And even exposure data isn't a reliable measure because it can vary highly by such factors as age, gender, the body part exposed to radiation and the patient's size, says James A. Brink, chief radiologist at Massachusetts General Hospital and co-chairman of Image Wisely.

WSJ: New Tracking of a Patient's Radiation Exposure

If American technology develops as expected, by 2018 the new shield would cover almost all of NATO's European members against an Iranian attack only a small part of Turkey would be exposed.

ECONOMIST: Missile defence in Europe: The next salvo | The