A hybrid optical and ultrasound system to measure localized oxygenation, blood flow and oxygen consumption in the human body

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A patient's health is in great danger when there is a prolonged lack of oxygen delivery to meet the metabolic demand of the tissue. This will eventually lead to cell death and organ failure. Therefore, it is very important for clinicians to monitor oxygenation in the body especially in critically ill patients or those undergoing major surgery. For example a measure of the oxygen levels in the venous system (venous oxygen saturation) has been shown to be very useful in reducing the death rate of patients with severe symptoms of whole body infection (sepsis). Also, measurements of venous oxygen saturation have been shown to be a good predictor of post-operative complications. Currently, clinical monitoring of venous oxygen saturation involves inserting an invasive catheter into a vein near the neck to perform measurements directly on the blood. However, the invasive procedures required to make these measurements demand considerable surgical skill and are associated with risk such as infection and bleeding. These procedures are only carried out in patients deemed sick enough to justify the risk, e.g. patients in the intensive care unit. These practicalities preclude many patients who can potentially benefit from the diagnostic value of the venous oxygen saturation measurement. The main objective of this work is to develop a new clinical monitor which can measure venous oxygen saturation non-invasively by combining optical and ultrasound technologies. The new clinical monitor has a probe containing both optical and ultrasound components which can be placed on the skin surface over the measurement site and target a localised region beneath. For example, it can be placed over the chest and measure the venous oxygen saturation in the pulmonary artery which contains the blood that has circulated through the whole body. This non-invasive venous oxygen saturation measurement can replace its invasive catheter based counterpart for clinical monitoring. The new monitor can also be used to target a vein draining the blood from the brain (jugular vein) so that the condition of the brain can be monitored. Other applications include the monitoring of limbs with poor circulation, recovery after surgery and the functioning of transplanted organs. Apart from venous oxygen saturation, the new monitor can also be used to measure blood flow and oxygen consumption, which indicates oxygen delivery to the tissue and the amount of oxygen used up by the tissue respectively. The principle of the new monitor is based on the phenomenon that ultrasound waves can cause periodic movement within a specific tissue region changing the way light travels through it. When light passes through this region, the intensity of the light will be altered and can be detected by a surface mounted optical detector. In other words, the light is tagged by the ultrasound waves which are the strongest in the target region. The detected tagged light is known as the acousto-optic signal and can be used to derive localized oxygenation, blood flow and oxygen consumption.In this work, different ways of combining the optical and ultrasound techniques will be systematically investigated, including the enhancement of the acousto-optic signals using short bursts of high energy ultrasound and microbubbles (an ultrasound contrast agent often used in modern ultrasound scan to improve image quality). The investigation will be conducted by both laboratory based and human experiments. For a thorough understanding, computer models will also be developed to explain the different mechanisms that generate the acousto-optic signals. These investigations will allow the design of a reliable hybrid monitor optimized for clinical use in a range of different settings.