Chest X-Ray Positioning: PA, AP & Lateral Projections
Introduction
Chest radiography is the most frequently performed radiographic exam and a high-priority ARRT topic. Mastery of positioning criteria, technical factors, and radiation protection principles is essential for exam success.
Definition: The three standard chest projections tested on the ARRT Radiography exam are the PA (Posteroanterior), AP (Anteroposterior), and Lateral projections. Each has distinct positioning criteria, technical parameters, and clinical indications.
Every projection requires a clear understanding of:
- Patient and CR positioning
- Correct technical factors (kVp, mAs, SID)
- Evaluation criteria for a diagnostic image
- Common errors that are frequently tested
Projection Positioning & Criteria
Definition: The PA (Posteroanterior) projection is the preferred standard chest view. The x-ray beam travels from posterior to anterior, with the patient's anterior chest wall against the IR.
Patient Positioning:
- Patient faces the IR
- Hands on hips, shoulders rolled forward to move scapulae out of the lung fields
- Chin elevated to prevent soft tissue superimposition over the apices
- Patient erect, full weight-bearing
CR & SID:
- CR perpendicular to T7
- SID: 72 inches
Technical Factors:
- kVp: 110–125 kVp
- mAs: 3–5 mAs
Evaluation Criteria (Diagnostic Image):
- Full inspiration: 10 posterior ribs visible above the diaphragm
- No rotation: clavicle heads equidistant from the spinous processes
- Scapulae projected outside the lung fields
- Chin and soft tissue clear of the apices
Why PA is Preferred: The 72-inch SID minimizes cardiac magnification by reducing OID (object-to-image distance). It also significantly reduces breast dose compared to the AP projection, making it the preferred view whenever the patient can stand.
Definition: The AP (Anteroposterior) projection is used when the patient cannot stand for a PA view — typically for portable or bedside exams. The IR is placed posterior to the patient and the tube is anterior.
Key Positioning Notes:
- IR placed posterior to the patient; tube is anterior
- SID: 40–48 inches for portable exams
- kVp: 80–90 kVp
- Must be documented as AP portable on the image
Inherent Limitations of AP:
- Heart appears more magnified due to shorter SID and greater OID
- Scapulae cannot be fully moved from the lung fields — patient positioning is limited in a supine or semi-recumbent state
- Increased breast dose compared to PA
ARRT Exam Tip: Always identify a portable chest as AP — never assume PA positioning for a bedside exam. The shorter SID causes cardiac magnification, which is a tested concept. If asked to compare image quality, PA at 72 inches always produces less magnification than AP at 40–48 inches.
Definition: The Left Lateral projection is the standard lateral chest view. The left side of the patient is placed against the IR to minimize cardiac magnification.
Patient Positioning:
- Left side to IR (left lateral is standard)
- Patient erect, arms raised above the head
- Coronal plane parallel to the IR
CR & SID:
- CR to T7 at the level of the axillae
- SID: 72 inches
Technical Factors:
- kVp: 110–125 kVp
Evaluation Criteria (Diagnostic Image):
- Posterior ribs superimposed — indicates no rotation
- Both costophrenic angles visible
- 10–11 posterior ribs visible on full inspiration
Why Left Lateral? The heart is a left-sided structure. Placing the left side closest to the IR reduces OID for the heart, minimizing cardiac magnification — the same principle as PA over AP.
Use this table for quick comparison of the three standard chest projections:
| Parameter | PA Chest | AP Portable | Left Lateral |
|---|---|---|---|
| Patient Position | Faces IR, erect | Supine/semi-recumbent, IR posterior | Left side to IR, erect |
| CR Level | T7 | T7 | T7 at axillae |
| SID | 72 inches | 40–48 inches | 72 inches |
| kVp | 110–125 | 80–90 | 110–125 |
| mAs | 3–5 | Higher (shorter SID) | Higher than PA |
| Cardiac Magnification | Minimal | Increased | Minimal (left side down) |
| Scapulae | Rolled out of lung fields | Cannot fully remove | Superimposed posteriorly |
| Inspiration Criteria | 10 posterior ribs | 10 posterior ribs | 10–11 posterior ribs |
Common Errors & Radiation Protection
The ARRT exam frequently tests recognition of positioning errors and their radiographic consequences. Know these cold:
Rotation on PA Chest: Unequal distances between the clavicle heads and spinous processes indicate rotation. Rotation can mimic mediastinal shift — a critical diagnostic error.
Expiration Film: If the patient does not take a full breath, the diaphragm is elevated, lungs appear denser, and fewer than 10 posterior ribs are visible. This can mimic pathology such as pulmonary edema or consolidation.
- Scapulae in lung fields: Patient did not roll shoulders forward — obscures lung anatomy
- Chin not elevated: Soft tissue superimposed over the lung apices — can obscure apical pathology
- Incorrect SID on portable: Using a shorter SID than documented magnifies the heart and distorts anatomy
Quick Error Recognition Summary:
| Error | Radiographic Sign | Consequence |
|---|---|---|
| Rotation | Unequal clavicle-to-spine distances | Mimics mediastinal shift |
| Expiration | Fewer than 10 posterior ribs | Mimics pathology |
| Scapulae in fields | Scapular shadows over lungs | Obscures lung detail |
| Chin down | Soft tissue over apices | Obscures apical anatomy |
| Short SID (portable) | Enlarged cardiac silhouette | Cardiac magnification |
Chest radiography technical factors are directly tested on the ARRT exam. Understand the rationale behind each parameter.
Technical Factor Summary:
| Projection | SID | kVp | mAs | Grid |
|---|---|---|---|---|
| PA Chest | 72 inches | 110–125 | 3–5 | Use if > 10 cm thickness |
| AP Portable | 40–48 inches | 80–90 | Higher than PA | Use if > 10 cm thickness |
| Left Lateral | 72 inches | 110–125 | Higher than PA | Use if > 10 cm thickness |
Radiation Protection Principles:
- High kVp technique (110–125 kVp) reduces patient dose by allowing lower mAs while maintaining image quality
- Collimate to the chest walls to reduce scatter and patient dose
- Use grids only when patient thickness exceeds 10 cm — unnecessary grid use increases dose
- ALARA: Use the minimum mAs that produces a diagnostic image
- PA is preferred over AP to significantly reduce breast dose — the breast is farther from the beam entrance in PA positioning
Dose Reduction Scenario: A female patient requires a chest x-ray. If she can stand, always perform PA at 72 inches rather than AP. The PA projection places the breast tissue farther from the x-ray source, reducing absorbed dose — a direct application of ALARA.
Key Concepts Summary
⚠ KEY CONCEPTS — Test-Ready Recall
- PA Chest: Patient faces IR, shoulders rolled forward, chin up. CR to T7. SID 72 inches. kVp 110–125, mAs 3–5. Full inspiration = 10 posterior ribs above diaphragm.
- AP Portable: Used when patient cannot stand. SID 40–48 inches, kVp 80–90. Heart is magnified. Must be documented as AP portable. Scapulae cannot be fully cleared.
- Left Lateral: Left side to IR, arms raised, coronal plane parallel to IR. CR to T7 at axillae. SID 72 inches. Posterior ribs superimposed = no rotation. Both costophrenic angles visible.
- Rotation error: Unequal clavicle-to-spine distances — can mimic mediastinal shift.
- Expiration error: Fewer than 10 posterior ribs — can mimic pulmonary pathology.
- PA preferred over AP to minimize cardiac magnification and reduce breast dose (ALARA).
- High kVp technique reduces patient dose. Use grids only when thickness exceeds 10 cm. Collimate to chest walls.
Sign up to track progress
Create a free account to mark lessons complete, track your learning progress, and access study tools