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Product Code: JLA_15_3_192


Authors:
Elizabeth Berry
Gillian C. Walker
Anthony J. Fitzgerald
Academic Unit of Medical Physics and Centre of Medical Imaging Research, University of Leeds, Wellcome Wing, Leeds General Infirmary, Leeds LS1 3EX, United Kingdom

N. N. Zinov’ev
Martyn Chamberlain
Institute of Microwaves and Photonics, University of Leeds, Leeds LS2 9JT, United Kingdom

Stephen W. Smye
Department of Medical Physics and Engineering, Leeds Teaching Hospitals NHS Trust, St. James’s University Hospital, Leeds LS9 7TF, United Kingdom

Robert E. Miles
Institute of Microwaves and Photonics, University of Leeds, Leeds LS2 9JT, United Kingdom

Michael A. Smith
Academic Unit of Medical Physics and Centre of Medical Imaging Research, University of Leeds, Wellcome Wing, Leeds General Infirmary, Leeds LS1 3EX, United Kingdom


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. © 2003 Laser Institute of America.

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