CT Pulmonary Angiography
CT pulmonary angiography (CTPA) is a contrast enhanced CT of the chest timed to opacify the pulmonary arteries, used to diagnose acute pulmonary embolism.
Contrast (50 to 80 mL of high concentration non-ionic medium at 4 to 5 mL each second, followed by a saline flush) is injected through an antecubital cannula of 18 to 20 G. Use bolus tracking in the main pulmonary artery (trigger 100 to 150 HU) or a fixed delay of 12 to 18 seconds.
The acquisition takes 3 to 5 seconds in a single breath hold, with slices of 0.625 to 1 mm, and reconstructions in axial, coronal and sagittal planes. Low tube voltage (80 to 100 kV) lowers contrast and dose.
Effective dose is about 3 to 7 mSv (lower with modern scanners); the foetal dose is under 0.1 mGy and the maternal breast dose is about 10 to 20 mGy.
Pulmonary embolism affects about 1 to 2 in 1000 people each year, and is the third commonest cardiovascular cause of death. Mortality is 25 to 30% untreated and under 5 to 8% with treatment.
CTPA has a sensitivity of about 83 to 95% and a specificity of 94 to 98% for pulmonary embolism, with a negative predictive value of over 99% when clinical probability is low or intermediate. It has largely replaced ventilation perfusion scanning for first line diagnosis.
It also shows alternative diagnoses in 25 to 50% of patients with a negative scan (pneumonia, pneumothorax, aortic dissection, pericardial effusion, malignancy, heart failure).
Indications. Suspected acute pulmonary embolism with a high clinical probability, or an intermediate or low probability with a positive D-dimer (adjusted for age). Unexplained dyspnoea, pleuritic chest pain, haemoptysis, syncope, or hypoxia with risk factors (recent surgery, immobility, cancer, previous venous thromboembolism, pregnancy and postpartum, oral oestrogen).
Suspected chronic thromboembolic pulmonary hypertension. Assessment of right heart strain to grade risk. Pulmonary artery aneurysm or sarcoma, pulmonary arteriovenous malformation, and congenital pulmonary vascular anomalies.
Follow up of known embolism with persistent symptoms. Not for patients with low probability and a negative D-dimer (no imaging needed), and not as a first test in pregnancy when chest radiograph and leg ultrasound can answer.
Contraindications and precautions.
- Previous severe contrast reaction (alternative: ventilation perfusion scan).
- Renal impairment (estimated glomerular filtration rate under 30 mL each minute: the risk of untreated embolism usually outweighs contrast risk; hydration).
- Pregnancy (acceptable, with leg ultrasound and the chest radiograph first, and ventilation perfusion imaging as an alternative with lower breast dose).
- Metformin, thyroid disease, and myeloma.
Unstable patients are scanned with a rapid acquisition; those too unstable should have bedside echocardiography for right ventricular strain and empiric treatment. A reliable cannula is needed because the bolus is fast, and the right arm is preferred.
Preparation and technique. No fasting is needed in an emergency.
The patient lies supine with arms raised over the head and is taught a quiet breath hold. Deep inspiration produces inflow of unopacified blood from the inferior vena cava, a transient interruption of contrast, so a mid-inspiration or shallow hold is preferred. The scan runs from the lung bases to the apices (caudocranial) to reduce breathing motion at the bases.
Bolus tracking triggers the scan. The lungs are reviewed with lung windows, and the arteries with a narrow window (width 700, level 100 HU) or with maximum intensity projection images.
The heart size, right ventricle to left ventricle ratio, septal position and contrast reflux into the inferior vena cava and hepatic veins are assessed. Dual energy CT can add iodine maps showing perfusion defects.
