Pulmonary emboli

What are the risk factors for pulmonary embolus?
What are the imaging procedures available to investigate PE?

There are several modalities of imaging available to investigate PE

What is the utility of each?
What is the appropriate sequence of imaging procedures?

Two options:

Describe the mechanism and limitation of VQ scan?

Concurrent images of the distribution of blood flow (perfusion scan) and the distribution of alveolar ventilation (ventilation scan) are obtained following the intravenous injection of 99m Tc-labelled particles of macroaggregated albumin, and the inhalation of a radioactive gas (for example, krypton-81 m or xenon-133) or -carbon particles known as ‘Technegas'.

Typically in PE, the perfusion scan shows segmental perfusion defects, while the ventilation scan remains normal, the so-called ‘mismatched perfusion defect'.

A properly performed perfusion scan, which includes at least six projections, is highly sensitive for the detection of small defects. However, this high sensitivity is not matched by a high specificity for the diagnosis of pulmonary embolism. There are many causes of positive perfusion scans, which could lead to a false positive diagnosis of pulmonary embolism. Various conditions can cause mismatched defects on ventilation-perfusion scanning including emphysema, tuberculosis, previous irradiation etc.

What is the need for baseline CXR for lung scan?
What do you understand from the terms normal, high probability and low probability scans?

Low probabilty and indeterminate scans do not rule out the possibility of PE. If there is a clinical suspicion for PE further studies should be done.

What is the sensitivity and specificity of lung scan in diagnosing PE?
What are the imaging findings of PE in CT angiogram?

CT Angiogram Showing Filling Defects Consistent With PE

Patient had a low probability lung scan. Clinically there was high index of suspicion for pulmonary embolism.

Arrowheads: Intra luminal thrombi in right pulmonary artery branches


Pulmonary Embolus

Arrow: Filling defect in pulmonary artery.


What is the sensitivity and specificity of CT?
What is the role of Doppler exam?
What is the role of pulmonary angiogram in evaluating PE?