CT Scan doses vary wildly depending on the type of X-ray (chest, head, limb, etc.), the size of the person, and a lot of other things. The bigger the cross-section of the thing you put in a scanner, the greater the exposure. CT machines themselves also vary considerably. Newer machines are often more sensitive and employ better imaging software, so they can do their job while delivering a much smaller dose. Anyone who quotes a solid number is probably using one that's absolute worst-case, which probably means a chest scan on a fat dude with a CT machine that's been around as long as disco music.
Personally, I like to think of medical imaging in terms of how long I'd have to work as a flight-attendant to get the same dose. When you fly at high altitudes (as most passenger jets do) there is less atmosphere to block cosmic radiation, which is the source of background radiation that we're exposed to every day. You get a higher dose of radiation at high altitudes amounting to a few tens of microsievert's for a decently long flight. Medical X-Rays that consist of a single photo are approximately in the same ball-park as an intercontinental flight or two, or working as a flight-attendant for a day or so. CT scans are in the ballpark of several months working as a flight-attendant. It sounds bad at first, but flight-attendants aren't known to have a massively increased rate of cancer after doing the job for decades.
In general, people fear radiation to an extent that is out of proportion with reality. To add more perspective, the background radiation in your living room will add up to a CT scan, most likely, in less than a year (unless you live in very deep mine). You can marginally improve your odds of getting cancer by avoiding medical scans, but only at the cost of massively increasing your odds of dying from something those scans might have helped diagnose. It's not an intelligent trade-off. Yes, unnecessary CT scans are bad, but not as bad as the article claims. The greatest cost of an unnecessary CT scan is probably to those who don't get scanned due to high demand on the machine. This article, by stoking irrational fears of medical imaging, is more likely to harm people than help them.
Note: Medical imaging is generally not done very often, is tracked, and is usually performed by trained technicians with quality apparatus. The same is not true of security X-Ray scans, which people can be subjected to much more frequently, are frequently performed by the near-unemployable, and use machines that are not designed with the primary goal of helping those it scans. The U.S. is also starting to use truck-based scanners to scan parked vehicles and people on public streets. You may be scanned by these not only without consent, but without knowledge. I do find this to be objectionable.