Skip to content

Spatial Calibration

Jump to bottom
tdowrick edited this page Dec 18, 2020 · 1 revision

Spatial Calibration Using A Tracked Stylus

This method computes the transformation between corresponding fiducials placed in the image coordinate system and the probe coordinate system to estimate the position of the ultrasound image relative to the ultrasound tracker (Image-to-Probe transformation). It requires the tracked ultrasound probe (e.g. with the SureTrack attached), a water tank and a tracked Stylus (e.g. Medtronic pointer stylus). The temporal calibration has to be performed prior to this. To facilitate the fiducial point collection, one can record a sequence (i.e. using the sequence module) on 3D Slicer to acquire images of the tracked stylus in the water tank.

Hardware requirements:

  • Ultrasound probe with tracker
  • Configuration file that specifies both the probe and depth of water used in the calibration
  • Tracked stylus
  • Water bucket with approximately 50 degrees (Celsius) temperature water (Sound velocity in room temperature water is approximately 1490 m/s and this will create distortions in the image)

Open the Sequence browser module (available in the slicer extension manager).

Select the proxy node you want to record : Ultrasound video stream + tracked stylus tip position.

Click on the record button and move the tracked stylus tip within the corners of the image to estimate the full image size and shape.

Turn the pointer tool to 180 degrees (to compensate for systematic error) and re-positioned the pointer tool in the four corners

Open the Fiducial registration wizard module in Slicer-IGT

Create a new “From fiducials” list called “Image Fiducials”

Create a new “To fiducials” list called ProbeFiducials

Create a new transform under registration result and name it ImageToProbe

Select the Similarity transform type and switch to manual update

In the “Place fiducial panel” select the StylusTip transform

Replay the previously recorded sequence and start collecting points on the ultrasound image

To create a fiducial pair:

  • Click on the fiducial icon under the “From fiducials” panel
  • Click on the ultrasound image at the stylus tip
  • Click on the “Place To” button in the “To fiducials” panel to create the matching fiducial

Collect first one point on each corner of the image

Collect four other points when the pointer tool has been turned to 180 degrees

Press the Update button to compute the calibration

Save the ImageToProbe transform or directly copy the calibration matrix and paste it in the Plus server config file

Clone this wiki locally