Photoacoustic imaging consists in light absorption by a sample generating acoustic waves due to photoacoustic effect. If temporal stress confinement is satisfied a broadband pressure wave will be generated and photoacoustic images can be acquired using a clinical ultrasound machine. Photoacoustic imaging combines contrast of optical images and spatial resolution of ultrasonic images. Due to the capacity of encoding optical proprieties distribution in acoustic waves, this image technique can be used to imaging blood vessels to diagnostics and monitoring of pathologies that change vasculature. Hemoglobin is an endogen chromophore that highly absorbs light, allowing photoacoustic imaging of vasculature. The specific optical absorption of hemoglobin is influenced by oxygenation and present absorption peak in 850 nm for oxy-hemoglobin and 750 nm for deoxy-hemoglobin. Thus, photoacoustic images generated using a LASER with different wave lengths can use this contrast mechanism to investigate oxygen heterogeneity in tumors. Other studies have shown increase in oxygenation in tumor after mild hyperthermia (heating the tissue to around 40°C) due to enhancement in perfusion and decrease of oxygen consumption. Mild hyperthermia in tumoral region can reduce the amount of hypoxic cells, increasing radiosensitivity and improving radiotherapy eficience. Therefore, it is proposed photoacoustic images to evaluate tumor oxygenation in an animal murine model. This evaluation will also be performed in tumors exposed to mild hyperthermia using high intensity focal ultrasound.
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