Fiducial Registration Wizard transforms

Support
1

I’m a beginner, I already have a calibrated pointer. I have a tracker at my object in real life, the pointer should be relative to the tracker on my real life object/reference, right? So my pointer transform will be calculated with the reference or anchor of the tracker that I attached to my object. How do I do it? Because I see that there is only one in the fiducial registration wizard, so I only created it for my dcom/ctscan data.

2

I’m not sure what you mean by “tracker” (your tracking camera? a tracker marker attached to your “patient”?) and “at my object” (is the marker actually attached - clamped, screwed - to the object?). If you can describe very precisely what tracking markers you have and what transforms do you stream via PLUS then we have a better chance of answering. Just in case you have not seen it already, the SlicerIGT-U12_LandmarkRegistration tutorial gives a very detailed explanation of the whole process.

3

Hi Andras,

Thank you for your reply.

Let me clarify my setup more precisely.

By “tracker”, I mean a rigid body with optical markers (OptiTrack) that is physically attached to my phantom skull (like a reference marker, similar to a patient reference in navigation systems). It is fixed to the object and moves together with it.

So in my setup I have:

  • A PhantomTracker (rigid body attached to the skull → serves as reference frame)
  • A StylusTracker (rigid body attached to the pointer tool)
  • Both are streamed via PLUS (OptiTrack → PLUS → OpenIGTLink → Slicer)

What I want to achieve is:

  • The stylus position should be computed relative to the PhantomTracker (object reference), not relative to the global tracking coordinate system.

So effectively:

  • StylusToPhantomTracker = StylusToTracker × inverse(PhantomTrackerToTracker)

Currently, I have:

  • Performed pivot calibration for the stylus
  • Performed fiducial registration between CT (DICOM) and the tracked object (phantom)

But I am unsure about:

  • The correct way to define and use this object reference (PhantomTracker) in Slicer
  • Whether this relative transform should be handled via PLUS configuration or within Slicer (e.g., transform chaining or scripting)

Also, in the Fiducial Registration Wizard, I only see one transform being generated (CT to tracker), so I’m not sure how to properly incorporate the PhantomTracker as a reference frame in this workflow.

Any clarification on the correct pipeline for this setup would be very helpful.

Thank you!

Best regards,
Zaky

4

What you describe is the basic patient registration use case, so you can follow the tutorial that I referenced above. I would recommend to use standard naming conventions (e.g., rename PhantomTracker to Reference, and pay attention to small differences, such as Stylus vs. StylusTip).