Photoacoustic imaging, which images the distribution of optical energy deposition (or optical absorption distribution) in tissue based on the detection of ultrasonic wave induced by absorption of pulsed laser energy, presents intrinsic physiologic specific absorption of tissues.28,39 In this paper, the maximum-amplitude projection (MAP) along the axis was used for 2-D image reconstruction. For functional parameter monitoring, a series of continuous images were acquired by sequential dual-wavelength line scanning of the same selected blood vessels. Before calculating the value, the threshold method was applied to the MAP image to eliminate the pixels with low signal-to-noise ratio. The threshold was selected as the intensity of the half-maximum of the chosen vessels, and then the pointwise value was calculated with the multi-wavelength method. Consequently, the value of each vessel was defined as the average value of located in this vessel. Here, we describe the multi-wavelength method for calculation simply. In brief, when oxyhemoglobin () and deoxyhemoglobin () are treated as the dominant absorbing compounds in blood, the image could be calculated using least-squares fitting as:34Display Formula
(1)where Display Formula
(2)where Display FormulaHere, and are the concentrations of and , respectively. is the proportionality coefficient, and and are the molar extinction coefficients () of the two forms of hemoglobin at wavelength , respectively. is the collected photoacoustic signal with wavelength . The wavelengths of 576 and 584 nm were selected for dual wavelength continuous line scans for calculation. The HbT value of each vessel was calculated from single-wavelength line scan at 584 nm for it is an isosbestic wavelength of and . Before calculating the HbT value, the same threshold method was used as it is applied in calculation to define the pixels included for calculation of the HbT value of each vessel. The HbT value of each pixel included in the chosen vessel was defined as the intensity of the MAP image at 584 nm. Consequently, the HbT value of each vessel was defined as the average value of HbT located in this vessel. The or HbT value of each rat was achieved by averaging the or HbT value of the chosen vessels.