Thursday, March 15, 2012

Computed tomography (CT) of a sinus

Computed tomography (CT) of a sinus

The CT is the principal imaging technique that aids sinus disorder diagnosis and helps in planning the surgery. The CT scan furnishes best possible anatomical details. The high-quality scans taken close together are necessary for correct anatomy visualization. Based on the plane along which scanning is done, there are three types of scans: axial, parasagittal and coronal. Usually, first the sinus region is scanned along the coronal plane to reconstruct the anatomy. Then the parasagittal scans are taken and analyzed, and thereafter axial plane scans are reviewed.
The coronal plane scan offers optimal view of the ostiomeatal unit (OMU). The coronal scans should be taken carefully to ensure that a cell on one slice can be followed on the next slice. From these scans, 3-dimensional anatomy of the surgery site can be constructed and visualized. With axial scan, you can identify the frontal sinus’ drainage pathway. This is very helpful if frontal recess is to be touched while surgery. The parasagittal plane scan improves access to the frontal recess and therefore ensures better understanding of the anatomy.

The CT scans can simulate the sinonasal cavity for endoscopic surgeries.  Since bone anatomy is crucial for an endoscopic surgery, high window level and wide window width are used for bone algorithm technique during the CT scanning of uncomplicated cases of sinusitis. While scanning a complicated sinusitis, iodinated contrast agents are used.  The axial plane scans use soft-tissue algorithm with soft-tissue windows and bone windows.
The tissue density in the CT scan is represented in the Hounsfield unit or number (HU). The tissue density of acute blood (normal hematocrit) is about 80 HU. The unit decreases with fall in hematocrit.
However, CT scans have some limitations. Polyposis may obstruct the OMC views. Claustrophobia may create problems. Inappropriate windows for diagnosing intracranial or intra-orbital disorders are another disadvantage. Differentiating between inflammatory disease and granulation tissue or scarring is difficult. The scan offers poor view of the frontoethmoidal recess. Overcoming these limitations require additional techniques.
For instance, the helical scanning is the best method for the patients having dental restorations and patients unable to stand / bear hyperextension of the neck. The technique acquires axial data that enables multiplanar or three-dimensional reformatting. The helical scanning requires less time. Since helical scanning facilitates multiplanar view in real time, it is always used for intra-operative usage. High-definition multislice helical CT scans of the sinus shall be taken in three planes: axial, parasagittal and coronal.

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