Design of digital beamformers for high-frequency ultrasound transducer arrays using field-programmable gate array (FPGA)

Abstract

Ultrasound imaging is a well-established and widely used clinical technique that shows the cross-sectional image of human tissues. Conventional ultrasound systems are targeted at imaging the heart or abdominal organs and the spatial resolution at this frequency range is on the order of a few millimeter. An ultrasound system will be capable of providing a better spatial resolution if higher frequencies are used. High frequency ultrasonic imaging (> 20 MHz) using single element transducers has been shown to be clinically useful in ophthalmology, dermatology, small animal and intravascular imaging. In recent years, more studies have been carried out on the development of high frequency array transducers. Due to the fact that beamforming electronics is not yet commercially available for the high frequency arrays, prototypical digital beamformers for the annular and linear arrays were specifically developed for the purpose of testing the annular array and linear array developed at the NIH Transducer Resource Center at University of Southern California (USC). Field Programmable Gate Arrays (FPGAs) have been demonstrated to be an ideal platform for beamformer development. They provide the processing speed necessary for real-time beamformer with high frequency array transducers. Two digital beamformers were developed for the 8-element annular array and 64-element linear arrays. An imaging system composed of an annular array transducer, an eight-channel analog front-end, a field programmable gate array (FPGA) based beamformer, and a DSP microprocessor based scan converter was deveolped. A PC computer is used as the interface for image display. The beamformer that applies delays to the echoes for each channel is implemented with the strategy of combining the coarse and fine delays. The coarse delays that are integer multiples of the clock periods are achieved by using a First-In-First-Out (FIFO) structure and the fine delays are obtained with a Fractional Delay (FD) filter. Using this principle, dynamic receive focusing is achieved. The image from a wire phantom obtained with the imaging system was compared to that from a single element transducer using one channel of the system. The improved lateral resolution and depth of field from the wire phantom image were observed. Images from an excised rabbit eye sample were also obtained, and fine anatomical structures were discerned. A real-time digital beamformer for high frequency (>20 MHz) linear ultrasonic arrays was designed and developed. The system can handle up to 64-element linear array transducers and excite 16 channels and receive simultaneously at 100 MHz sampling frequency with 8-bit precision. Radio frequency (RF) signals are digitized, delayed and summed through a real-time digital beamformer, which is implemented using a Field Programmable Gate Array (FPGA). Using fractional delay filters, fine delays as small as 1ns can be implemented. A frame rate of 30 frames per second was achieved. Wire phantom (20 µm tungsten) images were obtained and –6 dB axial and lateral widths were measured. The results showed that using a 30 MHz, 48-element array with a pitch of 100 µm produced a -6 dB width of 68 µm in the axial and 270 µm in the lateral direction while using a 35 MHz, 64-element array with a pitch of 50 µm produced the –6 dB width of 65 µm and 550 µm in the axial and lateral direction, respectively. Images from an excised rabbit eye sample were also acquired, and fine anatomical structures, such as the cornea, lens and iris, were resolved.