Abstract
Independent component analysis (ICA) is an unmixing method based on a linear model. It has previously been applied in in vivo multiwavelength photoacoustic imaging studies to unmix the components representing individual chromophores by assuming that they are statistically independent. Numerically simulated and experimentally acquired two-dimensional images of tissue-mimicking phantoms are used to investigate the conditions required for ICA to give accurate estimates of the relative chromophore concentrations. A simple approximate fluence correction was applied to reduce but not completely remove the nonlinear fluence distortion, as might be possible in practice. The results show that ICA is robust against the residual effect of the partially corrected fluence distortion. ICA is shown to provide accurate unmixing of the chromophores when the absorption coefficient is within a certain range of values, where the upper absorption threshold is comparable to the absorption of blood. When the absorption is increased beyond these thresholds, ICA abruptly fails to unmix the chromophores accurately. The ICA approach was compared to a linear spectroscopic inversion (SI) with known absorption spectra. In cases where the mixing matrix with the specific absorption spectra is ill-conditioned, ICA is able to provide accurate unmixing when SI results in large errors.
Highlights
Biomedical photoacoustic imaging has been a rapidly growing field of research in the last two decades, with significant development in the instrumentation, reconstruction algorithms, and exogenous contrast agents
The image contrast depends on the absorbed optical energy, which is nonlinearly related to the chromophore concentrations, because it is a product of the local light fluence and the absorption coefficient
But practical, fluence correction [Eq (6)], it was shown that Independent component analysis (ICA) results in smaller unmixing errors compared to spectroscopic inversion (SI) in two circumstances: first, when the absorption level of the contrast agents is ∼0.4 to 0.5 mm−1, ICA is more robust against nonlinearities caused by inaccurate fluence estimation than SI, because it allows the mixing matrix to vary in order to produce the most independent components
Summary
Biomedical photoacoustic imaging has been a rapidly growing field of research in the last two decades, with significant development in the instrumentation, reconstruction algorithms, and exogenous contrast agents. To acquire a photoacoustic image, the tissue is illuminated with nanosecond laser pulses, and the optical energy is absorbed by the chromophores in the tissue. This leads to a small temperature and pressure rise, generating broadband acoustic waves that propagate to the surface of the tissue where they are detected and used to reconstruct images of the pressure rise. The light fluence is spatially and spectrally nonuniform
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