Design and development of a 30 MHz six-channel annular array ultrasound backscatter microscope

A 30 MHz annular array imaging system and beamforming algorithm were developed as an intermediate step to improve focusing, and therefore image resolution, throughout the image field via dynamic focusing in 2D. Transmit and receive beamforming was accomplished with the full aperture by applying delays to each element. The annular array with six equal area elements was designed and fabricated for use with the beamformer. The array incorporated a fine-grain lead titanate, which provided good sensitivity with reduced lateral coupling. Each transducer element provided over 40% bandwidth and less than 18 dB insertion loss. The hardware system included six channels, each incorporating a pulser/receiver, time-gain compensation (TGC), and an amplifier. In this study, during reception, the RF data for each channel was digitized sequentially, due to the limited number of 500 MHz A/D converters, stored for off-line calculation and displayed with software. A model was used to evaluate the point-spread function of the system throughout the image field. Simulation showed that the point spread function in the axial direction was about 65 μm (-6 dB) and 105 μm (20 dB) throughout a distance of 10 mm to 18 mm. The lateral resolution was 150 μm (-6 dB) from 10-18 mm, whereas the -20 dB lateral resolution increased from 150 μm to 300 μm in the same range. Tests with an 8 μm diameter tungsten wire phantom showed an axial resolution of 55 μm (-6 dB) and 145 μm (-20 dB) at a depth of 10 mm to 17 mm. The -6 dB lateral resolution increased from 100 μm to 240 μm in the same range. The -20 dB lateral resolution was approximately 440 μm to 640 μm from 10 mm to 17 mm. The simulated and experimental results showed that improved resolution can be attained using cross-fade method. Preliminary images of an excised eye were acquired with the imaging system in vitro.