Real-time target localization in ultrasound is useful in many clinical and scientific areas. For example in radiation therapy tumors can be localized in real-time and irradiated with a high accuracy. To measure the position of an ultrasound target in a global coordinate system or to extend the tracking volume by moving the ultrasound transducer an optical marker is attached to it and observed by an optical tracking system. The necessary calibration matrices from marker to ultrasound volume are obtained using hand-eye calibration algorithms which take sets of corresponding observations of the optical marker and an ultrasound target as input. The quality of these calibration matrices is highly dependent on the measured observations. While the accuracy of optical tracking systems is very high, accurate tracking in ultrasound is difficult because of the low resolution of the ultrasound volume, artifacts and noise. Therefore accurate hand-eye calibration is difficult between ultrasound and optical tracking systems. We have tested different phantoms, matching- and sub-pixel strategies to provide highly accurate tracking results in 3D ultrasound volumes as basis for hand-eye calibration. Tests have shown that - using the described methods - calibration results with RMS errors of less than 1mm between observed and calibrated targets can be reached.
With the recent enhancements in imaging quality and speed three-dimensional
ultrasound has become attractive for automatic guidance during robotized
interventions and high-accuracy target tracking applications. We use a modified
GE Vivid 7 dimension 3D cardiovascular ultrasound station for real-time volume
processing and direct target localization in radio-surgery [
To map the obtained target positions to world coordinates the ultrasound
transducer is itself tracked using an attached optical marker and an accuTrack
250 (atracsys LLC, Bottens, CH) optical tracking system to locate the marker.
The position of the ultrasound target in world coordinates can be calculated
using both tracking results and the static translation between ultrasound
transducer and optical marker. Among other strategies the best results for this
translation are usually obtained [
One problem of hand-eye calibration is that the quality of the measured position sets highly influences the calibration result. While the measurements of optical tracking systems are highly accurate, tracking in ultrasound is difficult because of the low resolution of the volume data, artifacts and noise. To obtain an accurate calibration this tracking has to be optimized. 2
A calibration setup is used (Fig. 1). An application has been developed to capture both ultrasound and optical tracking results. To obtain corresponding datasets from both systems the capture time of both measurements is calculated using high-accuracy timestamps and an estimate for the system latencies. Then the optical tracking result is chosen to meet the capture time of the corresponding ultrasound volume. For an intuitive handling during hand-held sample acquisition an automatic capture trigger algorithm has been implemented besides the manually triggered capture process. This algorithm triggers in phases with minimal movements in both tracking results. In this way motion artifacts in the measured tracking results are further minimized. After completion of a predefined sample count the result of the hand-eye calibration is calculated and the distance error between the calibration and the measured points is computed. Depending on these values an iterative post-processing algorithm identifies up to ten percent of the samples with high distances, eliminates these samples and recalculates the tracking result. In this way errors such as wrong target detections in ultrasound are eliminated.
Two types of phantoms are used for tracking in ultrasound: A single target (Figure 1a) is used to maximize tracking accuracy while lateron a complex phantom (Fig. 1b) with multiple features is uses to increase targeting accuracy and reduce the amount of necessary count of measured transducer positions. 2.1
A lead bulb on a nylon wire in a liquid tank is used as target. The lead bulb reflects a high amount of ultrasound and can be easily detected in the ultrasound volume using a maximum intensity search. As our ultrasound volume has a worst-case resolution of more than 1.5mm per pixel this method is not sufficient for high-accuracy tracking. To overcome this limitation we use cubic splines to interpolate the target region around a detected position with maximum intensity. The extreme value of the interpolated volume is used as sub-pixel approximation of the lead bulb position. Another tracking possibility is template matching. Using a predefined pattern of the lead bulb, the sum of square distances (SSD) is calculated to find an optimal match between the pattern and the lead bulb. With sub-pixel enhancement through iterative interpolation of the target region a high accuracy tracking result can be optained.
Figure 3b shows the different tracking positions for a two-dimensional slice of the target. To test the sub-pixel quality of each strategy a lead bulb is moved through a liquid tank on a linear trajectory while being continuously tracked (Figure 3c). While the tracking accuracy is increased with both strategies, template matching shows the best linearity and highest accuracy. 2.2
Three-dimensional ultrasound offers the possibility to track multiple targets inside one volume simultanously. We use a complex wire phantom (Dansk Fantom
Service, Fig. 4a) to find a defined geometry of twisted nylon wires inside the volume. The wires are clearly visible in the ultrasound volume and can be easily tracked using a repeated maximum intensity search with an additional distance criterium, so that features in close proximity to already located points are not detected. Alternatively template matching can be used with multiple, predefined patterns for wires in different angles to the beam.
The obtained feature positions are matched against a predefined virtual
phantom or the first captured phantom using the ICP algorithm [
To measure the calibration quality we compute the RMS distance error between all calibrated and measured target points. Table 1 shows this quality value for the different calibration scenarios. For the single target an automatic calibration with 100 captured points is performed without manual interaction. Measurements at three different positions with 50 located targets in each volume are performed for the complex phantom. Both calibration methods show acceptable results with RMS errors of less than 2mm with standard resolution. With high-accuracy ultrasound target localization both methods can be significantly enhanced. For single targets the RMS errors can be reduced to less than 1mm using only half of captured points. As ICP is very sensitive to position errors, the accuracy gain for complex phantom calibration due to better target localization is even higher. 4
Two calibration methods are implemented and enhanced with sub-pixel target localization. Especially with complex phantoms a stable calibration can be performed in only three steps which makes the method attractive for common use.