Track images using Electromagnetic Tracker

Hi,

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 : https://www.slicer.org/w/images/e/ed/OpenIGTLinkTutorial_Slicer4.1.0_JunichiTokuda_Oct2012.pdf

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

Thank you!