Photoacoustic tissue characterization using envelope statistics and ultrasonic spectral parameters

Photoacoustic (PA) tissue characterization relies on the analysis of ultrasound (US) signals generated through the PA effect. The probability distributions of PA signal amplitude as well as the frequency content of the PA signals are typically not considered. We present a phantom study where we introduce the combined use of US/PA signal envelope statistics along with analysis of the frequency content of the US/PA signals for the purposes of monitoring physical changes in the absorbers. The phantoms were constructed using black polystyrene beads (radius 1.77 μm and 7.36 μm). Tissue microenvironment was emulated by homogenously mixing 10 beads/imaging-transducer-resolution-volume in order to accommodate large numbers of sub-resolved beads. The phantoms were imaged with the Vevo LAZR US/PA integrated imaging system (Fujifilm-VisualSonics) using a 40 MHz linear array probe and 680 nm illumination. US/PA signals from the same region of interest were analyzed by mapping the signal amplitudes distributions from 5 phantom locations, fitting the data to the Generalized Gamma (GG) distribution and extracting the fit parameters while computing the normalized power spectra to retrieve the spectral slope (SS) and midband fit (MBF) spectral parameters. The GG scale parameter increased by 500x for US images and 12x for PA equivalents as the size of the beads increased by ~4.5x. The SS decreased by 0.8x for US and 0.4x for PA. These changes can be attributed variations in size and spatial organization of the beads suggesting that combined US/PA statistical and spectral analysis can potentially monitor normal and abnormal tissue physiological changes.