Answer to Question #2745 Submitted to "Ask the Experts"

The fetal dose from diagnostic x-ray procedures is a difficult one to estimate without any data because of the multiplicity of factors for a given examination. Factors that can affect dose determination dose include field size, kVp, filtration, thickness of region being imaged, field location, use or nonuse of a lead drape over the abdomen/pelvis of the woman, image receptor sensitivity, etc. The best approach is to use a simple phantom and make exposure measurements at depths of interest using the same parameters as those of the examination. The phantom geometric deviations, such as square vs. body-shaped contours will have minimal effects on the reading. Be sure to record the results in a log book that allows you to build up a database of the most frequently used techniques at your institution and after a number of measurements you will be able to predict most doses based on previous measurements.

Being at a major medical center, we have all the phantoms we need that are anatomically correct. At a small facility phantoms may be limited to blocks of plastic such as lucite, polystyrene, and big bottles of water. Ionization chambers used will have to be calibrated to the x-ray energies of interest to give the correct energy response. Alternately, it is possible to measure the electrical charge on the ionization chamber and convert this to dose using the Bragg-Gray principle. This involves using the stopping power ratio of the phantom media, for example, water, lucite, etc., relative to the air in the chamber, and the W value or ionization energy for air. Generally the fetal dose is quite small and an error of 10% or so is acceptable because of the negligible dose in diagnostic x-ray procedures.

James B. Smathers, PhD, CHP