From shadow to slice
A CT scan spins an X-ray source around the body while a ring of detectors records from every angle. A computer then reconstructs the data into thin cross-sectional images — and, increasingly, into 3-D renderings. Because each slice avoids the overlap that plagues a plain film, CT separates structures that a chest X-ray superimposes, so a nodule hiding behind the heart or in the hilum becomes obvious.
Often an iodine-based contrast dye is injected into a vein. It brightens blood vessels and is essential when the question is vascular — most famously CT pulmonary angiography for a clot. For lung tissue questions, though, contrast is often unnecessary, and it carries a small risk to the kidneys and of allergy. The scan also delivers ionizing radiation — far more than a single X-ray — so each CT should answer a real question.
HRCT and the language of patterns
HRCT (high-resolution CT) uses very thin slices and a sharp reconstruction so the fine architecture of the lung — its tiny acini and the interstitium — comes into focus. It is the single most important imaging test for interstitial lung disease. Radiologists describe what they see in a shared vocabulary of patterns, and learning a handful of these turns a wall of grey into a story.
- Ground-glass opacity: a hazy increase in density through which you can still see the vessels — think inflammation, early infection, or fluid filling part of the air space.
- Consolidation: dense white where the air space is completely filled, and vessels disappear — classically pneumonia.
- Honeycombing: clustered, stacked cystic spaces in the periphery — a hallmark of established pulmonary fibrosis and a key clue toward usual interstitial pneumonia.
- Traction bronchiectasis: airways pulled open and distorted by surrounding scar — another sign that fibrosis has set in.