Imaging in Paranasal Sinus Disease

For many years rhinologists and radiologists have had to rely upon conventional x-rays for assistance in the diagnosis and management of paranasal sinus disease.  Information from these plain films is of most relevance to the general condition of the larger paranasal sinuses, in particular the frontal, sphenoid and maxillary sinuses.  X-rays are of little use in the study of delicate bony anatomical and mucosal changes that may be present in the ethmoid cells of the lateral wall of the nose.  As we have seen from the section on FESS, it is important to be able to visualize the entire paranasal sinus system to make an appropriate diagnosis and perform surgery with the least amount of complications.  X-rays have proved inadequate in this context.

Conventional x-rays (radiographs) can be taken from four angles or views, known as

Look at the following normal x-rays.  Can you see how much more information can be gathered by performing the correct projection?
 
A Caldwell view (PA) A Water's view.  Note the maxillary sinus cavities on either side of the nasal cavity. 

Views such as these also have a role in the diagnosis of acute sinusitis, where the features to look out for are the presence of an air-fluid interface or a totally opacified sinus.  Look at the following diagrams to get a feel for the role of conventional radiography in paranasal sinus disease.
 

Acute maxillary sinusitis.  This radiograph clearly demonstrates an air-fluid level in the right maxillary sinus (arrow).  There is also some mucosal thickening in the left maxillary sinus.  This is a Caldwell view. A lateral film demonstrating sphenoidal sinusitis.  Look at the fluid level within the sphenoid sinus seen on this radiograph.

In a CT scan, a slit-like beam of rays is produced from an x-ray tube.  The tube detector unit encircles the patient being examined.  Projected rays pass through the patient, some are absorbed, others are changed.  An array of detector receive the x-ray signals that have passed through tissues and become weakened.  A computer uses this information to produce a series of images which resemble slices through the patient.

Patients should not undergo CT during the acute phase of an uncomplicated sinus infection, as the acute inflammatory swelling prevents the ostiomeatal complex from being clearly visualised.  The maximum information is gleaned from scans when a patient is in a quiescent phase.

Magnetic Resonance imaging (MRI) involves rapidly changing magnetic fields.  MRI depends upon the physical and biochemical properties of the tissue examined.

CT and MR imaging have added a third dimension to the radiological imaging of the paranasal sinuses.  It is now possible to detect early disease, be it inflammatory or neoplastic, and the spread of such disease, either intracranially or into the orbit.  CT and MRI each have particular advantages.  The following table compares and contrast the relative benefits of each procedure.
 

ADVANTAGES OF CT OVER MRI
ADVANTAGES OF MRI OVER CT
BONY DETAIL: CT identifies bony details such as the extent of pneumatization of the ethmoid air cells, the location of the natural maxillary ostium, septations of the sphenoid sinus and the position of the internal carotid artery. SUPERIOR SOFT TISSUE DISCRIMINATION: This permits better assessment of the interface between a pathological mass and surrounding normal structures
COST: Presently, CT is significantly less expensive than MRI.  It is also more readily available. MUTLIPLANAR: Any anatomical plain must be viewed without a patient having to take up several uncomfortable positions.  The coronal plane is mostly used in the study of the head and neck.
EXAMINATION TIME: CT takes only seconds to take sliced images.  MRI requires several minutes per slice. VASCULAR ANATOMY: Vascular structures are readily seen without contrast material
PATIENT COMFORT: MRI scanners are noisier and more confining than CT scanners ARTIFACTS: Dental artifacts are not seen on MRI, but are seen on CT scans.

Compare these two scans of the normal maxillary sinus.  Which one gives more information?
 

A normal MRI scan through the middle of the maxillary sinus.  This demonstrates the middle (M), inferior (IT) and superior turbinates.  The intraorbital muscles (black arrows) are in the bright hyperintende orbital fat.  The ethmoid sinus (E) and the uncinate process (white arrow) are readily seen.  The lamina papyracea and the air in the ethmoid sinuses appear as void or blank areas against the bright orbital fat Normal maxillary sinus.  This CT scan demonstrates the anterior portion of the maxillary sinus (MS) draining through the maxillary ostia into the infundibulum (open arrow).  The infundibular channel is bounded superolaterally by the bulla ethmoidalis (EB) and inferomedially by the uncinate process (UP).  The infundibulum opens into the semilunar hiatus, which is a slit-like opening that connects the infundibulum to the middle meatus.  The infraorbital foramen (IOF), inferior turbinate (IT) and inferior meatus (IM) are shown.

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