Location of ferromagnetic foreign bodies by the PF-02 ferroprobe pole indicator

Abstract

UDC 615.47.03:616-003.6-089.878 The Institute for Physics of Metals, Ural Division, Russian Academy of Sciences has a leading place in the Russian Federation in the development of medical ferroprobe pole indicators. Several models of ferroprobe pole indicators have been developed at the Institute over the last two decades, including FP-2, PFT-2, PF-1, and PF-01 devices [3, 7, 8, 10]. Besides, various comprehensive methods of preoperative and operative ferroprobe diagnosis were developed for general and ophthalmological surgery, boundary location of ferromagnetic foreign body in orbital cavity, etc. [2, 4, 5, 7]. These methods have been successfully tested in clinics and hospitals of Ekaterinburg, Moscow, and St. Petersburg. In the present state of surgery, it is not enough to merely remove the foreign body, but it is important to remove it with minimum possible damage to surrounding tissues. Therefore, maximum accuracy of foreign body location during the preoperative period and surgery itself is very important. In technical terms, this can be achieved by increasing the sensitivity of ferroprobe pole sensors, by eliminating false signals caused by a uniform static geomagnetic field and low quality of magnetosensitive elements of the ferroprobe sensor (FS) [1, 3, 7], and by improving the circuitry base of the ferroprobe pole indicator. Consider the problem of increasing the sensitivity of a ferroprobe pole sensor in more detail. Theoretically, the sensitivi- ty of a ferroprobe pole sensor can be increased by decreasing the dysbalane between the magnetosensitive elements of the sensor or by increasing the gain of the electronic unit of the device. However, in practice, maximal sensitivity of a pole indicator is limited by the magnetic environment in the operating room (influence of devices, equipment, operating table, light sources, construction elements, etc.). Because the foreign body signal amplitude decreases as the fourth power of the distance from the free end of the probe, the problem of noise proofing places substantial limitations on further increase of the sensitivity of ferroprobe pole indicators. It should be noted that the sensitivity to gradient of static magnetic field strength of the PF-01 ferroprobe pole indicator, which is widely used in medical practice, reaches 4 mOe/cm full scale (0.0032 A/cm 2 in SI units). This allows sufficiently accurate preoperative location of ferromagnetic foreign bodies in living tissues, although if the depth of penetration of the foreign body exceeds 50 to 70 ram, such sensitivity proves insufficient for accurate preoperative location of the foreign body. Therefore, it seems that further increase in the accuracy of location of a ferromagnetic foreign body by significant increase in the sensitivity of the pole indicator is not a very promising approach to the problem. Consider the problem of increase of the accuracy of location of a ferromagnetic foreign body by means of elimination of false signals. Presently available models of pole indicators have a substantial disadvantage because variation of spatial orientation of the FS, e.g., rotation about its longitudinal (measuring) axis or slewing by 180 ~ induces a false signal which simulates the presence of an inhomogeneous magnetic field (i.e., the presence of a ferromagnetic foreign body), whereas the magnetic field is homogeneous (ferromagnetic foreign body is absent). These effects are due to: a) nonparallel orientation of magnetic axes (cores) of the FS magnetosensitive elements; b) lack of identity between the transformation coefficients of the FS magnetosensitive elements. In medical practice, a false signal may cause an unnecessary surgical incision which may have significant operative and postoperative complications. Both methodological and technical measures can be taken to eliminate or substantially reduce false signals. A ferroprobe pole sensor is considered as suitable for practical purpose if the amplitude of false signal measured at Institute for Physics of Metals, Ural Division, Russian Academy of Sciences. Ural State Medical Academy, Ekaterinburg. Translated from Meditsinskaya Tekhnika, No. 6, pp. 28-33, November-December, 1996. Original article submitted November 16, 1995. 0006-3398/96/3006-0337515.00 9 Plenum Publishing Corporation 337