Learning Radiology Chapter 2 - Recognizing Normal Pulmonary Anatomy

Technical Adequacy Parameters of a Chest X-Ray

• Five technical variables must always be checked before interpreting anatomy or pathology:

  • Penetration

    • Adequate: vertebral bodies and intervertebral discs are faintly seen through the cardiac silhouette.

    • Under-penetrated (film too light)

      • Spine and left hemidiaphragm disappear → can falsely mimic basilar disease or hide true pathology.

    • Over-penetrated (film too dark)

      • Lung fields appear excessively black → can mimic emphysema or pneumothorax.

  • Inspiration

    • Adequate when \ge 10 posterior ribs (ideal) or a minimum of 8\text{–}9 posterior ribs are visible above the highest point of the hemidiaphragm on the frontal PA view.

    • Poor inspiration crowds basilar vessels, exaggerating lung markings and falsely enlarging the cardiac silhouette.

  • Rotation

    • Spinous process should lie equidistant between the medial ends of the clavicles.

    • Rotation right ➔ left clavicular head approaches spinous process; rotation left ➔ right clavicular head approaches spinous process.

    • Rotation distorts heart contour, hila height, and diaphragm position.

  • Magnification

    • PA images (source → posterior, detector anterior) minimize magnification; AP (portable) adds modest magnification, especially of the heart.

  • Angulation (Lordotic tilt)

    • Occurs when the X-ray beam is angled cephalad (common in semi-recumbent portable films).

    • Clavicles project above posterior first ribs and lose their “S” curve; heart elongated; left hemidiaphragm partly obscured.

Normal Frontal Chest Radiograph – Key Landmarks (Fig 2.1)

• Trachea midline, slightly right of aortic knob.

• Hila: left hilar apex normally ≈0.5\text{–}1.5\,\text{cm} higher than right.

• Aortic knob, ascending aorta, SVC visualized.

• Cardiac borders: right atrium (right heart border); left ventricle (left heart border).

• Pulmonary vasculature: branching, tapering white lines throughout lung fields.

  • Vessels larger and more numerous at bases than apices in upright patient because gravity increases perfusion.

• Minor (horizontal) fissure on right projects as a thin horizontal line at level of 4^{th} anterior rib.

• Costophrenic angles (lateral) and cardiophrenic angles (medial) should be sharp and acute.

• Posterior vs anterior ribs:

  • Posterior ribs horizontal and more conspicuous.

  • Anterior ribs angled caudally toward costophrenic sulcus and less distinct.

Normal Pleura & Fissures

• Visceral pleura adherent to lung, folds to form fissures.

• Parietal pleura lines thoracic wall.

• Only the opposed visceral layers in a fissure can appear as a hair-line opacity \le 1\,\text{mm} thick.

• Normal pleural space contains a few mL of fluid but no air.

Normal Pulmonary Vasculature Principles (Fig 2.3)

• Upright gravity gradient: basal vessels > apical vessels (size, not number).

• All vessels taper smoothly from hilum toward pleura.

• Cannot distinguish arteries vs veins on plain CXR.

Normal Lateral Chest Radiograph (Fig 2.5)

Five routinely inspected zones (Box 2.1):

• Retrosternal clear space

  • dark, air-filled area behind sternum. Soft-tissue filling suggests anterior mediastinal mass (lymphoma, thymoma, thyroid goitre, teratoma).

  • Pitfall: patient’s arms kept down create apparent soft-tissue; identify humeral shafts to avoid false-positive.

• Hilar region

  • should merge imperceptibly with lung; no discrete mass seen.

  • Posterior to bronchus intermedius must remain lucent.

• Fissures

  • thin, uniform lines; major fissure courses obliquely T5 to anterodiaphragm; minor fissure horizontal at 4^{th} anterior rib (right only).

  • Thickening > 2 mm suggests fluid (CHF, Kerley B lines) or fibrosis.

• Thoracic spine

  • vertebral bodies rectangular, disc spaces equal/gradually widen caudally; density should increase (become “blacker”) caudally (Spine Sign).

  • Compression fracture: loss of height, superior end-plate collapse, osteophytes if degenerative.

  • Additional classic sign: Spine Sign – lower thoracic spine becomes whiter instead of darker on lateral film → suggests left lower-lobe opacity behind heart (e.g. pneumonia).

• Diaphragm & posterior costophrenic angles

  • Right hemidiaphragm usually higher and continuous front-to-back.

  • Left hemidiaphragm silhouetted anteriorly by heart.

  • Posterior costophrenic sulci sharply acute; blunted when effusion present.

    • Minimal detectable volumes: \approx 75\,\text{mL} blunts posterior sulcus on lateral; \approx 250\text{–}300\,\text{mL} blunts lateral sulcus on frontal.

Example Pathologic/Technical Scenarios Illustrated

• Anterior mediastinal adenopathy filling retrosternal space.

• Bilateral hilar adenopathy (sarcoidosis) casting lobulated mass on lateral.

• Fluid in both major fissures during congestive failure.

• Osteoporotic compression fracture with degenerative osteophytes.

• Case Quiz 2: Extreme patient rotation toward right mimicked dextrocardia; clavicular heads both overlie right hemithorax.

Chest CT – General Conventions & Viewing

• Patient viewed as if supine, feet to viewer: right → image left, anterior → top.

• Volumetric helical acquisition allows reconstruction in:

  • Axial (standard),

  • Coronal,

  • Sagittal planes.

• Windowing presets (single data set manipulated digitally, no re-scan):

  • Lung window – optimal for parenchyma & airway detail; mediastinum appears uniformly white.

  • Mediastinal (soft-tissue) window – mediastinum, hila, pleura shown; lung parenchyma black.

  • Bone window – cortex and trabeculae maximally contrasted.

CT Anatomy Highlights (Figs 2.13–2.20 & Video 2.1)

• Trachea: oval, \approx 2\,\text{cm} diameter; bifurcates at carina into R & L main bronchi.

• Aortopulmonary window (AP window) – fat space bounded by aortic arch (sup), left pulmonary artery (inf), trachea (med), left lung (lat). Common site of nodal enlargement.

• Right main bronchus gives off RUL bronchus then continues as bronchus intermedius (should have only aerated lung posteriorly, absence implies mass/adenopathy).

• Normal bronchus:artery ratio < 1 (bronchus < accompanying pulmonary artery). In bronchiectasis ratio reverses.

• Fissure representation on CT

  • Appears as

  • Thin white line when slice parallels fissure,

  • Relative avascular zone up to 2\,\text{cm} thick when slice crosses fissure obliquely.

  • Minor fissure visible best on sagittal/coronal because it lies horizontally.

• Lobar boundaries (sagittal):

  • Right: major fissure separates RLL from RUL & RML; minor fissure separates RUL from RML.

  • Left: major fissure separates LLL from LUL; lingula = middle-lobe analogue.

Comparative Detection Thresholds & Quantitative Facts

• Adequate inspiration: \ge 10 posterior ribs ideal.

• Posterior costophrenic blunt: \approx 75\,\text{mL} pleural fluid.

• Lateral costophrenic blunt on PA: 250\text{–}300\,\text{mL} fluid.

• Bronchus < artery in normal pair; reversed in bronchiectasis.

Practical Reading System (Author’s Suggested Checklist)

1. Technical adequacy (penetration, inspiration, rotation, magnification, angulation)

2. Soft tissues (neck, breast shadows, subcutaneous emphysema)

3. Bones (ribs, clavicles, scapulae, vertebrae, humeri)

4. Heart & mediastinum (size, contours, great vessels)

5. Lungs & hila (vessels, parenchyma, fissures)

6. Diaphragm & pleura (costophrenic/cardiophrenic angles, hemidiaphragm contours)

Key Take-Home Pearls

• Virtually all visible lung markings on radiograph = pulmonary vessels; normal bronchi are invisible because thin-walled & air-containing.

• Normal vessels taper peripherally; basal > apical calibre upright.

• Minor fissure usually visible on frontal; both fissures on lateral.

• Right hemidiaphragm higher, seen in entirety laterally; left masked anteriorly by heart.

• Retrosternal clear space, spine sign, and posterior costophrenic angles are rapid lateral-film checks for mediastinal mass, lobe opacity, and effusion, respectively.

• CT adds: airway detail, nodal stations (AP window), precise fissure/segment anatomy, multiplanar display, and distinct window options without extra radiation.

• System knowledge ≫ mnemonic; “You only see what you look for, and you only look for what you know.”