Group 6 - Frameless navigation

This topic deals with the frameless navigation during brain surgery. We will explain the state-of-the-art techniques as well as discussing accuracy of frameless navigation. Frame-based surgery includes many problems, as it is unpleasant for the patient and due to the frame the surgeon only has a limited view and working area. Therefore the first frameless stereotaxy was developed in the 1990s and is a topic of improvement up to now. [1]

State-of-the-art

In general, a frameless navigation system consists of a workstation, a position digitizer and a pointing device. A combination of these parts allows to track the head and surgical tools in real-time during surgery. Overall, there are many different systems available for frameless treatment, which rely on different tracking methods and different software planning types.

Procedure

Before the surgery, preoperative CT and/or MRI images are taken, loaded into the workstation and merged to a 3D model of the brain. On these images markers are already visible that were attached to the patients brain and that can be aligned with the position of the markers during surgery. With available software planning tools the surgeon then plans the surgery based on these medical scans. The planning includes definition of the entry point in the skull and laying trajectories to the area of the tumor. Right before the surgery, fiducial markers are attached on different places on the head and surgical tools are equipped with markers as well. By using the pointer on the fiducial markers, the position of each marker is registered in the workstation. This allows for the alignment of the preoperative scans and the position of the markers in the operation room space. During the surgery the tools can be tracked in different ways, which will be descriped later on. All methods enable a real-time tracking. Additional (mobile) monitors are integrated into the OR or in small ways integrated into the surgeons mircoscope (AR), so that the surgeon can see the planned trajectories in relation to the actual position of the tools. Intraoperative images can be taken to further improve the accuracy of the surgery. [2] [3]

Markers

The markers are used to co-register pre- and intraoperative positions of the patients brain, so they are also called fiducial markers. For preoperative CCT scanning, metal beads on self-sticking plates are used frerquently, while self-sticking ring markers are used for preoperative MRI imaging. In contrast to these markers, natural markers can be used as well. These markers are special areas on the brain, as the mastoid, the frontal and parietal tuber and the forehead. However these markers are less accurate than the fiducial ones due to the fact that they are often less discretely defined. An alternative to this point-matching is surface-matching. Instead of individual points, surface areas of the brain are digitized and aligned with preoperative images in the workstation. To allow for a correct registration of the brain, the distribution of the markers is subject to some restrictions: the markers need a large enough distance between each other in all three axis directions, the tumor should lay in the volume defined by the position of the markers and the CT slices should cover the whole brain. [3] [6] [7]

Figure 1: placement of markers: a) too small distance in z-direction, b) correct placement (lines describe one slices of imaging scans) [6]

Digitizer

The digitizer is used to track the position of the markers. Depending on the system, several methods exist for the tracking. The methods can be categorized into navigational microscopes or pointer-based methods and arm-based or armless devices. Furthermore active robots play a great part in research for more efficient digitization.

arm-based: Here, a mechanical arm is used to locate the position of the markers. The arm consists of joints with sensors attached to them, so that the position of the endeffector of the arm can be solely defined by the angle information of the joints. This method has a high accuracy, isn't influenced by the environment and doesn't need a line-of-sight. However only one object can be traced at a time.
armless: These systems can be further divided, depending on the technique that is used for the tracking. Some methods can be improved by attaching the receiver (i.e. the microphones) to the OR table, so that no re-registration is needed after moving the OR table.
- Sonic systems use sound for detecting the markers. The markers emit sound and their position is determined based on the time it takes for the sound wave to reach the microphones. For a sufficient 3D model of the operating space a minimum of three microphones are needed. This method however is rather unfeasible because it requires a line-of-sight between emitter and microphone and furthermore this technique is sensitive to parameters like air temperature and humidity inside the OR.
- Magnetic systems consist of a transmitter that generates a magnetic field and a receiver which is integrated into the pointer device. The location of the receiver then is determined by measuring the gradients of the magnetic field. This method doesn't need a line-of-sight, however the measurements can be distorted due to otehr metal objects inside the OR.
- Optical systems use sources and sensors. The sources can be either active (LEDs) or passive (i.e. infrared reflectors). The sensors are often charged-coupled device (CCD) camera arrays that scan the environment and determine the position of the sources from the takes. This technique shows a good accuracy and can track multiple objects in real-time. However it requires an unobstructed line-of-sight and only rigid tools can be tracked.
In a pointer-based method, the markers are tracked indirectly by a pointer. The freehand movable pointer device can for example be tracked magnetically and by pointing onto the markers, their positions are tracked as well.
A navigational microscope superimposes the preoperative images with the field of view through the mircoscope. This allows the surgeon to see the information directly without looking at an additional monitor. Advantages of these systems includes the absence of otherwise necessary arms, digitizers or magnetic fields. [10]

All methods can be improved by improving the registration algorithms and improving the quality of the tracking devices. [4] [7]

Figure 2: schematic illustration of arm-based (left) and armless (right) systems [14]

Accuracy

There are different definitions regarding the accuracy of frameless navigation. In general, the overall accuracy for frame-based systems was said to be higher than for frameless systems. However, new studies show that frameless systems can be as good as or even better than frame-based systems [8] [9]. Different reasons can be named for a difference in accuracy. First of all, in frameless navaigation only few markers are used to register the brain, instead of using a frame for the registration. Secondly, the position tracking has to pass several stages which can lead to computational inaccuracies. Other factors contributing to the overall accuracy during surgery ("application accuracy") are "mechanical" and "registration" accuracy. The former one is device-specific and describes the precision of the positions for devices in space. In arm-based systems this precision is based on technical parameters and in armfree ones the resoultion of the detector contributes to the precision, among others. The "registration" accuracy accounts for errors that occur due to matrix transformations in the registration process. Clinical factors, such as the occurence of brain shift, also contributes to a decreasing accuracy. A last contributer to inaccuracies lies in the type of coordinate systems. In some frame-based systems, the target point is the center of the frame, the movement axis is in the center and trajectories are radii of the coordiante sphere. In some frameless systems however, the movement axis is close to the skull, which accounts for a larger movement in the target area due to an increased spread of trajectories. [5] [6]

Figure 3: individual derivation from the planned target in frame-based and frameless surgery [5]

Figure 4: Schema of coordinate planning in frame-based and frameless systems [5]

Radiosurgery

Advantages of frameless navigation can also be applied to radiosurgery. Again, there are different systems used to perform frameless radiosurgery. One system described here uses an optical tracking of a dental tray. Here, infrared-light reflecting markers are attached to a biteplate. CT scans are used to perform treatment planning and during the treatment, the position of the head is tracked by infrared light in combination with a CCD optical system. This allows the system to report any displacement of the patient. The treatment is performed with conventional delivery mechanisms. If during the treatment the system notices a displacement that exceeds a certain threshold, treatment is interupted, the patient is repositioned and treatment is resumed. Tests show that this system has a similiar accuracy and reproducability to frame-based systems. However this system can't treat multiple isocenters and due to the conventional radiosurgery planning and delivery mechanisms, it's not possible to deliver doses to highly irregular target volumes. [11]

Another system for frameless radiosurgery uses a linear accelerator on a computer-modulated robotic arm to deliver the doses. Planning is done on CT images and X-ray is used during treatment to track natural or fiducial markers. The tracking allows for a repositioning of the robotic arm in case of a displacement that exceeds a certain threshold. However, due to using only one robotic arm, only one beam can be delivered at a time. This system shows similar accuracy to frame-based systems as well. [12]

Other studies confirm that modern frameless radiosurgery shows similar accuracy to frame-based systems and is more patient friendly. [13]