You ask an important question. I'm afraid that all the scans at doctors' disposal these days are overused for two reasons. The first is defensive medicine: doctors order all these scans not for medical reasons but to protect themselves from the ever-present threat of malpractice litigation. The percentage of diagnostic X-rays and computed tomography (CT) scans that fall into this category is high, which is yet another reason for the daunting cost of health care in the United States.
The second reason is, sadly, the profit motive. The machines are costly, and the owners of for-profit health care centers must use them a lot to recoup their investment. Doctors who work for them are often pressured to order scans against their best medical judgment.
You're quite right that all this testing is risky. X-rays and CT scans expose the body to ionizing radiation, which increases the risk of cancer by damaging genes. It is speculated that CT scans being done now will result in as many as two percent of the fatal cancers that will occur in the next 10 to 20 years.
Another danger is that indiscriminate use of high-tech scanners frequently turns up anomalous findings that have no clinical significance but still draw the attention of doctors, make patients anxious, and lead to more intensive and invasive testing. Patients in these situations are drawn deeper into the world of hospital medicine, where they may be persuaded to accept unnecessary treatments. The end result is more expense and sometimes grave physical harm.
As a patient, all you can do to protect yourself from an unnecessary test and the risk it carries to is carefully question doctors as to why a recommended test or scan is needed and make sure that you're satisfied that it isn't being done for reasons that don't concern your welfare. Bear in mind that high-tech medicine is effective for the diagnosis and treatment of severe illness and injury, for diseases involving vital organs, and for other dire conditions. On average, however, such cases represent a minority of the health problems that most physicians see. Rather than worrying about the tests and scans you've already had, I would urge you to:
Andrew Weil, M.D.