Ultrasonic annular array system for detecting tissue motion

We are developing ultrasonic strain-flow imaging instrumentation specifically to facilitate improved diagnosis of breast tissue disease. An 8-ring, 30-mm-diameter, f/1.5, spherically focused annular array was built to generate broadband, 10 MHz pulses at a rate up to 10 KHz. This transducer uses the synthetic receive aperture technique to record echoes while being mechanically steered by a linear positioner under microprocessor control. Specifically, a 200 Vpp signal is applied to all rings simultaneously on transmission. Individual rings are then sequentially multiplexed to a receiver. Echoes are dynamically delayed and coherently summed off-line to adjust the receive focus and extend the depth of focus. The aperture material is a 1-3 composite built by Imasonic SA, Besancon, France. One advantage of the design is that it provides a well-focused axisymmetric beam with an improved depth of focus to acquire images at a high spatial resolution. Pulse-echo simulations of our array using the Field II software package show a -6dB beam width at 0.24 mm and -6dB depth of focus at 2.4 mm that can be extended to 3.2 mm with dynamic focusing. These simulations agree with later measurements performed on the transducer. Our flexible aperture design allows us to drop outer rings significantly increasing the depth of focus (up to 56% increase by dropping 3 rings) with a tolerable decrease in lateral resolution (27% increase in beam width). We expect that our probe will enable us to examine detailed biological processes throughout the malignant growth period of a tumor tissue that exhibits elastic anisotropy, thus providing high resolution ultrasound images over an extended and adjustable depth of field.