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.