When a patient is placed in the magnetic field of an MR scanner, an elevation of the T-wave of the EKG is frequently noted. This elevation may be so marked that the T-wave actually becomes larger than the QRS-complex.The R-wave may also be reduced in amplitude and is occasionally inverted.
These effects on the T- and R-waves may result in faulty cardiac triggering, especially at higher fields.
In humans imaged at 1.5 T, the MHD-induced voltage is on the order of 5-10 mV and is even larger at 3.0T and above. The reason this voltage is superimposed on the T-wave is that this is the time of maximum flow in the descending aorta. The MHD-induced voltage distorts the recorded EKG but produces no untoward effects on the heart or blood flow. Its only potential clinical significance is that it may cause ST-segment elevation or depression and thus mimic or mask cardiac ischemia occurring during the MRI examination.
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The magnetohydronamic (MHD) effect arises from blood plasma ions (principally Na+, Cl−, and HCO3−) flowing in the descending thoracic aorta. In an external field (Bo) these ions experience a so-called Lorentz force (F) proportional to Bo, the velocity of blood flow (v), and the sine of the angle (θ) between them. F is a vector cross product and hence has direction perpendicular to the plane containing both Bo and v.
In cylindrical MR scanners the direction of Bo and v in the descending aorta are approximately collinear (i.e., directed from head to foot). Thus the ions tend to accumulate along the walls of the aorta, leading to a potential difference (E) across the lumen proportional to the magnitude of the Lorentz force (F) and the vessel diameter (d). This potential difference E, a voltage, is the MHD effect.
How this local MHD effect relates to the EKG recorded at the skin surface is a complex prediction, requiring assumptions about blood and tissue densities and conductivities, the shape, size, and composition of the body, as well as the location of the electrodes.
Orientation of the main magnetic field is important in determining where the MHD effect will be greatest. In cylindrical magnets the direction of the Bo field is generally collinear with the descending aorta. Hence the most dramatic EKG changes are typically noted in the inferior leads (II, III, aVF) and minimal in the precordial leads (V2, V3). The opposite effect is noted in vertical field scanners with leads III and aVF having the least artifact and precordial leads the most.
It should also be noted that the effect on the R- and T-waves will vary depending on whether the patient is placed head first or feet first in the scanner and whether the patient is prone or supine. In some orientations, inverted (rather than augmented) T-waves may be recorded.
Chakeres DW, Kanarlu A, Boudoulas H, Young DC. Effect of static magnetic field exposure of up to 8 Tesla on sequential human vital sign measurements. J Magn Reson Imaging 2003; 18:346-352.
Jekic M, Ding Y, Dzwonczyk R, et al. Magnetic field threshold for accurate electrocardiography in the MRI environment. Magn Reson Med 2010; 64:1586-1591.
Krug JW, Rose G. Magnetohydrodynamic distortions of the ECG in different MR scanner configurations. Computing in Cardiology, IEEE, 2011, pp 769-772.
Nijm GM, Swiryn S, Larson AC, Sahakian AV. Characterization of the magnetohydrodynamic effect as a signal from the surface electrocardiogram during cardiac magnetic resonance imaging. Comput Cardiol 2006; 33:269-272.
We often have trouble getting a clean EKG signal for cardiac gating. Any helpful hints?