Lung (VQ) Scan

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Indications:

  1. Diagnosis of pulmonary embolism.

  2. Quantitation of regional lung perfusion and ventilation.

Principle:

  • The ventilation study is performed first, this involves breathing in radioactive Technegas. Images are taken of the distribution of radioactive gas in the lungs.

  • The perfusion scan is then performed. This involves the I.V. administration of small (10 microns) radiolabelled particles of macro-aggregated albumin which are then trapped in the lung capillary bed. Images are taken of the distribution of the particles in the lungs, which is directly proportioned to regional perfusion.

Preparation and Procedure:

  • The patient should have a chest x-ray within 24 hours of the perfusion scan.

  • The patient's pre-test probability score should be written on the requisition.

  • The scan should be performed within 72 hours of a clinical suspicion of the diagnosis.

  • The procedure takes about 45 minutes.

Comment:

  • Although CTPA has become the preferred investigation for suspected PTE, in most centres, lung scanning is still a useful test. It is more sensitive than CTPA but not as specific. The lung scan is most useful when the CXR is normal and there is no evidence of COPD. It should also be considered when CTPA is negative and there is a high clinical likelihood. It should be used when there is a contraindication to CTPA.

  • A high likelihood lung scan is sufficient for the diagnosis of PTE in most patients. A normal perfusion scan excludes the diagnosis.

  • In pregnant and breast feeding patients a low dose perfusion scan only is preferred.

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Page last reviewed: 16 January 2015
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