibit intense enhancement, suggesting meningitis (arrow). There is an intracortial abscess within the left temporal lobe (arrowhead).

Fig 11 Tumour (arrowhead) and obstructed secretions (arrow) are easily distinguished on contrast-enhanced MR images. While the tumor enhances intensely, the obstructed secretions do not, and enhancement is limited to the thin, peripheral mucosal rim (double arrow). The intracranial extent of this aesthesioneuroblastoma is well visualized.

Tumour, normal tissue, inflammatory changes and retained secretions can be differentiated with MRI, based on differences in their proton mobility. Tumour appears intermediate in signal intensity on the T1 and T2 weighted images. On Gadolinium enhanced MRI tumour tissue shows diffuse enhancement, except in areas of intratumoral necrosis. The obstructed sinus shows typical peripheral, mucosal enhancement with central low signal areas, differentiating it from tumour. Melanotic melanomas exhibit characteristic MR appearances. They are hyperintense on T1 and hypointense on T2-weighted images. Primary Hodgkin or non Hodgkin lymphoma of the PNS or nose is observed in patients with disseminated lymphoma or AIDS. They appear as bulky masses that may cause expansion, erosion or infiltration.

CONCLUSION

HRCT is the modality of choice for evaluation of inflammatory sinus disease. Analysis of the ostiomeatal complex and of normal variations provides the surgeon with important information, preoperatively. MRI plays an important role in the evaluation of sinonasal tumours, with its ability to differentiate tumour from secretions, and allows optimal assessment of tumour spread, thereby directing appropriate therapy. Plain radiography retains its role as a cheap screening modality, but has several limitations in its ability to depict sinus pathologies accurately.

REFERENCES

Babbel R, Harnsberger HR, Nelson B, et al: Optimization of techniques in screening CT of the sinuses. Am J Neroradiol 1991; 12:849-854.

Dillon WP, Som PM, Fullerton GD: Hypointense MR signal in chronically inspissated sinonasal secretion. Radiology 1990 174:73-78,.

Gullane PJ, Conely J: Carcinoma of the maxillary sinus. J Otolaryngol 1983; 12:141-145.

Hudgins PA: Complications of endoscopic sinus surgery; role of the radiologist in prevention. Radiol Clin North Am 1993; 31:21.

Laine FJ, Smoker WRK: The ostiomeatal unit and endoscopic surgery: Anatomy, variations and imaging findings in inflammatory disease. Am J Roentgenol 1992; 159:849-857.

Mafee MF: Endoscopic sinus surgery: Role of the radiologist. Am J Neuroradiol 1991; 12:855-860.

Saunders SH, Ruff T: Adenocarcinoma of the paranasal sinuses. J Laryngol Otol 1976; 90:157-166.

Som PM, Shapiro MD, Biller HF, et al: Sinonasal tumours and inflammatory tissues: Differentiation with MR imaging. Radiology 1988; 167:803-808.

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fig1:	Plain radiograph - Caldwell view: the frontal and ethmoid (arrow) are well demonstrated on this view. There is haziness of the left orbit and maxillary sinus area due to a soft tissue mass overlying these structures (arrowhead)


Fig 2:	Plain radiograph -Water view: The maxillary sinuses are well demonstrated .The right sinus is hazy due to sinusitis.


Fig 3:	Coronal high resolution Ct scan. Normal ostiomeatal unit on the left side. The uncinate process (long row), maxillary infundibulum (arrowhead) and middle meatus (short arrow) are marked. On theright side, there is encroachment upon the OMU by the bulla ethmoidalis (arrow).


Fig 4:	The right middle turbinate (arrow) is pneumatised (concha bullosa) and encroaches on the maxillary infundibulum ( arrowhead). and ostium are obstructed. The maxillary infundibulum and ostium are obstructed by soft tissue. One of the right anterior ethmoid air cells is opacified.


Fig: 5A	Fig:5B
T1 - weighted axial MR image ( Fig. 5A).shows inflammatory secretions within the ethmoid and sphenoid sinuses (arrow) that are isointense (or similar in intensity ) to the temporalis muscle (arrowhead). the secretions are very bright (or hyper intense ) on the axial T2- weighted image( Fig. 5B).


Fig 6a :	Fig 6b:
Coronal CT (Fig. 6A) shows multiple ethmoidal polypi obstructing the ostia of both maxillary sinuses. The trubeculae are thinned out (short arrow). The polyps typically appear hyperdense centrally (long arrow), with a peripheral hypodense rim. The T2- weighted MR image (Fig. 6B)shows hyperintense polypys obstructing the sphenoethmoid recesses (arrow).


fig 7	Coronal CT.An antrochoanal polyp is seen filling the right maxillary sinus (arrow) and extending medially into the nasal cavity


Fig 8A :	Fig 8B :
Non contrast coronal (Fig. 8A ) and axial (Fig. 8B ) CT images in a patient with aggresive fungal sinusitis. The hyperdense soft tissue is seen to breach the cribriform plate (arrow in A) and the medial wall of the left orbit (arrow in B)


Fig 9:	A Soft tissue mass is seen filling the left maxillary sinus and expanding its walls (arrow) on this coronal CT image.

Fig10	Complicated sphenoid sinusitis. The intracranial complications are well shown on Gadolinium enhanced, T1-weighted MR images. The meninges along the floor of the temporal fossae are thickened and exhibit intense enhancement, suggesting meningitis (arrow). There is an intracortial abscess within the left temporal lobe (arrowhead).

Fig 11	Tumour (arrowhead) and obstructed secretions (arrow) are easily distinguished on contrast-enhanced MR images. While the tumor enhances intensely, the obstructed secretions do not, and enhancement is limited to the thin, peripheral mucosal rim (double arrow). The intracranial extent of this aesthesioneuroblastoma is well visualized.