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New Ways in the Field of Medical Imaging: Terahertz Radiation

24 November, 2011 | imaging equipment, MedWOW, Radiology, Terahertz

The History of Terahertz (THz) Radiation
Terahertz (THz) radiation refers to the region of the electromagnetic spectrum between 100GHz and 30 THz (wavelengths of 3 mm to about 1 µm). This encompasses the region from just beyond microwaves through the far infrared and some way into mid-infrared. In the past, generating THz radiation required bulky and expensive equipment like free electron lasers or alternatively, the use of thermal sources to produce weak, incoherent radiation. Detecting (Terahertz) THz radiation was not much easier, requiring liquid helium-cooled bolometers with poor noise performance.

Radio waves sent at terahertz (THz) frequencies usually travel in line of sight. These waves, known as terahertz radiation, are in a waveband that is the overlap of what is normally regarded as microwave radiation and far-infrared light. The Earth’s atmosphere is a strong absorber of terahertz (THz) radiation, so the range of terahertz radiation is quite short. However, recent technologies using terahertz radiation have been developed, which are intended for applications including medical imaging and surveillance.

The research in terahertz radiation is almost 15 years old and includes waves between 300 GHz to 10 THz. The first imaging device based on terahertz radiation was introduced in 1995 by Hu and Nuss. Terahertz imaging has applications in security screening systems, genetic engineering, pharmaceutical quality control and medical imaging.

Technological Advances in Terahertz (THz) Imaging
The lack of viable sources and detectors leads to this band of the spectrum to be referred to as the “THz Gap”. In recent times however, advances in technology have made possible the production and detection of THz radiation with solid-state devices operating at room temperature. In doing so, a previously unavailable region of the spectrum has been made accessible, and it is a region of great potential for medical science in particular. While other portions of the spectrum are already well-established in medical applications, the properties of the THz band allow it to occupy a new niche. THz quanta are far less energetic than those of x-rays and pose no ionization hazard for biological tissue. While this is also true of microwaves, the shorter wavelengths of the THz band allow for greater spatial resolution. The technology now exists to generate and detect coherent THz radiation at useful power levels, either as a continuous wave or as a series of pulses.

Tetrahertz (THz) Radiation Applications in Medicine and Biology
THz radiation has found two fundamental modes of application in medicine and biology – spectroscopy and imaging, though far more use has been made of the latter in medical applications. An unavoidable issue when imaging with THz radiation is absorption by water. The entire THz band is strongly absorbed by water (or any polar liquid) and consequently does not penetrate moist tissue to any significant depth. So, medical imaging can be done in two different ways, first by performing transmission imaging on thin, clinically prepared tissue samples, and secondly by imaging surface features using reflection geometry imaging. Transmission imaging, for obvious reasons, can only be carried out in vitro, while a reflection geometry set-up allows the possibility of in vivo imaging.

Imaging with THz radiation has been most successful when dealing with illnesses of the skin. Work in this area initially focused on identifying previously diagnosed basal cell carcinomas. At first this was accomplished in vitro using both transmission and reflection modes, where THz showed promising tissue differentiating abilities – being able to distinguish between diseased and normal, but inflamed tissue. More recently, reflection geometry techniques have been used to take in vivo THz images of skin cancers, showing surface features and depth information. In both the in vivo and in vitro tests, THz radiation has been shown to perform well when compared to the standard in vitro histological test.

Another area where THz imaging will likely find use is wound assessment. It is also conceivable that THz imaging could be of use in monitoring treatment of skin conditions (like psoriasis), since THz imaging is cheaper than MRI and does not necessitate contact with the skin like ultrasound.

Future Potential for Dental Applications
In dentistry, in vitro experiments using THz imaging and spectroscopy have already been carried out to determine the characteristic THz properties of enamel and dentine in human teeth. THz radiation has been shown to be capable of the early detection of dental caries. Since in vivo THz imaging is currently limited to surface features, it would seem that dentistry should be a suitable field of application. However, in practice THz imaging systems are large and cumbersome and even structures as observable as teeth can make a challenging target. In this respect, THz imaging is still some way off from offering a non-ionizing alternative to x-rays in dentistry.

Image Source: Ophir-Spiricon Beam Profiler