Biomedical Magnetic Measurement with SQUID Magnetometer
SQUID magnetometer has made it possible to measure extremely weak magnetic field produced by biological electromotive force or other mechanisms. Biomedical magnetic informations are divided into two categories. One is the fluctuating magnetic field evoked by ion current of excitative tissues, i.e. cardiac muscle, brain, skeletal muscle, etc. These magnetic informations are called magnetocardiogram (MCG), magneto-encephalogram (MEG) and magnetomyogram (MMG) respectively. The other is the steady magnetic field induced by the magnetic particles taken into the lung or the gastointestinal tract.
Levels of these biomagnetic informations are very low, generally less than 10-6 gauss. Taking into the consideration of background magnetic noise in urban environment, which is about 10-3 gauss, it is very difficult to detect these informations in good S/N ratio.
To solve this problem, usually two methods are taken. One is the magnetic shielding method and the other is to use nosie canceling flux transformer. The latter method is rather practical and effective. Especially, a flux transformer of second derivative type is able to detect MCG or other biomagnetic informations without magnetically shielded room.
As a medical-use system, materials and configuration of dewar containing liquid-He should be further studied. From a clinical standpoint, where and how to lead MCG or MEG is a important problem. So far, useful and standard method has not been established.
MCG, which has the highest level in the biomagnetic informations, has been investigated from the early stage, and even now, is the main theme in this area. Usually, normal components of magnetic B vector to the chest wall are recorded as MCG and their patterns are similar to electrocardiograms (ECG). But differences between them are clear and interpretation and meaning of them are to be studied according to various cardiac patients.
A significance of measuring biomagnetic informations has been discussed conceptually or theoretically. From these theoretical points of view, a meaning of magnetic measurement is definite. It is now very important to find practical usefulness based on lots of clinical data.
Another interesting method using SQUID magnetometer is a “magnetic tracer”. This method is to detect and follow the dynamics of innocent magnetic particles like magnetite inhaled artificially in the lung. A time-course of magnetic diminishing indicates “clearance function” of the lung. It has been reported that asbestos miners, arc welders or heavy smokers impair this function. Magnetic tracer method, like radioisotope, has a great possibility to apply to various medical fields.
A research on biomagnetic information is now only on the start line. The SQUID magnetometer is a strong and unique tool for measurement these informations. Development of reasonable, handy and stable meadical-use SQUID system is expected.
