Ultrasound super-resolution imaging with a hierarchical Kalman tracker

Microbubble (MB) tracking plays an important role in ultrasound super-resolution imaging (SRI) by enabling velocity estimation and improving image quality. This work presents a new hierarchical Kalman (HK) tracker to achieve better performance at scenarios with high concentrations of MBs and high localization uncertainty. The method attempts to follow MBs with different velocity ranges using different Kalman filters. An extended simulation framework for evaluating trackers is also presented and used for comparison of the proposed HK tracker with the nearest-neighbor (NN) and Kalman (K) trackers. The HK tracks were most similar to the ground truth with the highest Jaccard similarity coefficient in 79% of the scenarios and the lowest root-mean square error in 72% of the scenarios. The HK tracker reconstructed vessels with a more accurate diameter. In a scenario with an uncertainty of 51.2 mu m in MB localization, a vessel diameter of 250 mu m was estimated as 257 mu m by HK tracker, compared with 329 mu m and 389 mu m for the K and NN trackers. In the same scenario, the HK tracker estimated MB velocities with a relative bias down to 1.7% and a relative standard deviation down to 8.3%. Finally, the different tracking techniques were applied to in vivo data from rat kidneys, and trends similar to the simulations were observed. Conclusively, the results showed an improvement in tracking performance, when the HK tracker was employed in comparison with the NN and K trackers.

Automated summary

This study presents a new hierarchical Kalman (HK) tracker that can achieve better performance in scenarios with high concentrations of microbubbles and high localization uncertainty. The results show that the HK tracker is most similar to the ground truth in most scenarios and has the lowest root-mean-square error compared to the nearest-neighbor (NN) and Kalman (K) trackers. The HK tracker also provides more accurate vessel diameter reconstruction and better estimation of microbubble velocities. The tracking performance of the HK tracker is observed to be improved in in vivo experiments as well.