[Development of passive telemetry system for intracranial pressure measurement with corrector of errors caused by...

A new passive telemeter for the intracranial pressure monitoring have been developed. The completely implantable pressure sensor used in this system consists of a crystal, a coil and a ferrite rod attached on a diaphragm. The pressure on the diaphragm alters the volume of air in the receptacle and then the ferrite rod moves in and out of the coil and alters the resonance frequency of the sensor. Although the sensor doesn't have a battery or other energy storer, for example, a capacitor, the resonance frequency can be measured without contact. Therefore, at any time, we can measure the intracranial pressure with this sensor implanted under the scalp beforehand. However, not only pressure but also temperature alters the resonance frequency of the sensor, because the volume of air alters in proportion to temperature. Hence, we have developed a new passive telemetry pressure sensor which contains a passive telemetry temperature sensor. The temperature sensor consists of a coil and a special crystal whose resonance frequency varies with ambient temperature and its resonance frequency can be measured in the same way that we measure the resonance frequency of pressure sensor from outside of the body. With this system, we can measure the intracranial pressure about 60 times per second and the intracranial temperature every 8 seconds. The measured value of the pressure was automatically corrected by analog temperature correcting electric circuits. In animal experiment, the output of this system was similar to one of the catheter-tip type pressure transducer and we could observe the intracranial pressure altered synchronizing with respiration and with heart beat.