To troubleshoot low photoacoustic signal strength, let us take a closer look at the science behind the modality. In photoacoustic (optoacoustic) imaging, the received signal from a single point in the imaging plane can be described as:
The Grüneisen parameter is tissue-dependent and has little variation in water-based tissues at constant temperature, so as such it is often assumed to be constant.
If we assume that the photoacoustic system (transducer, receive electronics, laser, etc.) is operating correctly, then the amount of light delivered and the net optical absorption must be increased to get a stronger photoacoustic signal.
First, an appropriate laser wavelength should be selected that is within the near-infrared optical window of tissue (650 – 1350 nm). In this window the absorption by biological tissues is at a minimum, allowing more light to reach the nanoparticles. Second, select a nanoparticle with a maximum absorption wavelength that closely matches your source laser wavelength.
In addition to wavelength, the photoacoustic signal is dependent on the nanoparticle concentration within an imaging voxel. Other factors include:
- Make sure your rods are highly monodisperse and absorb at the appropriate wavelength of excitation. Verify with UV-Vis spectroscopy.
- Check if you are working in an optimized optical window where absorption from oxyhemoglobin and deoxyhemoglobin is minimal.
- Perform injections into tissue-mimicking phantoms to test signal at varying concentrations and depths.
- Improve circulation time and biodistribution by using PEGylated nanorods or conjugated nanoparticles.
- Use silica coated nanorods to maintain thermal stability at high fluences (Y-S chen et al, Enhanced thermal stability of silica-coated gold nanorods for photoacoustic imaging and image-guided therapy)
- Increase concentration of injection.
- Verify targeting strategy with other methods. E.g. By using gold nanoparticles conjugated with fluorescent secondary antibody.
Tell us more about your project and send us your current images. Our scientists can help you choose the right nanoparticle, select the optimal wavelength and adjust other parameters.
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