What scans diagnose a pulmonary embolism? 

A pulmonary embolism (PE) occurs when a blood clot, typically from the legs, travels to the lungs and blocks a pulmonary artery. In the UK, the primary scan used to diagnose this condition is a Computed Tomography Pulmonary Angiogram (CTPA). However, other imaging techniques, such as Ventilation-Perfusion (V/Q) scans or echocardiograms, are utilised depending on a patient’s specific health needs, such as kidney function or pregnancy. 

What We will cover in this Article 

• The mechanics of a CTPA and why it is the clinical ‘gold standard’. 

• How V/Q scans offer a safer alternative for specific patient groups. 

• The role of echocardiography in assessing heart strain from a clot. 

• Why chest X-rays are used to rule out other conditions rather than diagnose PE. 

• The use of bedside ultrasounds (POCUS) in emergency stabilisation. 

• Advanced imaging like MRI and the importance of the 24-hour diagnostic window. 

The Gold Standard: CT Pulmonary Angiogram (CTPA) 

The CTPA is the most common and accurate scan for diagnosing a pulmonary embolism in the UK. It is a specialised type of CT scan that focuses specifically on the blood vessels in the lungs. 

During the procedure, a contrast dye containing iodine is injected into a vein in your arm. This dye highlights the pulmonary arteries, allowing the scanner to take detailed X-ray images. If a clot is present, it will appear as a ‘filling defect’ a dark spot where the dye cannot flow because of the blockage. 

Why CTPA is Preferred 

• Speed: The scan itself takes only a few minutes, which is vital in emergency situations. 

• Clarity: It can identify very small clots and determine exactly which part of the lung is affected. 

• Alternative Diagnosis: It can simultaneously check for other issues like pneumonia or fluid around the lungs (pleural effusion). 

‘The CTPA provides a high-resolution map of the pulmonary vasculature. In the emergency department, its ability to quickly confirm or exclude a life-threatening clot allows us to initiate life-saving anticoagulation therapy without delay.’ Dr. Rebecca Fernandez. 

The Safe Alternative: Ventilation-Perfusion (V/Q) Scan 

While the CTPA is effective, the iodine dye and radiation levels may not be suitable for everyone. A V/Q scan is the preferred alternative for patients with kidney disease (who cannot process the dye) or those who are pregnant. 

A V/Q scan measures two things: 

1. Ventilation: How well air moves through your lungs (using a radioactive gas you breathe in). 

2. Perfusion: How well blood flows through your lungs (using a radioactive tracer injected into a vein). 

If air is reaching a part of the lung but blood is not, this is called a ‘mismatch’, which is a strong indicator of a pulmonary embolism. While less detailed than a CTPA, it provides a high level of safety for vulnerable patients. 

Supportive Imaging: Echocardiograms and X-rays 

Not all scans used in the diagnostic process are designed to ‘see’ the clot itself. Some are used to see how the body is reacting to the blockage or to rule out other causes of chest pain. 

Echocardiogram (Heart Ultrasound) 

In severe cases where a clot is large, it can put immense pressure on the right side of the heart as it struggles to pump blood through the blocked lung arteries. An echocardiogram can detect ‘right heart strain’. While it doesn’t always show the clot, it provides critical evidence of a PE’s impact on the cardiovascular system. 

Chest X-ray 

A standard chest X-ray cannot reliably see a blood clot in the lung. However, it is almost always performed first to see if symptoms like breathlessness are caused by something else, such as a collapsed lung (pneumothorax) or heart failure. 

Differentiation: When to Use Which Scan? 

The choice of scan is a tailored clinical decision based on a patient’s stability and medical history. 

Scan Type	Best Used For	Key Consideration
CTPA	Most emergency patients.	Requires healthy kidneys to process iodine dye.
V/Q Scan	Pregnant patients or those with dye allergies.	Takes longer to perform than a CT scan.
Echocardiogram	Critically ill or unstable patients.	Used to assess heart strain, not always to find the clot.
Leg Ultrasound	Patients with suspected DVT and PE.	If a leg clot is found, it supports the PE diagnosis.

Conclusion 

Diagnosing a pulmonary embolism requires a swift and structured approach using advanced medical imaging. Whether through the high-definition detail of a CTPA or the functional mapping of a V/Q scan, UK healthcare providers prioritise accuracy to ensure that treatment usually blood-thinning medication begins as soon as possible. Early detection remains the most critical factor in a successful recovery. 

If you experience severe, sudden, or worsening symptoms, call 999 immediately. 

You may find our free BMI Calculator helpful for monitoring your general health profile, as weight is an important factor in cardiovascular and vascular risk assessments. 

Authority Snapshot (E-E-A-T Block) 

This article was reviewed by Dr. Rebecca Fernandez, a UK-trained physician with an MBBS and extensive experience in cardiology, intensive care, and emergency medicine. Dr. Fernandez has significant clinical experience in the emergency diagnosis of pulmonary embolisms and the management of critically ill patients in acute hospital settings. This guide provides medically accurate, evidence-based information following current UK clinical standards.