13.1 Positioning & Projection Terminology

Speaking the Language of Positioning

Radiographic positioning is a precise technical language. Before you can produce a diagnostic image, you must translate a physician's order into a body plane, a body position, a projection, and a central-ray (CR) path. ARRT's Procedures category is the largest on the exam (66 scored items, 33%), and it assumes you have already mastered this vocabulary, because a single misused term changes the image entirely. Confusing a projection with a position, or an RAO with an RPO, will cost you points and, in the clinic, an unnecessary repeat exposure.

Body Planes and Reference Lines

Imaginary flat surfaces called planes slice the body for description. Four appear repeatedly on the exam.

The midsagittal plane (MSP) divides the body into equal right and left halves; keeping the MSP perpendicular or parallel to the image receptor (IR) is how you eliminate rotation. The midcoronal plane (MCP) divides the body into equal anterior and posterior halves and defines a true lateral.

Body Positions (General Body Attitude)

A position describes the patient's overall physical attitude. Do not confuse it with a projection.

• Supine (dorsal recumbent): lying on the back, face up.
• Prone (ventral recumbent): lying on the abdomen, face down.
• Erect (upright): standing or seated vertical.
• Recumbent: any lying-down position (supine, prone, or lateral).
• Lateral recumbent: lying on the right or left side.
• Decubitus: patient is recumbent AND the central ray is horizontal (parallel to the floor). Named for the dependent surface plus CR direction: dorsal decubitus (supine, horizontal beam), ventral decubitus (prone), and lateral decubitus (on the side). Decubitus positions are what reveal air-fluid levels.
• Trendelenburg: head lower than the feet (used for contrast studies and shock).
• Fowler: head higher than the feet (semi-erect).
• Sims: recumbent, semi-prone on the left anterior side with the right knee flexed; the classic position for inserting a barium-enema tip.
• Lithotomy: supine with knees flexed and thighs abducted.

Projections: The Path of the Central Ray

A projection is defined strictly by the path the CR travels through the body from entrance to exit.

• Anteroposterior (AP): CR enters the anterior surface and exits posterior.
• Posteroanterior (PA): CR enters posterior and exits anterior (the reverse of AP).
• Lateral: CR passes through the coronal plane; the projection is named for the side of the patient closest to the IR (a left lateral has the left side against the IR).
• Oblique: the body is rotated so the CR passes through it diagonally. Obliques are named for the body part closest to the IR: the PA obliques are the RAO (right anterior oblique) and LAO (left anterior oblique); the AP obliques are the RPO (right posterior oblique) and LPO (left posterior oblique).
• Axial: any projection with a longitudinal CR angulation of 10 degrees or more along the long axis of the body.
• Tangential: the CR skims (is directed tangent to) a body part to throw it into profile and free it of superimposition (for example, the sunrise patella or the zygomatic arch).

Oblique Naming Trap

Students lose points by mixing anterior and posterior obliques. Memorize: A = Anterior = part against IR is anterior, which requires the patient to face the IR (as in a PA-type setup). P = Posterior = part against IR is posterior, so the patient's back is toward the IR (an AP-type setup). An RAO (right anterior oblique) and an LPO (left posterior oblique) actually demonstrate the same anatomy because the same side of the body is closest to the IR.

Central-Ray Direction and Angulation

After you set the position, you aim the CR. Perpendicular means the beam strikes the IR at 90 degrees. Angling the tube toward the patient's head is cephalad (or cephalic); angling toward the feet is caudad (or caudal). Angulation is used to open joint spaces and to project overlapping structures apart — for example, an AP axial clavicle angles 15-30 degrees cephalad to throw the clavicle above the ribs, and an AP axial (Towne) skull angles caudad. Whenever you read "axial" in an ARRT item, expect a CR angle of 10 degrees or more along the long axis of the body.

Position vs. Projection vs. View — the Classic Trap

Exam writers deliberately blur these three terms. A projection is the CR path (AP, PA, lateral, oblique, axial, tangential). A position is the patient's body attitude (supine, prone, erect, RAO, left lateral, decubitus). A view, in the strict Merrill's convention, describes the image as the IR "sees" it and is the exact opposite of the projection — a term ARRT discourages for describing patient setup. So an order for a "PA chest, erect" combines a projection (PA) with a position (erect). If a question asks for the projection but you answer with a position name like "RAO," you have answered a different question. Read carefully: the word projection points to the CR path; the word position points to how the body is turned.

Surface Landmarks to Internal Anatomy

Because you cannot see internal structures, you center to palpable surface landmarks. High-yield correlations:

These landmarks reappear in every chapter of the Procedures domain. When an item asks where to center a chest, an abdomen, or a spine, you are being tested on this table.