Standardized accuracy assessment of the calypso wireless transponder tracking system

Abstract

Electromagnetic (EM) tracking allows localization of small EM sensors in a magnetic field of known geometry without line-of-sight. However, this technique requires a cable connection to the tracked object. A wireless alternative based on magnetic fields, referred to as transponder tracking, has been proposed by several authors. Although most of the transponder tracking systems are still in an early stage of development and not ready for clinical use yet, Varian Medical Systems Inc. (Palo Alto, California, USA) presented the Calypso system for tumor tracking in radiation therapy which includes transponder technology. But it has not been used for computer-assisted interventions (CAI) in general or been assessed for accuracy in a standardized manner, so far. In this study, we apply a standardized assessment protocol presented by Hummel et al (2005 Med. Phys. 32 2371-9) to the Calypso system for the first time. The results show that transponder tracking with the Calypso system provides a precision and accuracy below 1 mm in ideal clinical environments, which is comparable with other EM tracking systems. Similar to other systems the tracking accuracy was affected by metallic distortion, which led to errors of up to 3.2 mm. The potential of the wireless transponder tracking technology for use in many future CAI applications can be regarded as extremely high.

Monitoring of breathing motion in image-guided PBS proton therapy: comparative analysis of optical and electromagnetic technologies

“…More recently, electromagnetic tracking has also been applied in radiotherapy to localize wireless internal transponders using the ‘Calypso GPS system for the body ® ’ system (Varian Medical Systems Inc., Salt Lake City, US) [35, 36]. However, the need to maintain a large antenna close to the patient complicates its integration in proton therapy, where all the materials present in the treatment field have to be accurately qualified.…”

Assessment of Electromagnetic Tracking Accuracy for Endoscopic Ultrasound

“…2 (b)). The grid positions were the closest to the endoscope, thus more likely to affect the tracking accuracy [8,13,[18][19][20]. Additionally, the endoscope contained the shape tool in the working channel, and its position was measured as a reference of the distance between the two tracking tools.…”

“…The most common approach was proposed by Hummel et al [8] and has been used to assess new EM systems [12,13], the accuracy of sensors mounted on US probes [14,15], and also for optimization [16]. This assessment protocol employs a machined base plate to measure positional and rotational tracking data, offering simplicity, reproducibility, a high precision ground truth and accuracy.…”

“…Despite the growing popularity of EM tracking in interventional applications, significant tracking errors due to metallic objects (i.e., steel, aluminium and bronze) placed between the emitter and the sensor, and the use of some electronic devices, such as a C-arm unit, have been reported to cause disturbances to the magnetic field below 4.2 mm [8,13,[17][18][19][20]. Such errors can be particularly prevalent in clinical environments and their sources difficult to control.…”

Validation of the Calypso Surface Beacon Transponder

“…The ability to track a nonsite-specific, externally placed Beacon allows for noninvasive tracking of deep inhalation breath-hold (DIBH) and free breathing, as well as general patient motion. Calypso system positioning has been previously assessed for static accuracy and precision, (1,2,3) as well as the influence of motion on tracking accuracy. (4) Although previous work has demonstrated the Calypso 4D tracking system provides greater accuracy than alternative tracking methods, (5) these previous studies focus on tracking of three implanted Beacons.…”