CT sinus

Indication/Technique

Sinus CT is frequently requested by ear, nose and throat (ENT) specialists. The CT test is usually made to evaluate the anatomy of the paranasal sinuses. Information about the sinus anatomy of individual patients is essential prior to a FESS procedure (functional endoscopic sinus surgery).  Indications for a FESS procedure include chronic sinusitis that is unresponsive to medication, recurrent sinusitis, nasal polyposis and mucoceles. The objective of FESS is to improve mucociliary clearance from the sinuses by removing abnormal mucosa and bone that may obstruct the sinus outflow tracts. This can be achieved by infundibulotomy (= removal of the uncinate process) and anterior ethmoidectomy.
When indicated, the scan may be performed using intravenous contrast.

In order to evaluate malignancies, an MRI scan is generally opted for, in view of superior soft tissue visualization. However, ossal involvement in a malignancy can be evaluated effectively on a CT scan.

Read more about the CT technique in the X-ray/CT technique class.

Normal anatomy

Nose

The nose is divided in two by the nasal septum and reaches up to the nasopharynx. The nasal septum consists of both bone tissue and cartilage. 
The nasal conchae (= turbinals) are thin bony curled bulges covered with mucosa: inferior nasal concha, middle nasal concha and superior nasal concha.
The conchae create the various nasal passages: the inferior, middle and superior nasal meatus (fig. 1).

Figure 1. Coronal anatomy of the conchae.

Paranasal sinuses

The paranasal sinuses are air-filled cavities in the skull. There are four different sinuses (Fig. 2):

  • maxillary sinus
  • ethmoid sinus
  • sphenoid sinus
  • frontal sinus 

Figure 2. Schematic anatomy of the sinuses.

The sinuses can be subdivided into two groups, depending where they drain into. The anterior ethmoid cells, the frontal sinus and maxillary sinus drain into the middle meatus. The posterior ethmoid cells and sphenoid sinus drain into the superior meatus.
The nasolacrimal duct drains into the inferior nasal meatus.

Maxillary sinus

The first sinus to be developed is the maxillary sinus (fig.3).

 Figure 3. Maxillary sinus in the coronal direction (a) and transversal direction (b).

Drainage takes place through the ostium, through the infundibulum and semilunar hiatus into the middle meatus. The complex of structures into which the maxillary sinus, anterior ethmoid complex and frontal sinus drain is called the ostiomeatal complex (fig 4). It consists of:

  1. Maxillary sinus with ostium (= exit).
  2. Ethmoid bulla (= most posterior cell in the anterior ethmoid complex).
  3. Uncinate process (= medially oriented curving of the lateral nasal wall).
  4. Infundibulum (formed by the uncinate process and inferomedial wall of the orbita/ethmoid bulla).
  5. Semilunar hiatus (= passage through the anterior wall of the ethmoid bulla and the free posterior wall of the uncinate process).
  6. Middle meatus.

 Figure 4. Coronal direction. The ostiomeatal complex. 

In view of the joining of multiple drainage passages (maxillary sinus, frontal sinus and anterior ethmoid sinus), the ostiomeatal complex must be patent. Most infections occur here. The ostiomeatal complex can be visualized effectively in the coronal direction (fig. 4).

Ethmoid sinus

The ethmoid sinus is fully grown at about 12 years and contains 3-18 cells at each side.
The ethmoid sinus consists of a left and right part where the air cells are located. They are located roughly between the eyes. The lateral limits are also the medial orbital walls. The roof of the superior meatus is formed by the cribriform plate.  Immediately above it is the olfactory nerve (= smell nerve), which gives off small filaments through microscopic foramina (fig. 5).

 Figure 5. Schematic representation of the cribriform plate and the olfactory nerve (a). Coronal direction; upper limit and lateral limit (lamina papyracea) of the ethmoid sinus (b).

Finally, the ethmoid sinus is subdivided into an anterior complex and posterior complex. 
The anterior ethmoid cells drain into the middle meatus. The posterior ethmoid cells drain into the superior meatus (fig. 6).

 Figure 6. Sagittal direction. Drainage tract of the anterior/posterior ethmoid cells.
Note: The semilunar hiatus seems to reach above the middle concha on this sagittal image. However, this structure is the winding of the medial concha. The insertion of the concha is above this level, and therefore the area shown is the middle meatus.

Frontal sinus

The frontal sinus is not yet present at birth. Pneumatization takes place between the first and twelfth year (fig. 7). There is frequently a central septum, where the sinuses are asymmetric. There may also be multiple septae.

 Figure 7. Coronal direction. Normal frontal sinus.

Sphenoid sinus

The sphenoid sinus is separated by a septum, often not in the midline.  Multiple septations may occur, which are always oriented vertically. If horizontal lamellae are seen, these are posterior ethmoid cells.
Drainage is through the spheno-ethmoid recess (fig. 8).

 Figure 8. Transversal direction. Ethmoid sinus and sphenoid sinus with the spheno-ethmoid recess (arrows).

Key surrounding structures include the superior orbital fissure, the optic canal, and the cavernous sinus containing among other things the carotid artery. Importantly, any dehiscence (little to no bony coverage) of these structures should be noted, because damage may be done during the FESS.
 

Normal variations

The sinuses have frequent anatomic variations. Some variations may cause recurrent disease. The ENT specialist should be aware of the variations, allowing him or her to opt for another surgery technique. Finally, it is important to recognize the so-called ‘critical’ variations that may cause complications during surgery. Think e.g. of damage to the optic nerve or internal carotid artery, or leakage of cerebrospinal fluid.

  • Agger nasi cell: anterior ethmoid cell; it may obstruct the frontal recess.

 Figure 9. Agger nasi cell (*); most anterior ethmoid cell.

  • Ethmoid bulla: most posterior cell in the anterior ethmoid complex (see also fig. 4). There is significant variation in size and morphology.
  • Onodi cells: variation of posterior ethmoid cells located above the sphenoid sinus as a result of hyperpneumatization. There is a close relation with the optic nerve.
  • Haller cells (fig. 10): infraorbital ethmoid cells.  They may obstruct the infundibulum (part of the ostiomeatal complex - see maxillary sinus).
     
  • Concha bullosa (fig. 10): Extension of ethmoid cell in middle concha. There must be > 50% pneumatization in the coronal plane. The infundibulum or middle meatus may be obstructed. It often occurs bilaterally and is seen on CT in 14-53% of the cases. There is frequently also nasal septum deviation. Potential complications include the development of mucosal swelling, exudate, retention cysts or an osteoma.

 Figure 10. Coronal direction. Concha bullosa at left and Haller cell at right.

  • The cribriform plate can be not only asymmetric, but also developed more in the caudal direction (up to 16 mm). This will be associated with a deep olfactory fossa (fig. 10). If a fracture occurs during FESS, there is a risk of cerebrospinal fluid leakage or meningitis.

Figure 11. Coronal direction. Schematic representation of normal and deep olfactory fossa.

  • Sometimes bone is missing in the medial orbital wall (= lamina papyracea), creating a passage into the intraorbital space during FESS. This may give rise to intraorbital bleeding, fibrosis or damage to the ocular muscle.

Checklist

The following points may be used as a guide to assess a sinus CT.

If present, examine the coronal, axial and sagittal series. 

  1. Are all sinuses present?  
  2. Are the sinuses clear? Mucosal swelling or air-fluid level?
  3. What is the aspect of the bone? Bone thickening or destruction?
  4. Is the ostiomeatal complex patent?
  5. Is the cause for obstructed drainage visible?
  6. Are there any abnormalities outside the ENT area?

Pathology

  • sinusitis

Sinusitis

Because of the close association between the nose and the paranasal sinuses, persistent infection of the nasal mucosa (rhinitis) may spread through the mucosa to the paranasal sinus and cause sinusitis. Obstruction of the drainage passage secondary to mucosal swelling makes the sinus an attractive substrate for bacteria (fig. 12).

 Figure 12. Coronal direction. Bilaterally obstructed ostiomeatal complex secondary to mucosal swelling.

Other causes of sinusitis include: swollen concha, polyps, tumor, septum deviation.
Sinusitis is most common in the maxillary sinus, followed by the ethmoid sinus and the frontal sinus. The least common sinusitis is an infection of the sphenoid sinus.
In sinusitis, a CT scan may provide information on the extent and possibly the (mechanical) cause. Additionally, it provides useful preoperative information about anatomy. 
Sinusitis may be subdivided into acute (< 1 month), subacute (1-3 months) and chronic (> 3 months).
In acute sinusitis, air-fluid levels (fig. 13), mucosal thickening or full opacification of the sinus can be seen on CT scan.  
Mucosal thickening has the density (= whiteness) of soft tissue. Also polypoid soft tissue masses may be seen which may spread from the sinus into the nasal cavity.

 Figure 13. Transversal direction. Acute bilateral sinusitis of the maxillary sinus in a 12-year-old immunocompromised child.

In patients with chronic sinusitis, remodeling and bone thickening may develop secondary to osteitis (fig. 14). 

 Figure 14. Transversal direction. Thickening of the lateral wall of the right spenoid sinus secondary to chronic sinusitis.

Comments on air-fluid level:

  • Take gravity into account; the patient is scanned in the supine position (so examine the transversal series in particular).
  • Know that not each patient with acute sinusitis presents with a classic air-fluid level. 

In patients with chronic sinusitis, remodeling and bone thickening may develop secondary to osteitis. Additionally, chronic infection may be associated with polyp formation (nasal polyposis) in 2-16% of the cases. These tear-shaped polypoid soft tissue masses develop in the paranasal sinus outlet and may spread from the sinus into the nasal cavity.

Important:
In extensive disease, always examine the lamina papyracea. This is the thin bony structure forming the medial orbital wall. If the bone is affected, the infection may spread to the orbita.
CT is suitable to evaluate preseptal cellulitis, subperiostal inflammation and intraorbital spread. MRI can be used to evaluate any vascular complications such as thrombosis of the cavernous sinus.

Sources

  • Baert A. L., Sartor K., Maroldi R., Nicolai P. Medical Radiology - Diagnostic Imaging; Imaging in Treatment Planning for Sinonasal Diseases; ISBN 3-540-42383-4 Springer, 2005.
  • Larheim T.A., Westesson P-L A. Maxillofacial Imaging; ISBN-10: 3540786856 Springer, 2006.
  • Hoang JK, Eastwood JD, Tebbit CL, Glastonbury CM. Multiplanar sinus CT: a systematic approach to imaging before functional endoscopic sinus surgery. AJR Am J Roentgenol. 2010;194(6):W527-36.
  • Jenny K. Hoang1 Multiplanar Sinus CT: A Systematic Approach to Imaging Before Functional Endoscopic Sinus Surgery. 
  • H. Mulder Keel-, neus- en oorchirurgie. 2005.
  • Rochita V Ramanan; Sinusitis Imaging. Medscape. 

Author

  • R.L. Widya, resident radiology LUMC

  • A. van der Plas, MSK radiologist Maastricht UMC+

13/02/2014 (translated: 30/08/2016)

Copyright
All the work (text, illustrations, visual elements) seen on this website is copyright by Annelies van der Plas.
It may not be used without written permission of Annelies van der Plas.

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