Publisher Summary

The medical community has been sensitized as a result of experience with ionizing radiation, which has a known genotoxic and oncogenic effect. The potential adverse bioeffects of ultrasound, which is now routinely used in early pregnancy, have come under scrutiny. Therefore, any new technology, such as magnetic resonance imaging (MRI) with potential widespread use in reproductive medicine, elicits appropriate caution and enquiry. This chapter examines the bioeffects of the individual modalities within nuclear magnetic resonance (NMR) for likely endpoints, and also combinations in extreme conditions of their applications. The potential adverse effects of static magnetic field include flashes, induced currents that affect muscle and nerve conduction, and bone healing. The chapter also provides an overview of the bioeffects of NMR, from research using cell cultures, on human populations. The photon energy changes that are associated with nuclear excitation and relaxation are lesser than those associated with ionization by many orders of magnitude. There is low probability of long-term ill effects of MRI because it is based on a weak radiofrequency phenomenon.

Introduction

Over the last 10 years, there has been increasing concern over the potential adverse bioeffects from different imaging modalities. In particular, the medical community have been sensitized as a result of experience with ionizing radiation, which has a known genotoxic and oncogenic effect. More recently, the potential adverse bioeffects of ultrasound, which is now in routine use in early pregnancy, have come under scrutiny (Frigoletto, 1984). Therefore any new technology such as magnetic resonance imaging (MRI) with potential widespread use in reproductive medicine will elicit appropriate caution and enquiry.

During clinical imaging with magnetic resonance, a subject is exposed to a static magnetic field which may vary in field strength from 0.08 to 2.0 T, both static and as pulses in the form of field gradients. The radiofrequency (RF) pulses also vary over a wide range (3.5–100 MHz). Examination times are presently long, in comparison to other imaging modalities: 30–60 min is typical. In addition magnetic resonance scanners are a source of acoustic noise emanating from movement of the gradient coils. Psychological effects such as patient claustrophobia and anxiety also need to be addressed. More recently, another health consideration in the form of an intravenous contrast agent [Gadolinium diethylenetriaminepentaacetic acid (DTPA)] has been introduced.

The bioeffects of the individual modalities within nuclear magnetic resonance (NMR) have been examined for likely endpoints, and also combinations in extreme conditions of their applications. The potential adverse effects of static magnetic field include flashes (Lovsund et al. 1980), induced currents that affect muscle and nerve conduction (Roy, 1980; Polson et al., 1982), and bone healing (Bassett et al., 1977).

There is now a vast literature on the bioeffects of NMR from research using cell cultures to those on human populations. Initial concerns are diminishing, as additional and reproducible data are becoming more available. Most of the early studies focused on mature rather than developing systems, and so are not directly applicable to the potential effects on the reproductive cycle. The photon energy changes that are associated with nuclear excitation and relaxation are many orders of magnitude less than those associated with ionization. It is because MRI is based on a very weak RF phenomenon that there is the expectation of a very low probability of long-term ill-effect. Each of the three components of exposure during MRI are discussed with reference to the possibility of adverse effects, together with the rationale of the current guidelines governing exposure in the UK.

Static magnetic fields

Initially at a practical level, magnets will attract ferromagnetic objects with a force proportional to the strength of the applied field. The highest fields are within the volume encompassed by the windings of the magnet in which imaging takes place, but there is an extended field around the scanner that decreases as the cube of the distance. To avoid a ‘missile’ injury to the patient, all personnel working close to the magnet should not carry any metallic objects on their person. It is also important to ensure that unauthorized personnel cannot gain access to the scanning room during examinations. The potential hazard of objects such as scissors or tools in the vicinity of the magnet cannot be overstressed because they could become dangerous projectiles. Patients can be anaesthetized for magnetic resonance examinations, but the endotracheal tubes and connectors must not contain any metal parts. With a resistive system the magnetic field can be shut down in an emergency and cardiopulmonary resuscitation equipment used in the vicinity of the scanner. For other systems the patient can be moved on a non-ferromagnetic trolley, whilst maintaining an airway and external cardiac massage if required, to outside the 5 Gauss line where full resuscitation can then take place.

There have been numerous studies to assess the ferromagnetic qualities of different metallic implants, summarized by Shellock (1988). Certain metallic implants and non-ferromagnetic prostheses may be safely imaged without the danger of moving the implant or prosthesis. The technique currently popular for female sterilization in the UK is the Filshie clip. This is made from titanium, with silicon on the inner aspect and is therefore nonmagnetic; therefore the presence of such a clip within a patient is not a contraindication to magnetic resonance examination. Of greater importance is the effect of the stray magnetic field around the magnet which could influence those cardiac pacemakers that contain reed relays which are activated by magnetic fields. Certain pacemakers would be rendered inoperative in a sufficiently high field: this could be fatal in a patient with an underlying complete heart block. The most sensitive pacemaker reed relays can switch in a field of 7 Gauss. No person wearing a pacemaker should be allowed to enter a field higher than 5 Gauss and prominent warning notices should be displayed outside any room or area in which the field could exceed this level.

Superconducting magnets, once energized, require no external power source, but they have to be replenished periodically with the cryogens, liquid helium and liquid nitrogen. If for any reason the superconductive properties of the magnet are lost, the current in the winding decays rapidly as the windings become resistive. The heat generated in this process will cause a large volume of liquid helium to boil off within a period of a few minutes. This phenomenon of quenching can on rare occasions occur spontaneously and is accompanied by a loud noise (Bore et al., 1985). Venting systems can allow the helium gas generated to be directed away to the atmosphere.

Biological effects

There is a possibility that the weak forces experienced by certain large molecules in a magnetic field could result in a change in the conformation or orientation of enzymes influencing enzyme reaction rates. The threshold at which this effect could become significant is probably above 10 T (McLaughlin, 1981). Other mechanisms for influencing enzyme kinetics could be through interaction with paramagnetic centres or with free radical intermediate products (Kapstein, 1985). At the fields currently used in imaging, no effects have been found; for example in one experiment a field of 1 T did not influence the activity of the bacterial enzyme (β-galactosidase (Thomas and Morris, 1981). Moreover, the levels of metabolites measured by NMR P-spectroscopy in living systems parallel that of direct assay: this provides additional evidence against any significant effect on enzymes.

Studies to date have failed to show any association between exposure to the conditions experienced during MRI and any carcinogenic or mutagenic effect. A number of different cell-culture test systems have been employed with examination of the cells for chromosome aberrations and sister chromatid changes (Cooke and Morris, 1981; Wolff et al., 1985). Field strengths as high as 2.7 T have been used for extended periods without any effect having been detected (Geard et al., 1984).

Mutagenic effects have also been investigated by exposing rats to simulated imaging for up to 28 days before mating. There was no detectable increase in the incidence of dominant lethal mutations when compared with a control group (Mahlum et al., 1978). Similar exposures in mice at varying times during gestation showed no evidence of visceral or skeletal abnormalities, and the rate of maturation of offspring did not differ from that of a control group (Sikov et al., 1978).

Behavioural changes in experimental animals and reduced performance in trained tasks have been reported at very high fields (5–10 T; De Lorge, 1979).

High static magnetic fields are associated with abnormalities in the T-wave of the electrocardiogram. The explanation for this is that blood is a conductor and, as it moves in relation to the magnetic field, an electric current is produced by Faraday’s law of electromagnetic induction. This current is...