Synthetic aperture ultrasound
Compared with the conventional ultrasound image formation where one transducer or linear array is used, SAU imaging has achieved higher lateral resolution and deeper penetration, which will enable a more accurate diagnosis in medical applications, with no obvious loss in frame rate and without a large burden in computational complexities.
These are processes whereby the pulse-echo responses from individual pairs of elements are synthesized to reconstruct the formation and focusing, relying on the rule of linear superposition.
It thus saves large computational requirements for delay-and-sum beamforming and leaves space for increasing the frame rate.
Also the single transmission and receiving requirement for each firing significantly reduces the hardware complexity for system implementations.
[3] The basic idea of SAU originated from synthetic aperture radar (SAR) and sonar, where the motion of antenna is used over a region around the target to generate a higher resolution image of the object.
[5] However, at that time, SAU implementations were restricted due to the lack of powerful computational machines at a reasonable cost and size.
[9] Assuming the lth focal line in the low-resolution image acquired from the ith element's firing is represented as: Here,
[3] Passman and Ermert proposed using transmit focal points as virtual sources and this was further investigated in SAU by later researchers.
SAU imaging can be performed without regard to whether or not the element actually exists, after the aperture angle of the transducer was determined.
[11][12] Kortbek has put forward a sequential beamforming method to reduce the complexity and improve the simplicity of the hardware implementation.
For a linear array transducer with multiple elements, the lateral resolution of sequential beamforming SAU can be made more range independent and significantly improved compared to conventional dynamic transmit and receive focusing.
[13][14] The bi-directional pixel-based focusing (BiPBF) method was proposed to solve the problem that SAU imaging suffers from low SNR as the transmission is done by a small part of the array.
The pixel-based time delays used for compounding are calculated using the distances between pixels and virtual sources located at the successive lateral positions of the transmission focus.
[1] A better performance of these methods and devices will be achieved as the technology is improving in terms of higher computation speed and smaller size.
