Malekzadeh-Najafabadi Jaber, Prakash Jaya, Razansky Daniel, Ripoll Jorge, Gujrati Vipul, Ntziachristos Vasilis
Chair of Biological Imaging, Central Institute for Translational Cancer Research (TranslaTUM), School of Medicine and Health & School of Computation, Information and Technology, Technical University of Munich, Munich, Germany.
Institute of Biological and Medical Imaging, Bioengineering Center, Helmholtz Zentrum München, Neuherberg, Germany.
Light Sci Appl. 2025 Mar 27;14(1):142. doi: 10.1038/s41377-025-01772-7.
Optoacoustic signals behave nonlinearly at light fluences above a few mJ/cm, which may affect the interpretation and quantification of measurements. It has been proposed that optoacoustic nonlinearity arises from the heat-induced formation of nanobubbles or changes in local thermo-physical parameters. However, such explanations are only valid at much higher fluences than typically used in biomedical optoacoustic imaging (> 20 mJ/cm) or in the presence of materials with high absorption coefficients such as gold nanoparticles. We propose herein that electromagnetic permittivity changes in response to photon absorption are major source of optoacoustic signal nonlinearity at low fluences. We provide theoretical and experimental evidence that supports this postulation and show that optoacoustic pressure responses due to permittivity changes, which are function of thermally excited third-order nonlinear susceptibility, can explain the nonlinear behavior of the optoacoustic signal. Since different materials exhibit different thermally excited third-order nonlinear susceptibility, this property could function as a new contrast mechanism that can identify the sensitivity of a substance's dielectric constant to photon-induced temperature changes. Consequently, we propose an imaging method based on nonlinear optoacoustic signals that exploits this newly identified contrast mechanism. These findings may have far-reaching implications for improving the accuracy of optoacoustics and utilizing the proposed new contrast mechanism would advance our understanding of cellular and tissue functionality.
在光通量高于几毫焦每平方厘米时，光声信号表现出非线性，这可能会影响测量的解释和量化。有人提出，光声非线性源于热诱导纳米气泡的形成或局部热物理参数的变化。然而，这种解释仅在比生物医学光声成像中通常使用的通量（>20毫焦每平方厘米）高得多的情况下有效，或者在存在具有高吸收系数的材料（如金纳米颗粒）的情况下有效。我们在此提出，响应光子吸收的电磁介电常数变化是低通量下光声信号非线性的主要来源。我们提供了支持这一假设的理论和实验证据，并表明由于介电常数变化引起的光声压力响应（它是热激发三阶非线性极化率的函数）可以解释光声信号的非线性行为。由于不同材料表现出不同的热激发三阶非线性极化率，这一特性可以作为一种新的对比机制，用于识别物质介电常数对光子诱导温度变化的敏感性。因此，我们提出了一种基于非线性光声信号的成像方法，该方法利用了这种新发现的对比机制。这些发现可能对提高光声技术的准确性具有深远意义，并且利用所提出的新对比机制将增进我们对细胞和组织功能的理解。
