CT scans may be being over-used on children. See the report in the Los Angeles Times, where you will read that "CT scans are first-rate diagnostic tools, but they rely on radiation -- and children are more vulnerable than adults to the risks of radiation. Their smaller organs are more sensitive to it and they have more years in which to develop cumulative radiation damage."
I personally believe that X-rays and CT scans are routinely being used more than is necessary or desirable - e.g. mammography screening for breast cancer - and I decline them when I judge it to be the safer option.
You may be interested to look at this Radiation Chart and at this extremely informative webpage about the use of diagnostic technologies.