A high frequency array-based photoacoustic microscopy imaging system

Photoacoustic microscopy is an imaging technique which draws from the specific strengths of two imaging modalities by capturing the contrast of optical imaging, while retaining the high resolution of ultrasonic imaging. It provides great promise for studying the structure and dynamics of tissue micro-vasculature in development and pathogenesis. Previous work in photoacoustic imaging has been mostly limited to single element transducers. This thesis presents results of a novel photoacoustic microscopy system using a 30MHz linear array and a custom receive electronics. There are two versions of the system, PAM I and PAM II. Both systems are comprised of three main components, a short pulsed laser, a high frequency transducer, and a custom multi-channel electronics system. The attraction towards high frequency arrays over single element transducers is natural; they offer the same resolution advantage of higher frequencies, while diminishing the need for mechanical scanning through steering of the beam, delivering aperture flexibility, tighter focusing capabilities through beamformation, and the capability to image in real-time.; The PAM I system includes an Nd:YAG pumped tunable dye laser, delivering a 6.5ns pulse duration, and a 10Hz pulse repetition rate to the sample via an optical fiber. Furnishing an incident energy of approximately 6mJ/cm 2 at 584nm, the laser induced acoustic waves via thermoelastic expansion. Using a 30MHz linear array and a custom multi-channel receive system, both phantom and in situ photoacoustic images were obtained. The receiving transducer array is a piezo-composite 48 element linear array, with an 8mm focal depth, and a -6dB fractional bandwidth of 50%. Multi-channel receive electronics were developed to include multiplexing and signal processing stages. Four-to-one multiplexers are used to select between elements. The signals are passed through filtering stages, followed by variable and fixed gain stages. The system receiver gain varies from 33dB-73dB, with a -3dB system response between 8MHz and 55MHz. The channels are further multiplexed to acquire data from a 4 channel oscilloscope. Using offline delay and sum beamforming, initial results provided phantom images from an 80mum hair in water, and a 6mum carbon fiber in an optically scattering medium similar to biological tissue. Photoacoustic images in situ clearly showed subcutaneous vessels less than 100mum in diameter imaged at depths of 3mm below the skin surface in a Sprague Dawley rat.; The PAM II system is a 16 channel fully automated parallel multi-channel system that acquires data from all elements on the receive board at once. Controlled by the user interface and the PC, the PAM II system uses the receiver front end analog board and is complemented by custom digital electronics. The digital portion of the system is a backplane motherboard/channel board scheme. Individual channel boards simultaneously digitize the echoes from the receiver at a 100MHz sampling rate. Digital data are then stored in temporary memory and transferred via the PCI bus to the PC with an NI-6534 (National Instruments, Houston, TX) acquisition board. A Labview program was developed to handle system triggering, and control signals to the digital board and receiver multiplexers. PAM II uses an Isonnolab Edgewave laser pumping 6ns pulses at 598nm using an electro-optic Q-switch, delivering an incident energy of below 15mJ/cm2. Phantom images in water and Intralipid solution were formed to characterize the system. Photoacoustic images of micro-vessels in a human hand and 3D images of vasculature in two Sprague Dawley rats were obtained in vivo. The axial and lateral spatial resolutions for both systems were found to be 45+/-5mum and 100+/-5mum, respectively. Ongoing research is also presented for development of a real time PAM system. Initial experiments provided in vivo rat images differentiating micro-vessels in systole and diastole.