Terahertz cancer imaging: Challenges and opportunities

Terahertz (THz) radiation has unique characteristics for cancer imaging compared with those of other electromagnetic spectral regions. Most significant properties are the non-ionization due to low photon energy, the high sensitivity to water molecules, and the capability of spectroscopic analysis [1]. Except the radiation safety, these advantages are not fully utilized in cancer imaging. The sensitivity to water molecules helps probe the dynamics of biological samples but also limits the penetration depth of THz radiation into water-abundant tissues. This difficulty can be overcome by using several techniques such as paraffin-embedding [2,3], freezing [4,5], and the use of penetration enhancing gels [6]. The spectral fingerprints of biomedical samples by the motions of biological molecules are not readily observed because of two major reasons. One is due to the relaxational absorption of THz radiation by water molecules, which can be somehow circumvented by the techniques described above. Another comes from the inhomogeneous broadening of resonances in macromolecules. Distinct spectral peaks have been measured with nucleobases, which are components of DNAs and RNAs, but the resonant features of DNAs and RNAs are hardly discovered. Nevertheless, the resonances of bio-macromolecules can be observed during the progress of biochemical changes such as getting diseased with cancer. The mechanisms of showing fingerprints are to be discussed and THz resonance markers of several cancers will be presented. The contrast enhancing techniques for cancer imaging using nanoparticle contrast agents will be briefly explained [7-9]. In conclusion, the author believes that there is a high potential for THz imaging to contribute to the diagnosis of cancers in the real-world medicine [10].