The relationship between CT scout landmarks and lung boundaries on chest CT: guidelines for minimizing excess z-axis scan length.

ABSTRACT

OBJECTIVES: As the relationship between CT scout landmarks and chest CT boundaries is not known, the selected scan length is often greater than necessary for the CT scan, resulting in increased radiation dose to the neck and upper abdomen. The purpose of this study is to establish the relationship between CT scout landmarks with the superior and inferior boundaries of the lungs on chest CT. METHODS: Retrospective comparison of the location of the top of the first rib on frontal scout and the most inferior costophrenic angle on lateral scout to the chest CT slice just above and below the lungs. The percent of scans that would exclude part of the lung based on CT initiated at several distances above or below these landmarks was calculated. RESULTS: There was 2.7 times greater variability between scout landmarks and lung boundaries inferiorly than superiorly on chest CT (p < 0.001). Initiating CT at the top of the first rib on scout did not exclude any lung on CT. Initiating CT 0, 1, 2, 3, and 4 cm inferior to the CPA on lateral scout excluded part of the lung in 45.7%, 12.9%, 4.3%, 1.9%, and 0.8% of CTs. CONCLUSIONS: Chest CT to include the lungs should be performed from the top of the first rib to 3 or 4 cm below the costophrenic angle on lateral topogram. KEY POINTS • There is a greater motion at the inferior lung than at the superior lung. • Chest CT acquisition from the top of the first rib on scout would not exclude the lung. • Chest CT acquisition from CPA on lateral scout would exclude the lung 46% of time.