Methods have recently been developed that make use of electromagnetic radiation at terahertz (THz) frequencies, the region of the spectrum between millimetre wavelengths and the infrared, for imaging purposes. Radiation at these wavelengths is non-ionizing and subject to far less Rayleigh scatter than visible or infrared wavelengths, making it suitable for medical applications. This paper introduces THz pulsed imaging and discusses its potential for in vivo medical applications in comparison with existing modalities.
Techniques for the coherent generation and detection of electromagnetic radiation in the far infrared, or terahertz, region of the electromagnetic spectrum have recently developed rapidly and may soon be applied for in vivo medical imaging. Both continuous wave and pulsed imaging systems are under development, with terahertz pulsed imaging being the more common method. Typically a pump and probe technique is used, with picosecond pulses of terahertz radiation generated from femtosecond infrared laser pulses, using an antenna or nonlinear crystal. After interaction with the subject either by transmission or reflection, coherent detection is achieved when the terahertz beam is combined with the probe laser beam. Raster scanning of the subject leads to an image data set comprising a time series representing the pulse at each pixel. A set of parametric images may be calculated, mapping the values of various parameters calculated from the shape of the pulses. A safety analysis has been performed, based on current guidelines for skin exposure to radiation of wavelengths 2.6 μm–20 mm (15 GHz–115 THz), to determine the maximum permissible exposure (MPE) for such a terahertz imaging system. The international guidelines for this range of wavelengths are drawn from two U.S. standards documents. The method for this analysis was taken from the American National Standard for the Safe Use of Lasers (ANSI Z136.1), and to ensure a conservative analysis, parameters were drawn from both this standard and from the IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields (C95.1). The calculated maximum permissible average beam power was 3 mW, indicating that typical terahertz imaging systems are safe according to the current guidelines. Further developments may however result in systems that will exceed the calculated limit. Furthermore, the published MPEs for pulsed exposures are based on measurements at shorter wavelengths and with pulses of longer duration than those used in terahertz pulsed imaging systems, so the results should be treated with caution.
The first demonstrations of terahertz imaging in biomedicine were made several years ago, but few data are available on the optical properties of human tissue at terahertz frequencies. A catalogue of these properties has been established to estimate variability and determine the practicality of proposed medical applications in terms of penetration depth, image contrast and reflection at boundaries. A pulsed terahertz imaging system with a useful bandwidth 0.5-2.5 THz was used. Local ethical committee approval was obtained. Transmission measurements were made through tissue slices of thickness 0.08 to 1 mm, including tooth enamel and dentine, cortical bone, skin, adipose tissue and striated muscle. The mean and standard deviation for refractive index and linear attenuation coefficient, both broadband and as a function of frequency, were calculated. The measurements were used in simple models of the transmission, reflection and propagation of terahertz radiation in potential medical applications. Refractive indices ranged from 1.5 ± 0.5 for adipose tissue to 3.06 ± 0.09 for tooth enamel. Significant differences (P<0.05) were found between the broadband refractive indices of a number of tissues. Terahertz radiation is strongly absorbed in tissue so reflection imaging, which has lower penetration requirements than transmission, shows promise for dental or dermatological applications.
The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Abstract. Recently published studies suggest that terahertz pulsed imaging will have applications in medicine and biology, but there is currently very little information about the optical properties of human tissue at terahertz frequencies. Such information would be useful for predicting the feasibility of proposed applications, optimising acquisition protocols, providing information about variability of healthy tissue and supplying data for studies of the interaction mechanisms. Research ethics committee approval was obtained, and measurements made from samples of freshly excised human tissue, using a broadband terahertz pulsed imaging system comprising frequencies approximately 0.5 to 2.5 THz. Refractive index and linear absorption coefficient were found. Reproducibility was determined using blood from one volunteer, which was drawn and measured on consecutive days. Skin, adipose tissue, striated muscle, vein and nerve were measured (to date, from one individual). Water had a higher refractive index (2.04 ± 0.07) than any tissue. The linear absorption coefficient was higher for muscle than adipose tissue, as expected from the higher hydration of muscle. As these samples came from a single subject, there is currently insufficient statistical power to draw firm conclusions, but results suggest that in vivo clinical imaging will be feasible in certain applications.
We report on magneto-optical studies of excitons confined in Si nanocrystallites. As a result of Zeeman splitting of the exciton ground level the whole resonantly excited photoluminescence spectrum shifts linearly to lower energy with rising magnetic field. The recombination decay time depends on magnetic field due to the change in the thermal populations of singlet-triplet sublevels of the exciton ground state. The effect is stronger for weakly confined excitons when the value of the Zeeman splitting becomes comparable with the electronhole exchange energy. The experimental results strongly support the exciton nature of luminescing states responsible for the light emission from porous silicon. ͓S0163-1829͑99͒15631-5͔
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