Step 11 Tracking Solutions

Tracking devices are electrical/ mechanical components attached to the surgical instruments and/or the simulator, which can track and locate where your instrument or model is in space.

Depending on the application, they can provide multiple degrees of freedom (Fig. 9). The most basic tracking device is an "on and off" switch that is one degree of freedom. A simple example of two degrees of freedom tracking device is the mouse. The mouse moves only in one plane and yields position coordinates in terms of X and Y.

In general, most spatial movement can be defined with six degrees of freedom. This includes spatial positioning in the horizontal (X), vertical (Y), and depth (Z) directions. In addition, three degrees of orientation (rotation, yaw, and pitch) can be defined.

In some cases, more than the standard six degrees of freedom will be required. For example, suturing involves seven degrees of freedom for each hand (translation in the X, Y, and Z direction, rotation about the X-, Y-, and Z-axis, and grip). Another example of a simulator requiring additional degrees of tracking freedom is transurethral resection of the prostate simulation. To track a resectoscope, it is necessary to include the rotation of the stopcocks, loop extension, camera rotation, foot pedals, etc.

In order to provide visual realism, tracking devices need at least 30-60 Hz (or updates per second). Most people will not perceive an upgrade in animation quality beyond this update level. Force feedback requirement are much higher. To simulate smooth force feedback with soft tissue, it is usually necessary to update position tracking at no less than 300 Hz. Requirements are even higher when modeling hard tissue

FIGURE 9 ■ Degrees of freedom defined for an instrument working on a fulcrum, is present with endoscopy. In many endoscopic applications, there is a fixed entry port into a body cavity, and therefore movement along the X- and /-axes does not usually occur. Most endoscopic applications need at least four degrees of freedom (movement along the Z axis, pitch, roll, and yaw). In open procedures and laparoscopic simulations where port placement may vary, tracking along the X- and Y-axes would also be desirable.

A crucial element of virtual reality simulation is determining when a surgical tool has collided with tissue structures or other tools.

Orientation and calibration can be accomplished using in vivo or ex vivo techniques.

The difficulty with surgery simulation is that there are usually a number of deformable object in the scene.

For the purposes of interactivity and data acquisition, any analog data (measured in terms of voltage) need to be converted into a digital format.

When at all possible, it is important to try and avoid relying on subjective calibration of metrics, even from the most experienced subject matter experts. Haptics calibration is notoriously unreliably consistent between subjects.

structures, such as bone. Typically a tracking should be sampled at over 1000 Hz to accurately model the feel of hard objects.

A number of companies sell tracking hardware, such as Ascension and Polhemus®. Building your own tracking systems using optical encoders and fiduciary markers is also possible and can be much more affordable. Many force feedback systems include their own tracking capability, such as the Phantome and the Mantis.f

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