Systematic analysis of volumetric ultrasound parameters for markerless 4D motion tracking

Abstract

Objectives: Motion compensation is an interesting approach to improve treatments of moving structures. For example, target motion can substantially affect dose delivery in radiation therapy, where methods to detect and mitigate the motion are widely used. Recent advances in fast, volumetric ultrasound have rekindled the interest in ultrasound for motion tracking. We present a setup to evaluate ultrasound based motion tracking and we study the effect of imaging rate and motion artifacts on its performance.

Methods: We describe an experimental setup to acquire markerless 4D ultrasound data with precise ground truth from a robot and evaluate different real-world trajectories and system settings toward accurate motion estimation. We analyze motion artifacts in continuously acquired data by comparing to data recorded in a step-and-shoot fashion. Furthermore, we investigate the trade-off between the imaging frequency and resolution.

Results: The mean tracking errors show that continuously acquired data leads to similar results as data acquired in a step-and-shoot fashion. We report mean tracking errors up to 2.01 mm and 1.36 mm on the continuous data for the lower and higher resolution, respectively, while step-and-shoot data leads to mean tracking errors of 2.52 mm and 0.98 mm.

Conclusions: We perform a quantitative analysis of different system settings for motion tracking with 4D ultrasound. We can show that precise tracking is feasible and additional motion in continuously acquired data does not impair the tracking. Moreover, the analysis of the frequency resolution trade-off shows that a high imaging resolution is beneficial in ultrasound tracking.

Keywords: Image guidance; Motion estimation; Radiotherapy; Tracking; Ultrasound.

Fig. 1

Our experimental setup (a). The US probe (1) is mounted to the robot (2) and positioned above the water tank. The liver is fixated to foam to prevent it from moving or floating (b)

Fig. 2

General setup for US tracking during radiotherapy. The US probe is connected to a robot and placed on the patient for contact during image acquisition

Fig. 3

Exemplary trajectory for patient liver motion during free breathing in radiotherapy. The motion for x (blue), y (yellow) and z (red) is shown (a) as well as the main motion component of the three dimensions after applying a PCA (b)

Fig. 4

Slices from US volumes showing the spherical marker (a and b) and exemplary bovine tissue (c and d) with 8 $\times$ 8 beams and 16 $\times$ 16 beams, respectively. The red boxes indicate the crop used for tracking

Fig. 5

Results for NCC (red, dotted) and MOSSE (green, dotted) for step-and-shoot data set with the ground truth (blue). The main motion component of the trajectories and tracking results are shown for trajectory 4 for $16\times 16$ beams (a) and $8\times 8$ beams (b)

Fig. 6

Results for NCC (red, dotted) and MOSSE (green, dotted) for continuous data with the ground truth (blue). The main motion component of the trajectories and tracking results is shown for trajectory 4 for $16\times 16$ beams (a) and $8\times 8$ beams (b)

Fig. 7

Results for NCC (red, dotted) and MOSSE (green, dotted) for continuous data with the ground truth (blue). The main motion component of the trajectories and tracking results are shown for trajectory 3 for $16\times 16$ beams (a) and $8\times 8$ beams (b)