Thoracic helical CT:: influence of subsecond scan time and thin collimation on evaluation of peripheral pulmonary arteries

The objective of this study was to analyze the influence of collimation on the identification of peripheral pulmonary arteries on helical CT scans. Three hundred sixty of 370 consecutive helical CT angiograms of the pulmonary circulation obtained during an 18-month investigation period were considered as technically acceptable for the detection of acute pulmonary embolism and were retrospectively analyzed. Patients in group A (n = 274) underwent CT with 2-mm collimation and pitch of 2; those in group B (n = 86) underwent CT with 3-mm collimation and pitch 1.7; a 0.75-s rotation time was systematically used. A total of 2160 segmental (six arterial zones per patient) and 2160 subsegmental (sii arterial zones per patient) arterial zones were assessed. Whereas the percentage of segmental arteries was not significantly different between group A (86 %) and group B (89 %), the percentage of analyzable subsegmental arteries was greater in group A (65 %) than in group B (43 %) (P < 0.001). The causes of inadequately depicted subsegmental arterial zones were partial-volume effects (group A, n = 302; 52 %; group B, n = 197; 67 %; P < 0.001), suboptimal enhancement (group A, n = 145; 25 %; group B, n = 43; 15 %; P < 0.05), motion artifacts (group A, n = 113; 20 %; group B, n = 30; 10 %), and unincluded arteries (group A, n = 20, 3 %; group B, n = 25; 8 %). Helical CT with 3-mm collimation at 0.75 s per revolution enables marked improvement in the analysis of subsegmental arteries in routine clinical practice.