Synthetic Aperture Imaging in Medical Ultrasound

Synthetic Aperture (SA) ultrasound imaging is a relatively new and unexploited imaging technique. The images are perfectly focused both in transmit and receive, and have a better resolution and higher dynamic range than conventional ultrasound images. The blood flow can be estimated from SA images with high precision, and the imaging is easily extended to real-time 3D scanning. This paper presents the work done at the Center for Fast Ultrasound Imaging in the area of SA imaging. Three areas that benefit from SA imaging are described. Firstly a preliminary in-vivo evaluation comparing conventional B-mode imaging with STAI is presented. In this study 7 male volunteers were scanned abdominally, and resulting images were compared by 3 medical doctors using randomized blinded presentation. All 3 examiners scored a significant better image quality in STA images (P-value <0.001). Secondly the performance of a flow estimator is evaluated in a flow rig and in-vivo scans are shown. The method uses beamforming along the flow durection, which has the advantage that the signals are strongly correlated. A 7.5 MHz commercial transducer was used to measure the flow in a plastic tube with an entrance length of 1 m an a diameter of 17 mm. The velocity profile was measured for flow angles of 60 and 90 degrees. The standard deviation of the measurements was 0.36% (0.65 mm/s) for 60 degrees and 1.2 % (2.1 mm/s) for the purely transverse flow. Finally the application of SA imaging to real-time 3D scanning is presented. The method uses a combination of a rotating transducer and STA imaging. Due to the rotation of the transducer, a 2D array is synthesized. Images from point targets in a tissue mimicking phantom show that the -6 dB resolution is 1.5 deg. in both elevation and azimuth planes, and that the side-lobes are 50 dB below the peak. The system can potentially acquire 10 fully focused volumes per second.