THz medical imaging: from in vitro to in vivo

Terahertz (THz) radiation has attracted considerable attention in medical imaging owing to its nonionizing and spectral fingerprinting characteristics. To date, most studies have focused on in vitro and ex vivo objects with water-removing pretreatment because the water in vivo excessively absorbs the THz waves, which causes deterioration of the image quality. In this review, we discuss how THz medical imaging can be used for a living body. The development of imaging contrast agents has been particularly useful to this end. In addition, we also introduce progress in novel THz imaging methods that could be more suitable for in vivo applications. Based on our discussions, we chart a developmental roadmap to take THz medical imaging from in vitro to in vivo. Terahertz (THz) medical imaging has been widely explored in vitro and in vivo, and has demonstrated the potential to be a unique tool to complement existing medical imaging methods.A fundamental limitation for THz medical imaging in vivo is the strong THz absorption by water. Some smart nanoparticles have been proposed to work as imaging contrast agents, which suggest a way to overcome the deteriorated THz imaging contrast in a living body.The recent development of novel THz imaging methods should promote in vivo applications, such as THz holographic imaging and near-field imaging.