Static Magnetic Field

The suspicion that exposure to high magnetic fields could be associated with some kind of risk dates from 1970, when MR diagnostic imaging was first introduced. However, unofficial data indicate that about 150,000,000 MR diagnostic examinations have been performed in the world from 1980 to 1999 (ca. 20,000,000 a year, or 50,000 a day) with very few accidents. The few documented cases of severe injury directly ascribable to the static magnetic field were due to magneto-mechanical effects on patient implants or devices introduced into the scan room by mistake [3, 5].

Translation and Rotation Forces

All materials that magnetize transiently have a susceptibility value other than 0; in particular, materials whose values range from -1 and 0, designated as dia-

magnetic, are repelled by the magnetic field. In fact, although all materials have a diamagnetic field, in some of them the presence of ions of transition elements tends to raise the susceptibility value. Materials with positive values are attracted to the magnetic field and are called paramagnetic. Ferromagnetism is an extreme form of paramagnetism and characterizes some materials having high levels of susceptibility, which are strongly attracted to magnetic fields. The presence of such objects in the magnet room is extremely dangerous and must be avoided. Diamagnetic and paramagnetic materials, whose susceptibility is about 0.01, respond weakly to the magnetic field and are often considered non-magnetic. To this group belong biological tissues, whose susceptibility is in the range of -9 x 10-6 ± 20%, i.e. the susceptibility of water. For instance, when patients are slid into the tunnel, the magnet exerts a force that contrasts this movement, although it is so weak as to be negligible [5].

The magnetic flow density B, expressed as tesla (T), represents the magnetic induction deriving from exposure to a magnetic field H, according to the relationship:

where the magnetic susceptibility;^ = M is the ratio of induced magnetization to the source field, and p0 is the magnetic permeability in vacuum.

Although the distinction between source and induced magnetic field tends to be neglected in practice, in MR diagnostic imaging it has an important role when the susceptibility has an appreciable positive value, as is the case of paramagnetic contrast agents and ferromagnetic objects.

When an object, such as the human body, a metal prosthesis or an erythrocyte, is introduced into an MR tomograph, it is subjected to translation forces in the gradient zones and to rotation forces with respect to the direction of the static field B. Such forces depend on the susceptibility of the material from which the object is made, on its shape and volume, and on the intensity of the magnetic field, and in relation to them they may be negligible or have potentially lethal effects.

The force of translation Ftrasl = • B depends on the volume V, the susceptibility v0alue and the mag netic field gradient, whereas the force of rotation required to oppose the object's rotation Frot = jjrV ■ B2 also depends on its length L and volume, thus o0n its shape.

In general, Frot>> Ftrasl; thus the tissue damage induced by the rotation force generated by the field of an implanted metal object will be greater than the damage caused by the force of translation, especially in the presence of high magnetic fields, due to its quadratic dependence on B [5].

In human tissues the magneto-mechanical effects are negligible due to their weak susceptibility; for instance, the alignment of erythrocytes parallel to the magnetic field due to their ellipsoid shape is negligible because of the quadratic dependence of the torque on susceptibility. The force acting on 100 g of water in an 8.0 T magnetic field with a peak gradient of 7.9 T/m and a product B ■ ^, equalling 43 T2/m, is negative and equal to -3.4 x10-3 N.

By contrast, a 100 g stainless steel tool with susceptibility 0.01 and a volume of 12.5 cm3 will be attracted to the same field with a force of 4.3 N and an acceleration of 43 m/s2, i.e. more than four times the force of gravity, turning it into a projectile [3, 5]. The magnitude of the force acquired by the paramagnetic and ferromagnetic objects would be greater by a factor of 102 to 105, posing serious hazards for workers and patients in the magnet area.

People with splinters lodged in their bodies or wearing metal prostheses are also at high risk, since the movement of these objects, induced by the magnetic field, may cause tissue or vessel tears.

Inside the magnet the field gradient is zero and so is the force of translation exerted on a ferromagnetic object, which will thus remain trapped in the tunnel. Attempts to extract it forcibly in case of an accident are often vain and can impair the magnet's stability in the cryostat. Recovery of the object usually requires inacti-vation of the magnet. By contrast, the rotation forces, which depend solely on B2, continue to be active. To prevent this type of risk, metal objects must never be introduced into the scan room. This is clearly stated in notices and is prevented from happening by taking an accurate patient history.

Since most of the data on the compatibility of several implanted devices and prostheses concern 1.5 T static magnetic fields, MR investigations of such patients at higher fields are to be avoided.

However, information on the safety and compatibility of 26 aortic aneurysm clips tested at 8.0 T and of 109 implants and devices tested at 3.0 T has recently been published. With regard to safety the objects have been shown to carry no additional patient risk save a reduction in image quality, whereas compatibility demonstrates the property of not interfering with image quality. The distinction between long- and short-bore mag nets of equal field intensity lies in the differences in the respective position and height of gradient peaks at the tunnel aperture [6-9].

In general, patients wearing pacemakers should avoid undergoing MR imaging, especially at a high magnetic field. The several effects that may derive from exposure are due to the static magnetic field (e.g. pacemaker shifting, remote control switch-off and ECG changes), to the RF field (e.g. heating, alteration of pace frequency and device reset), and to the gradients (e.g. induction of voltage, heating and remote control switch-off). However, these effects still present controversial aspects because on the one hand early compatibility studies assessed models that are no longer in use, and on the other, recent data indicate that small groups of patients underwent MR imaging for vital reasons without experiencing adverse effects [10, 11].

In most materials, the magnetization induced is parallel to H; in such cases M, B and H all point in the same direction and the material is said to have isotropic susceptibility. If, however, the susceptibility varies within the volume of the material exposed to the magnetic field, the object becomes magnetized along directions that may not be parallel to the magnetic field; in aniso-tropic cell structures, this may lead to structure reorientation or distortion. In such cases this effect, which has been documented in vitro, is stronger than the one induced by object shape [5].

Another biological effect connected with a potential risk, which in the past has been attributed to abnormal myocardial repolarization, is in fact induced by the blood flow in the large vessels within a strong magnetic field. The Lorentz force, F, induces on vessel walls an electric field E = f= vBsenQ, where v is blood velocity, the angle between the flow direction and the direction of the magnetic field, and q the charge of the ions in the blood.

This induced electric field causes a reversible increment in T-wave amplitude in the ECG. In vivo investigation of this effect at fields greater than 8.0 T has demonstrated that the induced potentials are below the threshold of nerve or muscle stimulation. The migration of opposite charges to vessel walls induced by the magnetic field also causes a current in a direction perpendicular to the flow velocity in vessels and thus an additional Lorentz force in a direction opposite to the blood flow that could contrast it appreciably. However, it has been demonstrated that this effect is negligible even in the presence of magnetic fields exceeding 10T [12].

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