18-02-01 Incidental Hazards: External Objects
Projectiles. The most imminent danger for both patients and personnel in the magnetic field of an imaging system may result from ferromagnetic objects such as scalpels, scissors, pens, and even sand bags (not filled with sand but with iron shot) and gas dewars, which can be attracted by the magnet and thus behave like projectiles (Figure 18-06).
To prevent such accidents, the installation of a metal detector through which everybody has to pass before entering the MR suite has been recommended, but is rather cumbersome. Instead, automatically locking doors with badge access are recommended.
Monitors and respirators. The dependence on physiological monitoring, on mechanical respiration, and electric infusion pumps during MR examinations renders difficulties, and in certain instances does not allow such an examination.
However, with the development of appropriate monitoring and life-support equipment during the last few years, dependence is no longer a contraindication of MR imaging. Details on monitoring can be found, for instance, in an official Canadian publication [⇒ Medical Advisory Secretariat].
Contrast agents. Magnetic resonance contrast agents or other substances which have to be injected or applied in another way may present risks similar to those in any other invasive technique such as x-rays, particularly in patients with kidney diseases. In patients with kidney diseases certain contrast agents are contraindicated (see late adverse events, NSF).
The clinical experience of administering gadolinium-based or other agents intravenously to patients has shown that these agents are generally safe and well tolerated. Still, all necessary precautions for intensive-care treatment have to be considered when injecting such contrast agents, particularly in patients with a history of allergy, drug reactions. When contrast agents are used according to the given guidelines and regulations such side effects are extremely unlikely.
A precautionary 24-hour suspension of breast-feeding was generally recommended following the administration of gadolinium-containing contrast agents. However, it has been proposed that this suspension be reduced to 12 hours [⇒ Hylton].
18-02-02 Incidental Hazards: MR Equipment
Field Strength and Hazards. Many of the incidental hazards increase with field strength and the existing research on safety cannot straightforwardly be extrapolated to field strengths beyond 1.5 Tesla. Further studies are required to confirm the hazard-free use of MR imaging at ultra-high fields. Until proven otherwise, patient studies at ultra-high fields should be considered carefully.
Noise. The noise created by the switching of the gradients is an additional source of inconvenience and can cause ear damage to the patient and, occasionally, personnel [⇒ Radomskij]. This noise is comparable to very heavy traffic. Sound pressure levels (SPL) increase with field strength. The noise levels at 3 Tesla can approach twice those of those at 1.5 Tesla. Sound pressure levels at the center of a head coil can exceed 130 dB(A). For protection against damaging noise exposure, British and US American guidelines stipulate that the maximum permitted daily noise dosage is equivalent to 90 dB(A) for 8 hours; the guidelines for Europe allow only 80 dB(A) for 8 hours [⇒ Foster].
To minimize the acoustic burden, disposable earplugs for the patient are recommended in high-field MR machines. Noise-cancelling systems and special earphones are available, and active acoustic control systems are being developed [⇒ Mansfield 2000]. At fields of 3 Tesla and higher earplugs and headphones are strongly recommended.
Cooling gases. In superconductive magnet systems, helium and occasionally still nitrogen are used as cooling gases. In the case of a quench, gases are released to the outside. Under normal circumstances, the gases should escape through a pipe system and not reach the magnet-room atmosphere. However, accidentally some gas could be released into the magnet room.
In this case, there are two potential dangers. Frostbite can be induced because the gases are extremely cold. Secondly, nitrogen is to be considered hazardous, in particular under pressure (whereas there is no danger of direct intoxication from helium). All personnel and patients must evacuate the area immediately and return only after proper ventilation of the magnet room. Oxygen monitors with an audible alarm, situated at an appropriate height within the magnet room were proposed as safety devices [⇒ Shellock 1994].