For cardiovascular MR imaging, most systems use magnetic field strengths of 1 to 3 T. Research systems are being installed for human use that are 8 T and higher. Short-term exposures to static magnetic fields have not been shown to result in harmful biological effects in humans. The FDA has categorized clinical MR systems with a static magnetic field of up to 8 T as posing “nonsignificant risk” for patients >1 month old. (Current FDA guidelines can be found at their Web site: www.fda.gov.)

The fringe field around the MR scanner decreases with distance from the magnet. The static magnetic field strength that is thought to be totally risk-free for all individuals is 5 gauss (recall that 1 tesla = 10,000 gauss, so 5 G is 0.0005 T), or about 10 times the strength of the earth’s magnetic field. For safety purposes, the 5 G line is demarcated with signage and physical barriers that exclude access by members of the public who have not been screened appropriately.

The hazards to a subject resulting from the static magnetic field within the 5 G line primarily result from metallic projectiles and implanted or foreign metallic objects, including defibrillators and pacemakers. Ferromagnetic material behaves like a magnet when placed in a magnetic field, B. If the field is uniform, the material will tend to align itself with the direction of B and remain stationary. In the presence of a gradient, such as the fringe field outside of the magnet, ferromagnetic items will travel toward increasing B.

The attraction of objects such as non-MR-compatible oxygen tanks, intravenous fluid poles, floor polishers, and medical instruments to the MR scanner can result in their transformation to deadly projectiles. Several incidents of ferromagnetic projectiles causing injury or death have been reported. Careful testing, verification, and labeling of external devices form a significant component of any safety management program in a clinical MR facility.

Note also that damage to magnetic data storage devices (computer disks), credit cards, cameras, watches, and other electronic devices can occur in the magnetic field area within the 5 G line. As mentioned in Chapter I-1, it is
important to realize that once the MR system is installed and brought to field, the superconducting magnet is always on. These risks exist 24 hours a day, regardless of whether the MR computer console or system lights are on or off.

Specific considerations about implanted medical devices are reviewed in subsequent sections of this chapter.

Exposure of flowing blood, which is essentially a fluid filled with charged particles, to the static magnetic field causes a magnetohydrodynamic effect. The resulting voltage changes that occur across the diameter of a vessel are not considered physiologically significant, although they can manifest as an elevated T wave on the electrocardiogram. This effect does have the potential to interfere in the monitoring for ischemia in subjects inside the MR system, as will be discussed in Chapter III-1.

RF pulses result in power deposition in human subjects that is transformed into heat. MRI-related heating depends on a variety of factors including the nature of the exposure, the subject’s thermoregulatory system, certain underlying health conditions and medication use, and ambient conditions such as temperature, humidity, and airflow. The measurement used to describe the absorption of RF energy is the specific absorption rate (SAR), which is expressed in watts per kilogram. The SAR for an MR sequence is usually expressed in terms of whole-body averaged SAR or peak SAR. Current U.S. FDA guidelines limit whole-body SAR exposure to 4 W/kg for subjects with normal thermoregulatory function and 1.5 W/kg for all subjects regardless of their condition. Roughly speaking, the guidelines are based on levels that produce a maximum change in tissue temperature of less than 1°C. It is important to realize that the RF power deposition increases with the square of the B₀

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