Medical applications of terahertz imaging: a review of current technology and potential applications in biomedical engineering <s> I. INTRODUCTION <s> The use of terahertz pulses for imaging has opened new possibilities for scientific and industrial applications in the terahertz frequency range. In this article, we describe the technology necessary to perform terahertz "T-ray" imaging, novel imaging techniques, and commercial applications of T-ray imaging. <s> BIB001 </s> Medical applications of terahertz imaging: a review of current technology and potential applications in biomedical engineering <s> I. INTRODUCTION <s> The rapidly developing field of terahertz (T-ray) imagin promises to provide a non-invasive method of identifying the composition of various objects. Biomedical diagnostics, semiconductor device diagnostics, trace gas analysis and moisture analysis for agriculture are among the growing number of important T-ray applications. We present results using an electro-optical sampling method and discuss how this can be simplified by sign a micro antenna array realized by MEMS technology. The challenges and advantages of both approaches are compared. <s> BIB002
Terahertz (THz) radiation, or 'T-Rays', in its broadest description refers to the region of the electromagnetic spectrum between 100 GHz 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 THz radiation was not much easier, requiring liquid helium cooled bolometers with poor noise performance. Indeed the lack of viable sources and detectors lead to this band of the spectrum being 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 so doing, 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. When interacting with heterogeneous biological tissue, the comparatively long THz wavelengths can penetrate much further than visible light or the near infrared -the longer THz wavelengths being less susceptible to effects such as Rayleigh scattering. Furthermore, THz radiation excites rotational and vibrational modes of some biological molecules and THz radiation has already been shown to have good tissue differentiating abilities BIB001 . We are in the midst of a revolution in noninvasive imaging technologies for biomedical science and optical/quasi-optical techniques are forming the vanguard in this area. THz technology is well placed to provide the impetus for the development of the next wave of noninvasive biomedical instruments and it is important that biomedical engineers and scientists embrace this technology early in its development. To this end, this paper is an exposition of THz imaging technology with an emphasis on its current applications and future directions. Our purpose here is to prime the field of biomedical engineering for this increasingly relevant technology. The layout of the paper is as follows. Section II briefly summarizes technologies required to generate and detect T-Rays. The current range of imaging applications in medicine and biology is largely determined by the absorption of T-Rays by water. In section III we give a concise account of these applications. Section IV discusses shortcomings of the technology and techniques used to address these. In Section V we propose some improvements to quasi-optical design of delivery and measurement systems that will allow THz to attain its true potential in a clinical environment. We also include a description of a proposed experimental THz Fourier optics system for probing theoretical concepts. We conclude with a brief synopsis of the future viability of the technology in the biomedical field as well as describe some of the open problems in the area. While there are numerous ways of generating THz radiation, most of the useful sources and detectors can be divided into two groups -those based on electronics and those based on photonics BIB002 . The electronic sources, which include Gunn diodes, backward wave oscillators (BWO) and super-lattice electronic devices, for example, are extensions of high frequency electronics and as such can only operate at the microwave end of the THz region. The upper frequency limit for electronic sources is approximately 300 GHz (up to 1 THz for a BWO) and they usually provide limited power at THz frequencies. Their use is not currently widespread in medical applications.