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Co-registering ultrasound (US) and photoacoustic (PA) imaging is a logical extension to conventional ultrasound
because both modalities provide complementary information of tumor morphology, tumor vasculature and hypoxia for
cancer detection and characterization. In addition, both modalities are capable of providing real-time images for clinical
applications. In this paper, a Field Programmable Gate Array (FPGA) and Digital Signal Processor (DSP) module-based
real-time US/PA imaging system is presented. The system provides real-time US/PA data acquisition and image display
for up to 5 fps* using the currently implemented DSP board. It can be upgraded to 15 fps, which is the maximum pulse
repetition rate of the used laser, by implementing an advanced DSP module. Additionally, the photoacoustic RF data for
each frame is saved for further off-line processing.
The system frontend consists of eight 16-channel modules made of commercial and customized circuits. Each
16-channel module consists of two commercial 8-channel receiving circuitry boards and one FPGA board from Analog
Devices. Each receiving board contains an IC† that combines.
8-channel low-noise amplifiers, variable-gain amplifiers,
anti-aliasing filters, and ADC's‡ in a single chip with sampling frequency of 40MHz. The FPGA board captures the
LVDSξ Double Data Rate (DDR) digital output of the receiving board and performs data conditioning and subbeamforming.
A customized 16-channel transmission circuitry is connected to the two receiving boards for US pulseecho
(PE) mode data acquisition. A DSP module uses External Memory Interface (EMIF) to interface with the eight
16-channel modules through a customized adaptor board. The DSP transfers either sub-beamformed data (US pulse-echo
mode or PAI imaging mode) or raw data from FPGA boards to its DDR-2 memory through the EMIF link, then it
performs additional processing, after that, it transfer the data to the PC** for further image processing. The PC code
performs image processing including demodulation, beam envelope detection and scan conversion. Additionally, the PC
code pre-calculates the delay coefficients used for transmission focusing and receiving dynamic focusing for different
types of transducers to speed up the imaging process. To further speed up the imaging process, a multi-threads technique
is implemented in order to allow formation of previous image frame data and acquisition of the next one simultaneously.
The system is also capable of doing semi-real-time automated SO2 imaging at 10 seconds per frame by changing the
wavelength knob of the laser automatically using a stepper motor controlled by the system.
Initial in vivo experiments were performed on animal tumors to map out its vasculature and hypoxia level, which
were superimposed on co-registered US images. The real-time system allows capturing co-registered US/PA images free
of motion artifacts and also provides dynamitic information when contrast agents are used.
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