Physicians frequently use the term GPS, when explaining electromagnetic navigation bronchoscopy to the patient. We will explain what global positioning system (GPS) is, then elaborate more on electromagnetic science and show how a tracking system works.
So, how does GPS work? It is a worldwide radio-navigation system formed from a constellation of several satellites circulating around earth and their ground stations. A satellite sends or broadcasts radio signals at a specific time and location. The receiver takes the signal and determines how far the satellite is. We need 3 to 4 satellites to narrow down our position and come up with an exact location. The GPS helps us determine our location, or a location of our choice. A navigation system in the other hand, is a special software that makes use of GPS signals. Once the coordinates are located by the GPS, the computation device looks up the map database and take over the routing. The navigation system combines the real time position data with electronic mapping technology and formulates direction.
With vehicle GPS navigation, a map supplies a 2D travel route, and the car is moving towards a fixed destination. The movement is along the longitude and latitude lines, altitude is missing, rotational data are also missing, only two degrees-of-freedom are reported.
Now that we reviewed how GPS and navigation work, let’s move on to the electromagnetic navigation. If you ask: what is electromagnetism? the answer would be; it is one of the four fundamental forces of nature (gravity, electromagnetism, weak force, strong force). Earth is a giant magnet; it is constantly generating magnetic field. If a small compass is used to map the magnetic field around a bar magnet, like this one shown below, it will point in the direction away from the north pole, toward the south pole. By connecting the dots obtained by moving the compass, we can delineate the magnetic field lines.
A permanent magnet is an object made from a material that is magnetized and creates its own permanent magnetic field. Whereas an electromagnet is made from a coil of wire, often wrapped around an iron core and connected to an electrical current creating a magnetic field. An electromagnet can easily be assembled into different configurations and can be switched off and on. Electromagnetic tracking provides a display of position and orientation of the instruments we are using within previously acquired three-dimensional imaging datasets. It also allows us to navigate inside patient’s body and track the tool we are using. To apply the GPS analogy to procedural navigation applications, patient imaging datasets represent the roadmap, the target lesion is the destination and the instrument equipped with a sensor is the vehicle. The most important key to any navigation system is to have an accurate roadmap, no matter how sophisticated is the system or the tool we are using.
Electromagnetic tracking system consists generally of three components: The field generator, the sensor and the central unit. The magnetic field generator contains a certain number of coils of wire, positioned in a specific configuration. It emits low-intensity electromagnetic fields at different frequencies. The electromagnetic sensors capture those different signals at a set refresh time, the sensors are embedded into the medical instrument and when capturing the electromagnetic fields, they produce electrical signal (or voltage) that is transmitted to the control unit. The system control unit controls the field generator and collects information from the sensors. It calculates the sensor’s position and orientation and displays it on a monitor.
To our knowledge, there is no head-to-head published data that compared different electromagnetic navigation platforms. So, no platform has shown to be superior to the others. To illustrate what we just reviewed, we decided to pick one of the commercially available navigations systems and go over the procedural workflow.
The proclaimed advantages in using the spin thoracic navigation system, is its mobile field generator, and the fact that fluoroscopy is not needed when the platform is used in isolation, it also allows the proceduralist to access pulmonary lesions percutaneously.
Electromagnetic navigation bronchoscopy relies on a pre-procedural CT to create the 3D airway map. After the decision is made, that the patient will benefit from navigation bronchoscopy, perioperative computed tomography images are obtained and transferred to a computer, where a roadmap is generated. 2 scans are obtained, based on an inspiration and expiration CT scan protocol.
The V pads are a set of sensors that are placed on patient’s chest before obtaining the CT scans, they remain in place during the procedure, they allow automatic registration, which means, they help match CT chest data with patient’s anatomy. If automatic registration is not optimal, manual registration is performed. Target lesion movement during patient's breathing can be a challenge, V pads technology helps differentiate between inspiration and expiration and tracks nodule during patient’s breathing.
After the images data are loaded on laptop computer, the software constructs 3D images of patient’s lung. The anatomical landmarks, then the target lesion using the inspiration scan images are identified and a roadmap is automatically generated from the targeted lesion all away back to the trachea.
Before navigation, the CT images and the virtual structures need to be aligned with the patient’s anatomy. The anatomical landmarks (the main carina, and the secondary carina) are reviewed and confirmed with the bronchoscope. In matching the main carina, there is a translation shift, in matching the secondary carina, we are looking for rotation.
The steerable navigation instruments are directed towards the target location. The Always-on tip tracked instruments feature electromagnetic sensors, aiming to eliminate the need for fluoroscopy during guided therapy.
Conclusion:
There is a continuous development of a variety of navigational bronchoscopy platforms. It is important to understand the performance, and limitations of the navigation system that is being used. Knowledge of its fundamental principles is essential to successful use.
References:
IOSR-JAP 2015;7(3):01-07
AME Med J 2018;3:117
J Vas Interv Radiol. 2005;16(4):493-505
AJR 2011;196:1194-1200
Adv Ther (2016) 33:580–596
J Bronch Intervent Pulmonol 2014;21:242-264
Images, courtesy of Veran Medical Technologies