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The AAPM/RSNA Physics Tutorial for Residents¹

MR Imaging Safety Considerations

¹ From the Department of Radiology and Radiological Sciences, Vanderbilt University Medical Center, R-1308 Medical Center North, 1161 21st Ave South, Nashville, TN 37232-2675. From the AAPM/RSNA Physics Tutorial at the 1998 RSNA scientific assembly. Received July 13, 1999; revisions requested August 23 and received September 23; accepted September 24. Address reprint requests to the author.

View larger version (17K): [in a new window]   Figure 1.   Diagram illustrates how flowing blood behaves as a moving conductor within a magnetic field. Blood flowing with velocity (V) within a vessel oriented at an angle () with respect to the magnetic field (B) will produce a force (F) on the charge carriers in the blood. This force in turn produces a voltage difference across the vessel in a direction perpendicular to the blood flow. The magnitude of this electromotive force (emf) is equal to the product of the vessel diameter (d), the velocity (V), the field strength (B), and the sin of the angle between the direction of the blood flow and the magnetic field.

View larger version (13K): [in a new window]   Figure 2.   Diagram illustrates the magnetohydrodynamic effect of blood on an electrocardiographic tracing. The largest electromotive force (voltage) induced in a vessel will occur during that part of the cardiac cycle that produces the highest velocity blood flow. Because this flow occurs during the T-wave portion of the cycle, the effect causes the largest additive voltage to occur during the T wave, which results as an enhanced T wave in the electrocardiographic tracing.

View larger version (15K): [in a new window]   Figure 3.   Diagram illustrates the induced magnetic moment of a metallic object placed in a magnetic field. The strength of the magnetic moment (µ) will depend on the strength of the magnetic field in which the object is located and the susceptibility of the object. The induced magnetic moment of the object will be attracted by the magnetic field of the MR imager. The force of the attraction will depend on the magnitudes of the induced magnetic moment and of the field gradient in which the object is located. The object will be attracted in the direction of the higher field strength region of the MR imaging unit. In general, the magnitude of the field gradients increases as the distance from the imaging unit isocenter decreases. Thus, the force of attraction is greater as the object comes closer to the magnet. The number of magnetic field lines per unit area is used to indicate the region of stronger fields.

View larger version (20K): [in a new window]   Figure 4.   Diagram illustrates the inductive coupling of the RF field to a wire loop. The quantity B1 represents the strength and direction of the RF field with frequency () in the presence of a conductive loop with area A. The time-varying magnetic field produced by the RF coils will induce an electromotive force (emf) in the coil, which will in turn cause a current flow. The magnitude of the force is given by: emf = B1 A sin , where is the angle between the direction of the B1 field and the plane of the coil. The maximum induced electromotive force occurs when the B1 field is perpendicular to the plane of the coil. For maximum patient safety, nonconducting fiber-optic cables should be used instead of conductors whenever possible.

View larger version (21K): [in a new window]   Figure 5.   Diagram shows an inadvertent body loop, which can cause RF induction and result in burns. Inadvertent loops can be created in the upper body (when the skin of the two hands touch) and the lower body (when the skin of the two calves touch). Inadvertent loops can be avoided by eliminating bare skin contact points.

View larger version (23K): [in a new window]   Figure 6.   Graph demonstrates thresholds of stimulation of cardiac and peripheral nerves from rapid gradient switching in terms of dB/dt as a function gradient ramp time. The graph also shows the current FDA guidelines for allowable gradient switching rates. The FDA guideline values are the lowest of the three curves. The middle curve is the mean peripheral nerve stimulation threshold, which is approximately three times higher (3x) than the FDA levels. The top curve is the mean threshold for cardiac stimulation. The cardiac stimulation threshold is approximately 30 times (30x) higher than the FDA guideline. The vertical axis is a log scale rather than linear.

View larger version (46K): [in a new window]   Figure 7a.   Sample patient screening questionnaire for MR imaging procedures. (Reprinted, with permission, from reference 1.)

View larger version (44K): [in a new window]   Figure 7b.   Sample patient screening questionnaire for MR imaging procedures. (Reprinted, with permission, from reference 1.)