Control Device Thermal Modeling in MRI Receive Coil Q-Spoiling Circuits
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These semiconductor devices can see the full RF power during operation, and will exhibit significant heating, potentially leading to device failure. Even if the device does not reach a destructive heating level, the resistance of the device can increase and effectively degrade the level of blocking in the RF coil. A useful tool for MRI circuit designers would be a thermal model that predicts this change in resistance, and hence degradation in MRI coil blocking, as device temperature rises. This paper presents a method for modeling thermal effects in MRI Q-spoiling control devices. A PIN diode resistance-temperature model is reviewed, and two approaches for field effect transistors that model the temperature rise and subsequent device resistance variation with temperature using SPICE-compatible equivalent circuit elements are covered. Experimental results are presented that validate the form of the models and show that increased device resistance due to temperature rise will degrade MRI blocking by several dB. The ability to model the control device’s thermal characteristics will lead to improved understanding of circuit operation.
Image Registration for Microwave Tomography of the Breast Using Priors from Non-Simultaneous Previous Magnetic Resonance Imaging
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For the neoadjuvant therapy application, it would be desirable to achieve the most accurate possible characterization of the tissue properties. One method to achieve increased resolution and specificity in microwave imaging reconstruction is the use of a soft prior regularization. The objective of this study is to develop a method to use magnetic resonance (MR) images, taken in a different imaging configuration, as this soft prior. To enable the use of the MR images as a soft prior, it is necessary to register the MR images to the microwave imaging space. Registration fiducials were placed around the breast that are visible in both the MRI and with an optical scanner integrated into the microwave system. Utilizing these common registration locations, numerical algorithms have been developed to warp the original breast MR images into a geometry closely resembling that in which the breast is pendant in the microwave system.
A First Evaluation of the Realistic Supelec-Breast Phantom
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An iterative algorithm is then used to solve the inverse problem and reconstruct a 2-D plane transecting the phantom. The reconstructed images are compared to the ones recovered from a cylindrical phantom of equivalent phantom media. The results show that both phantoms are possible to reconstruct, although the interior of the Supelec phantom is more challenging.
A Novel Approach for Determining the Electromagnetic Properties of a Colloidal Fluid with Magnetic Nanoparticles for Hyperthermia Applications
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In particular, it makes use of indirect equations based on the single order Debye model combined with a punctual set of in vitro SAR measurements. The procedure has a general validity and it can be easily applied in the up-growing field of magnetic hyperthermia studies.
Dual-Band Skin-Adhesive Repeater Antenna for Continuous Body Signals Monitoring
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The proposed adhesive repeater is low profile, and it is designed for dual-band operation at the ISM band (2.4 GHz) and in the ultrawideband (UWB) spectrum (4-10.6 GHz) using a single excitation port. The ISM band is used for in-body communication with the implants and the UWB band for off-body burst communication with the BS. The antenna consists of three layers that grant it compactness and performance robustness. We assess the in-body link between the repeater and a custom-designed miniaturized implantable probe antenna. The study is performed in the frequency-domain with results showing adequate input impedance matching (s11 ≤ -10 dB) and robustness to different body parts. We extend the analysis to time-domain by transmitting a synthetized medical signal between the two antennas. In addition, we evaluate the feasibility of an off-body link for burst communication. Again, the results indicate very good performance by the repeater antenna both in terms of impedance matching (s11 ≤ -10 dB) and preservation of time-signals (fidelity ≥ 75%), which is relevant in impulse radio systems. To the authors’ best knowledge, this is the first time this kind of dual-band adhesive antenna is being proposed for BANs, complete with performance tests using frequency- and time-domain figures-of-merit.
Channel Characteristics and Wireless Telemetry Performance of Transplanted Organ Monitoring System Using Ultra-wideband Communication
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Ultra-wideband technology is well suited for wireless implanted applications since it allows physical size reduction as well as increased longevity of the implanted devices. However, because of the high attenuation of UWB signals inside the human body, it is necessary to examine their propagation characteristics and demonstrate the feasibility of such application. In this paper, an investigative study involving the liver implanted wireless telemetry link using UWB technology is presented. Measurements using multilayer phantoms and simulations using a human digital model have been conducted within the frequency band of 4.5-6.5 GHz. Multilayer phantom measurements have demonstrated the attenuations ranging between -50 dB and -100 dB over the considered frequency band. A path loss model at the liver area has been obtained from the simulations. Also, the numerical results have shown that the attenuation variation due to respiration-induced organ movements was within 30 dB range with respect to the largest organ movement distance of 40 mm which emphasized the influence of organ movements on the in-body attenuations. Our preliminary link budget evaluation of liver-skin surface communication link including the effect of shadowing and organ movements estimated that it is possible to achieve a high data rate of 10 Mbps with a bit error rate of 10−3 at a distance of about 40 mm.
