Ultrasound (US) imaging, in contrast to other image guidance techniques, offers the distinct advantage of providing volumetric image data in real-time (4D) without using ionizing radiation. The goal of this study was to perform the first quantitative comparison of three different 4D US systems with fast matrix array probes and real-time data streaming regarding their target tracking accuracy and system latency. Sinusoidal motion of varying amplitudes and frequencies was used to simulate breathing motion with a robotic arm and a static US phantom. US volumes and robot positions were acquired online and stored for retrospective analysis. A template matching approach was used for target localization in the US data. Target motion measured in US was compared to the reference trajectory performed by the robot to determine localization accuracy and system latency. Using the robotic setup, all investigated 4D US systems could detect a moving target with sub-millimeter accuracy. However, especially high system latency increased tracking errors substantially and should be compensated with prediction algorithms for respiratory motion compensation.
Ultrasound (US) imaging is a fast, low-risk imaging modality with excellent soft tissue contrast not relying on ionizing radiation. It is therefore an ideal candidate for image guidance of therapeutic interventions such as needle biopsies, tissue ablations or radiotherapy. Most US guidance systems to date are limited to using 2D images, mainly due to the accessibility of systems and data. However, US imaging also offers the unique capability to acquire true volumetric data in real-time (4D US) – a distinct advantage compared to other imaging modalities, especially when monitoring dynamic tissue or instrument motion often affected by 3D translation, rotation and even deformation .
In radiotherapy, the only 4D US guidance system in clinical use  utilizes a US probe with mechanical image acquisition and thus limited temporal resolution. While this system has been shown to be suitable for monitoring slow-moving structures such as the prostate  or the liver during deep inspiration breath-hold , the slow acquisition speed prohibits tracking of fast motion, such as breathing or pulsation, in real-time . Matrix array probes, on the other hand, do not rely on mechanical parts and can therefore achieve much higher volumetric framerates . With previous studies showing the important role of low latency for high-accuracy motion compensation , , matrix array systems are ideally suited for image guidance tasks. Several systems have already been investigated in isolated proof-of-concept studies, e.g., , , yet a quantitative comparison of different potentially suitable systems – especially regarding their real-time tracking capabilities – has not been conducted to date.
The aim of this study is to assess and compare three different 4D US systems which can be accessed via real-time streaming interfaces, the prerequisite for online image guidance. The quantitative comparison between these systems could potentially reveal their individual strengths as well as indicate the relevance of different parameters for real-time tracking performance.
For the quantitative comparison of different 4D US systems regarding their target localization accuracy and overall latency, a dynamic phantom study was conducted.
Three 4D US systems with matrix array probes shown in Figure 1 were included in this study: (1) Vivid 7, 3V-D probe (GE Healthcare), (2) Vivid E95, 4Vc-D probe (GE Healthcare) (3) Epiq 7, X6-1 probe (Philips Healthcare). The respective volume sizes were chosen to approximate the typical clinical setting of 58 × 45° in  as closely as possible, depending on the available system settings (see Table 1).
|System||Acquisition setting (framerate)||Volume size (16 cm depth)||Voxel size, mm³|
|Vivid 7||N/a (17 Hz)||60° × 25°||1.0 × 1.0 × 1.0|
|Vivid E95||R (8 Hz)||60° × 40°||1.0 × 1.0 × 0.9|
|S (38 Hz)||60° × 40°||1.0 × 1.0 × 0.9|
|Epiq 7||R (3 Hz)||60° × 45°||0.6 × 1.0 × 0.6|
|S (9 Hz)||60° × 45°||1.0 × 1.4 × 0.7|
Two different acquisition settings (see Figure 1) were used to investigate the impact of spatiotemporal resolution: (R) high spatial resolution, low acquisition speed, and (S) low resolution, high speed. Vivid 7 only offered a single setting. All systems featured software interfaces (Vivid 7: custom, E95 & Epiq 7: provided by manufacturer) for streaming the volumetric US data via Ethernet to an external computer.
As shown in Figure 2, the US probes were attached to a collaborative robotic arm (Panda, Franka Emika) with seven degrees of freedom. The probes were moved relative to a static US phantom (CIRS Inc., see Figures 1 and 2) using four sinusoidal motion types with amplitude A and period length T: I) A=10 mm, T=2.5 s; II) A=10 mm, T=5.0 s; III) A=35 mm, T=2.5 s; IV) A=35 mm, T=5.0 s, covering a typical liver motion range . The main axis of motion was in lateral direction relative to the US probes.
Data acquisition and analysis
Robot positions and US volumes were acquired for a duration of 60 sec and stored on an external computer, using a common clock for synchronized timestamps, for retrospective analysis. Robot positions were logged at 100 Hz.
The position of a spherical target embedded within the US phantom, shown in Figure 1, was detected in the US data with template matching (mean processing time 100 ms/volume, unoptimized Matlab implementation). Robot and US motion traces were then spatially aligned using a rigid landmark transform to explicitly exclude the effects of calibration inaccuracies on tracking performance. By defining the robot positions as ground truth for the tracking experiments, the mean root-mean-squared error (RMSE) between the corresponding data points was calculated for two scenarios: (i) excl. errors from system latency (‘best case’), i.e., latency between robot and US compensated by shifting US trace and (ii) incl. errors from latency, i.e., overall tracking accuracy.
Results and discussion
Continuous monitoring of target motion resembling a respiratory pattern was feasible with all investigated 4D US systems and acquisition settings. Figure 3 shows a typical target trajectory and the corresponding RMSE over time. Compared to the relatively smooth robot trajectory, the target trajectories from US follow discrete steps mainly due to the temporal resolution but also the finite voxel size of the US volume data (see Table 1), causing the saw-tooth shape of the RMSE. Using a sub-voxel matching approach could reduce this effect, however at the cost of higher runtime.
The quantitative results, summarized in Table 2, show that target localization in 4D US is possible with sub-millimeter accuracy if system latency is compensated, confirming prior studies on US image guidance in radiation therapy (e.g., , ). With latency compensated, Vivid 7 achieved the lowest mean RMSE of 0.6 ± 0.1 mm, followed by 0.7 ± 0.3 mm (Epiq 7) and 0.9 ± 0.5 mm (E95). The following sections discuss the individual impact of system latency, motion type and image quality on the overall tracking accuracy.
|System||Acqu. setting||Latency, ms||RMSE, mm Lat. comp.||RMSE, mm No lat. comp.|
|Vivid 7||n/a||199 ± 8||0.6 ± 0.1||2.8 ± 1.6|
|E95||R||343 ± 22||0.9 ± 0.5||4.5 ± 2.7|
|S||207 ± 35||1.1 ± 0.6||2.7 ± 1.5|
|Epiq 7||R||487 ± 13||1.3 ± 0.7||6.5 ± 4.1|
|S||211 ± 26||0.7 ± 0.3||2.4 ± 1.6|
Impact of latency
Overall system latency includes the acquisition and reconstruction time of the US volume as well as data transfer over the network to the control PC and conversion into Cartesian coordinates. The impact of latency was assessed by comparing tracking errors (i) with and (ii) without latency compensated. Using retrospectively aligned traces, (i) was calculated by removing the temporal offset between robot and US trajectories, while (ii) was based on the original trajectories and timestamps.
Latency had by far the strongest impact on US tracking accuracy in this study, with Vivid 7 achieving the lowest overall latency and the lowest RMSE. With latency left uncompensated (ii), Vivid 7’s RMSE increased by 340% to 2.8 mm. Comparable latency values of ∼200 ms (resolution setting ‘S’) caused a mean RMSE increase of 150% (2.7 mm) and 200% (2.37 mm) in E95 and Epiq 7, respectively. For slower acquisitions (‘R’), mean latencies of 340 ms/485 ms and error increases of up to 370%/395% were measured for E95 and Epiq 7, clearly highlighting the central role of latency in tracking scenarios.
It was also observed that, despite showing the fastest raw volumetric framerate among the three systems (38 Hz, ‘S’), E95 only reached 16 Hz via network streaming in this study. The additional latency was likely caused by a combination of network transfer delays, the event handling in our custom control software and the integrated Cartesian conversion of the volume data in the proprietary streaming software, requiring further investigation. While a custom streaming interface (as developed for Vivid 7 ) offers the advantage of being completely accessible and freely modifiable for optimization, it also has the clear disadvantage of not being an official product endorsed by the manufacturer.
Impact of motion type
Aside from system latency, motion amplitude and speed also substantially affected US tracking accuracy. The detailed results illustrated in Figure 4 show that slow target motion with a low amplitude (motion type ‘II’) led to lower RMSE values compared to fast, high-amplitude motion (type ‘III’) for all systems.
A more detailed analysis showed that an increase of A from 15 to 35 mm led to a rise in RMSE between 25 and 215%, while decreasing T from 5.0 to 2.5 s (i.e., increasing target speed) led to an increase in tracking errors of 20–80%. In both cases, RMSE increase was higher when the US framerate was lower since a higher amount of target motion would be missed between frames.
Impact of image quality
Image quality did not affect tracking accuracy substantially in this phantom-based study. The highest tracking accuracy was achieved by the oldest of the three systems (Vivid 7), despite the superior image quality of Epiq 7 indicated in a prior study . At the same time, the experiments conducted with the low spatial resolution setting (‘S’) generally outperformed the high-resolution results (‘R’), indicating that latency played a much bigger role than image quality. However, it should be noted that image quality might have a stronger impact in a more realistic anatomical tracking scenario with real patients.
This is the first study to directly compare different 4D US systems regarding their tracking performance using real-time streaming interfaces for volumetric data acquisition. In our phantom experiments, sub-millimeter tracking accuracy could be achieved with all investigated systems, even for fast target motion. Higher overall latency (i.e., lower US framerates), faster target motion and larger motion amplitudes increased the tracking error by up to almost 400%. Since system latency showed the most pronounced impact on tracking errors, especially for fast motion with high amplitudes, 4D US guidance should ideally be combined with prediction algorithms to account for latency-induced errors, especially when compensating for fast respiratory or pulsatory motion.
Funding source: BMBF
Award Identifier / Grant number: NavEVAR
Funding source: MVWATT
Award Identifier / Grant number: E95
Research funding: The following funding sources were involved in this study: BMBF grant ‘NavEVAR’ (Epiq 7), MWVATT of the State of S-H (E95, Panda).
Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
Competing interests: The 4Vc-D probe was provided by GE Healthcare.
Informed consent: n/a.
Ethical approval: The conducted research is not related to either human or animals use.
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© 2020 Svenja Ipsen et al., published by De Gruyter, Berlin/Boston
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