By Alan Macy, BIOPAC Systems, Inc.
When performing MRI and simultaneously recording time-series physiological data, there are two basic types of artifact associated with MRI processes. The first type of artifact is associated with the presence of the large constant MRI magnetic field. This type of artifact is typically called “magnetohydrodynamic effect” (MHD). MHD artifact results from movement of conductive fluid (blood) and from the movement of charge inside the body, when exposed to a magnetic field. A motional electromotive force (Vemf) is generated when a conductor moves through a magnetic field. Also, a charge moving in a magnetic field may have its travel path affected by the field. This two phenomenon can distort, and superimpose with, an existing biopotential signal. The most prominent MHD effects occur in the vicinity of the heart. The aortic arch is aligned perpendicularly to the MRI magnetic field and this geometry is conducive to creating large magnitude MHD artifacts in the ECG signal. Aortic blood flow is highest during systole, which corresponds to the ST segment in the ECG signal, so the largest MHD artifacts occur during this period. MHD effects are easily seen when measuring ECG inside the MRI, even when scanning is off. In this situation, the ECG signal will usually distort to the point of being non-diagnostic, such as lengthening QRS time and increasing T-wave amplitude.
The second type of artifact is associated with the time-based scanning processes occurring within the MRI. During scanning, two sources generate artifacts during biopotential measurements in the MRI. The first source is associated with magnetic field gradient switching during EPI or other scanning sequence. As the magnetic field changes orientation with respect to the biopotential electrode lead-subject-amplifier loop, a current is induced in the loop. The magnitude of induced current is proportionally linked to the loop area and number of turns in the loop. The second source is associated with radio-frequency (RF) pulsing coincident with gradient switching during the scanning sequence. The RF is at the Larmor frequency and couples to the biopotential loop conductors, which include electrodes, electrode leads and subject. RF energy can circulate in this loop because the biopotential signal conductors will travel through the cable harness to the patch panel connectors separating chamber room from control room. RF energy will couple throughout the cabling harness due to distributed capacitance in the cable, thus establishing an RF conducting loop that includes subject, electrodes, electrode leads and distributed capacitance in cable harness.
Download a related and important application note that describes the relationship between MRI imaging sequences and associated physiological measurements, when subjects are monitored in the MRI. In particular, this note addresses newfound insights on the impact that imaging sequence types have upon simultaneously recorded electrocardiography (ECG) and impedance cardiography (ICG) subject measurements.
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