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

Overview

Multispectral photoacoustic imaging enables the measurement of the optical absorption of various tissue components or exogenous contrast agents in vivo. The dominant, near infrared absorbing chromophores in human tissue are oxy- and deoxyhemoglobin followed by collagen, melanin and lipids. The multispectral measurement of the absorption of hemoglobin shows changes in blood oxygen saturation and blood volume. The high resolution of photoacoustic imaging also enables the vascular structure to be displayed. The aim of this exploratory study is to generate hypotheses by applying photoacoustic imaging to the field of head and neck tumor therapy. The next step is to investigate whether and how photoacoustic imaging can help improve diagnostics and better planning of treatments in the future. In particular, the differences between normal and tumor tissue and the changes in the tissue due to radiation therapy using photoacoustic imaging are examined. In the quantitative analysis of the images, measured chromophores, primarily oxygen saturation, blood volume and collagen concentrations at different measuring points are used in the course of the therapy.

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.