Ask an Expert: I was able to image gold nanorods in a phantom but I couldn't see them in vivo after an animal injection. What could be the problem?
Photoacoustic imaging requires light energy to be absorbed by nanorods, converted to heat energy, and then converted to pressure waves; in vivo imaging problems can manifest in a number of ways.
If we assume that the ultrasound receiver and pulsed light sources used in the phantom setup are operating correctly and the ultrasound properties of the phantom mimicked tissue, then the problem is likely a function of light delivery or photoacoustic contrast.
Troubleshooting common issues
Amount of light delivered: One of the most common issues encountered during photoacoustic imaging is delivering sufficient light. Skin and blood are the major sources of light attenuation, which can attenuate >90% of the incident light within the first couple of centimeters. To mitigate this problem, you can increase the incident light fluence, or intensity, up to the limits set by ANSI during your imaging session (For more info, visit: https://www.lia.org/store/product/ansi-z1361-2014-safe-use-lasers-electronic-version).
Factors related to nanorod delivery/accumulation/targeting: Another issue could be that there aren't enough nanorods in the tissue of interest. This can stem from an insufficient amount of nanorods introduced into the animal, or the nanorods not accumulating in the tissue of interest. The latter can be addressed by coating the nanorods with molecules, peptides, proteins, antibodies, DNA, aptamers, etc. which have a specific affinity to the cells or structures in the tissue of interest.
Imaging technique & customization: If nanoparticle accumulation is not the problem, then contrast can be improved with spectroscopic imaging techniques. Nanorods can be tailored to have a unique spectral signature, which can be differentiated from endogenous sources of contrast (namely hemoglobin and melanin) by employing spectral unmixing algorithms of photoacoustic images acquired at different wavelengths of light. Providers of photoacoustic imaging machines (e.g. VisualSonics, iThera) can provide spectral unmixing modules or you can post-process the imaging data yourself using published methods (Luke et al. & Namen et al.).
Need additional help?
Our team has more than two decades of experience working with photoacoustic imaging and nanoparticle contrast agents and we would be happy to answer any further questions you have.
Once we learn more about your project and take a look at your current images, our scientists can help you design your study, choose the right nanoparticles, select the optimal wavelength, assist with custom conjugations and adjust other parameters.
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