Magnetic tracking systems

Magnetic tracks are laid down in concentric circles. These tracks are where information is eventually encoded. These tracks, in turn, are split into pieces of 512 bytes called sectors. Some space is reserved in between the sectors for error correction information, referred to as CRC, or Cyclic Redundancy Check, information. The operating system may use CRC information to re-create data that has been partially lost from a sector. 

To achieve accurate 3D reconstruction, electromagnetic interference must be minimized, the transmitter must be close to the receiver, and there should be no ferrous or highly conductive metals in the vicinity (Downey et al. 2000 Kelly et al. 1994). Magnetic field tracking systems can be used with standard and endocavitary transducers. These systems have been used successfully for fetal (Kelly et al. 1994 Pretorius and Nelson 1994) and vascular (Hodges et al. 1994) 3D imaging. Recently, there has been some development with a miniature magnetic position sensor suitable for use with intra-vascular transducers. 

Tracking System System that locates an object s or subject s position and orientation through optic, ultrasonic, magnetic, electric, and mechanical sen-... 

Figure 4.7 Schematic of navigated endoscopic tracking using (a) extrinsic and (b) intrinsic (vision) tracking systems. The ability to fuse ultrasound (US) into video depends on the accurate estimation of the transition Tus- The local coordinate system of the extrinsic tracking system (often magnetic) is used as the world coordinate system (W), where the WTDRB denotes the tracker transformations. DRB USTUS denotes the US calibration matrix and VTDRB V denotes the camera calibration.

The motion of the needle tip was measured experimentally and compared with predictions of the analytical model. The tip motion is measured with a magnetic position sensor (miniBIRD model 80 from Ascension Technology). This electromagnetic tracking system measures the three positions and three orientations of a small sensor (1.3 mm in diameter, 6.5 mm in length) with RMS accuracies of 1.4 mm and 0.5°. The sensor is small compared to the manipulator and does not affect its performance. 

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