Answer to Question #2800 Submitted to "Ask the Experts"

There are a number of textbooks that provide information on Computed Tomography (CT), such as Christensen's Physics of Diagnostic Radiology (Curry, et. al.), The Essential Physics of Medical Imaging (Bushberg, et. al.), and Radiation Protection (Euclid Seeram). The latter text is written for x-ray technologists and is a little easier to understand than the textbooks written for physics students.

According to Seeram, patient dose is affected by x-ray beam parameters and image parameters. The x-ray beam parameters include items that can be chosen or adjusted by the technologist such as kilovoltage (kVp) (the voltage supplied to the x-ray tube—determines how penetrating the x rays are), milliamperage (mA) (the electrical current provided to the x-ray tube—determines how many x rays there are), filtration (added to remove the low-energy x rays that only increase patient dose without contributing to image quality), collimation (adjusting the x-ray beam to the area desired), and detection efficiency (how many of the x rays that "hit" the detector are "seen" by the detector). Image parameters cannot be controlled by the operator and include noise (makes the image appear "grainy"), spatial resolution (how well the system produces images of very small objects), and slice thickness (which can be controlled by the operator).

Multislice CT is simply newer technology allowing the scanner to acquire more (thinner) slices in a single rotation of the gantry, leading to faster scan times. The "cardiac package" that you refer to is probably a version of computer software that allows different image reconstruction for the cardiac studies and would then have no effect on scan parameters other than slice thickness. CT fluoroscopy is used for tasks such as biopsies and needle localization. Whether the CT x-ray tube is held stationary or if a second tube is attached to the scanner (like a fluoroscopy c-arm), the dose per unit time is much lower because fluoroscopy is typically operated at a much lower tube current (mA) and is used to look at dynamics.

A CT scan will give a higher radiation dose to the patient than a standard x-ray film examination due to the increased values of the operating parameters (tube voltage [kVp] and mainly the product of tube current and time [milliamp-seconds or mAs]). The increased operating parameters are needed in order to accomplish the purpose of a CT scanner, which is to scan many very thin slices of the body while providing much more detail than a typical x ray (much easier to see small objects).

Since I am not a physician, I cannot tell you how to convince your roommate to have (or not to have) a CT scan. I can, however, recommend that your roommate (or anyone else) follow the advice of his physician. The physician can explain the benefit vs. risk for this procedure (or many others). Diagnostic studies such as this are usually not recommended unless the physician believes that the benefit to the individual far outweighs the risk involved.

If you have further interest in CT, the Web site for the Food and Drug Administration has some information, much of which pertains to offering CT for screening exams. There are a number of links that can lead you to more information on doses, risks, etc.

Ken "Duke" Lovins, CHP