Non-invasive Detection System For Metallic Foreign Body

In daily life or clinical surgery, metallic foreign body may enter or remain in vivo, causing a lot of harm to human bodies, and most of time it must be removed. The exact location of metallic foreign bodies is the primary condition for fast removing. Among the existing positioning methods, metal detector is a simple, portable and non-invasive way. In addition, the metal detector can detect hidden metal objects, which is widely used not only in safety inspection in national defense, public security, customs, but also in building construction. But most of the metal detectors at present have poor anti-interference ability, low measurement sensitivity, and can not locate the target. To solve these problems, we use a variety of weak signal detection methods to explore and design a non-invasive detection system of metal objects in vivo with high sensitivity, high stability, simple to implement and convenient to employment, which is based on eddy current sensing method. It can also be used as a metal detector instrument.Metal detector is based on the eddy current effect of metal conductor; use the non-contact displacement measurement methods of eddy current sensor to achieve. Preliminary experiments of the eddy current sensor show that the eddy current effect of the little metal objects are very weak, and the key of the system is how to achieve high-resolution and high-sensitivity detection of weak change signal. Therefore, the system was designed combining the digital lock-in detection, over-sampling techniques with systemic scaling method. And the implementation plan was developed according to the design requirements for the system. On this basis, non-invasive detection system of metal in vivo was designed.The non-invasive detection system of metal objects in vivo was improved from many aspects, including the excitation source, the sensor itself, the measurement circuit and the signal acquisition and processing method, to improve the sensitivity and reliability of detection. The system synchronized the excitation with the acquisition and processing of the signal by the control unit to improve the stability; using home-made detection coil and combining constant current source circuit with the resonant circuit to simplify the measurement circuit, and to improve the sensitivity of detection; using digital lock-in detection technique in the processing of weak changes of AC signal to filter out a variety of noise other than the tested frequency, to improve signal to noise ratio; furthermore, using over-sampling technique to improve the conversion accuracy of ADC, reduce the ontology noise of the system and improve the detection accuracy and resolution; using systemic scaling measurement method to reduce the influence on measurement accuracy caused by the fluctuations of the excitation signal amplitude and the ADC reference voltage.As the eddy effect of the metal in vivo was very weak, there were higher requirements for the measurement circuit and signal acquisition and processing methods. According to the different requirements of digital lock-in detection algorithm, systems using ADuC841 and ADSP-BF506F as the control units were built, then the detection experiments on different metal objects were carried out. The assessment of sensitivity, stability, and accuracy of the system show that the detection system had an accuracy of nearly 5mm in the range of 0 ~ 60mm for the iron block, the copper block of 20mm×15mm×0.8mm, and a dollar coin. Compared with the existing metal detectors, this system is simple to use,has a good anti-jamming ability and reproducibility, and provides a convenient and practical means for fast detection and location for metallic foreign bodies in vivo.