The dual-modal system consists of a second-generation optical-resolution PAM (OR-PAM) system7 and a home-made ECG recorder, as seen in Fig. 1. In OR-PAM, the outputs of a solid-state laser (SPOT, Elforlight) and a wavelength-tunable laser system (pump laser: INNOSLAB, Edgewave; dye laser: CBR-D, Sirah) were combined using a beam splitter to provide pulse-to-pulse wavelength switching for the measurement of hemoglobin oxygen saturation (). The combined laser beam was spatially filtered by an iris (ID25SS, Thorlabs; aperture size: 2 mm), focused by a condenser lens (LA1131, Thorlabs) through a 50-μm-diameter pinhole (P50C, Thorlabs), and then coupled into a single-mode fiber (PA-460A-FC-2, Thorlabs). A tunable neutral density filter (NDC-50C-2M, Thorlabs) was placed before the fiber coupler to regulate the intensity of the incident beam. The fiber output was collimated by a microscope objective (RMS4X, Thorlabs), reflected by a mirror, and re-focused by a second identical objective to achieve nearly diffraction-limited optical focusing (focal diameter: 2.6 µm). A home-made acoustic-optical beam combiner, containing an oil-filled interface, was placed beneath the objective to align the optical excitation and ultrasound detection coaxially and confocally. The generated photoacoustic wave was focused by an acoustic lens, detected by an unfocused ultrasonic transducer (V214-BB-RM, Olympus-NDT), and amplified by two 24-dB cascaded electrical amplifiers (ZFL 500LN, Mini-Circuits). Electrocardiograms were simultaneously recorded with three electrodes, one connected to the ground, one to a front leg, and one to a hind leg of a mouse. The ECG signals were then amplified by a high-gain differential amplifier (Model 3000, A-M systems). The acquired photoacoustic and ECG signals were digitized by a dual-channel high-resolution digitizer (NI PCI 5124, National Instruments Corporation) and stored in a computer for offline data processing.