Measurement of Minute Changes in Magnetic Flux Density by Means of a Novel GMR Needle Probe for Application in Hyperthermia Therapy

The accuracy of heat deposition, which is proportional to magnetic fluid weight density, appears to be a major factor in the quality of magnetic-fluid-based hyperthermia. An innovative giant magnetoresistance (GMR) needle probe is proposed as a means of estimating very-low-concentration magnetic fluid weight density in a low-invasive way. The method of estimation is based on the difference between magnetic flux densities inside and outside a magnetic-fluid-filled area in the body, given that the applied magnetic flux density is uniform. Since, for very-low magnetic fluid weight densities the percentage change in magnetic flux density is in the order of 1/100, it is important that the applied magnetic flux density be at least 1/10th more uniform. For this purpose a Lee-Whiting type coil was designed and fabricated, producing a 0.001% fluctuation from the center of the coil in approximately 35% of the outer coil spacing along the axial direction and 25% of the diameter of the coils in the radial direction. Concentrations as low as 0.03% weight density (a change in magnetic flux density in the order of nanotesla) were estimated by means of a GMR needle probe.