Carbon Nanotube Microscale Fiber Grid as an Advanced Calibration System for Multispectral Optoacoustic Imaging

Optoacoustic (photoacoustic) imaging has gained tremendous attention in research and in clinical practice as a point-of-care system for noninvasive, fast, and safe tests. The first optoacoustic (OA) tomograph has recently passed the Food and Drug Administration (FDA) approval stage for clinical applications aimed at early breast cancer diagnostics. Furthermore, a broad application of OA imaging for Biomedical and Materials Science fields requires a proper tool to test the equipment and verify the quality of the measurements on a daily basis. In the present work, we propose fibers based on single-walled carbon nanotubes (SWCNTs) as a material for designing a stable and reliable calibration grid. The main advantage of the developed test system is the broad optical absorption of SWCNT-based fibers, ranging from visible to mid-infrared regions. Inspired by stringed instruments, we elaborate a grid to calibrate and verify spatial resolution in three projections and sensitivity of OA imaging systems. Thus, the real calibration grid parameters, such as fiber length and diameter, could be translated to the OA signal measurements. This proof-of-the-concept study evaluates the geometry of fibers, that is, the length/diameter and design of fibers, such as free-standing/twisted, and shows the fabrication procedure of the calibration grid prototype toward the successful validation of the OA imaging system, including raster-scanning optoacoustic mesoscopy (RSOM) at one wavelength and tomography at several wavelengths, which have grand prospects in preclinical and clinical practices. Besides, the more advanced geometry based on double-twisted fibers, or twistrons, applied here provided us with a chance to reach the lower resolution limit for RSOM because of the difference in diameter between the thin and thick parts in the morphology is verified by scanning electron microscopy.