A system and method of imaging tissue includes administering a contrast agent having charged gold nanoparticles in suspension into a vessel of the subject, such that the nanoparticles are carried into the tissue and performing microwave imaging of the tissue after administering the contrast agent. In embodiments, the nanoparticles have a tissue-selective protein tag. In embodiments, images are taken prior to administering the contrast agent, and further images may be taken during an agent-washout period after imaging with contrast agent. The contrast agent is injectable, with the nanoparticles suspended as a colloid in a biocompatible, isotonic, carrier. In particular embodiments, the nanoparticles have median diameter of less than fifty nanometers, or less than five nanometers, and may have a tissue-selective protein tag. A microwave imaging system has injection apparatus with the gold-nanoparticle agent, and is configured to take, and difference, pre and post contrast images as well as washout images.
