Guidelines on Exposure to Electromagnetic Fields from Magnetic Resonance Clinical Systems - Safety Code 26

4. Health Effects of MR Fields (continued)

4.4 Magnetic Resonance Fields^*

A few studies have been performed on cells and animals using MR fields. Practically none of these experiments have been corroborated by studies in more than one laboratory. No detectable mutagenic or cytotoxic effects were found in Chinese hamster ovary (CHO) cells exposed to MR fields of 0.35 T, 4.6 T/s and a peak SAR of 2.9 W/kg at 15 MHz (4 pulses of 5 ms duration)⁽²⁶⁾. Under the same exposure conditions no chromosomal damage was found in CHO cells in culture exposed for 14 h⁽³⁴⁾. Mice were exposed to MR fields of 0.7 T at an average SAR0.087 W/kg (estimated) for 1 h. No differences in chromosomal aberrations in bone marrow cells were found between the exposed and control mice⁽²¹⁾.

Various bacterial strains were exposed to 1 T, 1 T/s and an average RF power of 0.097 W with no mutagenic or lethal effects found⁽³¹⁾. Human hymphocytes were exposed under the same conditions and no significant adverse chromosomal effects were observed⁽⁸⁾. Rats and guinea pigs were exposed to fields of 0.16 T and 2 T/s and a lack of changes in the blood pressure, heart rate and ECG was reported⁽³³⁾.

On the other hand, mice exposed to MRI fields (a 0.15 T system) failed to exhibit the normal nocturnal enhanced morphine analgesia during the mid-dark period. Animals exposed during the mid-light period had weaker response to morphine-induced analgesia. These results may reflect field-induced alterations in neuronal binding and/or changes in the pineal gland activity⁽¹⁹⁾.

Experience with humans clinically exposed to MR fields is relatively small, as the devices have not been in use long enough to provide the opportunity for a long-term medical assessment of patients and volunteers^(5,6,30). A six-month follow-up of 181 patients and 70 volunteers did not find any changes in cardiac and neurological functions. However, the MRI device used in these studies had a static field of only 0.04 T. No visual or central nervous system effects were found in 118 patients whose heads were imaged by MRI⁽³⁰⁾.

4.5 Cardiac pacemakers and Metallic Implants

Cardiac pacemakers can be affected by each of the three types of fields produced by MR clinical devices. The static magnetic field affects the reed switch in programmable demand pacemakers and reverses them into asynchronous operation. The static field also exerts forces (torque) on ferromagnetic components within the pacemaker, which may result in a movement of the pacemaker. Six representative pacemakers from different manufacturers were investigated in MRI fields up to 0.5 T⁽²⁰⁾. Minimum flux densities of the static magnetic field that altered reed switch position ranged from 1.7 to 4.7 mT depending on the pacemaker type. In this investigation the pacemaker was outside the human body. The reed switch returned to the original position when the pacemakers were removed from the field for magnetic flux densities of 1.3 to 3 mT. All six pacemakers experienced forces and torques when placed inside MRI systems operating at 0.5 T. The authors considered the torque on two pacemakers sufficient to result in significant movement of the pacemaker within the chest wall unless a sufficient degree of fibrotic tissue was present⁽²⁰⁾.

The time-varying magnetic field and radiofrequency field can interfere with the pacemaker circuitry. Most pacemakers employ protective measures against electro-magnetic interference (e.g. a titanium casing and a low-pass input filter⁽³⁰⁾. When such interference occurs the pacemakers reverts into asynchronous operation.

For instance, 20 out of 26 pacemaker models investigated reversed to an asynchronous mode or exhibited abnormal pacing in 60 Hz magnetic fields of 0.1 to 0.4 mT (this corresponds to about 0.04 to 0.15 T/s)⁽³⁰⁾. Furthermore, a time variation of 3 T/s caused unipolar pacemakers to recognize the induced voltage as a valid cardiac electrical signal⁽²⁰⁾.

Tests were performed on several pacemakers of various types in a 0.15 T MR system with a 6.4 MHz RF field produced by a transmitter operated at a maximum power of 1 kW with a pulse period from 130 to 500 ms^(12,15). In all pacemakers tested the static magnetic field caused reed switch closure resulting in asynchronous pacing at the programmed rate. An exception was a pacemaker which can be programmed to "magnet off" mode. This pacemaker continued normal operation in a magnetic field of 0.15 T for the in vivo tests (the pacemaker implanted in a dog)⁽¹⁵⁾. The authors conclude that conversion to asynchronous pacing is usually not a problem, but in some patients it may produce an arrhythmia.

Effects of RF pulsed fields varied for different pacemaker models and types. For some pacemakers the pacemaker rate was affected by the pulse rate of RF field, causing either a decrease in the rate⁽¹²⁾ or an increase⁽¹⁵⁾. However, the operation of some models of pacemakers remained totally unaffected by the RF fields when tested in vivo^(12,15). In all cases the RF field caused artifacts in the ECG recording. However, these artifacts were found inconsequential for the operation of the heart. None of the pacemakers tested showed any alterations in programmed parameters or in the ability to be reprogrammed after they were removed from the RF field⁽¹⁵⁾. The authors recommend that patients with cardiac pacemakers should have their pacing activity monitored continuously during tests in a 0.15 T MRI device⁽¹²⁾.

Metallic implants made of ferromagnetic and even diamagnetic materials experience force and torque in magnetic fields. All metallic implants are heated by the RF field and to a negligibly small degree by the time varying magnetic field used in MR systems.

Twenty-one aneurysm and other hemostatic clips and a variety of other materials were investigated for forces and torques experienced in MRI systems operating at 0.147 T and 1.44 T⁽¹⁸⁾. Sixteen clips were deflected by the fields, and for five aneurysm clips, forces and torques were considered sufficient to produce risk of hemorrhage from dislocation of the clip from the vessel or aneurysm, or cerebral injury by clip displacement. The level of risk depends on the degree of ferromagnetism and geometry of the clip, the field strength and gradient, as well as other factors such as the clip orientation relative to the field, the clip closing force, the condition of the vascular wall, tissues and structures close to the clip. Stainless steel alloys containing high percentages of nickel (10-20%) do not exhibit significant ferromagnetic properties. However, some stainless steels used for aneurysm clips and other clips have considerable ferromagnetism⁽¹⁸⁾. Clips made of tantalum or titanium are non-ferromagnetic⁽³⁰⁾. In another study⁽³⁾, 54 different types of surgical clips were characterized in a 0.15 T and 1.5 mT/m field. Nonmagnetic properties of tantalum and various austenitic stainless steel alloys and silver alloys were confirmed⁽³⁾. Several other types of aneurysm clips were examined, and recommendation was made against use of clips having a high martensite content⁽¹⁰⁾. Several types of magnetometers and metal detectors were investigated as possible pre-imaging screening devices⁽¹³⁾. Both types of devices are capable of detecting ferromagnetic clips imbedded in a patient⁽¹³⁾.

Heating effects of time-varying magnetic fields and RF fields were investigated for surgical clips (steel and copper) and hip prostheses⁽⁹⁾. It was concluded that heating of surgical clips in MR systems is not significant, but large implanted metallic prostheses and rods might cause a problem due to heating under some circumstances and when very high RF fields are useds⁽⁹⁾.

A total of 305 MR examinations were performed in 236 patients with metallic implants⁽¹⁶⁾. Most examinations were in a 0.3 T system. Patients with cardiac pacemakers, electrical implants, prosthetic cardiac valves and aneurysm clips were excluded from the study. The study was aimed at evaluation of image artifacts and possible adverse effects due to the metallic implants. The types of metallic implants were: surgical clips, central nervous system (CNS) shunting devices, tantalum mesh, craniotomy, sternotomy and other wire sutures, skin staples, and orthopedic devices (hip prostheses, knee prostheses, rods, plates, screws, pins and wires). Only two patients expressed complaints that could possibly, but not necessarily, be attributed to MR examination. In one case, a child with a CNS shunting device complained of pain behind the ear. In the other case a patient with a hip prosthesis complained of a burning sensation in the hip, knee and calf⁽¹⁶⁾.