Answer to Question #11074 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
I am a certified radiologic technologist. I am being trained in a small-town hospital and in the use of their equipment. I am being told by the current computerized technology (CT) technicians that the size of the scout box that you set up in the computer does not affect the overall patient dose for the study. My understanding is that the size of this scout box is directly correlated to the kVp and mAs emitted by the scanner to perform the study. If I am correct than it stands to reason that the patient is being subjected to a higher radiation exposure. The current technicians merely cone in the data from the scan to do their formatting prior to the radiologist seeing the scan. I believe that this results in unnecessary exposure to the patient. Please advise.
My understanding is that the scout image is acquired with the tube in a fixed location (PA or LAT). It is a bit like acquiring an x ray but the table movement enable as much (or as little) of the anatomy to be imaged. From this the scanner determines what the mAs (and for newer scanners also the kV) is to be for each rotation. So if you scan from neck to pelvis for the scout but only do the CT scan for a portion of the abdomen, the technique used will be what the scanner has determined it to be from the scout of that portion of the body (using auto mA). The extra radiation to the rest of the body is then only from the scout scan that was done on those areas. The "box" set should be set (in the cranio - caudal direction) to only include the areas of interest. So if the area of interest is the lungs it is overkill (and more dose to patient) to do a CT scan from neck to pelvis.
Harjinder Khaira, MS, DABR