Hispeed lx i

CT images of the studied heads were obtained by using a single-slice helical device HiSpeed LX/i (General Electric Company, Medical Systems, Boston, MA, USA). Images were acquired in a bone algorithm (window level between 200 and 2250 Hounsfield units (HU) and bone filter to reduce the noise).
Animals were placed in ventral recumbency, and their correct positioning was evaluated by performing topograms and corrected when necessary. They were induced with propofol (Propofol Lipuro 10 mg/mL, B.Braun Portugal, Barcarena, Portugal) at a dose of 3–4 mg/kg intravenously, and the maintenance of anesthesia was carried out with volatile isoflurane mixed with oxygen.

All patients were positioned using a thermoplastic mask (Orfit Industries, Wijnegem, Belgium), a standard headrest and a knee support (Civco Medical Solutions, Kolona, USA). The planning CT (pCT) scan (Hispeed LX/I, General Electric Medical Systems, Milwaukee, WI, USA or Somatom Sensation Open, Siemens AG, Erlangen, Germany) was acquired with a slice distance and thickness of 3 mm from the cranium to the sternum. The patient setup on the linear accelerator (Elekta, Synergy, Crawley, UK) was verified by registration of the bony anatomy from the pCT to the CBCT using chamfer matching and was corrected using a regular offline protocol [14 (link)]. Additional CBCT scans were acquired for research purposes within the context of other studies. A total of 1170 CBCT scans were evaluated for this study with an average of 31 CBCT scans per patient. CBCT scans were acquired over a 360° arc in 1 min (16 mA – 40 ms – 120 kV, per projection) with an isocenter dose of ~1 cGy.