Final answer:
The reference line parallel to the Central Ray (CR) in a c-spine odontoid view with a collar is the interpupillary line. It ensures proper visualization of the odontoid process and correct positioning despite the immobilization caused by the cervical collar.
Step-by-step explanation:
When performing a cervical spine (c-spine) odontoid view in a patient with a collar, the reference line that should be parallel to the Central Ray (CR) is the interpupillary line. The odontoid process, also known as the dens, is part of the axis (C2) vertebra and is better visualized when the head is positioned correctly in relation to the x-ray beam. The interpupillary line is a horizontal line drawn between the pupils of the eyes when the patient's gaze is directed straight ahead. Correct alignment ensures that the odontoid process is properly displayed without rotational distortion, which is especially important when the patient is immobilized with a cervical collar due to a potential spine injury.