Photoacoustic imaging of rat knee joint

Osteoarthritis (OA) is the disease of articular cartilage which is a viscoelastic connective tissue with a smooth surface and serves as a cushion between joint bones. Among the diagnostic imaging modalities for knee OA including plain X-rays, CT, MRI and ultrasonography (US), US is quick, inexpensive and highest resolution imaging. Photoacoustic (PA) imaging is based on generation of the ultrasound by thermal expansion of the tissue when nano-second pulse laser is irradiated to the tissue. In the present study, PA system consisted of diode laser, ultrasonic concave transducer with the central frequency of 50 MHz, hollow optical fiber, amplifier, X-Y stage with stage controller to scan the sample, function generator, digitizer card with the sampling rate of 1 GHz and Windows-based PC was developed. The system can generate laser with the wavelength of 532 nm, the pulse width of 3.4 ns, the output power of 430 μJ and the repetition rate of 100 Hz. The hollow optical fiber used to transmit the laser enabled easy focusing by attaching lens cap on the fiber. The fiber was inserted through the center of the concave ultrasound transducer to be arranged coaxially. Cylindrical cartilage-bone complexes from adult male Sprague-Dawley rats were visualized by US and PA. In US image, strong echo is observed at the surfaces of the cartilage and subcartilage bone and low intensity echo is observed in the spongy bone. In PA image, strong signal is observed at the spongy bone which suggests high density of blood distribution. The power spectrum showed steep peak at 45 MHz in US and wide spectrum between 10 and 50 MHz in PA. The result suggests the possibility of PA imaging for detection of inflammatory change of the knee cartilage in clinical settings.