<P> CTPA is typically only requested if pulmonary embolism is suspected clinically . If the probability of PE is considered low, a blood test called D - dimer may be requested . If this is negative and risk of a PE is considered negligible, then CTPA or other scans are generally not performed . Most patients will have undergone a chest X-ray before CTPA is requested . </P> <P> After initial concern that CTPA would miss smaller emboli, a 2007 study comparing CTPA directly with V / Q scanning found that CTPA identified more emboli without increasing the risk of long - term complications compared to V / Q scanning . AV / Q scan may still be recommended when a lower radiation dose is required . </P> <P> CTPA is less desirable in pregnancy due to the amount of ionizing radiation required, which may damage the breasts, which are particularly sensitive during pregnancy, and because of concerns of the effects of iodine on the fetus' thyroid gland . V / Q scans can offer lower radiation doses, and may be adapted to further reduce the dose by omitting the lung ventilation portion of the exam . They are therefore recommended to be preferentially applied to pregnant patients . Diagnostic algorithms for pulmonary embolism in pregnancy vary; however, a common compromise is to perform ultrasound testing for deep vein thrombosis of the legs, and if this is positive, make the diagnosis of pulmonary embolism on the basis of symptoms and presence of the DVT . CTPA would then only be performed if exhaustive non-radiation based testing could not make a positive diagnosis . </P> <P> CTPA is contraindicated in known or suspected allergy to contrast media or in kidney failure (where contrast agents could worsen the kidney function). </P>

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