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Terahertz Imaging System for Biomedical Applications: Current Status A. K. Panwar, Abhisek Singh, Anuj Kumar, and Hiesik Kim
Abstract—
Terahertz (THz) radiation lies between the infrared and microwave regions of the electromagnetic spectrum. The THz technology has important opened up many opportunities in the area of medical. Advances in THz medical imaging field are immature. In this paper presents a review of the status of the THz imaging system and application in area of biomedical such as breast and colon cancer tissue. We give the main focus of this study of mapping margins of tumors in earlier stage based on terahertz imaging system such as terahertz pulse imaging, terahertz time domain spectroscopy, continuous wave terahertz, and THz generation with schottky diode and without beam stop.
Index Term— Terahertz radiation, biomedical imaging system, breast and colon cancer, THz-TDS , Terahertz pulse imaging. I. INTRODUCTION Over the past quarter century terahertz (THz) imaging technology has become increasingly important for biological applications such as breast tumors, skin cancer, cervical cancer and colon cancer. The THz frequency range is particularly interesting for biosensing applications because numerous characteristics vibrational modes of macro-molecules, like proteins or DNA [1-6]. Recently cancer is a leading cause of death worldwide. According to World Health Organization (WHO), an estimated data of people worldwide were diagnosed with cancer in 2008 is given in Table I. About 70% of all cancer deaths occurred in low- and middle-income groups. Deaths from cancer worldwide are projected to continue to rise to over 13.1 million in 2030 [7]. Breast Cancer is the second leading cause of death in US and India. Breast Cancer patients diagnosed are characterized into three cases, first case 90% patients diagnosed will undergo surgery to treat the disease, in second case, 60% will undergo breast conserving surgery, according to breast conserving surgery the primary tumor is removed with a margin of normal tissue around it and remaining mastectomy.
A. K. Panwar is with the Department of Applied Science, Delhi T echnological University, Shahbad Daulatpur, Main Bawana Road, Delhi42, India (
[email protected]). A. Singh is with the Department of Electronics Engineering, Radha Govind Engineering College, Meerut, UP, India (
[email protected]). A. Kumar and Hiesik Kim are with the Department of Electrical, Electronics, and Computers Engineering, University of Seoul, South Korea (Corresponding Author:
[email protected] and
[email protected])
Around 10 – 15% of patients will require a second operation, as the margins are not free of cancer on histopathology [8]. T ABLE I DEATH DATA VS CANCER (WHO STANDARDS 2008 ) Cancer Name Cases Lung Cancer 1.37 Million Stomach Cancer 0.736 Million Liver Cancer 0.695 Million Colorectal Cancer 0.0608 Million Breast Cancer 0.458 Million Cervical Cancer 0.275 Million
Most women with breast cancer will have some type of surgery. Surgery is often combined with other treatments such as radiation therapy, chemotherapy, hormone therapy, and/or targeted therapy. Surgery and radiation therapy are associated with each other. Radiation is used to destroy cancer cells remaining in the breast, chest wall, or underarm area after breastconserving surgery. Radiation may also be needed after mastectomy in patients with either a cancer that is larger than 5 cm in size or when cancer is found in the lymph nodes. There are two types of radiation therapy. External beam radiation is the usual type of radiation for women with breast cancer. Radiation is focused from a machine outside the body on the area affected by cancer. This usually includes the whole breast and, depending on the size and extent of the cancer, may include the chest wall and underarm area as well. External beam radiation therapy is typically administered over a period of 5 to 6 weeks; however, in recent studies, shortening the treatment to 3 weeks appears to be just as effective. Internal radiation therapy, known as brachytherapy, uses a radioactive substance sealed in needles , seeds, wires, or catheters that are placed directly into or near the cancer. Some patients are treated with both internal and external radiation therapies in combination. The way the radiation therapy is given depends on the type, stage, and location of the tumor being treated [9,10]. The third common cancer in both woman and man is a colon (gastrointestinal) cancer. As with most cancers, early detection is essential for improved survival rates. The other method is endoscopy, endoscopy is the accepted go ld standard for screening and surveillance of these cancers, but the technique is far from perfect. Because during routine endoscopy, multiple random biopsies are often required, during this procedure increases the risk of bleeding. These biopsies are processed, cut into thin slices, and observed under a microscope (histopathology). A large proportion may turn out normal and thus were not required. There is a need for better endoscopic visualization in specific circumstances such as the detection of dysplastic lesions (precancerous tissue), with the ultimate goal of improving sensitivity and specificity compared with
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International Journal of Engineering & Technology IJET-IJENS Vol:13 No:02 34 histopathology [8]. There is a clinical need to be able to used for 3D imaging of teeth and may be more accurate than accurate defines the margins of tumor and accuracy during conventional X-ray imaging in dentistry [20-22]. surgery, conserve normal tissue and minimize the number Experiments with THz radiation date back to measurements of of second surgical procedures/unnecessary biopsies [11,12]. black body radiation using a bolometer by Heinrich Reubens and Researchers and Scientist presented many imaging Ernest Fox Nichols in the 1890s [23,24]. In 1975, David Auston technique for mapping margins of tumors [13] but currently at AT&T Bell Laboratories developed a photoconductive emitter this field is immature. In view of ever increasing the cas es gated with an optical pulse that accelerated progress towards of breast cancer, skin cancer, cervical cancer and colon bridging the THz gap [25-27]. cancer these system should have the facilities to detect and quantify the cancer tissue in primary stage. Hence, there is a III. TERAHERTZ IM AGING SYSTEM growing demand for automatic, low cost, and easy to Medical imaging is the technique and process used to create operate THz imaging System. In this paper, introduce a images of the human body for clinical purposes (seeking to terahertz imaging with breast cancer and provides a short reveal, diagnose or examine disease) or medical science. review of recent advances in terahertz imaging system and Although imaging of removed organs and tissues can b e system for biomedical applications . performed for medical reasons. Measurement and recording techniques which are not primarily designed to produce images, II. TERAHERTZ RADIATION such as electroencephalography (EEG), magnetoTerahertz (THz) radiation (frequency: 0.1-10THz, encephalography (MEG), electrocardiography (EKG) and others, wavelength: 3mm - 30µm, wave-number: 3.3 – 333cm-1 , but which produce data susceptible to be represented as maps energy: 0.41 – 41MeV, color temperature: 1 – 100K) lies (i.e. containing positional information), now a days can be seen between infrared light and microwave in the as forms of medical imaging. Currently medical imaging electromagnetic spectrum, and it shares some properties dependents on terahertz imaging, Terahertz imaging is still a very with each of these [14]. Like infrared and microwave immature field, with the majority of research focused on radiation, terahertz radiation travels in a line of sight and is Instrumentation and hardware. The THz imaging system are non-ionizing. Like microwave radiation, terahertz radiation classified into two categories active (imaging or spectroscopy) can penetrate a wide variety of non-conducting materials. and passive (incoherent), active further classified into two Terahertz radiation can pass through clothing, paper, categories such as pulsed and continuous wave (CW). Many cardboard, wood, masonry, plastic and ceramics. The research groups have explained the THz imaging technique and penetration depth is typically less than that of microwave system [28-44]. radiation. Terahertz radiation has limited penetration In 1995, Hu and Nuss at Bell laboratories presented a first through fog and clouds and cannot penetrate liquid water or terahertz imaging system and this system based on optoelectronic metal [15]. terahertz time-domain spectroscopy as shown in Fig 1. Terahertz Terahertz radiation is emitted as part of the black body time-domain waveforms are down converted from the terahertz radiation from anything with temperatures greater than to the kilohertz frequency range, and the waveform for each pixel about 10K. While this thermal emission is very weak, is frequency analyzed in real time with a digital signal processor observations at these frequencies are important for to extract compositional information at that point. As first characterizing the cold 10-20K dust in the interstellar applications demonstrate for industrial inspection, in a similar medium in the Milky Way galaxy, and in distant starburst manner X-rays inspection, an image is formed from the galaxies. Telescopes operating in this band include the differences in transmittance through different materials inside a James Clerk Maxwell Telescope, the Caltech Sub- concealed package Fig 1 shows a THz image of a semiconductor millimeter Observatory and the Sub-millimeter array at the integrated circuit chip package and they also performed a Mauna Kea Observatory in Hawaii, the BLAST balloon biomedical application such as meat absorbs THz radiation borne telescope, the Herschel Space Observatory, and the because of its moisture content, whereas fat is nearly transparent Heinrich Hertz Sub-millimeter Telescope at the Mount to THz radiation. The terahertz image of a piece of bacon and are Graham International Observatory in Arizona. The able to map out the fat distribution and other some possible Atacama Large Millimeter Array, under construction, will applications to food inspections. This technology could also be operate in the sub-millimeter range. The opacity of the extended to medical tissue and biomedical studies such as skin Earth's atmosphere to sub-millimeter radiation restricts cancer detection, as the water retention of tumors may be these observatories to very high altitude sites, or to space different from that healthy tissue [27]. Loffler et al 2001 present [16-19]. a dark-field imaging in the terahertz (THz) frequency regime Contrary to X-rays, terahertz radiation has relatively low with the intention to enhance image contrast through the analysis photon energy for damaging tissues and DNA. Some of scattering and diffraction signatures. A gold-on-TPX test frequencies of terahertz radiation can penetrate several structure and an archived biomedical tissue sample are examined millimeters of tissue with low water content (e.g., fatty in conventional and dark-field transmission geometry. In tissue) and reflect back. Terahertz radiation can also detect particular, the capability of the technique for tumor detection is differences in water content and density of a tissue. Such addressed [28]. methods could allow effective detection of epithelial cancer with a safer and less invasive or painful system using imaging. Some frequencies of terahertz radiation can be 131502-5757-IJET-IJENS © April 2013 IJENS
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International Journal of Engineering & Technology IJET-IJENS Vol:13 No:02 35 temporal and spatial resolutions of this imaging system are mainly limited by the laser pulse duration and the diffraction limit of the THz beam, respectively [47]. Chan et al describe a terahertz imaging system that uses a single pixel detector in combination with a series of random masks to enable high -speed image acquisition. The image formation is based on the theory of compressed sensing, which permits the reconstruction of a N-byN pixel image using much fewer than N2 measurements. This approach eliminates the need for raster scanning of the object or the terahertz beam, while maintaining the high sensitivity of a single-element detector and demonstrated the concept using a pulsed terahertz time-domain system and show the reconstruction of both amplitude and phase-contrast images [48]. Chen et al. Fig. 1. First schematic of the T Hz imaging system proposed by Hu and presented optical gating beam on a semiconductor wafer, nearNuss [27]. field terahertz (THz) imaging with a dynamic aperture. The spatial resolution is determined by the focus size of the optical Wallace et al developed a pulses generates instrument of gating beam and the near-field diffraction effect. THz imaging broadband terahertz radiation in the range 0.05 to 4 THz with sub-wavelength spatial resolution (better than 50μm) is with a spectral resolution of 0.03 THz. The broadband demonstrated [49]. Mittleman et al. present a terahertz ―T-ray‖ signal-to-noise ratio is typically around 4000:1. The optics imaging system as shown in Fig 2. This relatively new imaging is raster scanned in the x-y plane to collect a grid of pulses. technique, based on terahertz time-domain spectroscopy, has the The dataset is three-dimensional with time as the third axis. potential to be the first portable far-infrared imaging The sample is placed on the quartz imaging plate and a 2 spectrometer and give several examples which illustrate the cm2 area can be scanned in a few minutes [40]. Dobroiu et possible applications of this technology, using both the amplitude al present an imaging system designed for use in the and phase information contained in the THz waveforms. terahertz range. As the radiation source a backward-wave Advanced signal processing tools are exploited for the purposes oscillator was chosen for its special features such as high of extracting tomography results, including spectroscopic output power, good wave-front quality, good stability, and information about each reflecting layer of a sample. They also wavelength tune-ability from 520 to 710 GHz. Detection is described the application of optical near-field techniques to the achieved with a pyro-electric sensor operated at room THz imaging system. Substantial improvements in the spatial temperature. The alignment procedure for the optical resolution are demonstrated. Terahertz time-domain spectroscopy elements is described, and several methods to reduce the (THz-TDS) is a spectroscopic technique in which the properties etalon effect that are inherent in monochromatic sources are of a material are probed with short pulses of terahertz radiation. discussed. The terahertz spot size in the sample plane is 550 The generation and detection scheme is sensitive to the sample µm (nearly the diffraction limit), and the signal-to-noise material's effect on both the amplitude and the phase of the ratio is 10,000:1; and are also measured other terahertz radiation. In this respect, the technique can provide characteristics. A number of preliminary applications are more information than conventional Fourier-transform also shown that cover various areas : nondestructive real- spectroscopy, which is only sensitive to the amplitude [50,51]. time testing for plastic tubes and packaging seals; biological Ostmann et al. presented THz imaging spectrometer based on terahertz imaging of fresh, frozen, or freeze-dried samples; continuous-wave THz radiation for investigation of a human paraffin-embedded specimens of cancer tissue; and liver with metastasis. The developed setup is less expensive than measurement of the absorption coefficient of water by use conventional time-domain imaging systems that comprise of a wedge-shaped cell [45]. K. Kawase et al developed a Femtosecond lasers. The system uses a two-colour externalnovel basic technology for terahertz imaging, which allows cavity laser diode. Hence it is much more compact as compared detection and identification of drugs concealed in to systems based on optically pumped solid-state lasers [52]. envelopes, by introducing the component spatial pattern Usami et al. developed a real-time THz imaging system based on analysis. The spatial distributions of the targets are obtained the two dimensional (2D) electro-optic (EO) sampling technique. from terahertz multispectral trans -illumination images, The 2D EO-sampling technique is explained THz images using a using absorption spectra measured with a tunable terahertz- CCD camera at a video rate of up to 30 frames per second. A wave source. As a reference, methamphetamine and spatial resolution of 1.4 mm was achieved. This resolution was MDMA samples are used because these samples are illegal reasonably close to the theoretical limit determined by drugs in Japan, and aspirin [46]. Wu et al presented a novel diffraction. We observed not only static objects but also moving electro-optic sampling system for real-time terahertz (THz) ones. To acquire spectroscopic information, time-domain images imaging applications. By illuminating a 6×8 mm2 ZnTe are collected [53]. crystal with a 300 μW optical sampling beam and detecting the beam with a digital CCD camera, we achieved timeresolved images of pulsed far-infrared radiation emitted from an unbiased GaAs wafer. At the focal point of the peak far-infrared field, the THz beam diameter is approximately 0.75 mm (full width at half-maximum). The 131502-5757-IJET-IJENS © April 2013 IJENS
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International Journal of Engineering & Technology IJET-IJENS Vol:13 No:02 36 sometimes only the intensity information is of interest, the data structure and post-processing are quite simple [58]. Verghese et al. also demonstrated that coherent detection is possible in the reverse scheme [59]. Menikh discussed a primary goal of current research in this field is to improve the THz sensor dynamic ranges, achieve faster data acquisition, and reduce water vapor absorption, THz bio-sensing capabilities, progress and limitations. THz-Generation System without beam-stop is shown in Fig 4. THz generation system used a pair of photodiodes measures the difference in current intens ities. A photodiode is a type of photo-detector capable of converting light into either current or voltage, depending upon the mode of operation. Photodiodes are similar to regular semiconductor diodes and Fig. 2. Schematic of T Hz-T DS System [50,51]. advantageous of photodiodes are fast response time, Excellent Lee et al. presented a real-time, continuous-wave terahertz linearity of output current as a function of incident light, low noise, low cost, and long life time [60]. Yu et al. discussed a imaging with a 10 mW, 2.52 THz (118.8 µm) far-infrared gas laser and a 160×120 element micro-bolometer camera. recent advances in terahertz imaging, spectroscopy techniques for cancer diagnosis. In Fig 5, the source is frequency tunable The micro-bolometer camera is designed for wavelengths of 7.5-14 µm but retains sensitivity at terahertz (THz) THz oscillator module and the detector is a schottky diode. In this case, the schottky diodes work as a receiver to detect the frequencies. The setup has no moving parts, and power of the transmitted through the sample. A schottky diode is transmission-mode THz images can be obtained at the a type of semiconductor which has a metal-semiconductor video rate of 60 frames/s. The peak signal-to-noise ratio is barrier; it is fast switching action, very low forward voltage drop, estimated to be 13 dB for a single frame of video, acquired in 16 ms. With this setup, THz imaging through a FedEx low junction capacitance, less temperature dependence, and envelope is demonstrated, showing the feasibility of real- highly sensitive at the low frequency region for power detection . time mail screening [54-56]. Knobloch et al. presents a THz The main advantage of schottky diode is the switching speed controlled by thermalization of hot injected electrons across the investigation of histo-pathological samples including the barrier picoseconds. [61]. Tonouchi presents an overview of the larynx of a pig and a human liver with metastasis. Knobloch measurements show that different types of tissue status of the technology, its uses and its future prospects are presented [62]. Tewari et al developed a reflective THz imaging can be clearly distinguished in THz transmission images, system sensitive to small variations in water concentrations. either within a single image or by a comparison of images Biological tissues such as skin, eyes and teeth are imaged to obtained for different frequency windows. This leads to the ascertain the systems response to tissue hydration. Difference in problem that images obtained for different frequencies water concentrations translated to contrast in the THz images. inherently have a different spatial resolution. An image obtained from two such images by a simple mathematical Contrast is also seen in THz images of skin cancer and burns suggesting the potential diagnostic capability of THz imaging operation may contain arte facts and discussed measures to deal with this problem. Finally, presents The possibility of system in clinical settings. All specimens analyzed were freshly excised ex-vivo tissues. These encouraging preliminary results improving the spatial resolution of THz images and the CW THz imaging system based on a photomixer and an external have motivated us to explore the in vivo potential of our imaging system [63]. Joseph et al presents a continuous wave terahertz cavity semiconductor laser that allows for simultaneous imaging system of non-invasive medical imaging modality for two-mode operation [57]. Gregory et al. presented a detecting different types of human cancers [64]. continuous-wave all-optoelectronic terahertz imaging system based on diode lasers. The coherent detection scheme is phase sensitive and operates at room temperature. Continuous-wave terahertz (CW-THz) radiation can be produced by photo-mixing two CW lasers in a photoconductor. The difference in frequency of the two lasers is tuned to the THz region, and monochromatic CW THz is emitted at the beat frequency. This can be achieved using diode lasers, addressing the issue of cost. The schematic diagram of CW-THz system as shown in Fig 3 and shows a combining beam-splitter, optical delay line and imaging optics. A typical CW system that mixing of two wavelength using two CW lasers in a photoconductor produces beating, which can modulate the conductance of a photoconductive switch at the THz difference frequency, since the source spectrum of the CW system is narrow and
Fig. 3. CW-T Hz schematic diagram (presented by I. S. Gregory) [58].
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Fig. 5. T Hz system with schottky diode (proposed by Calvin Yu) [61]. Fig. 4. T Hz-Generation system without beam stop (A. Menikh) [60]. T ABLE II AVAILABLE T HZ SYSTEM DEVELOP ED BY WELL KNOWN GROUP [65] Mode l Name
Fre que ncy or Spectral Range and Resolution
0.06THz – 4 THz (2cm-1 – 120cm-1 ), resolution 7.5GHz, SNR 70dB ≥3 T Hz (resolution upto 250MHz, dynamic range 60 dB Reception rate-250MHz, time measurement window – 4ns, wavelength – 1560nm, scan duration – 1s to 0.1ms ASOPS Dual Reception rate – 100MHz, time Color measurement window – 10ns, scan 1560/780nm duration 1s to 0.1ms, two color – 1560nm, 780nm T ERA – ≥3 T Hz, scane range – 300ps, K15 wavelength 1560nm T ERA – K8 ≥3 T Hz, wavelength 780nm T PS Spectra 3000 OSCAT T ERA Image ASOPS-THz (T win -250)
Measurement
Groups Anteral T eraview
T DS
Menlo-system
an 8 hour period. These images clearly show the formation and progression of edema in and around the burn wound area. For cornea, experimental data measuring the hydration of ex vivo porcine cornea under drying is presented demonstrating utility in ophthalmologic applications [67]. Ajito and Ueno present a THz-TDS spectroscopy beneficial for analytical chemistry and improved the sensitivity of THz time-domain (TDS). THz chemical imagings are very powerful tool in biology, pharmacology, and life sciences hydrogen bond distributions. THz-TDS is also promising for the quantitative chemical analysis and detection of molecules and clusters in nano-space and ice [68-70]. Special-purpose instruments have been demonstrated for imaging [71,72], spectroscopy [73,74], radio-astronomy [75], sensing [76], biology, etc. Calibration techniques of the instrument are presents in [77].
Upto 3.5THz (116cm -1), excellent spectral resolution better than 5GHz (0.17cm-1 ), non-dest. measurement PB-7100 0.1 – 1.9T Hz, 0.1GHz resolution, SNR 80dB @200GHz, SNR 60dB@1T Hz T -Spec
IV. CANCER STUDY THz imaging system should then be able to detect the early cancer before it is visible or sensitive to any other means. The Emcorepterahertz imaging system means terahertz radiation is nonhotonic system ionizing and not highly scattered in tissues (unlike optical FDS T Hz-CW 0.05 – 1.8THz, Accuracy – 0.1GHz, T optica emission), thus making it elegant for use in biomedical resolution 1MHz, SNR – applications [8, 13]. Researchers were studied to the terahertz 70dB@500GHz properties such as absorption coefficient and refractive index CW-400 0.05 – 1.5THz, resolution – 0.1GHz, T eraview of the images of the cases of breast and colon cancer. The SNR- 50dB@500GHz absorption coefficient and refractive index of breast T ray – 400 0.02 – 2THz, scan range 2.8ns, SNR – T DS Picometrix tumor/colon cancer tissue are higher in comparison to the 70dB normal tissue. These changes are dependable of higher water content and structural changes, like increased cell and protein Mini-Z 0.1 – 4 T Hz, resolution < 50GHz, Zomegaterahert density. The absorption spectrum of water exhibits a very SNR –
[email protected] z strong, broad peak centered at 5.6 THz attributed to resonant stretching of the hydrogen bond between water molecules. The effect of this absorption peak, which extends down to the Popovic and Grossman presented an overview of measurement frequency range used in terahertz pulse imaging, makes this techniques used in the THz region of the electromagnetic technique highly sensitive to water concentration. Thus, water spectrum, from about 100 GHz to several THz and also absorption is evident in the terahertz properties measured for described components of measurements for THz metrology, soft tissues, which explains the contrast seen between, for such as sources, detectors and available instrumentation for example, muscle and adipose tissue [40]. THz metrology. Table II represents the available THz imaging system developed by well known group [65]. Friederich et al V. CONCLUSIONS investigated of five active THz imaging modalities during the THz imaging is an initial stage of development for breast last few years for real-time imaging [66]. Taylor et al cancer and colon detection technique. However, THz imaging presented a THz imaging based two medical applications such has great potential to be a valuable imaging technique in the as skin burns and cornea. For burns, images of second degree, future, particularly for cancer diagnosis. Mostly THzpartial thickness burns are obtained in rat models in vivo over 131502-5757-IJET-IJENS © April 2013 IJENS
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International Journal of Engineering & Technology IJET-IJENS Vol:13 No:02 38 [13] A. J. Fitzgerald, V. P. Wallace, M. L. Jimenez, L. Bobrow, R. J. Pye, generation systems are classified into two parts such as A. D. Purushotham, and D. D. Arnone, ―Terahertz pulsed imaging of sources and detectors. Sources are further classified into two human breast tumors‖, Radilogy, vol. 239, no. 2, pp. 533-540, 2006. categories: pulsed broadband and continuous narrowband. The [14] G. Chattopadhyay, E. Schlecht, J. S. Ward, J. J. Gill, H. H. S. Javadi, most common approaches for generating broadband THz F. Maiwald, and I. Mehdi, ―An all solid state broad band frequency multiplier chain at 15 GHz‖, IEEE Trans. Microw. Theory Tech., vol. pulses are photoconductive antenna and optical rectification, 52, no. 5, pp. 1538-1547, 2004. while voltage controlled oscillators or dielectric resonators are [15] P. A. George, W. Hui, F. Rana, B. G. Hawkins, A. E. Smith, and B. J. the two widely used sources for generating low power Kirby, ―Microfluidic devices for terahertz spectroscopy of bio narrowband continuous THz waves. A THz-Generation molecules‖, Optics Express, vol. 16, no. 3, pp. 1577-1582, 2008. [16] J. A. Zeitler, P. F. Taday, D. A. Newnham, M. Pepper, K. C. Gordon, System without beam stop is a highly sensitive detection and T . Rades, ―T erahertz pulsed spectroscopy and imaging in the system. Some THz generation system used the broadband pharmaceutical setting – a review‖, J. Pharmacy and Pharmacology, detectors. The broadband detectors based on thermal vol. 59, pp. 209-223, 2007. absorption are commonly used to detect low THz signal. In [17] A. Rostami, H. Rasooli, and H. Baghban, ―T erahertz technology: fundamentals and applications‖, Lecturer notes in Electrical this reason, they require cooling units, to reduce thermal Engineering, vol. 77, 1 st Edition, Springer, 2011. background. For pulsed THz detection, in THz time domain [18] R. Bogue, ―T erahertz imaging: a report on progress‖, Sensor Review, systems, coherent detectors are required. The whole CW vol. 29, issue 1, pp. 6 – 12, 2009. system dependent of laser diodes and thus it can be made [19] L. f. Rodriguez, ―Radio astronomy: the achievements and the challenges‖, Physics and Astrophysics, 1-18, pp. 199-213, Springercompact and inexpensive and CW systems are provides some Verlag Berlin Heidelberg, 2009. limited information and give the information of particular [20] H. P. Siegel, ―T erahertz Technology‖, IEEE Transactions On Microw. frequencies. The Continuous-wave terahertz (CW-THz) Theo. and Techniq., vol. 50, no. 3, pp. 910-928, March 2002. system is less expensive and more compact than conventional [21] M. Nagel, F. Richter, P. H. Bolivar, and H. Kurz, ―A functional T Hz sensor for marker-free DNA analysis‖, Phys. Med. Biol., vol. 48, pp. time-domain imaging systems. The THz-TDS compared to 3625-3636, 2003. other techniques are that it has a smaller spectral range than [22] H. P. Siegel, ―Terahertz Heterodyne Imaging‖, Inter. J. of Infrared and fourier transform spectroscopy (FTS) and provides lower Millimeter Waves, vol. 27, no. 5, pp. 631–656, May 2006. resolution than narrowband THz spectroscopy. The THz [23] M. C. Beard, G. M. T urner, and C. A. Schmuttenmaer, ― T erahertz spectroscopy, J. Phys. Chem. B, vol. 106, pp. 7146-7159, 2002. beams which can be produced are still very low power. [24] M. C. Beard, G. M. T urner, and C. A. Schmuttenmaer, ― Liquid More work is required to the improvement and development dynamics: experiment, simulation and theory‖, Oxford University of THz-imaging system for will be added advantages such as Press, pp. 44-57, 2002. prototype; low cost; fully functionalized and easy to operate; [25] D. H. Auston, ― Picosecond optoelectronic switching and gating in silicon‖, Applied Physics Letters, vol. 26, no. 3, pp. 101-103, 1975. energy efficient, and contrast between diseased and healthy [26] H. Ahlborn, X. Ji, B. Space, and P. b. Moore, ― A combined tissue. instantaneous normal mode and time correlation function description REFERENCES
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