Introduction
This topic covers the imaging methods used for the respiratory system (chest X-ray, CT, CT angiography, ultrasound, MRI, PET/CT), a systematic way to read a chest X-ray, normal chest X-ray findings, and the basic radiological abnormalities of the lung: consolidation, atelectasis, nodules, linear opacities and the differential diagnosis of pathological shadows. The respiratory tract is divided into the upper tract (URT, down to the trachea) and the lower tract (LRT, from the bronchi onwards).
Imaging modalities
- Key modalities are X-ray and CT. The chest X-ray (CXR) is the quickest and most cost-effective test.
- Ultrasound (US) assists biopsy and evacuation of pleural effusion.
- MRI is not used for lung pathologies; only for the cardiovascular system, mediastinal lymph nodes, the thymus, and tumours/metastases of the soft tissue of the chest. It is best saved for specialised situations and allows study of the heart and great vessels in great detail.
- PET/CT is used only for diagnosis in oncology: evaluating a focal pulmonary abnormality (such as a solitary pulmonary nodule), staging of bronchogenic carcinoma and recurrent tumours.
Chest X-ray
- Gives information about the lung parenchyma, abnormalities, soft tissues and the skeleton; it can be PA, AP or lateral (PA and lateral are standard).
- Order an AP film when the patient is unable to move.
- Expiratory PA: pneumothorax and foreign body aspiration.
- Lateral decubitus CXR: pleural effusions.
CT
CT has high resolution, detail and 3D reconstruction. It gives information about diffuse parenchymal changes and tumours. It can be done with or without contrast media; it takes multiplanar images from the axial view and builds them into 3D. Typical views: axial view with mediastinal window, axial view with parenchymal window, and axial view with mediastinal window and contrast.
- Emergency indications: trauma (rib fracture, pneumothorax), aortic pathologies (dissection), pulmonary embolism, and complications after thoracic surgery.
- Non-emergency indications: diagnosis and staging of lung cancer, detection of metastases, interstitial lung disease, evaluation of nodules/masses.
- High-resolution CT (HRCT) is used for the differential diagnosis of interstitial abnormalities.
- CT angiography is CT with IV contrast, used for aneurysms, dissection, stenosis, malignancies and abnormal variations.
When to order a chest CT: remember VITAMIN D and C:
- V - Vascular diseases; I - Infection, inflammation, inhalation diseases; T - Trauma; A - Allergic/autoimmune diseases; M - Metabolic diseases (goitre); I - Idiopathic diseases; N - Neoplasias; D - Developmental diseases; C - Cardiac diseases.