Читать книгу Orthodontic Treatment of Impacted Teeth - Adrian Becker - Страница 78

The X‐ray source, emitting a pyramidal‐shaped beam, is mounted on a gantry facing a detector. Unlike the rotation centre in a panoramic machine, which slides during its rotation, the CBCT gantry axis is fixed. The patient is positioned with the centre of the region of interest (ROI) in close proximity to the gantry's rotation axis. The gantry performs 180–360° rotation around the patient’s head, during which, in most machines, multiple exposures (150 to about 1000) are taken. The number of exposures taken depends on the protocol chosen in the specific machine, but certain machines radiate continuously during the exposure. The 2D images taken in this single rotation of the patient during the scan are collectively known as raw data, which is then reconstructed into a 3D volume. This procedure is called a primary reconstruction and is carried out by a software algorithm. The algorithm will calculate and create the volume according to the protocol chosen and is depicted in the form of a cylindrical stack of axial slices, reminiscent of thin slices of salami, one on top of the other.