CT pulmonary angiography

CT pulmonary angiography. It is a specialized computed tomography imaging technique used to visualize the pulmonary arteries and their branches. The primary clinical application is the diagnosis or exclusion of pulmonary embolism, a potentially life-threatening condition. The procedure involves the rapid intravenous injection of iodinated contrast material timed to achieve high opacification of the pulmonary vasculature during image acquisition.

Indications and clinical use

The principal indication is the evaluation of patients with suspected pulmonary embolism, often presenting with symptoms like dyspnea, chest pain, or hemoptysis. It is frequently employed following an inconclusive ventilation/perfusion scan or when clinical probability is high. Guidelines from organizations like the American College of Radiology and the European Society of Cardiology recommend it as a first-line imaging test in many scenarios. It is also used to assess other pathologies of the pulmonary vasculature, such as pulmonary arteriovenous malformations, pulmonary artery aneurysms, and to evaluate congenital heart disease like tetralogy of Fallot. In oncology, it can help stage lung cancer by evaluating vascular invasion.

Technique and procedure

The examination is performed on a multidetector computed tomography scanner, typically with at least 16 detector rows, though modern 64-slice CT and dual-source CT systems are common. A bolus tracking or test bolus technique is used to synchronize the scan with the arrival of contrast in the main pulmonary artery. The patient receives an intravenous injection of a low- or iso-osmolar iodinated contrast agent via a power injector placed in an antecubital vein. The patient is instructed to hold their breath during the acquisition, which covers from the lung apices to the costophrenic angles. Parameters are optimized to minimize radiation dose while maintaining diagnostic quality, adhering to principles like ALARA.

Interpretation and findings

Interpretation involves reviewing axial images, along with multiplanar reconstructions and maximum intensity projection images, to systematically trace the pulmonary arteries. A positive finding for pulmonary embolism is a filling defect within a contrast-opacified vessel, which may be central or occlusive. The Westermark sign and Hampton's hump are ancillary findings of pulmonary infarction. Radiologists also assess for alternative diagnoses that may explain the patient's symptoms, such as pneumonia, pleural effusion, aortic dissection, or pericardial effusion. The Fleischner Society has published guidelines on the management of incidentally detected subsegmental pulmonary embolism.

Advantages and limitations

A major advantage is its high sensitivity and specificity for detecting pulmonary embolism, particularly in the main, lobar, and segmental arteries. It provides a rapid diagnosis, allowing for prompt initiation of anticoagulation therapy with agents like heparin or warfarin. The technique also enables simultaneous evaluation of the lungs, mediastinum, and thoracic aorta for other pathology. Limitations include reduced sensitivity for subsegmental pulmonary embolism, artifacts from patient motion or high heart rate, and challenges in patients with renal impairment due to contrast-induced nephropathy risk. The PIOPED II trial helped validate its diagnostic performance.

Risks and contraindications

Risks include exposure to ionizing radiation, with particular concern for younger patients and during pregnancy. Allergic reactions to iodinated contrast, ranging from urticaria to anaphylaxis, are possible. The risk of contrast-induced nephropathy is a significant concern in patients with pre-existing renal failure or diabetes mellitus. Absolute contraindications include a prior anaphylactic reaction to contrast media. Relative contraindications include severe renal impairment, where alternative tests like ventilation/perfusion scan or compression ultrasonography for deep vein thrombosis may be considered. The use of metformin in patients with diabetes requires careful management per guidelines from the American Diabetes Association.

Category:Medical imaging Category:Cardiovascular imaging Category:Radiology