Magnetic Marker Monitoring Using a Permanent Magnetic Sphere Oriented by a Rotating Magnetic Field

Magnetic marker monitoring (MMM) is a diagnostic technique known since about 1990 and mainly applied for motility assessment in the digestive tract. A particularly favorable MMM method uses a rotating marker, which can be aligned along an externally applied magnetic field HP. This novel method of rotating magnetic marker monitoring (RMMM) provides the starting point for the construction of small portable monitoring equipment. Friction effects and background fields, however, may cause deviations of the magnetically determined marker position from the actual location. In order to determine the magnitude of possible deviations, we measured the effect of friction acting on the marker in its bearing case by means of a realistic measuring principle and found a torque of static friction ranging from 10−7 N·m (air as bearing liquid) to about 10−6 N·m (special silicone oil). The torque linearly increases with increasing rotational frequency of the marker sphere. Furthermore, we estimated the influence of a background field HB by applying the method of look-up table and found that the operating distance D for monitoring mainly depends on the ratio HP/HB. Our preliminary set-up is working with pulsed HP fields and provides D ≈ 20 cm. However, D can be enlarged with increasing amplitude of HP. Taking into account practical demands of RMMM applications as well as technical restrictions, we propose means to enlarge the operating distance. It should be pointed out that the rotating marker can be used for remote controlled drug release as well.