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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.
Lung VQ scanning should be considered when CTPA is negative or indeterminate and there is a high clinical likelihood, and when there is a contraindication to CTPA (impaired renal function or iodine allergy).
It is the recommended investigation for young women and in women who are pregnant or breastfeeding as there is a lower radiation dose to the breasts from a VQ scan compared to CTPA. Investigations for Suspected PE in Pregnancy - CDHB Blue Book.
A high likelihood lung scan is sufficient for the diagnosis of PTE in most patients. A normal perfusion scan excludes the diagnosis.
Diagnosis of pulmonary embolism.
Quantitation of regional lung perfusion and ventilation.
The patient should have had a D-dimer test and chest x-ray within 24 hours.
The scan should be performed within 72 hours of a clinical suspicion of the diagnosis.
The ventilation (V) scan is performed first, this involves breathing in radioactive Technegas. SPECT(CT) images are taken of the distribution of radioactive gas in the lungs.
The perfusion (Q) scan is then performed. This involves the I.V. administration of small (10 microns) radiolabelled particles of Macro-Aggregated Albumin (MAA) which are then trapped in the lung capillary bed. SPECT(CT) images are taken of the distribution of the particles in the lungs, which is directly proportioned to regional perfusion.
The procedure takes about 45 minutes. A mismatch between the ventilation and perfusion images indicates PE.
note: SPECT-CT is not performed on pregnant women in order to minimise the radiation dose to the foetus.
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