CT data from 44 patients with pulmonary embolism were acquired using different protocols-Group A : 120 kV , filtered back projection , n = 12 ; Group B : 120 kV , 40 % ASIR , n = 12 ; Group C : 100 kV , 40 % ASIR , n = 12 and Group D : 80 kV , 40 % ASIR , n = 8 .