Track images using Polhemus Fastrak electromagnetic tracker


We are working on tracking images with Electromagnetic Tracker (Polhemus Fastrak) using PLUS/Slicer. We have position data from the emt and images from ultrasound system:
Q How do we obtain real-time transform matrices from the tracker ?

I referred the link :

But I would like to know how real time position/transform matrix can be obtained.

Thank you!