医療における人体磁界測定の有効性 心磁図の効力

Abstract

Action current of the heart muscle produces electric field on the body surface and also weak magnetic field around the thorax. Magnetocardiogram (MCG) is a record of this magnetic field by means of a very sensitive magnetometer. We usually record MCG at 36 points of the anterior chest wall by means of a SQUID with the transducer coil for the second gradient of the magnetic field. Electrocardiography has been used widely for the diagnosis of the cardiovascular diseases, but its limitations have also been known. MCG can not be expected to provide different kinds of informations from ECG, because both methods handle such informations related to the electromotive forces of the heart. But there may be some possibility that the MCG can give some diagnostic informations which can not be detected by ECG, because the magnetism is somewhat different from electricity in physical properties. For example diagnostic sensitivity of the right atrial overloading is 13.6% in ECG and 56.6% in MCG, showing higher diagnostic ability of MCG. As for the diagnosis of the right ventricular hypertrophy, diagnostic sensitivity is 31.3% in ECG and 73.3% in MCG, also showing the higher sensitivity in the latter than the former. Sensitivity for the diagnosis of the myocardial damage (abnormal ventricular repolarization) in hypertensive subjects was 20.0% in ECG and 42.5% in MCG. Sensitivity is especially higher in MCG in slight to moderate cases of hypertension. Multiple dipoles were detected in 7.5% in isopotential map and 27.5% in isomagnetic map at the peak of the T wave in lead II of the standard leads. The reason of the higher sensitivity of MCG for the diagnosis of the abnormal ventricular repolarization is thought to be due to the superiority of MCG in detecting multiple dipoles. At present, liquid helium is necessary to record MCG, and the multi-elements MCG system are not available with a reasonable price. It is expected that the MCG may become to be used widely in clinical medicine as a supplementary diagnostic method to the ECG, if these weak points are overcome.