I have heard numerous times when getting x-rays, MRIs, CAT Scans, etc. that each one is equivalent to a cross country airplane trip. Disregarding the different types of radiation as asked in this question, I was wondering if they are mixing total dosage over the entire body with the same dosage in a concentrated area.
To illustrate by example, if the total radiation received on a plane trip is 100 'rad' units and the body is 100 area units each body part would only receive 1 rad unit of exposure. Whereas if an x-ray is the same 100 rad units but your arm (for example) is only 10 area units then your arm is actually receiving 10 rad units of exposure - or 10 times the amount received on a plane trip.