To project the dorsum sella within the formen magnum in a skull of average shape.the orbito –meatal line is placed perpendicular to the film and the central ray angled cranially:
- a- 5 degrees
- b- 15 degrees
- c- 25 degrees
- d- 60 degrees
The correct answer is c. 25 degrees.
Here's why:
- Projecting the dorsum sella within the foramen magnum:
This requires angling the central ray cranially (tilting the head backwards) to visualize the structures below the base of the skull.
- Orbito-meatal line:
This imaginary line passes from the lowest point of the bony orbit to the external auditory meatus (ear opening). When positioned perpendicular to the film, it provides a reference for skull positioning.
- Angling the central ray:
The degree of cranial angulation depends on the individual skull shape and the desired level of detail for the dorsum sella. In an average skull, an angle of 25 degrees is typically sufficient to project the dorsum sella within the foramen magnum while maintaining clarity of the surrounding structures.
Other Options:
Here's a breakdown of the other options:
a. 5 degrees:
This angle might not be sufficient for visualizing the dorsum sella clearly within the foramen magnum in most average skulls.
b. 15 degrees:
This angle could be suitable for some average skulls, but it might not be necessary and could potentially obscure other structures in the base of the skull.
d. 60 degrees:
This is a significantly higher angle than necessary for projecting the dorsum sella in an average skull. It would likely result in excessive cranial tilt and potential distortion of other skull structures.
Therefore, considering the average skull shape and the need to visualize the dorsum sella within the foramen magnum, an angle of 25 degrees for the central ray is the most appropriate choice.
