A modified synthetic aperture imaging approach with axial motion compensation

Synthetic aperture (SA) imaging provides an alternate mean of obtaining ultrasound images. However, since this approach is based on coherent summation of low-resolution images (LRIs) acquired from different point sources along an array, its image quality may be degraded if motion is present in between firings. In this work, we report a modified SA imaging scheme that can compensate for the effects of axial motion on the image quality. The scheme first acquires data by making use of an interleaved firing sequence where a center-point firing is carried out in between each point-source firing. It then estimates the mean axial shift between two LRIs by performing a cross-correlation analysis on the raw channel data of successive center-point firings. The LRI of each point-source firing is then axially counter-shifted by the estimated shift value to compensate for possible aberration during SA image formation. To test out our proposed scheme, we conducted a tissue-cyst phantom imaging experiment, where raw channel data was acquired using our interleaved firing scheme for 97 virtual point sources laterally spaced apart at 0.3 mm and axially located at 10 mm behind the probe. This data acquisition procedure was repeated for a range of inter-firing probe displacements (15-90 mum) introduced via a motion stage. From the acquired data, motion-compensated SA images were formed using our modified image formation method, and their contrast was compared to those formed without motion compensation. Results show that our proposed scheme reduced the amount of blurring seen in SA images when uniform axial motion is present during data acquisition. Without motion compensation, the contrast level for the phantom cysts can drop by 15-20 dB (relative to the SA image taken without motion) for the range of inter-firing displacements examined. When our motion compensation strategy was applied, this contrast drop was less than 5 dB.