CT Chest — High Resolution (Interstitial Protocol)

Whole lung volumetric acquisition supine at full inspiration, with additional prone inspiratory and supine expiratory series.

Typical, not policy

Timings, volumes and delays here are representative values drawn from published guidance. Scanner generation, injector, cardiac output and local preference all move them. Confirm against your department's own protocol before you rely on a number.

When to use it

Suspected interstitial lung disease, including the pattern assessment that underpins a usual interstitial pneumonia diagnosis.
Suspected hypersensitivity pneumonitis or small airways disease, where the expiratory series carries the diagnosis.
Suspected bronchiectasis.
Serial assessment of known fibrotic lung disease.

Technique

The prone series exists to distinguish dependent atelectasis from genuine posterobasal fibrosis and should not be dropped for convenience.
The expiratory series exists to demonstrate air trapping and is the only way mosaic attenuation can be attributed to small airways disease.
Volumetric acquisition has largely replaced spaced axial sections; where dose is a dominant concern the prone and expiratory series may be acquired as interspaced axial sections instead.
Contrast is not part of an interstitial protocol and degrades assessment of the parenchyma by adding no information at extra cost.

Where it goes wrong

Omitting the expiratory series makes air trapping undiagnosable and is the commonest protocol failure in this study.
Omitting the prone series leaves dependent change indistinguishable from early subpleural reticulation, which is exactly the call that matters.
Sub-optimal inspiration mimics ground-glass opacity throughout.
Adding intravenous contrast because the request said "chest CT" converts an interstitial protocol into a mediastinal one.

Clinical questions that reach this study

Suspected pancreatic mass or painless obstructive jaundice Suspected cholangiocarcinoma Colorectal cancer — staging Incidental pulmonary nodule — characterisation and follow-up Known or suspected lung cancer — staging Suspected pneumonia and non-resolving consolidation Suspected empyema or complicated parapneumonic effusion Undiagnosed unilateral pleural effusion Suspected interstitial lung disease Suspected bronchiectasis Haemoptysis Suspected pneumothorax Suspected superior vena cava obstruction Staging or completion CT in a known solid tumour Liver assessment in a hypervascular primary Response assessment on systemic therapy Suspected or newly diagnosed lymphoma Metastatic disease with no identified primary Head and neck cancer — staging and post-treatment surveillance Indeterminate renal mass or suspected renal cell carcinoma Suspected phaeochromocytoma or paraganglioma Bladder cancer — pretreatment staging Acute scrotum and scrotal mass Suspected ovarian cancer or indeterminate adnexal mass

Contrast

None

Acquisition

Breathing: Full inspiratory breath-hold for the supine and prone series; a separate end-expiratory acquisition for air trapping.
Reconstruction: Thin sections — commonly not exceeding about 1.5 mm — in a high-frequency lung kernel, with coronal and sagittal reformats and minimum-intensity projections where mosaic attenuation is in question.
Preparation: Full inspiratory breath-hold, practised before the acquisition. Arms above the head; arms down doubles streak artefact through the upper thorax.

Phases

Each phase is authored once and shared across every protocol that uses it, so the physiology below is the same wherever you meet it.

Non-contrast (unenhanced)No injection. Acquired before any contrast is given.
Shows intrinsic tissue attenuation, and nothing else. The transferable principle is that contrast is anti-signal for anything that is already dense: calcification, acute haemorrhage, urinary and biliary calculi, iodine-containing or haemorrhagic fluid, and intrinsic fat all lose conspicuity, or become uninterpretable, once surrounding tissue enhances. It is also the only baseline against which enhancement can be measured, so any protocol that quantifies enhancement or washout (a lesion "enhances by X HU", adrenal absolute washout, renal mass characterisation) is arithmetically impossible without it. Conversely, an unenhanced series adds dose and no information whenever the question is purely about vascularity or perfusion.

Safety checks this protocol carries

Derived from the contrast agent and phases above, not authored here — which is why they cannot drift apart from what the protocol actually does.

Child-sized technique and contrast dose· radiographer at scan
Pregnancy status before an ionising exposure· radiographer at scan