As shown in Figure 2, a total of 6 landmarks were used in constructing the coordinate system. Four landmarks occupy the transverse section: the right/left internal acoustic pore (RIA/LIA) and the right/left ascending segment of the internal carotid artery in the posterior cavernous sinus (RAS/LAS). Around the nasopharynx, the positional relationship between the landmarks of the bilateral internal acoustic pore and the bilateral internal carotid artery is stable. In addition, these landmarks are often used for the construction of coordinate systems (26 (link),27 (link)). Furthermore, the 4 points are symmetrical and almost in the same plane. In our experimental data, the 4 landmarks were on the same slice in approximately 75% of the images. The 4 landmarks were not in the same slice in all of the images because the landmarks can be affected by a patient’s posture during image acquisition. In obtaining the parameters for 3D image registration, 2 landmarks in the sagittal section were used: the posterior clinoid (PC) process and the midpoint of the upper edge of the anterior arch of C1 (MC). The PC and MC are located in the central position of the posterior cranial fossa, closer to the nasopharynx.
The origin of the coordinate system is located at the midpoint of the connecting line between the landmarks of the LIA and RIA. The x-axis points from RIA to LIA. The y-axis is defined as the direction perpendicular to the x-axis in the transverse section and from back to front. The z-axis is defined as a top-down direction perpendicular to the transverse section. The unit of each coordinate axis is the millimeter, which was chosen to eliminate the influence of different resolutions in three dimensions.