Saqib Salahuddin

Effect of logarithmic and linear frequency scales on parametric modelling of tissue dielectric data

The dielectric properties of biological tissues have been studied widely over the past half-century. These properties are used in a vast array of applications, from determining the safety of wireless telecommunication devices to the design and optimisation of medical devices. The frequency-dependent dielectric properties are represented in closed-form parametric models, such as the Cole-Cole model, for use in numerical simulations which examine the interaction of electromagnetic (EM) fields with the human body. In general, the accuracy of EM simulations depends upon the accuracy of the tissue dielectric models. Typically, dielectric properties are measured using a linear frequency scale; however, use of the logarithmic scale has been suggested historically to be more biologically descriptive. Thus, the aim of this paper is to quantitatively compare the Cole-Cole fitting of broadband tissue dielectric measurements collected with both linear and logarithmic frequency scales. In this way, we can determine if appropriate choice of scale can minimise the fit error and thus reduce the overall error in simulations. Using a well-established fundamental statistical framework, the results of the fitting for both scales are quantified. It is found that commonly used performance metrics, such as the average fractional error, are unable to examine the effect of frequency scale on the fitting results due to the averaging effect that obscures large localised errors. This work demonstrates that the broadband fit for these tissues is quantitatively improved when the given data is measured with a logarithmic frequency scale rather than a linear scale, underscoring the importance of frequency scale selection in accurate wideband dielectric modelling of human tissues.

Open-Ended Coaxial Probe Technique for Dielectric Measurement of Biological Tissues: Challenges and Common Practices

Electromagnetic (EM) medical technologies are rapidly expanding worldwide for both diagnostics and therapeutics. As these technologies are low-cost and minimally invasive, they have been the focus of significant research efforts in recent years. Such technologies are often based on the assumption that there is a contrast in the dielectric properties of different tissue types or that the properties of particular tissues fall within a defined range. Thus, accurate knowledge of the dielectric properties of biological tissues is fundamental to EM medical technologies. Over the past decades, numerous studies were conducted to expand the dielectric repository of biological tissues. However, dielectric data is not yet available for every tissue type and at every temperature and frequency. For this reason, dielectric measurements may be performed by researchers who are not specialists in the acquisition of tissue dielectric properties. To this end, this paper reviews the tissue dielectric measurement process performed with an open-ended coaxial probe. Given the high number of factors, including equipment- and tissue-related confounders, that can increase the measurement uncertainty or introduce errors into the tissue dielectric data, this work discusses each step of the coaxial probe measurement procedure, highlighting common practices, challenges, and techniques for controlling and compensating for confounders.

Impact of Radial Heterogeneities of Biological Tissues on Dielectric Measurements

The dielectric properties of biological tissues are relevant in dosimetry studies and in radio-frequency and microwave medical applications. Broadband tissue dielectric data is commonly acquired using open-ended coaxial probe techniques. Generally, heterogeneous tissues are accurately characterised by conducting a meticulous histological analysis of the tissue types involved in the dielectric measurement. To this extent, it is fundamental to evaluate the tissue volume interrogated by the probe, also known as the probe sensing volume or histology region. In this study, early-stage experiments are presented to investigate how radial heterogeneities can impact the histology depth. The findings of this study aim to provide the basis for more accurate dielectric characterisation of heterogeneous tissues to support microwave medical device design.

Comparison of in-vivo and ex-vivo dielectric properties of biological tissues

Accurate knowledge of the dielectric properties of biological tissues are crucial for various applications such as assessment of specific absorption rate and safety of electromagnetic medical devices. Most of the availably dielectric data in literature is based on ex-vivo measurements on biological tissue samples obtained from various animal models. This study investigates the differences in the in-vivo and ex-vivo dielectric properties of biological tissues, and variance over several animal species. The dielectric data is obtained from literature and organized to a unified format for comparison. The analysis shows considerable variations not only between in-vivo and ex-vivo dielectric properties of various tissues but also between various animal species.

Effects of standard coagulant agents on the dielectric properties of fresh human blood

In this paper, the effects of coagulation and temperature on the dielectric properties of human blood are investigated over the frequency range of 400 MHz–20 GHz using freshly extracted blood samples. The dielectric properties are measured using blood in four different sample collection tubes (bottles): one containing pure whole blood, two containing different anticoagulant agents, and one containing clot activator and serum separator. The collected data indicates that additive agents can have a significant impact on the measured dielectric properties of blood, both immediately after the sample is taken, and over longer time periods. This is an important finding as it suggests that measurements of blood properties conducted on sample repositories, or tissue banks, may not be representative of natural blood properties. Further, the results demonstrate that the dielectric properties of normal blood vary over time due to coagulation. Different clotting rates lead to dielectric properties of female and male blood samples that vary distinctly over time. The results also show that the relative permittivity of the anti-coagulated blood decreases with increasing temperature, up to the cross-over point around 10 GHz where the trend reverses.