Rohit Chandra

On the Opportunities and Challenges in Microwave Medical Sensing and Imaging

Widely used medical imaging systems in clinics currently rely on X-rays, magnetic resonance imaging, ultrasound, computed tomography, and positron emission tomography. The aforementioned technologies provide clinical data with a variety of resolution, implementation cost, and use complexity, where some of them rely on ionizing radiation. Microwave sensing and imaging (MSI) is an alternative method based on nonionizing electromagnetic (EM) signals operating over the frequency range covering hundreds of megahertz to tens of gigahertz. The advantages of using EM signals are low health risk, low cost implementation, low operational cost, ease of use, and user friendliness. Advancements made in microelectronics, material science, and embedded systems make it possible for miniaturization and integration into portable, handheld, mobile devices with networking capability. MSI has been used for tumor detection, blood clot/stroke detection, heart imaging, bone imaging, cancer detection, and localization of in-body RF sources. The fundamental notion of MSI is that it exploits the tissue-dependent dielectric contrast to reconstruct signals and images using radar-based or tomographic imaging techniques. This paper presents a comprehensive overview of the active MSI for various medical applications, for which the motivation, challenges, possible solutions, and future directions are discussed.

A Receiver Architecture for Devices in Wireless Body Area Networks

A receiver architecture suitable for devices in wireless body area networks is presented. Such devices require minimum physical size and power consumption. To achieve this the receiver should, therefore, be fully integrated in state-of-the-art complementary metal-oxide-semiconductor (CMOS) technology, and size and power consumption must be carefully considered at all levels of design. The chosen modulation is frequency shift keying, for which transmitters can be realized with high efficiency and low spurious emissions. A direct-conversion receiver architecture is used to achieve minimum power consumption and a modulation index equal to two is chosen, creating a midchannel notch in the modulated signal. A tailored demodulation structure has been designed to make the digital baseband compact and low power. To increase sensitivity it has been designed to interface with an analog decoder. Implementation in the analog domain minimizes the decoder power consumption. Antenna design and wave propagation are taken into account via simulations with phantoms. The 2.45-GHz ISM band was chosen as a good compromise between antenna size and link loss. An ultra-low power medium access scheme has been designed, which is used both for system evaluation and for assisting system design choices. Receiver blocks have been fabricated in 65-nm CMOS, and a radio-frequency front-end and an analog-to-digital converter have been measured. Simulations of the complete baseband have been performed, investigating impairments due to 1/f noise, frequency and time offsets.

A Link Loss Model for the On-Body Propagation Channel for Binaural Hearing Aids

Binaural hearing aids communicate with each other through a wireless link for synchronization. An analytical propagation model is useful to estimate the ear-to-ear link loss for such binaural hearing aids. This paper presents an analytical model for the deterministic component of the ear-to-ear link loss. The model takes into account the dominant paths having most of the power of the creeping wave from the transceiver in one ear to the transceiver in the other ear, and the effect of the protruding part of the outer ear, called pinna. Simulations are done to validate the model using in-the-ear placement of antennas at 2.45 GHz on numerical heterogeneous phantoms of different age-groups and head sizes showing a good agreement with the model. The ear-to-ear link loss for a numerical homogeneous specific anthropomorphic mannequin (SAM) phantom is compared with a numerical heterogeneous phantom. The loss for the SAM phantom is found to be 30 dB lower than that for the heterogeneous phantom. It is shown that the absence of the pinna and the lossless shell in the SAM phantom underestimate the link loss. The effect of the pinnas is verified through measurements on a phantom where we have included the pinnas fabricated by 3-D printing.

Miniaturized antennas for link between binaural hearing aids

We have investigated the possibility of using the 2.45 GHz ISM band for communication between binaural hearing aids. The small size of a modern hearing aid makes it necessary to miniaturize the antennas to make this feasible. Two different types of hearing aid placements have been investigated: in the outer ear and in the ear canal. Both put strict demands on the size of the antenna, which have been miniaturized by applying disc loads and high permittivity materials. The investigations have been done by FDTD simulation of a modified SAM phantom head, where we have included a simple model of the ear canal. Simulations show that the outer ear placement is better, as it gives a total link loss of 48 dB. The placement in the ear canal gives a total link loss of 92 dB.

A Microwave Imaging-Based Technique to Localize an In-Body RF Source for Biomedical Applications

In some biomedical applications such as wireless capsule endoscopy, the localization of an in-body radio-frequency (RF) source is important for the positioning of any abnormality inside the gastrointestinal tract. With knowledge of the location, therapeutic operations can be performed precisely at the position of the abnormality. Electrical properties (relative permittivity and conductivity) of the tissues and their distribution are utilized to estimate the position. This paper presents a method for the localization of an in-body RF source based on microwave imaging. The electrical properties of the tissues and their distribution at 403.5 MHz are found from microwave imaging and the position of an RF source is then estimated based on the image. The method is applied on synthetic noisy data, obtained after the addition of white Gaussian noise to simulated data of a simple circular phantom, and a realistic phantom in a 2-D case. The root-mean-square of the error distance between the actual and the estimated position is found to be within 10 and 4 mm for the circular and the realistic phantom, respectively, showing the capability of the proposed algorithm to work with a good accuracy even in the presence of noise for the localization of the in-body RF source.

In-mouth antenna for tongue controlled wireless devices: Characteristics and link-loss

We have investigated the possibility of using a curved dipole antenna inside the mouth for the tongue controlled wireless devices in 2.45GHz ISM band. These devices can be interfaced with the wheelchair or the computer used by the paraplegic patients. Two antenna placement positions have been investigated: in front of the teeth and behind the teeth. The investigations were done through the FDTD simulations on a realistic heterogeneous phantom with the mouth closed and open. The link loss between the in-mouth dipole antenna and an external dipole antenna at 400mm from the center of the head was calculated. It was found that the radiation pattern changed according to the placement of the antennas inside the mouth and whether the mouth was open or closed. The link loss for the in front of the teeth placement was found to be 9dB–11dB lower than the behind the teeth placement depending on the open or the closed mouth. The variation in the link loss was 1dB–4dB for the open mouth when compared with the closed mouth depending on the antenna placement position. By using these results, a reliable wireless link for the in-mouth device can be designed.

Effect of frequency, body parts and surrounding on the on-body propagation channel around the torso

Wearable medical devices can be positioned around the torso for monitoring of critical health parameters. The signal transmission between them is through a wireless link over an on-body propagation channel. In this paper, the effect of some factors which could influence the propagation channel around the torso as: (a) frequency of operation (b) positions of the arms (c) material of a chair used, have been investigated. Moreover, a comparison between the link loss around the torso of a full body phantom and a truncated torso phantom has been done. It is found that the frequency of operation and the positions of the arms have a significant influence on the channel. The difference between the link loss of a full body phantom and a truncated phantom is found to be minimal, indicating a possibility of using a truncated torso for a faster simulation. The results presented in the paper gives an insight in to the influence of arms and the frequency of operation on the propagation channel around the torso and thus would be beneficial for designing a reliable wireless link.

A microwave imaging-based 3D localization algorithm for an in-body RF source as in wireless capsule endoscopes.

A microwave imaging-based technique for 3D localization of an in-body RF source is presented. Such a technique can be useful for localization of an RF source as in wireless capsule endoscopes for positioning of any abnormality in the gastrointestinal tract. Microwave imaging is used to determine the dielectric properties (relative permittivity and conductivity) of the tissues that are required for a precise localization. A 2D microwave imaging algorithm is used for determination of the dielectric properties. Calibration method is developed for removing any error due to the used 2D imaging algorithm on the imaging data of a 3D body. The developed method is tested on a simple 3D heterogeneous phantom through finite-difference-time-domain simulations. Additive white Gaussian noise at the signal-to-noise ratio of 30 dB is added to the simulated data to make them more realistic. The developed calibration method improves the imaging and the localization accuracy. Statistics on the localization accuracy are generated by randomly placing the RF source at various positions inside the small intestine of the phantom. The cumulative distribution function of the localization error is plotted. In 90% of the cases, the localization accuracy was found within 1.67 cm, showing the capability of the developed method for 3D localization.

Microwave Imaging of Circular Phantom Using the Levenberg- Marquardt Method

This paper presents our work on the reconstruction of the complex permittivity 2-D profile of biological objects simulated as circular phantoms. An iterative reconstruction algorithm called the Levenberg-Marquardt method developed by Franchois and Pichot is tested using synthetic data. Assumed permittivity profiles are generated for a simple circular phantom using the CST microwave studio software. Then, we reconstruct the permittivity profile of the object in MATLAB by using the data from CST microwave studio. The main work in this paper focuses on the realization of the inversion algorithm on three different circular phantoms. Our results show that the permittivity profiles can be very satisfactorily reconstructed, thereby indicating the usefulness of this approach for medical diagnosis.

Investigations on the effect of frequency and noise in a localization technique based on microwave imaging for an in-body RF source

Localization of a wireless capsule endoscope finds many clinical applications from diagnostics to therapy. There are potentially two approaches of the electromagnetic waves based localization: a) signal propagation model based localization using a priori information about the persons dielectric channels, and b) recently developed microwave imaging based localization without using any a priori information about the persons dielectric channels. In this paper, we study the second approach in terms of a variety of frequencies and signal-to-noise ratios for localization accuracy. To this end, we select a 2-D anatomically realistic numerical phantom for microwave imaging at different frequencies. The selected frequencies are 13:56 MHz, 431:5 MHz, 920 MHz, and 2380 MHz that are typically considered for medical applications. Microwave imaging of a phantom will provide us with an electromagnetic model with electrical properties (relative permittivity and conductivity) of the internal parts of the body and can be useful as a foundation for localization of an in-body RF source. Low frequency imaging at 13:56 MHz provides a low resolution image with high contrast in the dielectric properties. However, at high frequencies, the imaging algorithm is able to image only the outer boundaries of the tissues due to low penetration depth as higher frequency means higher attenuation. Furthermore, recently developed localization method based on microwave imaging is used for estimating the localization accuracy at different frequencies and signal-to-noise ratios. Statistical evaluation of the localization error is performed using the cumulative distribution function (CDF). Based on our results, we conclude that the localization accuracy is minimally affected by the frequency or the noise. However, the choice of the frequency will become critical if the purpose of the method is to image the internal parts of the body for tumor and/or cancer detection.