pages: Reproducibility of freehand calibrations    for ultrasound-guided needle navigation   Sydney Perrin, Zachary Baum, Mark Asselin, Grace Underwood, Saleh Choueib,  Hillary Lia, Tamas Ungi, Andras Lasso, Gabor Fichtinger  Laboratory for Percutaneous Surgery, School of Computing,  Queen’s University, Kingston,  Canada  ABSTRACT    PURPOSE : Spatially tracked ultrasound-guided needle insertions may require electromagnetic sensors to be clipped on  the needle and ultrasound probe if not already embedded in the tools. It is assumed that switching the electromagnetic  sensor clip does not impact the accuracy of the computed calibration. We propose an experimental process to determine  whether or not devices should be calibrated on a more frequent basis.   METHODS : We performed 250 calibrations. Of these, 125 were performed on the needle and 125 on the ultrasound.  Every five calibrations, the tracking clip was removed and reattached. Every 25 calibrations, the tracking clip was  exchanged for an identical 3D-printed model. From the resulting transform matrices, coordinate transformations were  computed. Data reproducibility was analyzed through looking at the difference between mean and grand mean, standard  deviation and the Shapiro-Wilks normality constant. Data was graphically displayed to visualize differences in calibrations  in different directions.   RESULTS : For the needle calibrations, transformations parallel to the tracking clip and perpendicular to the needle  demonstrated the greatest deviation. For the ultrasound calibrations, transformations perpendicular to the sound  propagation demonstrated the greatest deviation.   CONCLUSION : Needle and ultrasound calibrations are reproducible when changing the tracking clip. These devices do  not need to be calibrated on a more frequent basis. Caution should be taken to minimize confounding variables such as  bending the needle or ultrasound beam width at the time of calibration. 
  Canada  ABSTRACT   
  KEY WORDS:  Calibration, reproducibility, ultrasound, spatial tracking, freehand. 
  1.   INTRODUCTION 
  Ultrasound (US)-guided needle insertions are a common image-guided medical procedure. The US image is used to  visually guide the needle to the intended destination. Possible applications of US-guided needle insertions include biopsies  and drug deliveries. Benefits include increased work speed and fewer needle reinsertions. Spatial tracking of the needle  and US probe allows the needle tip to be readily located. Electromagnetic (EM) tracking systems may be used as the  tracking method. In EM tracking a transmitter generates an EM field that EM sensors detect. The needle and US probe  require spatial calibration for the tracking to be accurate. Calibration is the process of computing the position of the device  relative to the tracked sensors so the system can determine the spatial position of the device. Pivot calibration is the process  of computing the position of the needle tip relative to the needle-mounted EM sensor. US calibration is the process of  computing the position of the image relative to the EM sensor attached to the US transducer. 
  Electromagnetic sensors are not always embedded in clinical tools. In this case, tracking clips must be used to affix the  EM sensors in place (Figure 1). Devices are calibrated once and may be used for a prolonged period of time without  recalibration. Users of EM-navigated procedures may adjust the clip for a variety of purposes, such as swapping out broken  or damaged clips, or for sanitization. It is assumed that the computed calibration can be reused when an identical clip is  changed. There is no established recommendation for tracking clip calibrations. Below, we will present preliminary data  to aid in producing such recommendations.  Ultrasound calibration accuracy has been extensively studied. <sup>1,2</sup>  A particular study under EM tracking measured the  variance in spatial US calibrations with a pointer tool. <sup>2</sup>  This study found a variance of x: 0.10 mm, y: 0.10 mm and z: 0.43  mm. <sup>2</sup>  However, this study did not analyze the potential confounding variable of changing the tracking clip and can be used  as comparative data for our study. Data was not found on the consistency of open-source (3D-printed) or commercially  available general purpose tracking clips.  <fig rect=[63.02399826049805, 70.87210845947266, 551.5494384765625, 164.69879150390625]></fig>
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  <fig rect=(116.25, 201.989990234375, 495.6000061035156, 538.5399780273438)></fig> beam width and the focal point when calibrating to ensure reproducibility between calibrations. Consistency may change  across different types of tracking clips than those used in our experiments.  REFERENCES  [1] Mercier, L., Lango, T., Lindseth, F., Collins,  D.L., “A review of calibration techniques for freehand 3 -D ultrasound  systems.” Ultrasound Med Biol.,  vol. 2018, pp. 449-71 (2004).   [2] Welch, M., Andrea, J., Ungi, T., Fichtinger,  G., “Freehand ultrasound calibration: phantom versus tracked pointer”,  Proceedings of SPIE  –  The International Society for Optical Engineering, vol. 8671 (2013).  [3] Lasso, A., Heffter, T., Rankin, A., Pinter, C., Ungi, T., Fichtinger,  G. “PLUS: Open -source toolkit for ultrasound- guided intervention systems”, IEEE Transactions on Biomedical Engineering,  vol. 61, pp. 2527-2537 (2014).   [4] Chen,  T., “AN AUTOMATED ULTRASOUND CALIBRATION FRAMEWORK INCORPORATING ELEVATION  BEAMWIDTH FOR TRAC KED UTLRASOUND INTERVENTIONS” , Ph.D. thesis,  Queen’s University (2012).   [5] Sadjadi, H., Hashtrudi-Zaad, K., Fichtinger,  G., “Simultaneous Electromagnetic Tracking and Calibration for Dynamic  Field Distortion Compensation”, IEEE Transactions on Biomedical  Computing, vol. 63, pp. 1771-1781 (2015) 
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