Development of a 40-MHz annular array

The design and testing of a 5-ring, focused annular array is presented. The array has a total aperture of 6 mm, a geometric focus of 12 mm, and a nominal center frequency of 40 MHz. The active element of the transducer is a 9-Pm polyvinylidene fluoride (PVDF) membrane and the array pattern is formed on a copper clad polyimide film. A system has been developed to permit testing of the array. The system consists of a XYZ axis scanning component, a pulscr/recciver, a crosspoint switch module, a digital storage oscilloscope, and a PCI digital acquisition card. The system permits a single ring of the array to be pulsed, and then the radio-frequency echo of all five array elements to be simultaneously digitized. Data from all 25 transmit/receive ring pairs were collected and then post-processed with a synthetic focusing technique to establish an improved depth of field. To accomplish this testing, a wire phantom with nine 25-mu m diameter wires spaced in 1 mm by 1 mm increments was utilized. An improved depth of field was observed for data processed with the synthetic focus technique versus data with no focusing.