Real-time volumetric ultrasound imaging using free hand scanning

Collecting high quality volumetric ultrasound (US) data using freehand scanning is challenging. The quality of the final 3DUS image is highly related to the applied scanning protocol and the subsequently used reconstruction method. The protocol should ensure the sonographer collects sufficient data of satisfactory quality for an accurate reconstruction. In this study we developed a real-time reconstruction method that provides visual feedback during scanning. The feedback indicates the areas, of which the sonographer should collect more data. The method was tested by acquiring US data of a breast phantom in a setup mimicking freehand scanning which consisted of a linear transducer mounted in a translation stage that also allowed rotation. To reconstruct the volume in real-time on a target grid of 0.5x0.5x0.5mm, we applied a simplified Voxel Nearest Neighbor (VNN) method, i.e., only the closest to B-mode plane voxels were updated. Furthermore, voxels were updated only when their projection on the B-mode plane was closer to the transducer surface than in the previous scan planes. Interpolation was performed within the acquired volume to fill in the holes where sufficient data were available. Sub-volumes with insufficient data were visualized in the reconstructed volume (update rate 50 Hz). This visual feedback can guide the sonographer during freehand scanning to improve the quality of the reconstructed 3DUS images. Cross-sections of the reconstructed data were compared to the independently acquired B-mode images and confirmed that our real-time method of low computational complexity provided accurate volumetric ultrasound images.