Multiparametric Photoacoustic Imaging in the Course of Radiation Therapy of Malignant Head and Neck Tumors

Multispectral photoacoustics enable non-invasive, inexpensive and dose-free real-time imaging of light-absorbing molecules (absorbers), e.g. Deoxyhemoglobin and oxygenated hemoglobin in human tissue. This allows blood oxygen saturation (sO2) to be determined at depths of up to several centimeters. Measurements of correlates to blood volume and collagen concentration are also made possible. In photoacoustic imaging, the tissue to be examined is irradiated with nanosecond short, near-infrared (650 - 1300nm) laser pulses. If laser light is locally absorbed by a tissue structure, it expands thermoelastically, which triggers an ultrasonic pressure wave, which is measured with the aid of an ultrasonic head. The initial pressure distribution and thus the absorption in the tissue can then be reconstructed. Since different molecules show distinct absorption behavior depending on the wavelength in the near infrared, by acquiring several wavelengths it is possible to estimate which absorbers are in which concentration in a tissue structure. The effectiveness and tolerability of modern high-precision radiation therapy for head and neck tumors largely depends on the quality of the imaging. The potential diagnostic benefits of photoacoustics in the radiotherapy of patients with head and neck tumors principally concern the target volume definition, the implementation of image-guided, adaptive radiotherapy and imaging tumor follow-up as well as the early detection of tumors.

Multispectral photoacoustics primarily enable the analysis of tumor hypoxia, which has been associated several times with increased radio resistance and an unfavorable prognosis. In addition, other factors, e.g. the blood volume and the collagen content in the tissue are analyzed.