11.3 Radiographic Positioning and Normal Anatomy: Extremities, Thorax, and Abdomen
Key Takeaways
- The shoulder series includes AP (internal and external rotation) and a scapular-Y or axillary lateral to assess glenohumeral alignment and dislocation direction.
- Elbow fat pad signs (anterior and posterior) on a true lateral indicate joint effusion and suggest occult fracture even when no fracture line is visible.
- Weight-bearing knee and foot views reveal joint-space narrowing from osteoarthritis that may not appear on supine films.
- A PA chest radiograph requires the scapulae rotated off the lung fields, equal rib spaces, and 10 posterior ribs visible above the diaphragm on full inspiration.
- An erect abdominal radiograph (KUB) is preferred for detecting pneumoperitoneum (free air under the diaphragm) and air-fluid levels in bowel obstruction.
11.3 Radiographic Positioning and Normal Anatomy: Extremities, Thorax, and Abdomen
Quick Answer: Part II expects you to know the standard view series for major extremity joints, the chest, and the abdomen; the positioning criteria that confirm a technically adequate image; and the anatomic landmarks and soft-tissue signs that guide interpretation. Weight-bearing views, true laterals, and erect positioning are recurring themes.
Upper Extremity
Shoulder
| View | Positioning Key | Diagnostic Purpose |
|---|---|---|
| AP (internal rotation) | Arm at side, palm facing posteriorly; humeral head centered in glenoid | General osseous anatomy, greater tuberosity profile |
| AP (external rotation) | Arm at side, palm facing anteriorly | Lesser tuberosity profile, bicipital groove |
| Scapular-Y lateral | Patient rotated ~45° so the scapula is edge-on ("Y" shape) | Glenohumeral alignment, anterior vs. posterior dislocation |
| Axillary lateral | Arm abducted 90°; beam angled through axilla | Alternative for dislocation; also shows inferior glenoid rim |
On a properly positioned AP shoulder, the glenohumeral joint space should be uniform. The scapular-Y view profiles the coracoid, body, and spine of the scapula as a "Y" with the humeral head centered in the intersection — anterior dislocation places the head anterior to the Y; posterior dislocation places it posterior.
Elbow
| View | Positioning Key | Diagnostic Purpose |
|---|---|---|
| AP | Arm extended, palm up (supinated) | Radial head, coronoid process, medial/lateral epicondyles |
| Lateral (90° flexion) | True mediolateral with forearm and humerus superimposed | Fat pad signs, radial head, coronoid, olecranon |
The elbow fat pad sign is a critical soft-tissue finding on the true lateral:
- Anterior fat pad: normally visible as a thin lucent stripe anterior to the distal humerus; displacement upward ("sail sign") indicates joint effusion
- Posterior fat pad: normally not visible; any visible posterior fat pad is always abnormal and indicates effusion
Both fat pad signs together strongly suggest an occult fracture (radial head/neck most commonly) even when no fracture line is seen.
Wrist and Hand
| View | Positioning Key | Diagnostic Purpose |
|---|---|---|
| PA wrist | Forearm and hand flat, palm down | Distal radius/ulna, carpal rows, metacarpals |
| Lateral wrist | True lateral with radial styloid slightly anterior to ulnar styloid | Distal radius volar tilt, lunate position, carpal alignment |
| Oblique hand/wrist | Hand rotated 45° | Scaphoid profile, intercarpal joints |
| Scaphoid view | Ulnar deviation of wrist with beam angled toward scaphoid | Dedicated scaphoid visualization when fracture is suspected |
Normal carpal alignment on the lateral wrist: the radius, lunate, capitate, and third metacarpal should form a smooth arc (the carpal arcs of Gilula). Disruption suggests dislocation or fracture.
Lower Extremity
Hip
Covered in Section 11.2 (AP pelvis, frog-leg lateral, cross-table lateral). Additional points:
- Cross-table lateral: patient supine; affected leg remains neutral; the tube is angled horizontally with the cassette placed beside the hip. This is the safest view when fracture is suspected.
- Shenton's line: a smooth arc from the medial femoral neck to the inferior pubic ramus — disruption indicates fracture or dislocation.
Knee
| View | Positioning Key | Diagnostic Purpose |
|---|---|---|
| AP (weight-bearing preferred) | Patient standing, knees forward | Joint-space symmetry, osteophytes, alignment |
| Lateral | True mediolateral, 5–10° flexion | Patella, tibial plateau, suprapatellar effusion |
| Sunrise/tunnel (patellofemoral) | Knee flexed ~45°, beam angled under patella | Patellofemoral joint, patellar tracking |
| Oblique | Leg rotated 45° medially or laterally | Tibial plateau, fibular head |
Weight-bearing AP knee is preferred for assessing osteoarthritis because joint-space narrowing (loss of cartilage) is best demonstrated under physiologic load. A supine AP may show a normal joint space in a patient with significant weight-bearing compartment narrowing.
Ankle and Foot
| View | Positioning Key | Diagnostic Purpose |
|---|---|---|
| AP ankle | Leg internally rotated 15–20° | Tibiotalar joint, malleoli |
| Lateral ankle | True mediolateral | Tibiotalar alignment, calcaneus, Achilles tendon region |
| Mortise view | Leg internally rotated 15–20° (same as AP but named for joint profile) | Tibiotalar joint space uniformity (medial, superior, lateral clear spaces) |
| AP foot | Foot flat, plantar surface against receptor | Metatarsals, phalanges, tarsal bones |
| Lateral foot | True mediolateral | Calcaneal inclination angle, Lisfranc region |
| Oblique foot | Foot rotated 30–40° medially | Intertarsal and tarsometatarsal joints |
The ankle mortise view profiles the tibiotalar joint so the joint space appears uniform on all three sides (medial, superior, lateral). Asymmetric widening of the medial clear space suggests deltoid ligament injury or lateral malleolus fracture.
Thorax (Chest)
Standard View Series
| View | Positioning Key | Diagnostic Purpose |
|---|---|---|
| PA erect (standard) | Patient standing, back to receptor, arms hugging detector; beam at T7 | Heart size, lungs, mediastinum, pleura, bony thorax |
| AP erect/portable | Patient facing receptor (supine or semi-erect) | Bedside studies; heart appears magnified |
| Lateral | Arms raised, left side against receptor (left lateral standard) | Posterior costophrenic angles, retrosternal space, thoracic spine |
Positioning Criteria and Landmarks
A technically adequate PA chest requires:
- Full inspiration: 10 posterior ribs (or 6 anterior ribs) visible above the diaphragm
- No rotation: medial ends of the clavicles equidistant from the spinous processes
- Scapulae off lung fields: achieved by rolling shoulders forward and depressing them
- Adequate penetration: vertebrae faintly visible through the heart shadow
Key anatomic landmarks:
- Cardiothoracic ratio: heart width (widest point) divided by thoracic width (at diaphragm) on PA view; normal is less than 50% on a properly inspired film
- Costophrenic angles: should be sharp; blunting suggests pleural effusion (minimum 175–200 mL to be visible on erect film)
- Hilar regions: left hilum normally sits slightly higher than the right; abnormal enlargement may indicate lymphadenopathy or vascular congestion
- Diaphragm: right hemidiaphragm is normally 1–2 cm higher than the left due to the liver
Common Chest Faults
| Fault | Effect |
|---|---|
| Incomplete inspiration | Heart and mediastinum appear enlarged; lower lobes crowded |
| Rotation | Apparent mediastinal shift, asymmetric rib spaces |
| AP instead of PA | Magnified heart (increased cardiothoracic ratio artifact) |
| Scapulae over lung fields | Pulmonary pathology obscured |
Abdomen
Standard View Series
| View | Positioning Key | Diagnostic Purpose |
|---|---|---|
| Supine AP (KUB) | Patient supine, beam centered at iliac crest level | Bowel gas pattern, calcifications, organ outlines, foreign bodies |
| Erect AP abdomen | Patient standing or sitting upright | Free air under diaphragm (pneumoperitoneum), air-fluid levels |
| PA chest (lower included) | Often obtained with erect abdomen to evaluate diaphragm | Pneumoperitoneum (free air under right hemidiaphragm) |
Positioning Criteria and Landmarks
The supine KUB (kidneys, ureters, bladder region) visualizes:
- Bowel gas pattern: normal small bowel gas is scattered and non-dilated; dilated loops (>3 cm small bowel, >6 cm colon) suggest obstruction
- Psoas shadows: bilateral soft-tissue stripes lateral to the lumbar spine; obliteration may indicate retroperitoneal pathology
- Properitoneal fat lines: thin lucent stripes outlining the peritoneum; displacement suggests mass or fluid
- Calcifications: renal, ureteral, gallbladder, vascular, or phleboliths (pelvic, round, often with central lucency)
The erect abdomen is the view of choice for detecting pneumoperitoneum (free intraperitoneal air). On an erect film, free air accumulates beneath the diaphragm as a crescent-shaped lucency. As little as 1 mL of free air may be visible on an erect chest/abdomen film, compared with approximately 300 mL needed on a left lateral decubitus view.
Air-fluid levels on an erect abdomen suggest ileus or mechanical obstruction. Multiple air-fluid levels at different heights in dilated small-bowel loops are characteristic of obstruction.
Common Abdominal Faults
| Fault | Effect |
|---|---|
| Supine film for free air | Pneumoperitoneum not visible — free air rises and requires erect or decubitus positioning |
| Inadequate collimation | Unnecessary pelvic/hip anatomy exposed; increased scatter |
| Motion from respiration | Blurred psoas shadows and bowel margins |
| Exposure technique too low | Soft-tissue structures (psoas, properitoneal fat lines) not visible |
Protection and Quality Principles Across Regions
Regardless of the body region, three principles apply:
- Collimate to the anatomy of interest — a chest film does not need the entire pelvis exposed
- Shield gonads and thyroid when the shield does not obscure the diagnostic area (for example, do not place a gonadal shield over the sacrum on a pelvis view)
- Repeat only when necessary — identify whether the fault is positioning (reposition) or technique (adjust mAs/kVp) before re-exposing the patient
On a true lateral elbow radiograph, a visible posterior fat pad indicates:
Why is a weight-bearing AP knee radiograph preferred over a supine AP for assessing osteoarthritis?
Which positioning criterion confirms adequate inspiration on a PA chest radiograph?
An erect abdominal radiograph is preferred over a supine view for detecting: