In medical care, the correct placement of a medical device such as a catheter or a guide wire in a patient has become increasingly important for a number of reasons. In the case of an infusion catheter, for example, medications can need to be targeted to, or for, specific organs, or areas of the body. A catheter can need to be located sufficiently near the heart in a particular region where there is a particular blood flow rate; as for example, a particular high blood flow rate to ensure adequate dilution/mixing of infused fluids. Alternatively, a catheter or other internally-positioned medical device can simply need to be disposed in the right place to function; as for example, an enteral feeding tube within the stomach. Use of a medical device position location and/or guidance system can thus provide for less skilled practitioners to accurately and reliably position a medical device such as a catheter.
Accordingly, a variety of systems have been developed to attempt to indicate location or position of catheters within the body of a patient. Relatively reliable location devices have made use of x-ray or fluoroscopy; however, these devices expose the patient and/or caregiver to undesirable amounts of radiation. As a consequence, a variety of different systems have been attempted to more continuously and accurately indicate location of a catheter with a goal of replacing the use of x-rays. However, such systems still suffer from various problems.
Electromagnetic catheter position location devices have been in development. Some position location systems have made use of alternating current, AC, driven external coils with a sensor (sensor coil) in the catheter tip. Others have used an AC driven coil in the catheter tip with external sensor coils. A disadvantage of such a conventional catheter tip driven system has been the need for heavy or thick wires running into the catheter to carry sufficient drive current to generate a sufficient electromagnetic signal for the external sensors. This has precluded the use of such a system with smaller diameter catheters. Other position location systems have used a fixed (or DC) magnet on the catheter tip with external sensor coils. A significant disadvantage to such a fixed magnet location system has been that the magnet would necessarily be very small, and as such would generate a very small signal from the tip of the catheter. As a consequence, other magnetic fields in the vicinity can create significant interference problems for such a system. Furthermore, the field of such a magnet drops off extremely quickly over distance and thus cannot be sensed more than a few inches deep into the patient's tissue. Another AC drive system has been described including driving two coils simultaneously; however, those respective coils were specified as having been driven at two different frequencies so that the coil drives are not additive and the sensor demodulates the two different frequencies as two independent values. Yet another AC drive system has been described driving two coils simultaneously in quadrature which simulates a single spinning coil; however, this system can only indicate the orientation of the sensor in the x-y plane and its relative position in that plane.
This statement of background is for information purposes only and is not intended to be a complete or exhaustive explication of all potentially relevant background art.