Source: Modern Medicine.

Terahertz imaging has several properties that make it attractive for in vivo imaging, researcher says Research with terahertz imaging is about one step removed from development of a system for routine use in clinical practice, according to researcher Based on steady progress in laboratory studies, researchers are optimistic about the potential for noninvasive terahertz imaging systems to become a useful tool for intraoperative delineation of nonmelanoma skin cancers (NMSCs).

In a recent publication (Lasers Surg Med. 2011;43(6):457-462), Cecil S. Joseph, Ph.D., and colleagues from the Submillimeter-Wave Technology Laboratory, the Advanced Biophotonics Laboratory, at the University of Massachusetts, Lowell, and Massachusetts General Hospital, Boston, described positive results with continuous-wave terahertz transmission imaging for differentiating between cancerous and normal skin tissue.

At the 31st annual meeting of the American Society for Laser Medicine and Surgery, Dr. Joseph reported on the next step in development of the technology involving reflectance terahertz continuous-wave imaging.

¡°Terahertz imaging has several properties that make it attractive for in vivo imaging. At its present stage, our research with this technology is about one step removed from the development of a system that would be feasible for routine use in clinical practice,¡± Dr. Joseph says.

THz potential

An H&E histology of consecutive slice (far left), a 1.4 THz transmittance image (middle), and a 1.6 THz transmittance image, showing the cancerous area outlined by the black dotted line. (Photos: Cecil S. Joseph, Ph.D.)

Terahertz wavelengths lie between the microwave and infrared regions on the electromagnetic spectrum and are of interest for use in biomedical imaging because terahertz radiation has reasonable spatial resolution, is nonionizing, and a large number of biomolecules have characteristic resonance frequencies within the terahertz wavelength range.

For imaging of NMSCs, contrast with normal skin is intrinsic so that no exogenous contrast agent is needed and appears to be related to differences in water content and in the state of the water state (bound versus free) that exist between the malignant and normal tissue, Dr. Joseph says.

Wallace and Woodward, pioneers of terahertz imaging for NMSC, originally demonstrated its feasibility using a pulsed terahertz source, and they were able to show in in vivo experiments that it could be used to discriminate between basal cell carcinomas and noncancerous skin in reflection geometry.

Dr. Joseph and colleagues have been working to develop continuous-wave terahertz imaging, which would have a number of advantages compared with a pulsed system.

¡°A pulsed system typically needs a femtosecond laser to produce the desired frequency, whereas a continuous-wave system would likely be based on a solid-state source that would make it comparatively much less expensive to produce,¡± Dr. Joseph says.

¡°Furthermore, the signal-to-noise ratio is typically better with a continuous-wave source compared with a pulsed system, and the data-acquisition time is also faster because the data are collected at a single frequency,¡± he says.

Initial study

In an initial ex vivo study, Dr. Joseph and colleagues demonstrated transmission imaging of NMSC specimens using a continuous-wave terahertz imaging system. Using fresh tissue from Mohs surgeries performed at Massachusetts General Hospital, they first isolated the frequency that would produce contrast between cancerous and noncancerous regions, and then they identified the source receiver characteristics that would provide tumor demarcation.

The performance of the imaging system for cancer delineation was determined by correlating the terahertz images with hematoxylin and eosin histopathology.

A tool that could be used to identify cancer margins in vivo would have to work in reflection modality, however, so the researchers have gone on to generate reflection images with a continuous-wave terahertz imaging system. Now, they will be correlating those images with the transmittance images and histological findings, as well as evaluating the resolution.

¡°The next step will be to isolate specific parameters for frequency and amplitude stability that will be used to identify proper source technology. Once those specifications are determined, a clinically deployable source can be tested,¡± Dr. Joseph says.

¡°So far, we have used CO2 optically pumped far-infrared gas lasers as the terahertz source for our experiments,¡± he says. ¡°These lasers have several advantages, as they have high power, are stable and allow for easy frequency selection. However, they are only suitable for use in a laboratory setting.

¡°Solid-state transceivers that can be used in clinical practice work with room temperature sources and detectors, but for adequate sensitivity, the detector has to have a very narrow frequency window to eliminate noise,¡± Dr. Joseph says.

¡°A priori knowledge of this target frequency will enable more efficient development of a continuous-wave terahertz reflection imaging system. The technology exists to build the system, but it is a matter of getting the parameters right,¡± he says.