Measuring the oxygen saturation is of great value in a wide range of medical fields. For instance, tumor progression and malignancy is strongly dependent on tumor hypoxia. Monitoring of cerebral oxygen saturation can be used in the diagnosis of cerebral desaturations in stroke patients. In cardiovascular disease and diabetes the measurement of abnormalities of the microcirculation can monitor the progression of the disease. During plastic and reconstructive surgery, the oxygen saturation can be monitored to predict survival of flaps.
Several techniques have been developed to monitor oxygen saturation, but they all have limitations. For instance, functional magnetic resonance imaging monitors the blood oxygen level dependent (BOLD) contrast but is sensitive only to deoxygenated hemoglobin and the MRI machine is bulky. In positron emission tomography (PET), the use of ionizing radioisotopes is required. The most well-known technique for measuring oxygen saturation is pulse oximetry that uses the physiologic activity of the cardiac pulse, in combination with the difference in spectroscopic reflectance at wavelength of 660 nm and 940 nm defining the concentration of oxyhemo and deoxyhema, to determine oxygen saturation. Simultaneous plethysmography allows measurement of only the arterial saturation. Near infrared Spectroscopy (NIRS) and diffuse optical tomography (DOT) are spectroscopic methods that use light in the 690-nm to 850-nm range and can monitor changes in concentrations of oxygenated hemoglobin, deoxygenated hemoglobin, by using distinct absorption peaks at e.g. 850 nm and 760nm. The technique measures oxygenation in a mixture of venous and arterial blood. However, all these techniques lack the spatial resolution to differentiate heterogenic oxygenation.
Photoacoustic imaging provides high-resolution images containing information on the function and molecular composition of the tissue. PAI can thus both visualize anatomical structures, such as the microvasculature, and has the ability to detect haemoglobin, lipids, water and other light-absorbing chomophores. Relative oxygenation in a tissue can be measured using an automated sequence using two wavelengths (750 and 850 nm). There is also a possibility of recording a continuous spectrum, and therefore choosing other wavelengths when calculating tissue oxygenation. PAI thus has the potential of measuring oxygen saturation non-invasively with spatial resolution.
To date, PA imaging has mainly been developed for measuring oxygen saturation mainly in phantoms and numerous preclinical studies in animals have exploited the oxygenated and deoxygenated hemoglobin components to characterize tumour microenvironment. In our studies, the feasibility of using PAI for estimating the spatial distribution of oxygen saturation is examined, in models of occlusion reperfusion and adrenalin-dependent vasoconstriction in humans. The possibility of determining impaired microcirculation in diabetic patients is being determined. The system was validated in vivo against complementary techniques, including white light and diffuse reflectance spectroscopy.
Bunke J, Sheikh R, Reistad N, Malmsjö M. Extended-wavelength diffuse reflectance spectroscopy for a comprehensive view of blood perfusion and tissue response in human forearm skin. Microvasc Res. 2019 Jul;124:1-5.
Sheikh R, Bunke J, Thorisdottir RL, Hult J, Tenland K, Gesslein B, Reistad N, Malmsjö M. Hypoperfusion following the injection of epinephrine in human forearm skin can be measured by RGB analysis but not with laser speckle contrast imaging. Microvasc Res. 2019 Jan;121:7-13.