Quantitative imaging of microvasculature in deep tissue with a spectrum-based photo-acoustic microscopy.

We analyze photo-acoustic signals from capillaries and theoretically demonstrate the quantitative relationship between vascular diameter and spectral slope in a low-frequency band. Phantom experiments validate the theoretical analysis. Based on this finding, spectral slope is proposed as the imaging parameter of a photo-acoustic microscopy. This system effectively quantifies the microvasculature with diameters of 60 and 150 μm, which are smaller than the wavelength 342 μm at the central frequency 4.39 MHz of ultrasound transducer. The low frequency also guarantees the imaging depth in the order of centimeters. The proposed scheme could be potential for noninvasive diagnosis of diseases related to abnormal vasoconstriction or angiectasis.