Improving N-wire phantom-based freehand ultrasound calibration

  • Guillermo Carbajal
  • Andras Lasso
  • Álvaro Gómez
  • Gabor Fichtinger
Original Article



   Freehand tracked ultrasound imaging is an inexpensive non-invasive technique used in several guided interventions. This technique requires spatial calibration between the tracker and the ultrasound image plane. Several calibration devices (a.k.a. phantoms) use N-wires that are convenient for automatic procedures since the segmentation of fiducials in the images and the localization of the middle wires in space are straightforward and can be performed in real time. The procedures reported in literature consider only the spatial position of the middle wire. We investigate if better results can be achieved if the information of all the wires is equally taken into account. We also evaluated the precision and accuracy of the implemented methods to allow comparison with other methods.


   We consider a cost function based on the in-plane errors between the intersection of all the wires with the image plane and their respective segmented points in the image. This cost function is minimized iteratively starting from a seed computed with a closed-form solution based on the middle wires.


   Mean calibration precision achieved with the N-wire phantom was about 0.5 mm using a shallow probe, and mean accuracy was around 1.4 mm with all implemented methods. Precision was about 2.0 mm using a deep probe.


   Precision and accuracy achieved with the N-wire phantom and a shallow probe are at least comparable to that obtained with other methods traditionally considered more precise. Calibration using N-wires can be done more consistently if the parameters are optimized with the proposed cost function.


Freehand ultrasound Calibration  N-wire phantom  Accuracy 



This work was supported in part by Agencia Nacional de Investigacion e Innovacion (ANII, Uruguay) under grant (BE POS 2010 2236). Comision Sectorial de Investigacion Cientıfica (CSIC, Universidad de la Republica, Uruguay) supported the internship of G. Carbajal at Perklab. This work was co-funded as an Applied Cancer Research Unit of Cancer Care Ontario with funds provided by the Ontario Ministry of Health and Long-Term Care. G. Fichtinger is supported as a Cancer Care Ontario Research Chair.

Conflict of interest   None.


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Copyright information

© CARS 2013

Authors and Affiliations

  • Guillermo Carbajal
    • 1
  • Andras Lasso
    • 2
  • Álvaro Gómez
    • 1
  • Gabor Fichtinger
    • 2
  1. 1.Facultad de IngenieríaUniversidad de la RepúblicaMontevideoUruguay
  2. 2.Laboratory for Percutaneous Surgery, School of ComputingQueen’s UniversityKingstonCanada

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