Forearm X-Ray Positioning: A Complete Guide for Rad Techs
Master the AP and lateral forearm projections — from centering to evaluation criteria. The forearm (radius and ulna) is one of the most common extremity exams ordered in the emergency department. Getting it right means including both joints, choosing the right technical factors, and knowing how to adapt when your patient can't cooperate. Whether you're studying for the ARRT exam or heading into clinicals, this guide covers everything you need.
Why Forearm Positioning Matters
The forearm is unique among extremity exams because it requires you to include two joints — both the wrist and the elbow — on a single image. This makes centering and IR selection critical.
Key clinical scenarios:
- Suspected radius or ulna fractures (most common indication)
- Monteggia fracture-dislocation (fractured ulna with radial head dislocation)
- Galeazzi fracture-dislocation (fractured radius with distal radioulnar joint dislocation)
- Forearm foreign body evaluation
- Post-reduction or post-operative assessment
📋 ARRT EXAM TIP
A classic ARRT question tests your knowledge of what must be included on a forearm X-ray. The correct answer is both the wrist and elbow joints. This is because forearm fractures can involve the proximal or distal radioulnar joints, and excluding either joint means missing a potential dislocation.
Anatomy Review: Radius and Ulna
The forearm consists of two parallel bones:
- Radius — The lateral (thumb-side) bone. Proximally, the radial head articulates with the capitulum of the humerus at the elbow. Distally, it articulates with the carpal bones (scaphoid and lunate) at the wrist.
- Ulna — The medial (pinky-side) bone. Proximally, the olecranon process articulates with the trochlea of the humerus. Distally, the ulnar styloid process is palpable at the wrist.
- Interosseous membrane — A fibrous sheet connecting the radius and ulna along their length, maintaining the interosseous space.
Key landmark: The midpoint of the forearm is where you center — approximately halfway between the elbow (olecranon) and the wrist (styloid processes). In most adults this is roughly at the junction of the proximal and middle thirds.
AP Forearm Projection
Positioning
- Seat the patient at the end of the X-ray table
- Extend the arm fully onto the table
- Supinate the forearm (turn the palm facing up) — this is the true AP position
- Adjust the arm so the entire forearm is parallel to the IR
- The shoulder, elbow, and wrist should be in the same horizontal plane
IR and Centering
| Parameter | Specification |
|---|---|
| IR size | 14 × 17" (35 × 43 cm) lengthwise |
| Orientation | Portrait/vertical (long axis of forearm) |
| Centering | Midpoint of the forearm |
| SID | 40" (100 cm) |
| kVp | 55–65 |
| mAs | 2.0–5.0 |
| Grid | No (extremity technique); yes if arm thickness > 10 cm |
What to Include
- Proximal: Entire elbow joint — distal humerus, radial head, olecranon process
- Distal: Entire wrist joint — distal radius and ulna, proximal carpal row
- Soft tissues: Forearm musculature visible medially and laterally
Clark's Reference — AP Forearm
Clark's Pocket Handbook (p44) specifies: The arm is fully extended, the forearm supinated (palm up), and the IR includes both the wrist and elbow joints. The CR is directed perpendicular to the midpoint of the forearm. The image must demonstrate the radius and ulna in their entirety without rotation.
📋 CLINICAL PEARL
Getting a true AP of the forearm requires full supination. If the patient is in too much pain to rotate the arm, you may need to accept a slightly oblique position. In these cases, note "limited rotation due to patient condition" and consider a two-view trauma series — separate AP of the distal forearm (wrist) and AP of the proximal forearm (elbow) on smaller IRs if the patient truly cannot supinate the whole arm.
Lateral Forearm Projection
Positioning
- From the AP position, flex the elbow to 90°
- Rotate the arm into a true lateral position — the thumb points upward, the palm faces medially (toward the patient's body)
- The humerus should be horizontal (flat on the table), with the forearm angled upward
- Support the wrist with a sponge or sandbag if needed to maintain true lateral
IR and Centering
| Parameter | Specification |
|---|---|
| IR size | 14 × 17" (35 × 43 cm) lengthwise |
| Orientation | Portrait/vertical (long axis of forearm) |
| Centering | Midpoint of the forearm |
| SID | 40" (100 cm) |
| kVp | 55–65 |
| mAs | 3.0–6.0 (slightly higher than AP due to increased tissue thickness) |
| Grid | No (extremity technique) |
What to Include
- Proximal: Elbow joint in profile — olecranon process, radial head, trochlea. On a true lateral, the olecranon process should be seen in profile within the olecranon fossa
- Distal: Wrist joint in lateral view — distal radius and ulna superimposed
- Key check: The radius and ulna should overlap at the distal end on a true lateral
Clark's Reference — Lateral Forearm
Clark's Pocket Handbook (p46) specifies: The elbow is flexed 90°, the forearm is in a true lateral position with the thumb uppermost. The CR is directed perpendicular to the midpoint of the forearm. A true lateral projection demonstrates the radial head anterior to the coronoid process of the ulna, and the distal radius and ulna are superimposed.
📋 ARRT EXAM TIP
A frequent exam question: "What is the key anatomical relationship seen on a true lateral forearm?" Answer: The distal radius and ulna are superimposed. If they aren't superimposed, the forearm is rotated and not a true lateral. Also, the radial head should be anterior to the coronoid process of the ulna at the elbow.
Technical Factors Comparison
| Parameter | AP Forearm | Lateral Forearm |
|---|---|---|
| kVp | 55–65 | 55–65 |
| mAs | 2.0–5.0 | 3.0–6.0 |
| SID | 40" | 40" |
| Grid | Usually no | Usually no |
| Exposure time | Short (to minimize motion) | Short (patient may fatigue with arm flexed) |
| Breathing | Suspend respiration | Suspend respiration |
Common Positioning Errors
| Error | Cause | Fix |
|---|---|---|
| Joint not included | Poor centering or wrong IR placement | Palpate landmarks; ensure IR extends 1–2" beyond both elbow and wrist |
| Rotation on AP | Forearm not supinated fully | Check: palm should be facing UP; thumb should be on the lateral side |
| Rotation on lateral | Forearm not in true lateral | Check: thumb should point UP; radius and ulna should overlap distally |
| Foreshortening | Arm not parallel to IR | Adjust patient position or angle CR to remain perpendicular to forearm |
| Overexposure | kVp too high for small forearm | Use extremity technique (55–65 kVp); reduce for pediatric/atrophic patients |
| Underexposure | kVp too low in larger patient | Increase kVp by 5–10 for large/muscular forearms |
| Motion blur | Patient moved during exposure | Use shorter exposure time; immobilize with sandbags |
| Wrong IR size | Using 10 × 12" instead of 14 × 17" | Always use 14 × 17" for forearm — the only way to include both joints |
Evaluation Criteria
When reviewing your forearm images, check these criteria:
AP Forearm
- Both wrist and elbow joints included
- Radius and ulna visible in their entirety
- No rotation — both bones appear parallel with no superimposition
- Elbow joint: open joint space visible
- Wrist joint: distal radius and ulna clearly seen with no overlap
- Adequate penetration — trabecular pattern visible in both bones
- No motion — sharp bony margins and cortical outlines
- R or L marker visible and not overlying anatomy
Lateral Forearm
- Both wrist and elbow joints included
- Elbow flexed 90° — humeral condyles superimposed
- Distal radius and ulna superimposed — confirms true lateral
- Radial head anterior to coronoid process — confirms true lateral
- Olecranon process seen in profile
- Adequate penetration
- No motion
- R or L marker visible
📋 CLINICAL PEARL
The lateral forearm is the single best view for detecting subtle fractures. A nondisplaced radial head fracture, for example, may be invisible on the AP but obvious on the lateral as a "fat pad sign" — an elevated anterior or posterior fat pad of the elbow joint. If you see a fat pad on the lateral forearm, that's a positive fat pad sign indicating an occult fracture (most commonly of the radial head).
Trauma and Non-Routine Positioning
When the Patient Cannot Supinate
If the patient has a suspected fracture and is in too much pain to rotate the arm:
- Keep the forearm in a comfortable position (usually neutral or slightly pronated)
- Take two AP views: one centered on the wrist (distal forearm) and one centered on the elbow (proximal forearm)
- Take the lateral in the usual position if possible
- Document that the patient was unable to achieve full supination
Pediatric Forearm Considerations
- Include the contralateral forearm for comparison when evaluating for subtle fractures or growth plate injuries
- Lower kVp by 10–15 compared to adult technique
- Use shorter exposure times to minimize motion
- Immobilization: sandbags and parent assistance; for younger children, use acetate tape to secure the hand in position
ARRT Exam Quick Reference
Here's what the ARRT expects you to know about forearm positioning:
- IR size: 14 × 17" (35 × 43 cm) — portrait orientation
- Centering: Midpoint of the forearm
- Inclusion: Both wrist and elbow joints (ARRT tests this frequently)
- AP technique: Forearm supinated (palm up)
- Lateral technique: Elbow flexed 90°, thumb up
- Lateral check: Distal radius/ulna must be superimposed
- Fat pad sign: Positive on lateral = occult radial head fracture
- kVp range: 55–65 for adult forearm (extremity technique)
📋 ARRT EXAM TIP
Here's a scenario question you might see: "A patient arrives with a suspected forearm fracture and cannot supinate the arm for the AP projection. What should the technologist do?" The correct answer is to perform separate AP projections of the proximal and distal forearm centered on the elbow and wrist respectively, documenting the positioning limitation. Never force a fractured forearm into a position that causes pain.