A rad measures the amount of radiation absorbed by a gram of material where as a rem measures the biological effects of different kinds of radiation. To measure the degree of biological effects produced by radiation you would need to know the mrad value as well as the type of radiation because different types of radiation produce different degrees of damage. For example every 1 mrad of beta or gamma radiation produces 1 mrem, every 1 mrad of high energy protons and neurons produce 10 mrems and every 1 mrad of alpha particles produce 20 mrems.