Transformation of multichannel magnetocardiographic signals to standard grid form.

Multichannel magnetocardiographic (MCG) recordings with fixed sensor arrays are not directly comparable with single-channel measurements carried out at standard grid locations. In addition, comparison of data obtained with different types of magnetometers is difficult. We present a method for transforming multichannel measurements to the standard-grid format. The minimum-norm estimate (MNE) of the source current distribution in the body is calculated, and the desired field components in standard grid points are then computed from the MNE. We measured three subjects with both a 24-channel and a single-channel instrument. The signals extrapolated from the multichannel measurements corresponded quite well to the single-channel data registered at the standard grid locations, especially in those grid points that were covered by the 24-channel device. The signal-amplitude-weighted correlations between the extrapolated and directly measured signals were 0.73-0.87. In simulations with ideal measurement geometry but with a realistic amount of random noise in the signals, we obtained a 0.99 correlation. It was also found that the method is relatively tolerant to errors in the location and orientation of the multichannel magnetometer. For example, a simulated 20-mm displacement in the location of the sensor array caused only a 3% decrease in the correlation, and when it was rotated and tilted by 10 degrees C, the correlation decreased by 5%. The basic advantage of our extrapolation method is its physiologic nature: the method is based on the mathematical modeling of the source current distribution, rather than on direct constraints applied to the magnetic field.