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Microwave imaging is a science which has been evolved from older detecting/locating techniques (e.g., radar) in order to evaluate hidden or embedded objects in a structure (or media)using electromagnetic (EM) waves in microwave regime (i.e., ~300 MHz-300 GHz).〔(【引用サイトリンク】title=Synthetic aperture radar-based techniques and reconfigurable antenna design for microwave imaging of layered strutures )〕 Microwave imaging techniques can be classified as either quantitative or qualitative.〔L. Lianlin, W. Zhang, and F. Li, “Derivation and Discussion of the SAR Migration Algorithm Within Inverse Scattering Problem: Theoretical Analysis,” IEEE Trans. Geosci. Remote Sens. E, vol. 48, no. 1, pp. 415–422, Jan. 2010.〕 Quantitative imaging techniques (are also known as inverse scattering methods) give the electrical (i.e., electrical and magnetic property distribution) and geometrical parameters (i.e., shape, size and location) of an imaged object by solving a nonlinear inverse problem.〔M. Fallahpour, J.T. Case, M. Ghasr, and R. Zoughi, “Piecewise and Wiener Filter-Based SAR Techniques for Monostatic Microwave Imaging of Layered Structures”, IEEE Transactions on Antennas and Propagation, vol. 62, no. 1, pp. 1-13, Jan. 2014.〕 The nonlinear inverse problem is converted into a linear inverse problem (i.e., Ax=b where A and b are known and x (or image) is unknown) by using Born or distorted Born approximations. Despite the fact that direct matrix inversion methods can be invoked to solve the inversion problem, this will be so costly when the size of the problem is so big (i.e., when A is a very dense and big matrix). To overcome this problem, direct inversion is replaced with iterative solvers. Techniques in this class are called forward iterative methods which are usually time consuming.〔J. De Zaeytijd, A. Franchois, C. Eyraud, and J.-M. Geffrin, “Fullwave Three-Dimensional Microwave Imaging with a Regularized Gauss–Newton Method—Theory and Experiment,” IEEE Trans. Antennas Propag., vol. 55, no. 11, pp. 3279–3292, Nov. 2007.〕 On the other hand, qualitative microwave imaging methods calculate a qualitative profile (which is called as reflectivity function or qualitative image) to represent the hidden object. These techniques use approximations to simplify the imaging problem and then they use back-propagation (also called time reversal, phase compensation, or back-migration) to reconstruct the unknown image profile. Synthetic aperture radar (SAR), ground-penetrating radar (GPR), and frequency-wave number migration algorithm are some of the most popular qualitative microwave imaging methods(). ==Principles== In general, a microwave imaging system has two parts, namely: hardware and software (or post-processing). The hardware part is responsible to collect data from the sample under test. A transmitter antenna sends EM waves toward sample under test (e.g., human body for medical imaging). If the sample is made of only homogeneous material which extends to infinity, theoretically no EM wave will be reflected. Introduction of any anomaly which has a different properties (i.e., electrical/magnetic)in comparison with the surrendering homogeneous medium may reflect a portion of EM wave. The bigger the difference between the properties of the anomaly and the surrounding medium is, the stronger the reflected wave will be. This reflection will be collected and measured by the same antenna (in monostatic case) or a different receiver antenna (in bistatic case) To increase cross-range resolution of the imaging system, several antennas should be distributed over an area (which is called sampling area) with a spacing less than the operating wavelength. However, by having several antennas placed close to each other, the mutual coupling between antennas may degrade the accuracy of the collected signals. Moreover, transmitter and receiver system will become very complex. To address these problems, one single scanning antenna is used instead of several antennas. Later, when the antenna finishes scanning the entire sampling area, one can put together collected data all over the sampling area and process them together, simultaneously. In fact, a synthetic (virtual) aperture is produced by moving the antenna (similar to synthetic aperture radar principle〔M. Soumekh, Synthetic Aperture Radar Signal Processing, 1st ed. New York, NY, USA: Wiley, 1999.〕). Later, the collected data which is sometimes referred as raw data is fed into the software part to get processed. Based on applied processing algorithm, microwave imaging techniques can be categorized as quantitative and qualitative. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Microwave imaging」の詳細全文を読む スポンサード リンク
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