<prompt> Pulmonary embolism is best diagnosed byUSGX Ray ChestVentilation - Perfusion ScanCT Scan  <response>: D i.e. CT Scan Investigation of choice IOC is CT angiography contrast CT followed by pulmonary aeriography followed by a ventilation - perfusion scanQ. Advantages Disadvantages of various tests for DTE Dx Test Advantages Disadvantages 1. Plasm D-dimer ELISA Normal results make PE highly unlikely Level is elevated in various systemic illnesses 2. Lung V-P Scanning Standard initial imaging test for Most scans are neither high probability nor PEQ. High probability scans are reliable for detecting PE normalnear normal scans are reliable for precluding PE normalnear normal; most are equivocal 3. Pulmonary angiography Gold standard for diagnosisQ Invasive, Costly Uncomfoable ; so not a first line investigation 4. CT angiography helical - Noninvasive, widely available test - Initially it was a sensitive method of that has replaced pulmonary detecting central segmental, lobar main angiography conventional as the pulmonary aery emboli upto 4th order vessel main stay investigation for PEQ. -- 7mm diameter i.e., from aoic arch to - Stratifies risk, obtains images of inferior pulmonary vein. However, with RV LV advent of helical multidetector CT MDCT has improved resolution of peripheral aeries upto 6th order. 