We developed a reflection-mode broadband photoacoustic microscopy (PAM) based on surface plasmon resonance (SPR). Taking advantage of high-sensitivity refractive index sensing and ultrafast time response of the SPR sensor, photoacoustic (PA) transients can be measured accurately. The PAM system shows, experimentally, the available detection bandwidth of up to ~109 MHz, giving an estimated axial resolution of around 12.1 μm. A reflective objective is combined with a miniature SPR sensor, enabling the reflection-mode PAM with a lateral resolution of 5.0 μm. Using our PAM system, we image melanoma cells in vitro, providing the spatial distribution of melanin particles within melanoma cells. Further, the microvasculature is acquired in a mouse ear in vivo, delineating three-dimensional morphological characteristics of both the major blood vessels and capillaries.
Photoacoustic (PA) microscopy can measure the optical absorption properties of tissues with high specificity. However, most photoacoustic microscopy (PAM) systems use piezoelectric ultrasonic transducers for PA pressure detection. Due to the limited bandwidth, the axial resolution of PA imaging is low (generally lager than 20-μm), resulting in inaccurate positioning of light absorbing biomolecules. Moreover, the large difference in spatial resolution between the axial and lateral directions severely degrades the reconstruction of the three-dimensional image of the tissue. Surface plasmon resonance sensing (SPR) has an ultra-fast time response and thus is expected to increase the detection bandwidth of PA waves. The disturbance of the PA pressure wave causes the refractive index change of the medium near the sensing layer, which modulates the SPR field. By detecting the change of the optical reflectivity, wideband PA detection can be realized. Here, we combine the SPR detector and an acoustic cavity with the ellipsoid inner surface for PA detection, which not only enhances the signal detection sensitivity, but also realizes the reflection-mode PA imaging. The experimental results show that the imaging signal-to-noise ratio (SNR) increases by around three times, and the detection bandwidth is more than 70-MHz. High resolution and high contrast vascular imaging of mouse ear is acquired in vivo.