Answer to Question #6915 Submitted to "Ask the Experts"

Q

To rule out a pulmonary embolism I had a CT (computerized tomography) image of my chest (with contrast). I wasn't really put at ease by the technologist performing the CT; he seemed to be in a hurry. I was placed on the bed and they positioned me so my chest was inside the gantry of the CT machine. An automated voice asked me to hold my breath and the bed moved slightly until the automated voice told me breathe normally. At this point my chest was still inside the chamber of the CT machine and I lay there for about four minutes until the automated voice again asked me hold my breath, the bed moved slightly, and I felt the dye being injected. The machine was noisy during the four-minute wait. Do you think I was exposed to radiation during the four-minute wait, or are the x rays only emitted when the bed moves and the automated voice asks the patient to hold his/her breath? Hope this isn't a silly question . . . I don't like the idea of being exposed to x rays and the machine seemed to be making the same noise constantly while my chest was in the gantry. Hence, I was wondering if the tube in the machine was revolving around me and emitted x rays while I was waiting for the next command.

A

You ask a very good question. Typically, CT scanners involve x-ray exposure ("on" time) on the order of seconds. It sounds as if you received the first scan and then, a few minutes later, you received another scan. The estimated four-minute delay is probably the amount of time needed for the contrast (dye) to adequately circulate before the scan with contrast (the second one) was taken. The reason you heard the same continuous noise from the gantry during the entire time you were lying on the table was due to the fact that the x-ray tube continues to rotate in the gantry as part of the x-ray tube cooling process after the exposures.

Ken "Duke" Lovins, CHP