Philip F. Taday

Detection and identification of explosives using terahertz pulsed spectroscopic imaging

The absorption spectrum of the explosive 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) has been measured using a conventional Fourier transform infrared spectroscopy and by terahertz pulsed spectroscopy. Seven absorption features in the spectral range of 5–120cm−1 have been observed and identified as the fingerprint of RDX. Furthermore, the spatial distribution of individual chemical substances including RDX, has been mapped out using reflection terahertz spectroscopic imaging in combination with component spatial pattern analysis. This is the terahertz spectroscopy and chemical mapping of explosives obtained using reflection terahertz measurement, and represents a significant advance toward developing a terahertz pulsed imaging system for security screening of explosives.

Security applications of terahertz technology

Recent events have accelerated the quest for ever more effective security screening to detect an increasing variety of threats. Many techniques employing different parts of the electromagnetic spectrum from radio up to X- and gammaray are in use. Terahertz radiation, which lies between microwave and infrared, is the last part to be exploited for want, until recently, of suitable sources and detectors. This paper describes practical techniques for Terahertz imaging and spectroscopy which are now being applied to a variety of applications. We describe a number of proof-of-principle experiments which show that Terahertz imaging has the ability to use very low levels of this non-ionising radiation to detect hidden objects in clothing and common packing materials and envelopes. Moreover, certain hidden substances such as plastic explosives and other chemical and biological agents may be detected from their characteristic Terahertz spectra. The results of these experiments, coupled with availability of practical Terahertz systems which operate outside the laboratory environment, demonstrate the potential for Terahertz technology in security screening and counterterrorism.

Terahertz pulsed spectroscopy and imaging in the pharmaceutical setting--a review.

Terahertz pulsed spectroscopy (TPS) and terahertz pulsed imaging (TPI) are two novel techniques for the physical characterization of pharmaceutical drug materials and final solid dosage forms, utilizing spectral information in the far infrared region of the electromagnetic spectrum. This review focuses on the development and performance of pharmaceutical applications of terahertz technology compared with other tools for physical characterization. TPS can be used to characterize crystalline properties of drugs and excipients. Different polymorphic forms of a drug can be readily distinguished and quantified. Recent developments towards a better understanding of the fundamental theory behind spectroscopy in the far infrared have been discussed. Applications for TPI include the measurement of coating thickness and uniformity in coated pharmaceutical tablets, structural imaging and 3D chemical imaging of solid dosage forms.

Terahertz Pulsed Spectroscopy of Human Basal Cell Carcinoma

Good contrast is seen between normal tissue and regions of tumor in terahertz pulsed imaging of basal cell carcinoma (BCC). To date, the source of contrast at terahertz frequencies is not well understood. In this paper we present results of a spectroscopy study comparing the terahertz properties (absorption coefficient and refractive index) of excised normal human skin and BCC. Both the absorption coefficient and refractive index were higher for skin that contained BCC. The difference was statistically significant over the range 0.2 to 2.0 THz (6.6 cm−1 to 66.6 cm−1) for absorption coefficient and 0.25 to 0.90 THz (8.3 cm−1 to 30 cm−1) for refractive index. The maximum difference for absorption was at 0.5 THz (16.7 cm−1). These changes are consistent with higher water content. These results account for the contrast seen in terahertz images of BCC and explain why parameters relating to the reflected terahertz pulse provide information about the lateral spread of the tumor. Knowing the properties of the tissue over the terahertz frequency range will enable the use of mathematical models to improve understanding of the terahertz response of normal and diseased tissue.

Hidden object detection: security applications of terahertz technology

Recent events have led to dramatic changes to the methods employed in security screening. For example, following the failed shoe bombing, it is now common for shoes to be removed and X-rayed at airport checkpoints. There is therefore an increasing focus on new Recent events have led to dramatic changes to the methods employed in security screening. For example, following the failed shoe bombing, it is now common for shoes to be removed and X-rayed at airport checkpoints. There is therefore an increasing focus on new technologies that can be applied to security screening, either to simplify or speed up the checking process, or to provide additional functionality. Terahertz (THz) technology is a promising, emerging candidate. In previous publications we have shown how our THz pulsed imaging systems can be used to image threat items, and have demonstrated that explosive materials have characteristic THz spectra. We have also demonstrated that nonmetallic weaponry can be imaged when concealed beneath clothing. In this work we examine more closely the properties of barrier and potential confusion materials. We demonstrate that barrier materials have smooth spectra with relatively low attenuation. We further demonstrate that the terahertz spectra of several common chemicals and medicines are distinct from those of threat materials.

Development and Application of Terahertz Pulsed Imaging for Nondestructive Inspection of Pharmaceutical Tablet

Terahertz pulsed imaging (TPI) was evaluated for nondestructively characterizing the 3-D internal structures of pharmaceutical tablets. The structural information of a pharmaceutical tablet, such as coating thickness and interface uniformity, was obtained directly from the analysis of the time-domain terahertz waveform. The chemical map of a sample was obtained by using frequency-domain terahertz spectra, together with spectral matching techniques such as cosine correlation mapping. The axial spatial resolution achieved was 30 mum, limited by the confinement of terahertz pulses in time (pulse width); and the lateral spatial resolution was determined to be 150 mum at 90 cm-1 (2.7 THz), limited by the confinement of terahertz pulses in space (focus size, which is diffraction limited and thus frequency dependent). In addition, the buried structure within a tablet was mapped using the TPI, and its chemical composition was successfully identified through spectral-time analysis of the recorded terahertz waveform. We also present a rigorous electromagnetic theory for simulating the terahertz propagation in a multilayered sample, to facilitate terahertz data analysis and interpretation. In conclusion, the TPI is a powerful tool for assaying the tablet coating layer thickness and interface uniformity, and for identifying polymorphs.

Terahertz pulsed imaging and spectroscopy for biomedical and pharmaceutical applications

Terahertz (THz) radiation lies between the infrared and microwave regions of the electromagnetic spectrum. Advances in THz technology have opened up many opportunities in this scientifically and technologically important spectroscopic region. The THz frequency range excites large amplitude vibrational modes of molecules as well as probing the weak interactions between them. Here we describe two techniques that utilize THz radiation, terahertz pulsed imaging (TPI) and terahertz pulsed spectroscopy (TPS). Both have a variety of possible applications in biomedical imaging and pharmaceutical science. TPI, a non-invasive imaging technique, has been used to image epithelial cancer ex vivo and recently in vivo. The diseased tissue showed a change in absorption compared to normal tissue, which was confirmed by histology. To understand the origins of the differences seen between diseased and normal tissue we have developed a TPS system. TPS has also been used to study solids of interest in the pharmaceutical industry. One particularly interesting example is ranitidine hydrochloride, which is used in treatment of stomach ulcers. Crystalline ranitidine has two polymorphic forms known as form 1 and form 2. These polymorphs have the same chemical formula but different crystalline structure that give rise to different physiochemical properties of the material. Using TPS it is possible to rapidly distinguish between the two polymorphic forms.

Elimination of scattering effects in spectral measurement of granulated materials using terahertz pulsed spectroscopy

Spectral distortions are commonly observed in terahertz spectra of granulated materials. These spurious structures in the spectroscopy are caused by scattering due to the refractive index mismatch between the particles and their surrounding medium. We find that the scattering contribution is random across sample positions and could be eliminated by summing and averaging multiple measurements over a sample area. We present experimental results of both absorbing and nonabsorbing particles in the size range 50–250μm and also give an empirical expres-sion to describe the effect of grain size on the scattering-induced extinction as a function of frequency.

Simulating the response of terahertz radiation to basal cell carcinoma using ex vivo spectroscopy measurements

Studies of basal cell carcinoma using terahertz pulsed imaging have revealed a significant difference between regions of tumor and healthy tissue. These differences are manifested in the reflected pulse due to what is thought to be changes in refractive index and absorption. We present measurements of the refractive index and absorption coefficient of excised normal tissue and basal cell carcinoma using terahertz (THz) transmission spectroscopy. We extract Debye parameters from these data and enter them into a finite difference time domain simulation to predict the shape of the waveforms reflected off the normal tissue and basal cell carcinoma and compare them with published in vivo data. Simulating the interaction of terahertz radiation with normal and cancerous tissue is a key step toward understanding the origin of contrast in terahertz images of skin cancer.

Applications of terahertz pulsed imaging to sustained-release tablet film coating quality assessment and dissolution performance

The potential of terahertz pulsed imaging (TPI) to predict the dissolution performance in sustained-release tablets was investigated in this study. Batches of coated tablets with similar weight gain during the coating process at the lab and pilot scales were subjected to non-destructive imaging by TPI and subsequently analysed by dissolution testing. The results from the dissolution tests revealed significant differences in the product performance between the lab and pilot scales (Student t-test, P<0.05). The model-independent dissolution parameters in the pilot scale showed a prolonged mean dissolution time. This indicated that the pharmaceutical active ingredient was released at a slower rate in the pilot compared to the lab scale. While weight gain measurements (the traditional coating quality parameter), failed to provide an early indication of the product functional performance; terahertz parameters (terahertz electric field peak strength and coating layer thickness) provided insight into the subsequent dissolution behaviour. Correlations between terahertz parameters and dissolution were much stronger than correlations between weight gain and dissolution; with the R(2) value for terahertz correlations typically around 0.84 as opposed to 0.07 for weight gain correlations. This study presents the initial finding of correlations between terahertz parameters for assessing the coating quality to the dissolution performance of the coated tablet. The contributing factors for these particular correlations are also discussed.