An implantable medical device comprised of a lumen with a volume of from about 1.times.10.sup.-7 cubic meters to 1.times.10.sup.-5 cubic meters wherein, when said device is exposed to radio frequency electromagnetic radiation with a frequency of from 10 megahertz to about 200 megahertz, at least 90 percent of the electromagnetic radiation penetrates to the lumen of the device, and the concentration of the electromagnetic radiation that penetrates to the lumen of the device is substantially identical at different points within such lumen.