What is precision?

Precision is normally described in terms of the RMS (Root Mean Square) of a tracker within the defined space (typically a cube, cone or cylinder). Of equal importance in real life situations is the standard deviation of the RMS. The smaller the standard deviation with fewer outlying points the better is the effective precision of the tracking system. This apply as well to the confidence level. The number of allowable outlying points for a given RMS will be significantly higher for a confidence level of 95% versus one of 98%.

How does the size of the working volume or space impact on the precision?

Most tracking systems are designed to be very precise on the X and Y axis. However as you move further away from the tracker, the calculation of the Z axis becomes more difficult resulting in lower precision. Hence, if you are working at the furthest point in the specified volume, it is given that the tracking error you will see will be above the stated RMS.

As the tracking distance increases, so do the position errors. Both the accuTrack and the easyTrack system are therefore designed to be placed as close as possible to the working area to minimize precision compromises due to being too far away.

Static and dynamic precision

Static precision is the measurement of the position of an object in space that does not move. All measurement values for different manufacturers are presented in this manner. Dynamic precision is the precision of an object that is in movement. These values can be significantly different. Passive tracking systems, with their slower update speed can suffer from significant errors when tracking objects that move. The accuTrack with its ultra high speed, was designed especially for applications that involve any movement. In fact, the acquisition speed of the accuTrack can even compensate for a portion of the vibrations coming from various tools.

Temperature and warm-up time

Recent research has shown that during the warm up phase of traditional passive tracking systems (which may last up to 30 minutes), the tracking accuracy will be significantly inferior to the stated RMS accuracy. If using one of these machines, it is strongly advised to turn them on well before you need to take any critical measurements.

The accuTrack system has been designed with a state-of-the-art temperature compensation system that eliminates the worry of waiting for the machine to warm up before you can use it. Accuracy is available from the moment you turn it on.

Why is tracking precision so important?

The choice of tracking system is one of the most easily controlled factors in the design of an overall tracking solution, especially in areas such as Image Guided Surgery.

With the accuTrack and easyTrack systems, we start with the highest precision in our class. In fact, the precision of the Atracsys systems has been compared to systems costing several times more.

What is total precision?

In the example of Image Guided Surgery, we must keep in mind that the TOTAL precision is what is important to the surgeon. The following equation is typical of a tracking solution using a CT scan as input.

Total RMS = CT Scan + Patient Registration + Tool Registration + Tracking system