Imaging - chest CT

Choose the protocol before you order

Protocol	Use	Key point
Non-contrast	Nodules, ILD, follow-up, screening	Lowest dose, no renal or allergy issue
HRCT	Interstitial lung disease	Thin (1 mm) slices, inspiratory + expiratory + prone; expiratory phase detects air trapping = small airways disease; prone excludes dependent atelectasis mimicking fibrosis
CTPA	Pulmonary embolism	Timed to the pulmonary arteries
CT with contrast (portal venous)	Staging, mediastinum, pleura, infection	Assesses nodes, pleura, chest wall
CT bronchial/aortic angiogram	Haemoptysis, aortic disease	Maps bronchial arteries before embolisation
Low-dose CT	Lung cancer screening	~1-2 mSv

Ordering "CT chest" without specifying the question gets you the wrong protocol and a repeat scan

Radiation and screening

Typical doses: CXR ~0.02 mSv; low-dose CT ~1-2 mSv; standard chest CT ~5-7 mSv; CTPA ~3-10 mSv
- Annual Australian background ~1.5-2 mSv
Australia's National Lung Cancer Screening Program commenced 1 July 2025 - biennial low-dose CT for people aged 50-70 with a >=30 pack-year history who currently smoke or quit within 10 years
Pulmonary nodules are found in >30% of CTs performed for any reason - the great majority are benign
CTPA positivity rates in unselected practice are often <10%, indicating over-investigation

Density and why it matters

Hounsfield units: air -1000, fat ~-50 to -100, water 0, soft tissue 20-60, blood 40-60 (acute), calcium/bone >150
Windowing determines what you can see
- Lung window - parenchyma, nodules, emphysema
- Mediastinal/soft tissue window - nodes, masses, pleura, vessels
- Bone window - metastases, fractures
- A nodule invisible on mediastinal windows is not "absent" - always look at both

Patterns and what generates them

Pattern	Mechanism	Typical causes
Consolidation	Alveolar filling (air bronchograms)	Pneumonia, haemorrhage, oedema, adenocarcinoma, organising pneumonia
Ground glass	Partial alveolar filling or interstitial thickening	Infection (PJP), oedema, haemorrhage, early fibrosis, NSIP, hypersensitivity pneumonitis
Reticulation + traction bronchiectasis + honeycombing	Established fibrosis	UIP/IPF (basal, subpleural)
Tree-in-bud	Bronchiolar filling	Endobronchial spread of TB, MAC, aspiration, bronchiolitis
Mosaic attenuation with air trapping	Small airways or vascular disease	Bronchiolitis obliterans, hypersensitivity pneumonitis, CTEPH
Crazy paving	Ground glass + septal thickening	PAP, PJP, oedema, haemorrhage
Nodules - perilymphatic	Along lymphatics	Sarcoidosis, silicosis, lymphangitis carcinomatosa
Nodules - centrilobular	Bronchiolocentric	Hypersensitivity pneumonitis, infection, smoking-related
Nodules - random	Haematogenous	Miliary TB, metastases

Lung collapse - the worked example

CXR: opacification of the affected field, tracheal and mediastinal shift TOWARDS the collapse, elevated hemidiaphragm, crowded ribs, displaced fissure
- LLL: "sail sign" behind the heart with a double left heart border and loss of the medial left hemidiaphragm
- Contrast a large effusion: opacification with shift AWAY
CT chest: characterises the causative lesion - endobronchial tumour, mucus plug, extrinsic compression, node - and stages it
Bronchoscopy: direct assessment of an endobronchial lesion, washings +/- biopsy, and therapeutic clearance of plugs

Nodule management - Fleischner Society

Nodule management - Fleischner Society (incidental nodules, age >=35, no known cancer)

Solid nodule	Low risk	High risk
<6 mm	No routine follow-up	Optional CT at 12 months
6-8 mm	CT at 6-12 months, then 18-24 months	CT at 6-12, then 18-24 months
>8 mm	CT at 3 months, PET-CT or biopsy	Same

Subsolid nodules need longer follow-up (up to 5 years) - part-solid nodules with a solid component >6 mm carry the highest malignancy risk
*Fleischner criteria do not apply to lung cancer screening (use Lung-RADS), to patients with known malignancy, or to the immunosuppressed*

Pulmonary embolism

CTPA: filling defect, RV/LV ratio >1 signals RV strain
V/Q or perfusion scan preferred in: pregnancy, contrast allergy, renal impairment, young women (breast dose)
Subsegmental PE on modern scanners is of uncertain significance - not every filling defect requires anticoagulation

Limitations and harms

Contrast: AKI risk (lower than historically believed; hydrate and assess eGFR), allergy, thyrotoxicosis after an iodine load, extravasation
Cumulative radiation, especially in young patients and with repeated scans
Overdiagnosis: incidental nodules, adrenal and thyroid incidentalomas, coronary calcium
CT does not replace the clinical question - a scan ordered without a question generates findings without answers

What a CT changes

Lung cancer: diagnosis, TNM staging, target for biopsy (CT-guided vs EBUS vs bronchoscopy), response assessment (RECIST)
ILD: a definite UIP pattern on HRCT can make the diagnosis of IPF without biopsy in the right clinical context - subpleural, basal-predominant reticulation with honeycombing and traction bronchiectasis, no features suggesting an alternative
Infection: cavitation (TB, S. aureus, anaerobes), halo/reverse halo (invasive aspergillosis), empyema (split pleura sign, lenticular shape, septations) -> drain
Haemoptysis: locates the bleeding lobe and maps hypertrophied bronchial arteries before embolisation
Pre-operative: segmental anatomy, fissure completeness, nodal stations for EBUS targeting
Emphysema: distribution and fissure integrity for endobronchial valve selection
Pleural disease: distinguishes empyema from lung abscess, identifies nodular or circumferential pleural thickening and diaphragmatic involvement suggesting mesothelioma or malignant effusion

Practical rules

Give the radiologist the clinical question and the relevant history - the report quality tracks the request quality
Compare with prior imaging - 2 years of stability in a solid nodule is strong evidence of benignity
Check renal function and contrast allergy before contrast; metformin need not be withheld at normal eGFR
In women of childbearing age, exclude pregnancy or choose an alternative

Common indications

Lung cancer; National Lung Cancer Screening Program eligibility
Interstitial lung disease - IPF, NSIP, hypersensitivity pneumonitis, CTD-ILD, drug-induced (amiodarone, methotrexate, nitrofurantoin, bleomycin, checkpoint inhibitors)
Occupational lung disease - asbestosis, silicosis (including accelerated artificial-stone silicosis - a major current Australian problem), coal workers' pneumoconiosis
Pulmonary embolism and CTEPH
Bronchiectasis, cystic fibrosis, allergic bronchopulmonary aspergillosis
Immunocompromised host - PJP, invasive aspergillosis, CMV, nocardia
Sarcoidosis; lymphangioleiomyomatosis; pulmonary alveolar proteinosis
Incidental coronary artery calcification, thyroid and adrenal nodules, vertebral fractures

Screening and nodule stability

Low-dose CT screening reduces lung cancer mortality ~20% (NLST) and ~24% in men at 10 years (NELSON) - the evidence behind the Australian program
- Costs: false positives, invasive procedures for benign disease, overdiagnosis, incidental findings
A solid nodule stable for 2 years is almost certainly benign; subsolid nodules can grow slowly over 5+ years and need longer surveillance
Radiological progression in ILD often precedes symptoms and physiology - serial HRCT guides antifibrotic decisions
Post-infective changes (organising pneumonia, scarring) may persist for months - do not re-image too early and do not over-interpret residual opacity
Cumulative radiation risk is small per scan but real across a lifetime of repeated imaging - justify each scan and use the lowest appropriate protocol

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