Procedures · Positioning basics2 min read5-question check
Projections, positions and views
Projection is the path of the central ray; position is how the patient is placed; view is the image.
The rule
A projection describes the path of the central ray from where it enters the body to where it exits, such as AP or PA. A position describes how the patient is physically placed, such as supine, erect, left lateral, or RAO. A view describes the body part as seen from the image receptor and is used only to describe a finished image.
Anatomic position
Every term is based on anatomic position: standing erect, facing forward, palms forward. In an AP projection the ray enters the anterior surface and exits the posterior surface, so the back is against the receptor. In a PA projection the ray enters posteriorly and exits anteriorly, so the front is against the receptor.
Obliques and laterals
Lateral and oblique positions are named for the side of the body closest to the receptor. In a left lateral position the left side touches the receptor. In an RAO position the right anterior surface is closest, and the ray travels from back to front, so it is a PA oblique projection. LAO is the same with the left side. RPO and LPO place a posterior surface closest, so both are AP oblique projections. For cervical intervertebral foramina, anterior obliques show the foramina closest to the receptor, and posterior obliques show the side farther away.
Decubitus, axial, tangential
Decubitus means lying down with a horizontal beam, and it is named for the surface the patient lies on. In a left lateral decubitus position the patient lies on the left side and the projection is AP or PA. In a dorsal decubitus position the patient lies on the back, and in a ventral decubitus position the patient lies prone; both of those produce lateral projections. An axial projection uses a longitudinal central ray angle of 10 degrees or more along the long axis of the body. A tangential projection skims the ray across the edge of a curved part to show it in profile, such as the patella or the zygomatic arch.
Clinical Example
An order reads left lateral decubitus abdomen. You lay the patient on the left side on a firm pad and use a horizontal beam. The image is an AP or PA projection, even though the order names a position.
Common Mistakes
- Calling an RAO an AP oblique; with the anterior surface down, the ray travels posterior to anterior.
- Naming a decubitus by the side that is up instead of the side the patient lies on.
- Thinking a decubitus is a lateral projection; only dorsal and ventral decubitus give lateral projections.
Why It Matters
Wrong terms lead to the wrong projection, a repeat, and extra dose. The registry tests these definitions directly and inside almost every positioning item.
Key Relationships
- Lateral and oblique positions are named for the side closest to the receptor
- RAO and LAO = PA oblique projections; RPO and LPO = AP oblique projections
- Decubitus = recumbent with a horizontal beam, named for the dependent surface
- Axial = longitudinal CR angle of 10 degrees or more
- Tangential = ray skims the edge of a curved part