image: (a-b) Reconstructed image of absorption coefficient at (a) low and (b) high light fluence. The entire optoacoustic data (c) was generated using a modified model-based reconstruction algorithm designed to image the thermally excited third-order nonlinear susceptibility (TETONS). This algorithm processed optoacoustic signals acquired at two different light fluences, as shown in panels a and b. By analyzing the nonlinear variations in optoacoustic pressure and reconstructing the data with the modified algorithm, the image highlights differences in tissue TETONS and reveals structures within the kidney cross-section, particularly emphasizing the high-frequency components of the signal. Arrows 1 and 2 show the skin and muscle of the abdomen, respectively. Arrows 3-6 indicate structures within the kidney (Arrow 3: Capsule, Arrow 4: Cortex, Arrow 5: Medulla, and Arrow 6: Calyx). (d) Anatomical reference for the kidney structures.
Credit: Malekzadeh-Najafabadi et al.
Optoacoustic imaging has emerged as a powerful technique for visualizing biological tissues with high resolution and contrast. In a new study published in Light: Science & Applications, a team of scientists led by Jaber Malekzadeh-Najafabadi and Vasilis Ntziachristos explored the origins and applications of nonlinear changes in optoacoustic tomography at low light fluence.
The team identified that changes in electromagnetic permittivity, induced by thermally excited third-order nonlinear susceptibility, significantly contribute to optoacoustic signal nonlinearity at low light fluences. Using theoretical models and experimental validations, the researchers demonstrated that nonlinear variations are most prominent at high-frequency optoacoustic signals and can be harnessed as a novel imaging contrast mechanism.
The findings open the door to new imaging methodologies that improve the accuracy of tissue characterization. By mapping thermally excited nonlinear susceptibility, the researchers reconstructed the first images showcasing this unique contrast in phantoms and in vivo mouse tissues. The method also displayed potential for monitoring physiological and pathological changes in organs such as the kidney and liver, offering insights into diseases linked to tissue composition changes, including obesity and metabolic disorders.
"We were fundamentally interested in better understanding the source of non-linearity observed in optoacoustic signals" says Ntziachristos. "Our findings underscore the potential of using the non-linearity of the optoacoustic signal to offer a revolutionary new contrast mechanism in optoacoustic imaging," adds Malekzadeh-Najafabadi. "While further studies are required to corroborate our postulation on the sources of non-linearity, the new method can be widely employed in basic research and clinical translation applications” adds Ntziachristos.
Journal
Light Science & Applications
Article Title
Nonlinearity of optoacoustic signals and a new contrast mechanism for imaging