Radiographic Imaging & Chest X-ray Interpretation

Objective 1.4.1 — Evaluate radiographic imaging quality using penetration, rotation, and inspiration; identify standard projections (PA, AP, lateral neck, lateral decubitus); recognize normal anatomical structures and landmarks on a chest radiograph; and verify proper placement of endotracheal tubes, tracheostomy tubes, pacemakers, Swan-Ganz catheters, central venous catheters, chest tubes, and nasogastric tubes.

Overview & Indications

Chest radiography (chest X-ray / CXR) is one of the most frequently performed diagnostic examinations in critical care and pulmonary medicine. It provides essential baseline and dynamic information regarding the anatomical and physiological state of the thorax.

Primary Indications for a Chest X-ray

Detect alterations of the lung (e.g., infiltrates, atelectasis, masses, hyperinflation)
Determine the appropriate therapy (e.g., bronchodilators, diuresis, chest tube insertion)
Evaluate the effectiveness of treatment (e.g., resolving pneumonia or clearing pulmonary edema)
Determine tube and catheter positions (e.g., endotracheal tube, central lines, nasogastric tube)
Observe lung disease progression or improvement over time
Assess the patient after an invasive procedure (e.g., post-intubation, post-thoracentesis, or post-central line insertion to rule out pneumothorax)

Positions & Projections

Understanding the path of the X-ray beam relative to the patient and image receptor is critical for correctly interpreting organ size and localized pathology.

      PA PROJECTION                        AP PROJECTION
   (Posterior -> Anterior)             (Anterior -> Posterior)
   
     [X-ray Tube]                        [X-ray Tube]
          │                                   │
          ▼                                   ▼
    (Patient Back)                     (Patient Chest)
    (Patient Chest)                    (Patient Back)
          │                                   │
          ▼                                   ▼
   [Image Receptor]                    [Image Receptor]

Projection / Position	Description & Clinical Utility
PA Projection (Posterior – Anterior)	• X-rays travel from posterior to anterior. • Image receptor touches the patient's anterior chest with the patient's back facing the X-ray tube. • Standard position for a routine outpatient or stable chest X-ray. • Minimizes artificial magnification of the heart silhouette.
AP Projection (Anterior – Posterior)	• X-rays travel from anterior to posterior. • Image receptor is placed behind the patient's back. • Commonly used for bedridden patients or patients in the ICU using a portable X-ray machine who are too sick to be transported to radiology. • Note: Causes artificial magnification of the heart shadow because the heart is further from the image receptor.
Lateral Position	• Projection from either the right or left side against the image receptor. • Excellent for viewing structures behind the heart or base of the lungs.
Lateral Decubitus	• Patient lying on the affected side. • Extremely valuable diagnostic projection for detecting small pleural effusions (fluid layers out along the dependent lateral chest wall).
Lateral Neck	• Valuable diagnostic tool for identifying upper airway obstruction in children. • Croup (laryngotracheobronchitis) reveals subglottic narrowing known as the steeple sign. • Epiglottitis reveals a swollen, enlarged epiglottis known as the thumb sign. • Both cause localized swelling of tissues that may result in partial to complete airway obstruction requiring prompt therapy.

Upper Airway Obstruction Radiographic Signs

Here are representative classic images illustrating upper airway pathology on neck radiographs:

Croup - Steeple Sign showing subglottic narrowing

Epiglottitis - Thumb Sign showing an enlarged, rounded epiglottis shadow

Evaluating X-ray Technical Quality

Before interpreting diagnostic findings, the respiratory therapist must assess the technical quality of the film using three key factors: Penetration, Rotation, and Inspiration.

1. Penetration / Exposure

The degree to which X-rays have passed through the body tissues to reach the film.

Proper Penetration: The vertebrae are just visible behind the heart shadow. The spaces between the vertebrae should be equal, visible, and distinct.
Underexposed (Under-penetrated): Film appears too white. Does not allow visualization of the intervertebral discs through the heart shadow.
Overexposed (Over-penetrated): Film appears too dark/black. Shows black lung parenchyma without blood vessels (vascular markings are lost).

2. Rotation

The patient's orientation relative to the image receptor.

The patient should not be rotated.
Verification: The heads of the clavicles should be level and equidistant from the spinous processes of the vertebral column.

3. Inspiration

The degree of lung inflation during film exposure.

Films are routinely taken at full inspiration.
Verification of adequate inspiration:
- 5 – 6 anterior ribs should be visible above the diaphragm.
- 8 – 10 posterior ribs should be visible above the diaphragm.

The Normal Chest X-ray

A technically acceptable, normal PA chest radiograph demonstrates distinct anatomical relationships and clear lung zones.

       NORMAL PA CHEST X-RAY LANDMARKS
       
             (Trachea Midline)
                    │
            ┌───────┴───────┐
            │               │
       (R. Hilum)       (L. Hilum)
            │               │
            │           (Heart Shadow)
            │               │
     _______▼_______________▼_______
    / R. Hemidiaphragm               \  <-- L. Hemidiaphragm
   /  (Higher: 6th rib)               \     (Slightly Lower)
  └────────────────────────────────────┘

Normal General Characteristics

Both hemidiaphragms are rounded and dome-shaped.
Right hemidiaphragm is slightly higher than the left due to the underlying liver.
The right hemidiaphragm normally rests at the level of the sixth anterior rib.
Trachea is midline and appears as a vertical radiolucent (dark) column.
Bilateral radiolucency throughout the lung fields with clear vascular markings.
Sharp costophrenic angles bilaterally.

Standard Normal PA Chest Radiograph demonstrating clear lung zones and proper landmarks

Anatomical Landmarks

A systematic approach to evaluating anatomical structures ensures subtle pathology is not missed.

1. Trachea

Seen as a dark (radiolucent) area midline.
Approximately the same width as the vertebral column.
Pathology: A shift to one side indicates a unilateral lung problem (e.g., shifts away from a tension pneumothorax or massive effusion; shifts toward massive atelectasis or lobar collapse).

2. Mediastinum

The central area between the lungs containing the heart, lymphatics, major blood vessels, and mainstem bronchi.
Pathology: May shift with a significant pleural effusion, tension pneumothorax, or severe volume loss.

3. Hilum / Hilus

Consists primarily of the major bronchi, pulmonary arteries, and pulmonary veins.
Asymmetrical but contains the same basic anatomical structures on each side.
Left hilum is commonly higher than the right hilum.
Both hila should be of similar overall size and radiodensity.

4. A-P Diameter

The anterior-posterior depth of the thorax.
Pathology: Increased A-P diameter is a classic finding in COPD, severe asthma, barrel chest, and chronic hyperinflation.

5. Costophrenic Angles

The acute angle formed by the outer downward curve of the diaphragm and the lateral chest wall.
Pathology: Angles are obliterated (blunted or blurred) by pleural effusions.

6. Diaphragm

Normally rounded and dome-shaped.
Right hemidiaphragm is slightly higher than the left at the sixth anterior rib.
Pathology: Flattened diaphragm is a primary indicator of hyperinflation resulting from COPD.

7. Vascular Markings

Fine white branching structures representing blood vessels, lymphatics, and interstitial lung tissue.
Visible throughout the normal lung parenchyma. Absence of markings indicates hyperlucency (e.g., pneumothorax or severe emphysema).

8. Heart Size and Position

Should take up less than half of the internal thoracic cavity width on a PA film.
Cardiothoracic (C/T) ratio < 50%.
Positioned approximately 1/3 on the right side and 2/3 on the left side of the midline.
Right Cardiac Border: Consists of two distinct bulges — the superior vena cava and the right atrium.
Left Cardiac Border: Consists of three distinct bulges — the aortic knob, the main pulmonary artery, and the left ventricle.
Pathology: An increase in the cardiac shadow or silhouette (cardiomegaly) indicates congestive heart failure (CHF) or a significant pericardial effusion.

9. Soft Tissue

Tissue surrounding the chest wall and extending into the neck area.
Pathology: Subcutaneous emphysema occurs when free air (hyperlucency) tracks into the surrounding subcutaneous soft tissue planes.

10. Ribs

Inspect for normal smooth curvature, intercostal spacing, and intact cortical margins (rule out fractures).

11. Pleura and Pleural Spaces

The visceral and parietal pleura are normally thin and only visible when an abnormality is present.
Pathology visible in the pleural space:
- Pleural thickening
- Pleural effusions (fluid accumulation causing blunted costophrenic angles or a meniscus sign)
- Pneumothorax (air accumulation separating the visceral pleura from the chest wall, visible as a distinct visceral pleural line without peripheral vascular markings)

Chest Radiograph demonstrating a Pleural Effusion layering at the base

Verification of Tube & Catheter Placement

A primary ICU responsibility for the respiratory therapist is verifying that artificial airways, monitoring catheters, and therapeutic tubes are correctly positioned on the chest radiograph.

Device	Proper Radiographic Landmark & Positioning Criteria	Clinical Indications & Notes
Endotracheal Tube (ETT)	• Tip must be below the vocal cords. • Positioned below the superior clavicular margin and above the T5 spinous process. • 2 – 5 cm above the carina (tracheal bifurcation).	• Maintains advanced airway patency and mechanical ventilation support. • Too deep: Enters right mainstem bronchus causing left lung atelectasis.
Tracheostomy Tube	• Positioned centrally within the mid-trachea. • Does not pose the hazard of mainstem intubation, but requires monitoring to ensure placement inside the tracheal lumen (rule out false tract placement).	• Long-term mechanical ventilation or airway bypass.
Pacemaker	• Pacing wires and radiopaque electrodes should normally terminate positioned securely in the apex of the right ventricle.	• Cardiac conduction management.
Pulmonary Artery Catheter (Swan-Ganz)	• Radiopaque tip should appear positioned within the right lower lung field.	• Indication: Measuring pulmonary artery pressures, PCWP, and cardiac output.
Central Venous Catheter (CVC)	• Inserted via the subclavian or internal jugular vein. • Tip placement must reside in the lower portion of the superior vena cava (SVC), directly proximal to the right atrium.	• Indication: Long-term venous access, central pressure monitoring, and infusion of concentrated vasoactive medications.
Chest Tube	• Located securely within the pleural space surrounding the affected lung. • Tubes feature a radiopaque stripe along their length with a distinct gap/break identifying the most proximal drainage eyelet. • The break in the radiopaque line must be fully inside the pleural space to prevent subcutaneous air leaks.	• Indications: Evacuation of a Pneumothorax (air) or Hemothorax / Pleural Effusion (fluid/blood).
Nasogastric Tube (NGT)	• The radiopaque tip and feeding ports must cross below the left hemidiaphragm. • Securely positioned within the stomach 2 – 6 cm below the diaphragm.	• Indications: Administering enteral feedings or performing active aspiration of gastric contents. • Hazard: Accidental insertion into the tracheobronchial tree.

Online Radiology Gallery Reference

For extensive clinical review and comparison of normal and pathological chest radiographs across diverse clinical scenarios, consult external educational archives:

🔗 Radiology Masterclass Galleries — Explore specialized sub-galleries detailing acute thoracic conditions, hardware placement, and cross-sectional imaging fundamentals.

Quick Reference Summary

Projections at a Glance

PA: Standard · beam enters posterior · upright film · accurate heart size. AP: Portable ICU · beam enters anterior · supine/bedridden · magnified heart shadow. Lateral Decubitus: Patient lies on affected side · best for small pleural effusions. Lateral Neck: Croup = Steeple sign (subglottic narrowing) · Epiglottitis = Thumb sign (swollen epiglottis).

Film Quality Criteria

Penetration: Vertebrae just visible behind heart shadow. Underexposed = too white. Overexposed = too black/no blood vessels. Rotation: Clavicle heads level and equidistant from spine. Inspiration: 5–6 anterior ribs or 8–10 posterior ribs visible above diaphragm.

Key Landmarks & Pathology

Trachea: Midline column · shifts away from tension pneumothorax · shifts toward atelectasis. Diaphragm: Domes · Right higher than Left (at 6th anterior rib) · Flattened in COPD. A-P Diameter: Increased in COPD / Barrel chest. Costophrenic Angles: Sharp acute point · Blunted/obliterated by pleural effusion. Heart Size: C/T ratio < 50% · 1/3 right, 2/3 left · Cardiomegaly = CHF or pericardial effusion. Subcutaneous Emphysema: Air tracking into soft tissue planes.

Tube Positions

ETT Tip: 2–5 cm above the carina (between clavicles and T5). CVC Tip: Lower Superior Vena Cava (SVC) just above right atrium. Swan-Ganz Tip: Right lower lung field. Chest Tube Stripe Break: Fully inside the pleural space. NG/Feeding Tube Tip: Stomach 2–6 cm below diaphragm.