Signal Amplitude Ratio of Magnetocardiogram Between Sensor Assembly and Chest Surface

Magnetocardiography (MCG) using a superconducting quantum interference device (SQUID) sensor has been known for a noninvasive, noncontact, and safe diagnostic tool to detect ischemic heart disease such as angina and myocardial infarction. The magnetic distribution generated by the electrical activation of the heart reveals the abnormality of the myocardial status. The bust depth of people in 20s and 70s are much different, and the greatest difference is up to 154 mm. The purpose of this study is to identify whether MCG signals and parameter values are changed at the different locations of the SQUID sensor assembly. It would be an important reference for the standard measurement. Four healthy male subjects (33.3 plusmn 6.3 years) participated in this study. The distance between the chest surface and the bottom of the sensor assembly was 20, 40, 60, and 80 mm. Recording was made using 64-channel MCG system (axial type, first-order gradiometer) developed by Korea Research Institute of Standards and Science (KRISS). MCG were recorded for 30 s in a magnetically shielded room. As the sensor location is getting away from the chest surface, the each mean amplitude of R- and T-wave peak decreases to 70% (at 40-mm distance), 51% and 50% (at 60 mm), and 38% and 35% (at 80 mm) of the reference amplitude measured at 20-mm distance (
y
= -0.000002*
x

³
+ 0.0003*
x

²
- 0.0308*
x
+ 1.4961,
R

²
=1.00; where
y
= average amplitude remained after reduction},
x
= distance between the chest surface and sensor location). Interindividual difference was less than 5%. The regression equations may be used as a good reference to calculate how much strength will be decreased by the distance. In MCG parameters, most parameters decreased when the distance increased. As an example, the current moment at T-wave peak reduced to 52% (at 40-mm distance), 33% (at 60 mm), and 19% (at 80 mm). Results showed how MCG signal and parameter values decreased rapidly when the sensor assembly was located far from the signal source. The study results may be used as a useful reference to design MCG system and analysis program.