When did radiologists begin trying to minimize radiation from imaging? especially on children?have they been minimizing since the 70's or just recent?

Since 70's: The standard has been to minimize exposure since the dangers of radiation were discovered. This was the standard in the 1970's.

Since the beginning: The harm from ionizing radiation was apparent within a few years of its use in medicine. However, the science is still evolving. It is unclear what is a "safe" dose of radiation but the risk to benefit ratio is the key. The rule of thumb is to use alternative procedures if they can produce equivalent information for patient care. Use the smallest dose necessary. http://www.imagewisely.org