Axial Plane Acquisition
This scanning plane enables adequate demonstration of some anatomical structures difficult to assess on coronal plane due to their spatial orientation (such as posterior frontal sinus and sphenoid sinus wall, sphe-noethmoid recess). Nonetheless, axial acquisition is recommended only whenever the digital lateral view (topogram) shows excessive dental amalgam or metallic implants, or as a complement after coronal scanning whenever precise information on specific anatomic structures is required.
In the first condition, a data set of thin and contiguous (to avoid aliasing) slices may be acquired with the incremental technique (1-2 mm collimation) or the SS spiral technique (2 mm collimation; pitch 1-1.5; 1-2 mm reconstruction) (Table 1.2). The patient lies in supine position, both his/her sagittal plane and hard palate should be perpendicular to the gantry's scan line, in order to achieve, respectively, optimal symmetry of the anatomical structures on axial plane, and no/minimal gantry tilting. In fact, the quality of MPR is degraded by gantry inclination (stair-step artifacts). Direct scans should be oriented parallel to the hard palate and range from the upper border of frontal sinuses to the alveolar process of maxillary bones; nasal tip and petrous bone should be included in the field of view. Subsequent coronal MPR reformation is obtained for proper assessment of critical anatomical areas either at risk of iatrogenic damage (cribriform plate, fovea ethmoidalis) or playing a key role in mucous drainage (ostiomeatal unit).
When acquired as a complement to the coronal study, axial scans are basically focused on anatomical areas inadequately demonstrated in that orientation (i.e., anterior and posterior walls of maxillary, frontal, and sphenoid sinus, sphenoethmoid recess). Additionally, this scan plane is valuable for the detection of Onodi cells. In this setting, contiguous slices and full coverage of the paranasal area are generally unnecessary, as a consequence, 4- to 5-mm increments can be applied, decreasing both patient exposure and examination time.
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