Electric Field Measurement of the Living Human Body for Biomedical Applications: Phase Measurement of the Electric Field Intensity

Hindawi Publishing Corporation International Journal of Antennas and Propagation Volume 2013, Article ID 305362, 6 pages http://dx.doi.org/10.1155/2013/305362 Research Article Electric Field Measurement of the Living Human Body for Biomedical Applications: Phase Measurement of the Electric Field Intensity Ichiro Hieda1 and Ki Chang Nam2 1 2 National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8566, Japan Yonsei University College of Medicine, Seoul 120-752, Republic of Korea Correspondence should be addressed to Ichiro Hieda; Received 20 June 2013; Revised 30 August 2013; Accepted 8 October 2013 Academic Editor: Yifan Chen Copyright © 2013 I. Hieda and K. C. Nam. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The authors are developing a technique for conducting measurements inside the human body by applying a weak electric field at a radio frequency (RF). Low RF power is fed to a small antenna, and a similar antenna located 15–50 cm away measures the electric field intensity. Although the resolution of the method is low, it is simple, safe, cost-effective, and able to be used for biomedical applications. One of the technical issues suggested by the authors’ previous studies was that the signal pattern acquired from measurement of a human body was essentially different from that acquired from a phantom. To trace the causes of this difference, the accuracy of the phase measurements was improved. This paper describes the new experimental system that can measure the signal phase and amplitude and reports the results of experiments measuring a human body and a phantom. The results were analyzed and then discussed in terms of their contribution to the phase measurement. 1. Introduction The authors are developing a technique for conducting measurements inside the human body by applying a weak electric field at radio frequency (RF), typically 1–60 MHz [1, 2]. Technological advancements have led to the development of high-level diagnostic techniques, including X-ray computed tomography (X-ray CT), magnetic resonance imaging (MRI), and positron emission tomography (PET), which have contributed greatly to medical care and welfare. However, such high-level care and large-scale medical equipment represent financial burdens to taxpayers in most developed countries. Moreover, due to these costs, people in developing countries rarely benefit from these high-level diagnostic techniques. From this perspective, simple and easy-to-use equipment utilizing electric impedance and magnetic induction is expected [3–6]. The authors started to apply the radio imaging method, which was originally used for geological survey (RIM), to the measurement of the human body [2, 7, 8]. Later, the evolved technique was classified as an electric field method [9, 10]. There are several studies of biomedical measurements that use an electromagnetic wave. One example is microwave tomography. The basic principle of their projects is similar to our proposal. To obtain finer resolution less than 1 cm, pulse signals and multiple antennas for transmitting and receiving were implemented [11–14]. Because attenuation in the human body at the microwave frequency range is remarkable, an electromagnetic darkroom is necessary to prevent interference of an electromagnetic wave along indirect paths as well as to suppress emission of the microwave to the outside environment. The authors’ method is simple, safe, and cost-effective and leads to the expectation of two goals. One is the extension of the current experimental system, which can be applied to medical screenings such as abdominal fat CT. Another goal is smaller systems able to be used as wearable sensors or installed at home, such as urine volume sensors and dehydration alarms, for welfare and health care. 2 International Journal of Antennas and Propagation Transmitting antenna 40 mm SDR GNU radio 30 Probe Abdomen USRP2 30 400 RF generator or SDR (10 mW, 1–50 MHz) strength was simple subtraction of the loss from the increment caused by current leakage and permittivity, respectively [17]. Because the signal attenuation from the current leakage was dominant, the effect of the human body permittivity was buried in the measurement data. Improving the phase measurement capability of the electric field intensity would help to discriminate the effect of permittivity from signal attenuation caused by the current leakage. In this paper, a new experimental system that can measure the signal phase and amplitude is described. Experiments are also reported where the human body and a phantom were measured by the system. The results were analyzed and discussed in terms of their contribution to the phase measurement. 2. Method 650 mm Moving table Figure 1: Overview of the experiment. A subject stood still on the moving table. The transmitting antenna and the probe were set at the height of the abdomen of the subject. USRP comprised an RF front end and A/D converter that worked as a software-defined radio (SDR) in conjunction with GNU Radio software installed on a PC. Figure 1 shows an overview of the developed system. A portion of the human body was scanned by a weak electric field at radio frequency (RF), and the measured signals were analyzed to obtain the permittivity that corresponded to the moisture distribution inside the body. Experiments were performed in previous studies to determine basic characteristics of the method. To support the experimental results, the measurement system was numerically simulated using the finite-difference timedomain (FDTD) method [15–19]. One of the technical issues suggested by these previous studies was that when the human body was measured, the pattern of the electric field intensity differed from that of a phantom [1, 17]. When the system scanned the phantom, an acrylic water tank filled with water, the electric field intensity at the receiving antenna increased due to the high relative permittivity of the water, which was approximately 70. In contrast, human body tissues have a variety of permittivities. The permittivities of tissues containing much moisture, for example, muscles and internal organs, are as high as those of water at room temperature and pressure [20]. Therefore, water was used for the medium of the phantom to simulate a portion of the human body that contains much moisture. When the system scanned the living human body, however, the electric field intensity decreased. This was caused by RF current leakage through the human body, which had much larger dimensions and a much tighter electrostatic connection to the electric ground than the phantom. It was experimentally confirmed that measurement of signal 2.1. Experimental System. Figure 2 shows a schematic block diagram of the experimental system. The system (...truncated)