Minimizing interference from magnetic resonance imagers during electrocardiography

Magnetic resonance imaging (MRI) allows a physician to obtain images of internal organs noninvasively. Imaging a moving organ such as the heart requires a trigger so that successive scans can be synchronized. In the case of cardiac imaging this trigger is the electrocardiogram (ECG). When a patient is in an MRI scanner he/she is subjected to both static and dynamic magnetic fields which can cause interference in the ECG. The dynamic fields consist of 8- to 64-MHz radio frequency (RF) pulses and low-frequency magnetic gradient pulses with frequency components below 100 Hz. Conventional bandpass filters are only moderately effective because the passband allows magnetic gradient-induced interference to be superimposed on the ECG, causing distortion of the signal. This paper describes a technique which can be used to remove induced MRI gradient interference from an ECG recorded on a patient inside the bore of a MRI scanner. Induced signal from an external loop is subtracted from the ECG to minimize the low-frequency interference. The gradient induced low-frequency interference was reduced to approximately 20% of its magnitude when using conventional ECG amplifiers.