CT Chest

Computed tomography of the chest acquires a volume from the lung apices to the adrenal glands during a single breath hold of 5 to 10 seconds.

Slices of 0.625 to 1.25 mm are used for review and thin lung reconstructions, with multiplanar and maximum intensity projection reformats.

It shows the lungs, airways, pleura, mediastinum, great vessels, heart, chest wall, and upper abdomen.

Intravenous contrast (80 to 100 mL of non-ionic iodinated medium at 2 to 3 mL each second, with a delay of 30 to 70 seconds) is added to define the mediastinum, hila, vessels, pleural disease and masses.

Non-contrast scans are adequate for lung nodules, interstitial disease and many follow up studies.

Effective dose is about 5 to 7 mSv, and low dose settings reach 1 to 2 mSv.

CT chest is one of the commonest cross-sectional examinations. Lung cancer causes about 1.8 million deaths each year worldwide, pneumonia and tuberculosis remain leading infectious causes of death, and pulmonary embolism affects 1 to 2 in 1000 people yearly.

CT is far more sensitive than chest radiography, which misses about 20 to 25% of lung cancers visible on CT and misses most small nodules, early interstitial change and mediastinal disease.

Indications. Abnormal chest radiograph (mass, nodule, consolidation that does not resolve, effusion, widened mediastinum). Suspected or staged lung cancer and other thoracic malignancy, metastases and lymphoma. Haemoptysis.

Persistent cough, dyspnoea or weight loss with a normal radiograph. Suspected pneumonia complications (empyema, abscess, necrosis), tuberculosis, fungal and atypical infection, and immunocompromised patients with fever. Pleural disease and chest wall tumours.

Chest trauma (part of whole body CT). Mediastinal masses, thymic disease and lymphadenopathy (sarcoidosis, lymphoma). Aortic disease and pulmonary embolism (dedicated angiography techniques).

Suspected interstitial lung disease (high resolution technique). Planning of biopsy, radiotherapy and surgery. Post-operative and post-treatment surveillance.

Not for routine investigation of uncomplicated acute bronchitis or an asthma attack.

Contraindications and precautions. Pregnancy: avoid unless essential; shielding is not needed; acceptable with a clear indication because foetal dose is very low. Children: low dose, size adjusted settings.

Contrast: previous severe reaction (premedication or alternative), renal impairment (weigh benefit against risk of contrast nephropathy and hydrate), metformin (follow local renal function advice), thyroid disease, and extravasation risk with power injection through a poor cannula.

Patients unable to hold the breath need short scan times and sedation if appropriate.

Preparation and technique. Fasting for 4 hours for contrast studies where local policy requires; a cannula of 20 G or larger in an antecubital vein. The patient lies supine with arms raised over the head; deep inspiration and breath hold. Expiratory images are added for air trapping, and prone images for dependent atelectasis versus fibrosis.

Contrast timing is set for the question: 30 to 35 seconds for pulmonary arteries, 60 to 70 seconds for mediastinum, hila and pleura, and delayed scans for empyema or vascular tumour. Lung (width 1500, level -600 HU), mediastinal (width 350, level 40 HU) and bone windows are reviewed, along with multiplanar and thin slab minimum intensity images for airways.

Normal appearances. Lungs are uniformly aerated (density about -800 HU) with fine branching vessels tapering towards the periphery, thin airway walls, sharply defined fissures, and no pleural thickening or fluid. The trachea is midline with a diameter of 13 to 25 mm and the main bronchi divide at the carina.

Mediastinal lymph nodes are under 10 mm in short axis (up to 15 mm in the subcarinal station). The aorta is under 4 cm in the ascending segment and under 3 cm in the descending, the main pulmonary artery is under 29 to 30 mm, and the heart is under half the thoracic width.

The thymus is present in the young, fat replaced in adults. The pleural space has under 5 mm of fluid at most, and the oesophagus is collapsed with a thin wall.

Interpretation.

• Pulmonary nodule: solid, part solid or ground glass.
• Features favouring malignancy are size over 8 mm, spiculated or lobulated edge, upper lobe location, growth on follow up (doubling time 30 to 400 days), pleural tags, bubble like lucencies, and a solid component in a part solid nodule.
• Benign features are central, laminated or popcorn calcification, fat (hamartoma), and a stable size over 2 years for solid nodules.
• Lung cancer: spiculated mass, cavitation with thick irregular wall (squamous), peripheral adenocarcinoma, lepidic ground glass spread, central mass with lobar collapse, and obstructive pneumonia.
• Spread: direct invasion of chest wall, mediastinum or vertebra, enlarged nodes (short axis over 10 mm), pleural effusion or nodules, adrenal and liver metastases.
• Metastases: multiple well defined round nodules in the lower zones, cannonball pattern, cavitating (squamous primary), calcified (osteosarcoma, thyroid), lymphangitic spread (septal thickening with nodular beading), and miliary pattern.
• Pneumonia: lobar consolidation with air bronchograms, bronchopneumonia with patchy peribronchial opacities, cavitation and abscess (thick wall, air fluid level), necrotising pneumonia (non-enhancing parenchyma), empyema (split pleura sign, enhancing thickened pleura), and aspiration (dependent right lower and posterior upper lobes).
• Tuberculosis: upper lobe and apical cavities, tree-in-bud nodules, consolidation, miliary nodules, calcified granulomas and nodes (healed), necrotic nodes with rim enhancement, and pleural effusion or empyema.
• Fungal and opportunistic infection: halo sign and air crescent sign (invasive aspergillus), aspergilloma in a cavity, ground glass opacities (Pneumocystis, with cysts), and nodules with halos in the neutropenic patient.
• Interstitial lung disease: reticulation, honeycombing and traction bronchiectasis in fibrosis, ground glass in inflammatory or infective disease, and mosaic attenuation in small airway disease (more detail in the dedicated high resolution study).
• Airway disease: bronchiectasis (signet ring sign, airway larger than the accompanying artery), emphysema (low attenuation without walls), bronchial wall thickening, mucus plugging, foreign body, and tracheal stenosis or tumour.
• Pleura: effusion (simple fluid, 0 to 15 HU), loculation, thickening, pleural plaques (asbestos, calcified), mesothelioma (nodular circumferential pleural thickening with fixation of the hemithorax), pneumothorax (visible even when small), and haemothorax.
• Mediastinum: anterior mass (thymoma, teratoma, lymphoma, retrosternal thyroid), middle (lymphadenopathy in sarcoidosis with symmetrical hilar nodes, lymphoma, metastases, bronchogenic cyst), and posterior (neurogenic tumours, oesophageal lesions, paravertebral abscess).
• Vascular: aortic aneurysm and dissection (flap), pulmonary embolism (filling defect), pulmonary artery enlargement (hypertension), and venous collaterals in superior vena cava obstruction.
• Cardiac and pericardial: pericardial effusion, calcification (constrictive pericarditis), cardiomegaly, coronary and valve calcification, and left atrial enlargement.
• Chest wall and bone: rib fractures, lytic lesions, sternal and vertebral tumours, breast masses, and axillary nodes.
• Trauma: contusion, laceration, haematoma, pneumothorax, haemothorax, aortic injury (mediastinal haematoma, pseudoaneurysm), diaphragmatic rupture, and tracheobronchial injury (persistent air leak).
• Upper abdomen: liver and adrenal metastases, gallstones, and hiatus hernia.

Critical values and findings needing immediate action.

• Tension pneumothorax or large pneumothorax: immediate decompression and chest drain.
• Pulmonary embolism with right heart strain: urgent anticoagulation or reperfusion.
• Aortic dissection, rupture or traumatic aortic injury: emergency vascular or cardiothoracic surgery.
• Massive haemothorax or active bleeding: resuscitation and surgical control.
• Large pericardial effusion with tamponade: urgent drainage.
• Central airway obstruction or foreign body: urgent bronchoscopy.
• Lung abscess or empyema: drainage and antibiotics.
• Lung cancer with superior vena cava obstruction or cord compression: urgent oncology.
• Newly diagnosed suspicious lung mass or nodule: referral to a multidisciplinary lung cancer team within the local time limit.

Limitations and artefacts.

• Respiratory motion produces blurring and pseudo bronchiectasis, with ghosting of vessels near the diaphragm that mimic nodules.
• Dependent atelectasis simulates fibrosis (prone images clarify), and expiratory scans reveal air trapping missed in inspiration.
• Beam hardening from dense contrast in the superior vena cava and subclavian vein hides adjacent nodes; reduce the contrast load or use saline flush.
• Metal and pacemaker leads cause streaks; cardiac pulsation blurs the lingula and aorta.
• Small nodules under 4 to 6 mm cannot be characterised, and follow up depends on risk factors and growth; reporting must state size, position, density and prior comparison.
• CT cannot distinguish all benign from malignant nodes, and reactive nodes in infection are common; biopsy or PET gives better characterisation.
• Radiation dose adds up with repeated studies; use low dose settings for follow up of nodules and for young patients.
• Incidental findings (thyroid nodules, adrenal masses, coronary calcification, liver lesions) are common and need rules for follow up.
• Wrong contrast timing misses pulmonary embolism or aortic pathology.

Special groups.

• Children: use size specific low dose techniques; the commonest indications are infection complications, congenital lung malformations, foreign body and tumours.
• Pregnancy: low foetal dose; reserve for clear indications such as embolism not otherwise excluded.
• Immunocompromised patients (HIV, transplant, chemotherapy): lower threshold for CT, as the radiograph is often normal.
• Smokers and people with asbestos exposure: nodules, emphysema, pleural plaques and mesothelioma.
• Patients with renal impairment: non-contrast CT is adequate for many lung questions.
• Regions: tuberculosis, post-tuberculous bronchiectasis and cavities, hydatid cysts, histoplasmosis, paragonimiasis, silicosis, and HIV-related lung disease are common, and CT availability is limited; CT often shows disease that the radiograph does not, and helps decide on biopsy and surgery.

Follow up tests.

• High resolution CT for interstitial disease, and CT pulmonary angiography for embolism.
• PET-CT for staging of lung cancer and characterisation of nodules.
• MR for chest wall, apical and mediastinal invasion.
• Bronchoscopy with lavage and biopsy, and image directed percutaneous biopsy.
• Sputum for culture, acid fast bacilli and cytology, and pleural fluid analysis.
• Blood tests (full blood count, inflammatory markers, D-dimer, serology), spirometry and lung function tests, and echocardiography.
• Repeat CT at set intervals for nodules.
• Referral to respiratory medicine, thoracic surgery or oncology.