Towards Characterising the Size of Microscopic Optical Absorbers Using Optoacoustic Emission Spectroscopy

To assess the malignancy and progression of a tumour, parameters such as the size and number density of the microvessels are expected to be important. The optical absorption due to the blood that fills the microvessels can be visualised by optoacoustic imaging (OA). We have previously reported that increasing the inhomogeneity of absorption within a large absorbing volume produces evidence of reduced acoustic coherence which results in improved contrast and boundary detectability. Here we propose to take advantage of the expectation that the detailed nature of the inhomogeneity should influence the frequency spectrum of the OA signal. The overall aim of this work is to determine whether an analysis of the frequency spectrum of the emitted optoacoustic signal can be used to determine the scale of this absorption inhomogeneity, in particular parameters such as the characteristic size and separation of the absorbers (microvessels). In the preliminary study reported here, various gelatine-intralipid phantoms containing cylindrical wall-less tubes filled with an ink solution were measured in water with a linear array ultrasound detector, using pulsed-illumination that had been adjusted for an optimal distribution of light fluence with depth. Simulations of the experiments were also conducted, using a time domain acoustic propagation method. The results confirm that optoacoustic signals bear information on the sizes and distribution of the absorbers in their frequency spectra. It is shown that a simple way to determine the diameter of a single cylindrical absorber is to estimate the quefrency of the peak in the cepstrum of the measured signal. Further work is proposed to extend this to the statistical estimation of mean diameter and mean separation for an ensemble of similar absorbers and to absorbers with a diameter that is smaller than the axial resolution of the acoustic receiver.