Al-Armaghany, AM; (2015) Development of a hybrid microwave-optical system to monitor human thermoregulation (Al-Armaghany, A, Trans.). Doctoral thesis , UCL (University College London).

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Abstract

Warming of human tissue causes vasodilation and therefore, increase in blood volume. Such thermal responses allow the assessment of hemodynamics in the tissue, providing physiological and clinically important information of the diagnosed subject. Local warming is often accomplished on the skin because of its accessibility and simplicity. To allow the investigation into deeper tissue such as the muscle, an innovative hybrid microwave-optical system has been developed. This comprises of a microwave system, an optical monitoring and cooling system. The tissue warming is induced by a novel microwave applicator, which was based on microstrip patch design operating at 2.45 GHz with a superstrate interface layer to improve the coupling of electromagnetic (EM) waves into the skin. The active cooling was introduced to reduce skin heating. While the optical sensors based on Near-Infrared Spectroscopy (NIRS), was used to measure the changes in tissue oxygenation including the muscle. This thesis demonstrates the development procedure, covering the design and operation of the entire system. Moreover, the majority of the work is based on the four developed applicators, where each design was evaluated using EM and thermal simulation based on numerical phantoms. The study evaluates the distribution of absorbed EM energy in the tissue known as the specific absorption rate (SAR). The applicators are developed in the following order: (i) Applicator I was fabricated for preliminary study for general tissue heating with the integrated optical probes. This early study provided an insight to the importance of superstrate thickness and material. (ii) Applicator II, which introduces a new approach to skin cooling based on Thermoelectric Coolers (TEC) and high thermal conductive superstrate. This design could cool the skin and monitor tissue oxygenation, skin perfusion and temperature. (iii) Applicator III was an updated model of the predecessor, resolving cooling configuration and the discrepancy in operating frequency, and was capable of minimising skin heating effectively (iv) Circularly polarized (CP) Applicator aimed at reduction of the SAR in the superficial layer, and hence skin heating. The simulated thermal study of all developed applicators was validated with exvivo (mimicked phantom) and in-vivo experimental trials. The measurements and the simulation model were in agreement, apart from the CP applicator due to the complexity of measurement. The results from the phantom and human calf indicated superficial heating was reduced by about 5.0-6.0 ° C when skin cooling was applied, while the temperature change in muscle was not significantly affected. The measurement with mimicked tissue showed the applicator was capable of elevating muscle temperature by approximately 3.0-4.0 ° C. This is a sufficient increase to cause tissue dilation, and therefore, change in the thermal response. The hybrid microwave-optical system has been developed and examined on three human calves during in-vivo physiological study. The results using Applicator II illustrated that the device can successfully stimulate and measure thermal responses in terms of oxy/deoxy/total haemoglobin concentrations changes ( HbO2/ HHb/ HbT). The slope (rate of change) of HbT curve during microwave exposure is defined as the thermal response. This parameter is essential in studying physiological responses between different subject, particularly in vascular diseases, transplanted free flaps and other conditions, including chronic spinal cord injury. Subjects with such conditions will have a distinguishable response to tissue heating than a healthy subject. The monitored haemodynamic signals of Applicator II are primarily based on superficial responses. However, measurements with Applicator III showed the potential of the applicator. The measured thermal response was 0.83 10 3×10⁻³ μM/s without skin cooling, which was dedicated by skin heating. The introduced cooling system has reduced the skin temperature and maintained the local skin micro-circulation, which was monitored with the secondary optical system based on Laser Doppler Flowmetry (LDF). This probe measures blood flow at superficial depth, and consequently, was used as a validation tool to demonstrate the cooling efficiency. The measured thermal response with skin over-cooling was -0.08 10 3×10⁻³ μM/s. The negative response indicates arterial constriction, and therefore, the skin heat was eliminated while the simulations study to indicate the muscle temperature was elevated by 3 ° C. However, the response was dominant by the superficial response. Obtaining a response from muscle only was challenging and currently being solved in numerous applicator and cooling technique, which have been presented in the thesis.

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