3D Guidance Trackers
Q. What trackers make up the new 3D Guidance product line?
Q. How do they differ from Ascension’s earlier magnetic trackers?
A. They represent a new generation of Ascension trackers, designed with the latest advancements in digital signal processing, passive sensing of DC magnetic fields, Kalman filtering, dynamic noise suppression, and micro miniaturization of sensor components. They enable you to track the instant position and orientation of miniaturized sensors at unprecedented high speed and high accuracy with minimal noise. Each product contains self-diagnostics and run-time monitoring for improved tracker reliability and safety.
medSAFE is the first Ascension tracker to meet the highest medical compliant standards. It is a Class 1, Type CF, Defib Proof tracker, which means its sensors can be used in cardiac applications once users meet FDA/CE and/or IRB requirements.
driveBAY and trakSTAR are the first Ascension trackers to fully meet RoHS and WEEE standards as well as all pertinent medical, electrical and safety standards.
Q. Aren’t all magnetic trackers the same?
A. No, there are several generations of trackers on the market. These range from 1st generation AC electromagnetic technology, first patented by Polhemus Inc. in the 1970s, and later enhanced by medical device companies, including Biosense, GE, Medtronic, MediGuide, NDI, and SuperDimension; 2nd generation trackers patented by Ascension in the 1990s that employ pulsed DC magnetic technology; and now 3rd generation DC magnetic technology, also patented by Ascension. 3D Guidance technology offers the latest improvements and innovations in magnetic tracking technology.
First generation trackers are notoriously susceptible to distorted measurements in the presence of common metals, such as carbon steel, aluminum, and even stainless steel.
Second generation DC trackers exhibit only 1/5 the sensitivity to non-magnetic conductive metal as their earlier counterparts. As a result their sensors can be attached to ultrasound scanheads, titanium instruments, and non-magnetic stainless steel objects without discernable loss of accuracy. However, their sensors contained fluxgates that limited sensor miniaturization to 5 mm in diameter, were subject to ferrous metal distortion, and were too complex to achieve truly low cost.
Third generation magnetic tracking represents the state-of-the-art in real-time tracking. It includes six degrees-of-freedom sensors as small as 0.94mm in diameter, low cost, disposable sensor pricing, special transmitters that block distortions emanating below the tracking volume, and advanced processing and calibration techniques for robust performance in all kinds of environments.
Q. How do the new 3D Guidance trackers differ from one another?
A. Here is a chart of key differences:
Q. What are the differences between earlier 3D Guidance/microBIRD trackers and medSAFE?
A. Ascension’s original 3D Guidance tracker -- with full medical compliance for navigation of miniaturized sensors -- was released in 2007. It succeeded microBIRD that had been released in 2005 as a laboratory tool for medical researchers. microBIRD enabled one to track miniaturized sensors, but it did not carry a medical certification for use in internal medical procedures.
medSAFE tracks up to 8 six-degrees-of-freedom sensors simultaneously, and offers improved dynamic performance, hot swapping, time stamped records, on-board flash reprogramming, customizable settings, biocompatible sensors, and extensive self-test and run-time diagnostics for improved reliability and safety.
medSAFE also supports tracking of the world’s smallest six degrees-of-freedom sensor: 0.94mm in diameter X 7.6mm in length. It is small enough to fit into the distil tip of a 19-gauge hollow needle.
Q. Are 3D Guidance trackers compatible with legacy (older model) Ascension trackers?
A. Yes, 3D Guidance trackers are drop-in replacements for many Ascension magnetic trackers that you once used.
trakSTAR is fully compatible with Ascension magnetic trackers that use RS-232 and USB interfaces. trakSTAR supports Flock of Birds, miniBIRD and all other Ascension magnetic tracker using the RS-232 Interface, running winBIRD, CBIRD, SBIRD and SOCKET programs, and employing Windows drivers.
3D Guidance (older model), microBIRD, and pciBIRD users can substitute new USB and DLL (drivers) and continue to use old applications. Note, however, 3D Guidance trackers do not presently support the extended range transmitter that is available with legacy products. If you need extended range tracking, you can still obtain it by buying a Flock or MotionStar tracker.
Q. What is the difference between a short-range transmitter with an air core versus a short-range transmitter with a ferrite core?
A. The biggest difference is range. Both transmitters enable six degrees-of-freedom sensor tracking, but the ferrite version produces a stronger signal than the air core version. At 17 inches of separation, the position and angular noise for a ferrite transmitter will be about one half that of the air core. For this reason, we recommend that the air core transmitter be used in very short-range applications, such as tracking a sensor referenced to a transmitter that is no greater than 8 -12 inches away.
Q. Is the new short-range transmitter light enough to mount on a person?
A. As long as the transmitter is mounted on a bracket (and therefore not in direct contact with a person), it can be used for body tracking. The “air core “ version weighs just 230 grams. As stated above, it is specifically designed for close range applications. For some medical applications, it can be mounted on a swing arm or insulated band attached to the patient for tracking miniaturized sensors within the body. In at least one new application, this new transmitter is attached to a headband for tracking sensors and instruments on and near the face. Sensor measurements can be referenced relatively to one another or absolutely from their geometric center to the center of the transmitter.
Q. How close can we put two sensors?
A. 8mm sensors should maintain a center-to-center separation distance of approximately 1.3 inches (33mm) for best performance.
Q. What does hot swapping of sensors mean?
A. Each sensor is individually calibrated and can be used with any electronics unit. Sensors can also be unplugged and re-plugged into the electronics unit and data reporting will resume as soon as a new connection is made.
Q. What do you mean by time stamping of records?
A. A temporal stamp is applied at the time of data acquisition with 100-microsecond resolution. The time stamp is provided in UTC format and is synchronized with your host computer’s clock when initialized by our DLL.
Q. Do any of the trackers support a button mode?
A. Yes, trakSTAR allows you to append an external switch or button, to each data record. See below for additional information.
Q. How is button status reported in the data file?
A. Three data formats are provided in our API that include the button status as a member of the data format structure. (See: DOUBLE_POSITION_ANGLES_ TIME_Q_BUTTON_RECORD on pages 164 and 165 of the trakSTAR Operations Guide.)
With these formats, the state of the button is returned as a button value with each data record (regardless of state of button). When a switch/button is connected, a 1 in the button value indicates the contact or switch is CLOSED, and a 0 indicates the contact is OPEN. The button input is sampled by the electronics once per transmitter axis measurement (i.e. 3 times per system measurement cycle for a mid or short range transmitter).
Q. Is there any way to synchronize the trakSTAR with other data acquisition equipment in our lab? In other words, can the trakSTAR accept a TTL signal to either start the acquisition (with no delay) or to record this signal as a high-low signal synchronized with the recorded data?
A. Although we do not support use of an external sync to drive the acquisition, there are two ways to correlate time events with the sensor data records:
Timestamps – As noted above, several of our data formats include time stamped records. As the timestamp is synchronized to the host PC’s time clock during initialization, any other device connected to the PC using the PCs timestamp can be matched up to a given sensor’s data record.
Button Mode – We include a data format in the API that gives you the status of an external button/switch (simple contact closure - no TTL required) with each record. You can use this button input to represent an external event (i.e. from another device) and be reflected in the sensor’s data stream.
Q. Can we plug in any device and use your button?
A. Any contact closure switch or button that meets the electrical requirements stated in the trakSTAR Operations Guide can be used. Note: your device must be wired to mate with the external BNC connector on the rear panel of the trakSTAR.
Q. Power-line noise once was a problem with DC magnetic trackers. How do 3D Guidance trackers handle this problem?
A. 3D Guidance trackers support a new and improved wide notch filter that dramatically reduces 60Hz noise without exacting a noticeable penalty in dynamic performance.
Q. What is the difference between measurement rate and update rate?
A. With previous generation trackers, there was no difference. Measurement rate was the update rate. With 3D Guidance trackers, however, a new tracking solution (full 6DOF output) is iteratively computed after each transmitter axis excitation. With a mid-range or short-range transmitter, this means that by the time each transmitter axis has been energized, three solutions have been derived and output to your host computer. If the measurement rate is set at 80HZ and you are using one of our dipole (cubic) transmitters, you will receive 80Hz X 3 or 240 solutions in each measurement cycle. Update rate is independent of the number of sensors tracked so the update will remain constant regardless of the number of sensors tracked.
Q. Can you explain how measurement and update rates are calculated?
A. The calculations differ in the speed in which new tracking solutions are available for your use:
Measurement Rate is the rate at which the tracker acquires a complete measurement set from all available axes in the transmitter.
Update Rate is the rate at which a new (unique) position and orientation solution for the sensor is computed by the tracker.
Legacy (Older) Ascension Trackers:
In previous generation ATC trackers, a position and orientation solution would only be available after all transmitter axes had been energized.
Thus the update rate was equal to the system measurement rate.
Q. How do 3D Guidance trackers interface to host computers?
A. medSAFE and trakSTAR communicate via high speed USB or serial RS-232 ports. driveBAY outputs are available over your computer’s USB port.
Q. What software is provided?
A. You will receive a Windows API, which is compatible with APIs available for legacy trackers (pciBIRD, microBIRD and 3D Guidance) configuration utility, sample programs, and a number of demo utilities. The Cubes utility is a graphically based demo program that lets you checkout the tracker’s performance prior to incorporating it into your application. A virtual dice cube on your monitor mimics the motion of your sensor in free space. Cubes also present a numerical representation of the absolute position and orientation of each sensor as it moves. These programs are contained on a CD-ROM provided with your tracker for loading onto your computer.
Q. If I need a firmware upgrade, must I return my tracker or insert new proms in my tracker’s electronics unit?
A. No, we can provide on-board flash reprogramming of your unit for most upgrades. In some cases, a hardware upgrade will be required to add new features to a medSAFE tracker.
Q. Is accuracy the same throughout the tracker’s motion box?
A. No. If, under static conditions, you move one sensor throughout the motion box (i.e., the volume in which a magnetic tracker meets its accuracy specifications) and measure its position and angular accuracy, you will observe accuracy changes as a function of its location in the motion box. Similarly, if you rigidly fix two sensors near one another in a mechanical assembly and then move and rotate it in the motion box, you will observe the range between the two sensors varies as a function of the assembly’s location in the motion box.
Q. Are these variations in accuracy “normal behavior” or is there a hardware or software problem with the magnetic tracker?
A. You are observing normal behavior as long as the errors fall within the expected (specified range) of errors for a given tracker.
Q. What is the expected range of errors?
A. For an ideal magnetic tracker, there are no positional measuring errors. Wherever you move a sensor in the motion box, it measures position and orientation without error. That is to say errors do not vary throughout the motion box because each individual sensor perfectly measures its position coordinates.
In the real world, however, position and orientation errors vary in magnitude throughout the motion box. These errors are caused by the designer's inability to design a perfect measuring machine. Errors can also be produced by environment factors, such as metal in or near the motion box. Depending on the composition and size of this metal, it can distort the magnetic fields and cause varying errors in the motion box. These errors are static errors. In other words, if you move away from a location and return to it, the sensor will always measure the same error at this location. If you move to a different location, you will measure a different but still repeatable error. Errors do not jump from one value to another, but change smoothly as the sensor moves about.
Q. How can the accuracy be greater or less than the specified accuracy at different points in the motion box?
A. Ascension specifies the tracker’s accuracy using the Root-Mean-Square (RMS) error statistic. It is computed by moving the sensor to hundreds of locations throughout the motion box and recording the error at each location. The RMS value is then computed using all the error data throughout the motion box. RMS = square root of (sum of all errors squared / number of locations). Statistically speaking, if the errors are normally distributed then 68% of all errors fall within the band of +/- RMS and 99.7% of the errors fall within the band of +/- 3 x RMS.
For some Ascension trackers the RMS accuracy is specified as 0.07 inch or 1.8 mm. Other products are specified as 0.04 inches or 1.0 mm. A sensor placed at any one location in the motion box will read only the peak error only at that location, not the RMS error. As a result, some errors may be larger and some smaller than the specified accuracy. For example, if the sensor were located at the peak positive error then you would measure an error of 0.12 inches (3 mm) when using the sensor with a 0.04-inch (1 mm) RMS spec. If you had two sensors and you were unlucky enough to place one sensor at the positive maximum error and another at the negative maximum error then you would record a total difference in positional error of at least 0.24 inches (6 mm).
If you are developing a new product or doing research with a magnetic tracker, it is important to take this discussion into account when determining if the tracker is performing as specified.
Q. What regulatory standards do 3D Guidance trackers meet?
Medical Usage of Magnetic Trackers
A. medSAFE is a Class 1 medical device per IEC 60601-1. Its sensors are designed and tested to the Type CF, Defib Proof Applied Part designation. The system complies with electrical, safety, and electromagnetic compatibility requirements set forth in IEC 60601-1 and –2.
driveBAY and trakSTAR are Class 1 medical devices per IEC 60601-1. Their sensors meet Type B Applied Part standards. They also carry the CE mark and meet IEC 60601-1-2, Class B Limits for EMC.
Q. How are trackers used in medicine?
A. Motion trackers measure the position and orientation (X, Y, Z, Yaw, Pitch, Roll) of one or more sensors in 3D space. Measurements are used to track the real-time motion of medical instruments and devices for localization and targeting purposes.
In one current application, an Ascension 3D sensor is attached to an ultrasound scanhead for freehand data acquisition and subsequent viewing of 3D reconstructions of multiple 2D image planes. Ascension trackers are also used in minimally invasive procedures for measuring anatomy, guiding interventions, and navigating inside the body.
Q. What’s the benefit of using a magnetic tracker with an imaging system?
A. The combination of miniaturized magnetic sensors with imaging systems lets clinicians follow a real-time graphic display of the current and projected position of interventional tools onto real-time ultrasound or reconstracted CT images. Using it, clinicians can quickly and precisely guide biopsy needles or ablation tools to soft-tissue lesions within the human body. The risk of hitting delicate, adjacent anatomy is also minimized.
Magnetically guided intervention shortens procedures through pre-visualization of target trajectory and single needle insertion. When used with CT imaging, it reduces radiation exposure by minimizing the number of verification CT scans. In CT procedures, a spare position/orientation measurement sensor is placed on the patient so the true anatomical position of the target can be continuously monitored during procedure. Ascension's new 3D Guidance is the choice for such imaging procedures. It offers highly accurate tracking of multiple miniaturized sensors without performance loss due to the presence of nearby conductive metals.
Q. What is the difference between optical and magnetic trackers?
A. Optical trackers operate by emitting a light source that is sensed by one of more detectors. While highly accurate, they are obtrusive and require a clear line-of sight between source and detector at all times. If an occlusion occurs, measurements are lost. This precludes use within the body and limits usefulness in application in which heads and hands move freely within a tracking volume.
Magnetic trackers transmit magnetic fields that permeate all non-metallic surfaces. These sensors have been miniaturized for medical applications and can be placed inside the body and even inside some instruments. Its transmitter possesses a small form factor and can be draped to avoid sterilization. As a result, magnetic trackers minimize clutter in procedural fields.
Q. What should I look for when choosing a medical tracker?
A. First, seek a company that focuses on developing medical tracking products and presents a robust record of bringing products to market. Making a tracker work in medicine requires clear understanding of technology tradeoffs, interface issues, and customization requirements. It is one thing to make a tracker work in a vacuum; it is a far different issue to make it work successfully in a medical application.
Second, evaluate competing trackers before making a decision. At least one major medical company has come to regret not performing due diligence before making a major investment.
Finally, look for expertise and versatility in tracking solutions. With the recent expiration of first generation AC magnetic tracking patents, a number of start-ups have built AC trackers and rushed into the medical market. While their intentions are good, their track records are poor. Your safest bet is to deal with an experienced tracking company offering multiple trackers, and configurations that can be readily modified to meet your requirements.
Q. Is it true that magnetic trackers are hard to use in medical environments?
A. Yes, they can be, especially if the technology is first generation vintage. In the last 10 years many medical device manufacturers have evaluated these early trackers and found them inadequate, usually because of accuracy distortion and improper placement. When developing the 3rd generation tracking technology the focus was and is to address the previous generation’s inadequacies. One of the major improvements has been metal compensation techniques. The improvement of these techniques continues. Bottom line: don’t give up on electromagnetic tracking due to its previous limitations.
Q. Can Ascension customize a tracker to my needs?
A. Yes. Ascension has a long and successful record of modifying tracker configurations to meet stringent medical requirements. In 2001, Ascension designed and developed for General Electric Medical Systems a new BIRD tracker with a pci bus interface and custom cabling in under six months. In 2007, GE again turned to Ascension for 3D tracking requirement for electronics to be miniaturized to fit in the optical drive bay of a PC computer., That tracker, driveBAYTM ,was successfully integrated in GE’s new LOGIC E9 ultrasound system. It now provides volumetric measurement and fusion capabilities for GE’s current ultrasound platform.
Q. When should I start thinking about incorporating a tracker into my medical device?
A. During the development stage, if possible.
Integrating a tracker into an existing medical device without forethought can be the recipe for trouble. This approach can be expensive, time consuming, and will most likely not provide optimal system performance. For instance, the present configuration of the medical device may require the tracker to operate beyond its specified range, resulting in degraded performance.
The best approach is to design the tracker in during the development stage. The end result is a robust tracker specifically designed and optimized for your medical application.