Basic HRCT lung anatomy and bronchopulmonary segments

1. How to orient an axial HRCT image

• Images are conventionally viewed as if looking up from the patient's feet.
  • Patient's right is on the left side of the image.
  • Anterior is at the top, posterior at the bottom.
• On lung windows:
  • Air is black.
  • Normal lung is dark grey.
  • Vessels, bronchial walls, fissures, and interlobular septa are relatively white.
• HRCT uses thin sections and a high-spatial-frequency reconstruction to show fine lung structures, especially small airways, interstitium, and the secondary pulmonary lobule.

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2. Lobar anatomy on HRCT

Right lung: three lobes

Left lung: two lobes

• The oblique (major) fissure separates the lower lobe from the upper lobe on the left, and from the upper and middle lobes on the right.
• The horizontal (minor) fissure is usually present only on the right and separates RUL from RML. Gray's Anatomy for Students, p. 205.

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3. Bronchopulmonary segment: definition

1. One segmental (tertiary) bronchus
2. Its accompanying branch of the pulmonary artery

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4. Right bronchopulmonary segments

Right upper lobe

Right middle lobe

Right lower lobe

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5. Left bronchopulmonary segments

Left upper lobe

Left lower lobe

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6. HRCT landmarks for localizing a lesion

A. Upper lobe versus lower lobe

• Upper lobe disease tends to lie anteriorly at the upper chest.
• Lower lobe disease tends to lie posteriorly, particularly near the bases.
• In supine CT, dependent posterior lower-lobe ground-glass opacity may be positional atelectasis. Prone images can help distinguish this from genuine interstitial abnormality.

B. Middle lobe and lingula

• RML: anterior-medial, contacts the right cardiac border.
• Lingula: anterior-medial left upper lobe, contacts the left cardiac border.
• These are analogous locations. Bronchiectasis or nodularity involving both often suggests a small-airway distribution pattern.

C. Superior segment of lower lobe, S6

• Lies in the posterior upper portion of each lower lobe.
• It may look relatively high on axial images but is still a lower-lobe segment.
• This is a common localization trap.

D. Basal segments

• Seen more clearly on lower cuts.
• Posterior basal segments, S10, lie most posteriorly.
• Anterior basal segments, S8, are more anterior.
• Lateral basal segments, S9, are lateral.

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7. Secondary pulmonary lobule: the key HRCT unit

HRCT patterns based on lobular anatomy

• Centrilobular nodules: spare the pleural surface and interlobular septa; suggest bronchiolar disease.
• Perilymphatic nodules: along fissures, pleura, bronchovascular bundles, and septa.
• Random nodules: diffuse, including subpleural lung.
• Interlobular septal thickening: at the periphery of secondary lobules.
• Mosaic attenuation and air trapping: often reflect small-airway obstruction, assessed best on expiratory HRCT.

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8. Short reporting-style examples

• “Cylindrical bronchiectasis in the right middle lobe medial segment (S5) and lingula.”
• “Subpleural reticulation and traction bronchiectasis predominantly involve the posterior basal lower-lobe segments (S10).”
• “Tree-in-bud nodules are centred in the centrilobular region, indicating small-airway rather than random hematogenous spread.”
• “A focal opacity in the posterior upper part of the lower lobe is in the superior segment (S6), not the upper lobe.”

High-yield recall

• Right: 3 lobes, 10 segments.
• Left: 2 lobes, usually counted as 8-10 because S1+2 and S7+8 commonly fuse.
• Artery and bronchus are intrasegmental.
• Veins are intersegmental.
• Lower lobes are posterior; upper lobes are anterior on axial CT.
• RML and lingula are anterior-medial.
• S6 is posterior but belongs to the lower lobe.