Abstract: A medical or surgical device or tool that is designed to be implanted or inserted inside the human or mammalian body, having at least part of its surface coated whereby the ultrasound visibility of said device or tool in vivo is enhanced, wherein the coating comprises one or more of the following: (i) a matrix material containing a plurality of contrast enhancing elements; (ii) magnesium; (iii) a liquid or polymer which alters its ultrasound imaging properties upon elevating the temperature from ambient to physiological temperature; (iv) a liquid or polymer which alters its ultrasound imaging properties as a result of a change in pH; (v) an essentially non-polymeric bio-compatible compound which forms a discontinuous coating. Preferably the device or tool is a radioactive source for use in brachytherapy, and in particular a radioactive seed.