Contribution Invited Oral
Biomedical Applications of Terahertz Radiation
Early experiments by pioneers of THz imaging demonstrated that the technique could have a variety of applications, including medicine and dentistry - terahertz images of biological tissues showed contrast between lean meat and fat; changes in refractive index of teeth at THz frequencies allow for the detection of early decay.
Since then there have been many studies of tissues, from wax embedded tissue block, histopathological samples, excised tissues and even some in vivo imaging. Some of this most significant work revealed contrast between regions of healthy skin and basal cell carcinoma, the most common form of skin cancer, in vitro and in vivo. Further, other work on excised breast carcinoma has shown good contrast between tumours and normal tissues. This has led to the development of a prototype handheld (intra-operative THz) imaging probe for use during breast surgery.
The high absorption of THz by water in this range (23 mm−1 at 1 THz) makes transmission imaging through a body impossible, but it is the difference in absorption due to water content which explains the contrast seen between muscle and adipose tissue and between tumour and normal tissue. Terahertz spectroscopic measurements of carcinoma show significant differences in the frequency dependent refractive index and absorption coefficient. Such contrast on the surface is often obvious at optical frequencies but it is the ability of THz to penetrate below the surface that provides the potential for medical imaging, in particular, of epithelial tissues (cancer with origins in tissue surfaces both external and internal), which includes skin, breast and colon cancer, accounts for 85% of all cancers.
It is the ability of THz to penetrate just below the surface that provides the, as yet not fully realised, potential for tissue imaging, visualisation of the subsurface spread of tumours and other applications. Full understanding of the contrast mechanisms in tissues and their physiological significance will enable THz to fulfil its potential for medical applications.
To date in vivo measurements have been limited due to the restricted nature of imaging systems. Compact, mobile THz imaging systems are now being developed which allows for TPI measurements in a clinical setting.