Techniques

Patient preparation:

The timing of the CT scan study in relation to the disease process is critical, as many studies have shown that in the presence of a simple upper respiratory tract infection, 90% of patients will show mucosal thickening within the paranasal sinuses suggestive of sinusitis. It is ideal to perform the initial CT scan after a course of treatment with antibiotics and decongestants to eliminate the changes of acute sinusitis and for better evaluation of the underlying anatomic structures. A sympathomimetic nasal spray 15 minutes prior to scanning can also be used to reduce the nasal congestion. The spray minimizes the mucosal oedema, making possible an improved display of the fine bony architecture. The patient should blow his nose just before the procedure to clear out any mucus. A large blob of mucus in the nasal cavity could mimic inflammatory disease or a polyp.

Techniques:

In patients with inflammatory diseases, coronal scans are preferred because the anatomy and pathology are examined in a plane almost identical to that approached by the endoscopist and it is also the imaging plane that best displays the osteomeatal unit. Moreover, the exact situation and configuration of the two fundamental lamellae which form osteomeatal unit, the uncinate process and the bulla lie on a vertical plane and cannot be studied with axial CT scans without reviewing several series scans made in close proximity, which increases the patient's exposure to radiation and as well as the expense of the study However, complementary direct axial sections are helpful in displaying the position of the carotid arteries and optic nerve and to guide the approach to the sphenoid sinus and the posterior ethmoidal cells. Axial imaging is also important in the evaluation of sinus neoplasms and trauma patients.

Parameters for coronal CT scans Fig.1 and Fig. 2)

Parameters forAxial CT scans. Fig. 3 & Fig. 4)

For coronal sections, patient is placed prone on the scanner table with the chin hyper extended (Fig. 1) and the scanner gantry is angled to be perpendicular to the bony palate or at least within 10 degrees from the plane perpendicular to the palate (Fig. 2).  Scanning is performed anteriorly from frontal sinus to posteriorly through to the sphenoid sinus.

In patients who cannot tolerate prone position (children, patients of advanced age), the "hanging head" technique can sometimes be utilized (Fig.5).  The patient is placed in the supine position and the neck is maximally extended. A pillow placed under the patients's shoulder helps the positioning.  The CT gantry is angled to be as perpendicular as possible to the bony palate. It is not always possible to obtain direct coronal images with this technique.   When both these technique are not possible, as in patients like young children, intubated or tracheostomized patients or patients with severe cervical spondylosis, coronal reconstruction images can be done using thin, serial axial sections.

Section thickness of 3 or 4 mm with overlapping sections or sections with no intersection gap are advised with table advancement of 3 or 4 mm. Concern has been raised over the radiation exposure to the patient, in particular, with regard to radiation induced cataracts. All coronal CT examinations of the sinuses requested for the planning of functional endoscopic sinus surgery should use a low mAs technique. Because of the inherent contrast between air, soft tissue, and bone in the paranasal sinuses, a reduction in the radiation exposure parameter to as low as 40 mA can be used without adversely affecting the image quality.

There is dispute regarding the number of slices required among various authors. However, beginners should ask for a detailed study to understand the anatomy, anatomical variants and the pathology.   As one becomes more experienced, can ask fora limited cut study.  The limited CT examination may be useful for detecting inflammatory disease, but it is very difficult to reproduce the exact positioning on subsequent CT examination, which decreases the accuracy of the follow up examination and moreover a limited scan cannot provide a complete surgical road map.

The bone, soft tissues and air passages are visualized with appropriate window settings. For routine CT of the paranasal sinuses, the scanner is set with a window settings of 2000 Hounsfield units with a level of -200.   This setting will usually demonstrate both bone and soft tissue adequately and then pointometer can be manipulated manually for optimal display. When soft tissue pathologic conditions are to be emphasized, the scanner is set with a window of 300, centered at +65.  The same setting may be used to film the entire study.  The narrow window settings assist in characterizing tumors. If there is significant involvement of the surrounding soft tissue with loss of the fat plane, the lesion is more likely malignant. This situation can be more accurately assessed with narrow window settings. The bone window settings are excellent for assessment of bony erosion or destruction.

Taking true sagittal section is technically difficult (Fig.6).  Sagittal reconstruction (Fig. 7   &   Fig. 8) can be obtained for a morphologic orientation.  Various distances and angles can be measured on these views to aid in the passage of instruments during surgery.