This article provides not only a useful review of the ways in which nanotechnology can contribute to the field of THz medical imaging but also a review of the possibilities and shortcomings of the field in general. I believe that it will be of interest to both nanotechnology and THz researchers in general.  A good list of references is provided. The authors concentrate on issues related to contrast agents, sources and detectors and mention many of the most widely-used and promising of these. Highlighting the potential impact of nanotechnology marks this review out from other THz review articles. 

The abstract and conclusion sections are appropriate and provide a useful summary for anyone briefly scanning the article. The publication search method used by the authors is clearly described.

Minor comments

• The authors switch between THz and terahertz throughout the article.
• On page 4 (3rd paragraph). I find the sentence “The THz imaging systems can be separated into two main categories: Passive (also named incoherent) and Active (also named coherent) Pulsed or Continuous” confusing. Which are the 2 categories? Are all passive systems incoherent and pulsed? etc. Incoherent and coherent refer to the detection of the radiation, not whether the imaging set-up is active or passive. Should it be phrased “THz imaging systems can be separated according to whether they are active or passive, incoherent or coherent, continuous or pulsed”.
• Page 4 2nd column, 2nd paragraph “.. separated depending on whether …” or _“separated on the basis of...”  _Would it be worth a line in the text (it is in table 2) that an advantage to THz is that it can propagate through materials such as bandages.
• Towards end of p.8 change "capacity" _ to "_heat capacity".
• In the Conclusion, 1st line, “..emerging THz imaging modality..." _should be changed to “..emerging THz imaging modalities.”_