title: Sub-pixel localisation of passive micro-coil fiducial markers in interventional MRI.
abstract: OBJECTIVE
  Electromechanical devices enable increased accuracy in surgical procedures, and the recent development of MRI-compatible mechatronics permits the use of MRI for real-time image guidance. Integrated imaging of resonant micro-coil fiducials provides an accurate method of tracking devices in a scanner with increased flexibility compared to gradient tracking. Here we report on the ability of ten different image-processing algorithms to track micro-coil fiducials with sub-pixel accuracy.
  
  
  METHODS
  Five algorithms: maximum pixel, barycentric weighting, linear interpolation, quadratic fitting and Gaussian fitting were applied both directly to the pixel intensity matrix and to the cross-correlation matrix obtained by 2D convolution with a reference image.
  
  
  RESULTS
  Using images of a 3 mm fiducial marker and a pixel size of 1.1 mm, intensity linear interpolation, which calculates the position of the fiducial centre by interpolating the pixel data to find the fiducial edges, was found to give the best performance for minimal computing power; a maximum error of 0.22 mm was observed in fiducial localisation for displacements up to 40 mm. The inherent standard deviation of fiducial localisation was 0.04 mm.
  
  
  CONCLUSIONS
  This work enables greater accuracy to be achieved in passive fiducial tracking.
authors.affiliation: Radiological Sciences Unit (2N), Charing Cross Hospital, London, W6 8RF, UK. marc.rea@imperial.ac.uk
authors.forename: Marc
authors.initials: M
authors.lastname: Rea
authors.forename: Donald
authors.initials: D
authors.lastname: McRobbie
authors.forename: Haytham
authors.initials: H
authors.lastname: Elhawary
authors.forename: Zion T H
authors.initials: ZT
authors.lastname: Tse
authors.forename: Michael
authors.initials: M
authors.lastname: Lamperth
authors.forename: Ian
authors.initials: I
authors.lastname: Young
mesh_terms: D004867:Equipment Design; D019544:Equipment Failure Analysis; D007089:Image Enhancement / Q000295:instrumentation*; D007090:Image Interpretation, Computer-Assisted / Q000295:instrumentation* / Q000379:methods*; D053783:Magnetic Resonance Imaging, Interventional / Q000295:instrumentation* / Q000379:methods; D008280:Magnetics / Q000295:instrumentation*; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity