Right lateral thoracic radiographic view of a clinically normal Miniature Pinscher dog illustrating measurements of vertical tracheal diameters (VTDs) at caudal cervical (level A), thoracic inlet (level B), and intrathoracic (level C) tracheal regions, and measurements of manubrium length (ML), thoracic inlet distance (Ti-D), and proximal 3rd rib-width (PR3-W) for determination of manubrium (M-TI) and thoracic inlet-tracheal indices (Ti-TI) and proximal R3-tracheal scores (PR3-TS) for each absolute and average tracheal diameters