Magnetocardiogram recordings in a nonshielded environment--reproducibility and ischemia detection
Benjamin A Steinberg1, Ariel Roguin, Stanley P Watkins
1Department of Medicine, Division of Cardiology, Johns Hopkins Medical Institutions, Baltimore, MD, USA.
Background:
Magnetocardiography (MCG) is a noninvasive technology that measures the magnetic field of the heart by superconducting quantum interference devices (SQUID) sensors. The novelty of the present system is that the sensors can be operated without electromagnetic shielding of the examination room, thus allowing the system to be easily installed in the emergency department or chest pain unit. Studies in shielded rooms, found that this imaging modality may have better sensitivity as compared to ECG in detecting ischemia. We aimed (1) to assess the reproducibility, intra-observer, and interobserver interpretation variability and (2) to assess the MCG maps in the presence of coronary narrowings.
Methods And Results:
All measurements were performed in a nonshielded room. For the first part of the study, two MCG maps were recorded in 24 otherwise healthy volunteers (age 20-44 years, median 24, 16 male) in an interval ranging from 2 to 48 hours. The maps were interpreted using the CardioMag software for contour maps, averaged MCG time traces, and waveform morphology of repolarization by two observers blinded to each other. The parameters tested had low disagreement between repeated measurements. The correlations of the intra-observer and interobserver interpretation were excellent. Secondly, MCG maps were obtained in 29 patients referred for angiography due to suspected coronary artery disease. Nineteen of them had coronary narrowings defined as more than 50%. In this group, 16 (84.2%) had abnormal MCG maps as compared to only 5 (26.3%) who had abnormal ECGs (P < 0.01).
Conclusions:
MCG maps can be successfully obtained in a nonshielded room and allow feasible, accurate, and reproducible measurements with little intra-observer and interobserver variability. Ischemic changes in the heart's magnetic field may occur before electrical changes. Our pilot data suggests that this imaging modality may potentially offer better sensitivity as compared to rest ECG in detecting ischemia in a cohort of patients who had coronary narrowings identified by angiography.
