Let's first define some terms. I don't want to lose you in the detail of sievert (Sv) vs gray (Gy) as units of radiation measurement because there is no meaningful way you can calculate your individual risk based on adding up sources of ionizing radiation. Any assessment of risk must be done by statistical studies of large numbers of people. Technical accuracy and detail is more of a distraction to achieving my aims, so apologies in advance to the techies & quants in the community. For the obsessed, another rabbit hole of info: <http://en.wikipedia.org/wiki/Gray_(radiation)>
CT means “Computerized Tomography” and is the same as a CAT scan or “Computerized Axial Tomography” scan. There is some indication that there is a place for FDG-PET (Fludeoxyglucose Positron Emission Tomography) scan with some CLL patients showing indications of potential RT (Richters Transformation). More on that later. Been at NIH, done that. <http://en.wikipedia.org/wiki/Positron_emission_tomography>
CT scanners are produced by many companies and the technology is rapidly changing. A quick review of manufacturers lists GE with 9, Siemens with 9, Philips with 7 and Toshiba with 7 models. I kinda like the creative naming of the Siemens models with their “Somatom Spirit Dual” model, where I would expect all technicians to be in saffron robes and patients achieving transcendence after their 10th scan or the “Emotion Duo Dual” no doubt used in marriage counseling sessions(-; Names actually can hint at a lower radiation dose machine if it has “dual” in it. CT scanners cost big bucks and many older machines work well so are kept on which deliver much higher amounts of Ionizing radiation than the newest models. Do you know how old the machine is that you will be scanned on? Ask. Software updates for older machines can reduce radiation exposure but not all older machines can be successfully retrofitted. Ask about that. Relying on CT scan technicians and technical literature, the range of radiation, in terms of “chest X-Rays”, can vary from 30 to 450 per scan! If you are about to be scanned, which machine would you opt for? The CT that will give you 450 chest X-Rays per scan will not deliver better information than the CT that delivers 30 chest X-Rays of Ionizing radiation. Any machine described as a dual source machine will give some confidence that you are in the lower ranges of radiation exposure. On larger medical campuses you may have a choice of newer/better vs older/worse CTs.