Why a special kind of CT
A plain chest X-ray can hint at ILD but it blurs the fine detail. The workhorse is the HRCT — high-resolution computed tomography. It is an ordinary CT scanner run with thin slices and a sharp reconstruction so that the lung's finest architecture, down to the level of the secondary lobule, comes into focus. In ILD the HRCT is not just confirmatory; the pattern it shows often makes the diagnosis without ever needing a piece of tissue. Learning to read three or four key features takes you a long way.
The vocabulary of the scan
- [[ground-glass-opacity|Ground-glass opacity]] — a hazy grey veil where you can still see the vessels through it. It means the air spaces or interstitium are partly filled — by inflammation, fluid, or early fibrosis. On its own it is often a sign of active, potentially treatable disease.
- Reticulation — a fine net of lines, the radiological face of thickened interstitial scaffolding. It marks fibrosis that is more established than ground-glass alone.
- [[honeycombing|Honeycombing]] — clustered, thick-walled cystic spaces stacked like a bee's comb, usually at the lung bases and edges. This is end-stage, irreversible scar: the original architecture has been destroyed and replaced by these little air pockets.
- [[traction-bronchiectasis|Traction bronchiectasis]] — airways yanked permanently open and distorted because the surrounding scar is pulling on their walls. It is a reliable sign that real fibrosis, not just inflammation, has set in.
Patterns, not just features
The real skill is reading the distribution: where the abnormalities sit and how they cluster. The most important named pattern is usual interstitial pneumonia (UIP): honeycombing and reticulation that hug the bases and the outer rim of the lung, getting worse toward the periphery, with little ground-glass. When a radiologist sees a confident UIP pattern in the right clinical setting, that is essentially diagnostic of idiopathic pulmonary fibrosis — and a biopsy can often be avoided altogether.
Other distributions tell other stories. Disease concentrated in the upper lobes, or scattered as small nodules along the lymphatic routes, points toward sarcoidosis. A mosaic of ground-glass with patchy air-trapping and a mid-to-upper bias raises hypersensitivity pneumonitis. Because no single feature is unique, the modern standard is a multidisciplinary discussion — pulmonologist, radiologist, and pathologist together weighing the HRCT, the history, and sometimes bronchoalveolar lavage before settling on a name.