Magnetic eye tracking: a new approach employing a planar transmitter.

A new scleral search coil (SSC) tracking approach employing a planar transmitter has been developed theoretically and tested experimentally. A thin and flat transmitter is much more convenient in installation, operation, and maintenance than the conventional large cubic one. A planar transmitter also increases the mobility of SSC systems, simplifies their accommodation in a limited clinical space, enables bedside testing, and causes no visual distractions and no discomfort to the users. Moreover, it allows tracking not only the SSC orientation, but also its location, which is very important for many medical and scientific applications. The suggested approach provides the speed and precision that are required in SSC applications. The experimental results show that it can be used for the diagnosis of vestibular disorders. The tracking precision is in good agreement with its theoretical estimation.

Minimizing crosstalk in three-axial induction magnetometers.

A model for crosstalk in three-axial induction magnetometers has been developed theoretically and verified experimentally. The effect of crosstalk on the magnetometer accuracy has been analyzed. It has been found that the inevitable crosstalk in the transverse coils has two components: one due to the applied magnetic flux and the other due to the secondary flux produced by the electric current in the longitudinal coil. The first component has a constant magnitude. The phase of the second component, relative to the first one, is nearly 180° at low frequencies, 90° at resonance, and 0° at high frequencies. Its magnitude approaches zero at low frequencies, has the maximum at resonance, and then drops off by a factor equal to the coils' quality factor and approaches the first component value. As a result, the crosstalk due to the applied flux is dominant at low frequencies. At a frequency just below the resonance, the crosstalk is very low, if no magnetic feedback is applied. Just above the resonance, the crosstalk reaches the maximum because of the rapid increase in the secondary flux. Applying a strong enough magnetic feedback nearly flattens the crosstalk amplitude response. However, an undesirable effect of the feedback is that it significantly increases the minimum crosstalk value. A very low crosstalk at a single frequency can be beneficial for magnetometers tuned to a narrow frequency band. It can also be beneficial for wide-band magnetometers to measure their mechanical orthogonality with a minimum effect of crosstalk.

A new method for eye location tracking.

A standard scleral search coil (SSC) system is supplied with a single three-axial transmitter (or with a triad of transmitters) and the direct magnetic coupling between the transmitting coils and the SSC is used for computing the eye location. The SSC's orientation components measured by the SSC system are involved in the location-tracking algorithm as well. The suggested method differs from traditional ones where the eye location is computed indirectly, relative to the measured location and orientation of a transitional three-axial magnetic sensor attached to the subject's head. The new method eliminates systematic errors caused by inaccurate situating the transitional sensor relative to the eye. It also eliminates systematic errors caused by imperfect orthogonality of the transitional sensor. It is found that an accurate location tracking with a single transmitter is possible only if orientation freedom of the eye is limited. To track the eye location with no orientation limitations, a triad of transmitters is employed. The resulting accuracy can be as good as +/- 1 mm (in a 200-Hz bandwidth) over a relatively large (0.5 x 0.5 x 0.5 m) operating region in the center of a 1 x 1 x 1 m SSC system. The analytical results obtained are confirmed with the help of computer simulations.