The present embodiments relate to synthetic aperture beam forming. Broad transmit beams with synthetic aperture processing assist three-dimensional imaging.
Three-dimensional real-time imaging uses complex ultrasonic transducer arrays, transmit circuitry and/or receive circuitry. Simple array construction, sufficient power/acoustic transmission and density of electrical connection may be difficult to achieve.
In one approach for three-dimensional imaging, a one-dimensional array electronically steers along one dimension and is mechanically steered along another dimension (i.e., wobbler array). However, the mechanical scan may limit the speed for scanning an entire volume. The mechanism for moving the array may be large and/or complex, resulting in loss of comfort for the user or reduction in life of use.
In another approach, a one-dimensional array is translated or rotated by the user. This free hand scanning may degrade resolution. The position of the array is estimated or measured for rendering. The estimation may be inaccurate. The measurements may require additional hardware for use.
In another approach, elements of a two-dimensional array of discrete elements operate for both transmit and receive functions. Many limitations and complexities arise from such an approach. Each element is connected with transmit circuitry that is capable of supporting high drive voltages and power levels required to adequately insonify the area of interest. For a relatively small array of 32 by 32 elements, 1024 connections are needed. Direct connection of the elements with the system electronics via a cable bundle is impractical, so drive electronics may be positioned in the transducer housing. Positioning drive electronics in the transducer housing complicates packaging, and thermal dissipation issues limit the complexity of the drive electronics.
For receive operation, each element drives connecting cables or electronics. Pre-amplification near the elements may provide sufficient drive capability. Elements of an N by M array may be smaller than a typical one-dimensional array, making impedance matching of a single layer piezo-ceramic element to the transmission line very poor. Multilayered two-dimensional arrays may better match the impedance of the transmission line, but are difficult to construct. Additionally, the receive electronics must be isolated from the high voltage drive circuitry, requiring some type of transmit/receive switching at the array, diode isolation circuits or connections to both sides of the piezoceramic element, doubling the number of necessary connections.
Making a connection to each of several thousand transducer elements and placing beam-forming electronics in the transducer to reduce the number of cables may be achieved. Where partial beamforming within the transducer limits the number of cables, less channel information is provided. The available data for the imaging system is the partial beams. However, availability of the raw channel data may be important in a variety of clinical scenarios.
The speed of acquisition may be limited by requirements of spatial beam sampling and the sound speed in tissue. For a square transducer, if M beams fill a plane, at least M2 beams fill a volume. A typical beam is 2 wavelengths wide, and a typical transducer may be 200 wavelengths long, giving M=100. A typical beam requires 0.2 ms to acquire. If Z focal zones are needed to improve coherence, a total of M2Z2 firings make up a volumetric image. For typical imaging depths, this results in a maximum imaging speed of 0.5/Z2 volumes per second. This speed may be insufficient for real-time three-dimensional imaging.
One technique to increase acquisition speed is used for two-dimensional imaging. Synthetic transmit aperture imaging or co-array imaging uses broad transmit beams to insonify an entire area of interest. Receive beams within the area of interest are formed in response to the single transmission.