(E-174) Design of a Prototype Device to Detect Asymptomatic Peripheral Artery Disease

Introduction:: Peripheral Artery Disease (PAD) is associated with a high risk for serious cardiovascular events (heart attack, stroke). Delayed PAD diagnosis may be a factor in observed disparities in cardiovascular outcomes. A simple, fast and easy method for measuring skin blood flow rate (BFR) could be extremely useful in screening for asymptomatic PAD and allow for earlier diagnosis and intervention. A prototype REFLO (Rapid Electromagnetic Flow) device for measuring skin BFR is currently in development at the University of South Alabama [1].

REFLO is a novel technology for noninvasive measurement of microcirculatory flow. The method utilizes the relationship between flowrate and tissue temperature increase during exposure to radio frequency (RF) energy. At high frequencies (≥ 6GHz) absorption occurs almost entirely within the skin and can result in a measurable increase in skin temperature. Measurement of skin surface temperature during RF exposure provides an indication of blood flow rate in underlying tissue.

As the depth of RF penetration and absorptivity are functions of wavelength, the RF frequency is a critical design variable. The rate and spatial distribution of blood flow in the skin also plays a major role in resultant temperature profiles. The extent to which these factors may affect dermal temperature response is not well established. A detailed understanding of how device parameters, skin morphology and physiologic factors affect localized temperature response is essential for device design and optimization.


Materials and Methods:: The objective of this study is to determine the extent to which the magnitude and depth of heating are functions of frequency and BFR. Modeling tissue heating in response to RF exposure is a two-step process entailing (i) determination of the specific absorption rate (SAR) using a discretized model of the tissue(s) of interest and (ii) incorporating the local SAR values in a thermal model that relates RF absorption and other relevant factors to predict local tissue temperatures over time.

SAR values were determined using the xFDTD solver (Remcom, State College, PA) [2]. A heterogeneous tissue model (one-dimensional) consisting of four tissue layers: Stratum Corneum (SC), Epidermis (ED), Dermis (D) and Hypodermis (HD) was designed. Exposures were far-field, continuous wave with wave frequencies of 6, 35 and 94GHz and a power density of 250mW/cm². The resulting SAR values were then used as input to a thermal solver based on the modified Pennes bioheat equation [4]:

ρc(∂T/∂t) = k(∂²T/∂x²) - ωc_b[T(x,t) - Ta] + q(x) (1)


As metabolic heating is negligible, the q(x) source term represents the RF heating. The average perfusion rate (ω) was set to 0.008s^-1. Four levels of perfusion were tested (0xω, 0.5xω, 1xω, 10xω). A convective surface boundary condition was used to account for environmental heat loss. Tissue temperature at model depth was set to deep arterial temperature (37°C). The initial conditions were determined from Foster’s steady state solution [5].




Results, Conclusions, and Discussions:: Figure 2 shows temperature increase over a 120 second heating period for three frequencies under investigation. For the scenarios shown here, higher perfusion rates resulted in smaller surface temperature increases at each frequency. This indicates that the temperature increase is a function of BFR under the given exposure profiles.

In determining design operating frequencies and power levels for a device to detect PAD, the goal is to achieve temperature increases that will make it feasible to distinguish between normal and low blood flow rates without risking injury. The results at 35 GHz show a difference of 2℃ between the temperature increases for the baseline and zero flow conditions. This may be sufficient to provide a clinically useful measurement. The temperature increases at 6GHz resulted in surface temperature increases below 4°C at all BFR values. The 94GHz results showed an intermediate level of discrimination between baseline and low blood flow, but at base perfusion level the surface temperature increased by approximately 10°C. Temperature increases that large may risk pain and/or burns with prolonged exposure.

Published estimates of blood perfusion rates in human dermal tissue layers are highly variable. Thus, the results here do not necessarily imply response of a device in vivo. A study is underway, using volunteer subjects under an IRB (Institutional Review Board) approved protocol, to validate the model results using a prototype device.

In addition to device performance, practical considerations such as hardware availability and cost must be considered. RF transmitters that operate at high frequencies can be expensive, which may affect the viability of a commercial device. We are currently testing additional frequencies in the range 15-40GHz that may provide responses comparable to those seen at 35 GHz, but with potentially lower hardware costs. The results will be used to determine computational model accuracy and further develop the diagnostic device and measurement protocol.


Acknowledgements (Optional): : This work was supported by a grant from University of South Alabama, Office of Research and Economic Development, and by the Alabama Space Grant Consortium.


References (Optional): : [1] Latif, S. I, Austin, C. M., Chatham, J. W., Stewart, B. E. Stubblefield, T. H. and Nelson, D. A. 2019. “A Novel Transducer for Measurement of Skin Blood Flow Using Radio Frequency Energy.” Journal of Engineering and Science in Medical Diagnostics and Therapy 2 (4). https://doi.org/10.1115/1.4045063.

[2] XFdtd 3D EM Simulation. REMCOM, 2021. https://www.remcom.com/xfdtd-3d-em-simulation-software

[3] “104-2 Thin Skin, H&E, 40X.” University of Michigan Histology Department, n.d. https://histologyslides.med.umich.edu/Histology/Integumentary%20System/104-2_HISTO_40X.htm

[4] Pennes, H. H. 1998. “Analysis of Tissue and Arterial Blood Temperatures in the Resting Human Forearm.” Journal of Applied Physiology 85 (1): 5–34. https://doi-org.libproxy.usouthal.edu/10.1152/jappl.1948.1.2.93

[5] Foster, K. R., Kritikos, H. N., and Schwan, H. P. 1978. “Effect of Surface Cooling and Blood Flow on the Microwave Heating of Tissue.” IEEE Transactions on Biomedical Engineering BME-25 (3): 313–16. https://doi.org/10.1109/TBME.1978.326313.