Pulmonary embolism: ventilation/perfusion imaging

Ventilation/perfusion (V/Q) scintigraphy diagnoses or excludes acute pulmonary embolism by comparing two maps of the lung obtained in the same sitting: a perfusion map, made by injecting Tc-99m-labelled macroaggregated albumin (MAA) particles that lodge in pulmonary capillaries in proportion to blood flow, and a ventilation map, made with an inhaled agent such as Technegas or krypton-81m gas. A pulmonary embolus blocks a branch of the pulmonary artery, so the perfusion map shows a wedge-shaped defect in that lung segment while the corresponding ventilation map remains normal - a mismatch that is the scintigraphic hallmark of embolism, as opposed to a matched defect where both are reduced together, typical of parenchymal lung disease rather than embolism.

The 2019 EANM guideline for V/Q SPECT strongly recommends tomographic (SPECT) rather than planar imaging wherever available, since SPECT has higher sensitivity and specificity, and defines a positive study as a mismatch of at least one segment or two subsegments conforming to pulmonary vascular anatomy; a normal perfusion study, respecting the anatomical boundaries of the lungs, excludes pulmonary embolism outright regardless of any other finding. Combining V/Q SPECT with a low-dose CT further improves specificity, particularly in patients with coexisting chronic obstructive pulmonary disease, heart failure or pneumonia, all of which can otherwise complicate interpretation.

V/Q scanning has a substantially lower radiation dose than CT pulmonary angiography, no iodinated contrast requirement, and virtually no absolute contraindications, which makes it the preferred first-line test in pregnancy, in patients with renal impairment or contrast allergy, and in younger patients where lifetime radiation exposure is a bigger consideration. ACR appropriateness criteria and SNMMI/EANM/ASH/ACCP/ACEP appropriate use criteria both support V/Q scanning or CT pulmonary angiography as appropriate first-line tests for suspected pulmonary embolism with intermediate or high pre-test probability, with the choice between them driven by patient factors (pregnancy, renal function, chest radiograph findings, local availability) rather than one modality being universally superior.