Special Considerations for Cardiovascular Magnetic Resonance: Safety, Electrocardiographic Setup, Monitoring, and Contraindications

General Issues

CMR generally takes longer than other diagnostic modalities (although the time is significantly shortened with real-time imaging ), and the confined space in which the patient is placed is rather narrow, which some patients find uncomfortable. During CMR, communication with the patient may be difficult because of interfering noise from the gradient coils. On the other hand, CMR is entirely noninvasive. Overall, the safety issues during CMR that may pose potential safety concerns can be summarized as follows :

• 1.

  Biologic effects of the static magnetic field

• 2.

  Ferromagnetic attractive effects of the static magnetic field on certain devices

• 3.

  Potential effects on the relatively slow time-varying magnetic field gradients

• 4.

  Effects of the rapidly varying RF magnetic fields, including RF power deposition concerns

• 5.

  Auditory considerations of noise from the gradients

• 6.

  Safety considerations concerning superconducting magnet systems

• 7.

  Psychological effects

• 8.

  Possible effects of the intravenous use of magnetic resonance (MR) contrast agents

• 9.

  Patient safety during stress conditions

The safety concerns inherent to these issues are discussed.

Biologic Effects

Concerning the biologic effects of a static magnetic field, many structures in animals and humans are affected by magnetic fields. Many potential biologic effects and different magnitudes of magnetic fields have been examined, including the effect of the field on cardiac contractility and function. Gulch and colleagues concluded that static magnetic fields used in CMR do not constitute any hazard in terms of cardiac contractility. These magnetic fields do not increase ventricular vulnerability, as assessed by the repetitive response threshold and the ventricular fibrillation threshold. In one of the investigations, however, cardiac cycle length was shown to be altered. Two studies reported that lymphocytes from blood samples obtained in humans exposed to a 1.5 tesla (T) field demonstrated DNA double-strand breaks and changes consistent with mild inflammation. However, these studies have significant limitations and four recent reports have found no such DNA damage. Another area of concern is CMR during pregnancy, for example, teratogenicity (cellular effects, for example on differentiation, have been demonstrated in animals), damage to the fetal inner ear due to noise exposure and tissue heating effects, although none have been documented to cause harm. A recent study of CMR in pregnancy (including first trimester) found CMR to be safe. Furthermore, 7 T scanners are becoming more common (although currently very rarely used for CMR), and have been associated with transient dizziness upon movement of the subject into the scanner (postulated to be due to induced currents affecting vestibular hair cells in the inner ear), as well as a metallic taste in the mouth in up to 11% of patients. Numerous biologic effects on other systems have been investigated extensively, and it may be concluded that no deleterious biologic effects from static magnetic fields used in CMR have yet been established.

Ferromagnetism

The physical effect of the static magnetic field consists of a potential health hazard from the attractive effect on ferromagnetic objects. Ferromagnetic objects can be defined as those in which a strong intrinsic magnetic field can be induced when they are exposed to an external magnetic field. The existence of different kinds of scanners with different shielding makes the discussion about this topic even more crucial. When dealing with a static magnetic field, two types of physical concerns exist.

First, there are concerns about forces exerted on ferromagnetic objects within, on, or distant from the patient. These forces result in rotational (torque) or translational (attractive) motion of the object. Within the human body, a ferromagnetic metallic structure may be sufficiently attracted, or have a sufficient amount of torque exerted, to create a hazardous situation. These factors should be carefully considered before subjecting a patient with a ferromagnetic implant or material to CMR, particularly if the device is located in a potentially dangerous area of the body, where movement or dislodgment of the device could injure the patient. Another potentially injurious effect is known as the projectile, or missile, effect. This refers to the fact that ferromagnetic objects have the potential to gain sufficient speed during attraction to the magnet that the accumulated kinetic energy could be injurious or even lethal if the object were to strike a patient. Numerous studies have been performed to assess the ferromagnetic qualities of various metallic implants and materials. The results indicate that patients with certain metallic implants or prostheses that are nonferromagnetic or are minimally deflected by static magnetic fields can safely undergo CMR. The literature on this topic has been extensively reviewed and compiled. However, there are common misconceptions about what types of objects are ferromagnetic. The most important misconception is that stainless steel is ferromagnetic, when it is not. Patients with stainless steel implants can therefore be imaged safely, except for a small number of well-described exceptions, as discussed later. The implant will interfere locally with the images; for example, signal loss occurs around metallic prosthetic valves ( Fig. 10.1 ) and sternal wires ( Fig. 10.2 ) after bypass surgery, but this does not make the imaging hazardous. Nonstainless steel, which may be ferromagnetic, is not used for human implants, but is commonly used for oxygen cylinders, for example. Finally, batteries are typically attracted to the magnet, and this is one of the problems of imaging pacemakers.

The second type of physical concern deals with magnetically sensitive equipment, the functioning of which may be adversely affected by the magnetic field. The most common of these is the cardiac pacemaker. Most pacemakers include a reed relay switch whereby the sensing mechanism can be bypassed and pacing in the asynchronous mode can occur. This switch is activated when a magnet of sufficient strength is held over the pacemaker. In addition, the function of cardiac pacemakers may be influenced by field strengths as low as 17 gauss. In practice, reed switch closure can be expected in all pacemakers placed in the bore of the scanner. Pacemaker and ICD function is considered again later in this chapter.

Effect of Rapidly Switched Magnetic Fields

CMR exposes the patient to rapid variations of magnetic fields by the transient application of magnetic gradients during imaging. The effect of rapidly switched magnetic fields may be the induction of currents within the body or any other electrical conductor, according to Faraday's law. The current is dependent on the time rate of change of the magnetic field (d B /d t ), the cross-sectional area of the conducting tissue loop, and the conductivity of the tissue. Biologic effects of induced currents can be caused either by power deposition by the induced currents (thermal effects) or by direct effects of the current (nonthermal effects). Thermal effects as a result of switching gradients are not believed to be clinically significant. Possible nonthermal effects include stimulation of nerve or muscle cells. The threshold currents for nerve stimulation and ventricular fibrillation are known to be much higher than the estimated current densities induced under clinical CMR conditions. The echo planar imaging method, however, involves more rapidly changing magnetic field gradients, and peripheral muscle stimulation in humans has been reported. Such considerations have become more important as new technology has allowed the introduction of commercially available ultrafast gradient switching systems, and guidelines for maximum magnetic field variation are under development.

Radiofrequency Time-Varying Field

The transmitted RF time-varying field induces electrical currents within the tissue of the patient. The majority of this power is transformed into heat within the patient's tissue as a result of ohmic heating. The time-varying magnetic gradients have the potential to cause either thermal or nonthermal biologic effects. The distinction between these two is a matter of frequency, waveform shape, and magnitude. The discussion about nonthermal effects from RF magnetic fields is controversial because of questions about the relationship between chronic exposure to electromagnetic fields over many years and the causation of cancer or developmental abnormalities. The most recent evidence suggests that proximity to power lines is not injurious. Clearly, acute exposure of a patient to short-term RF fields for a diagnostic CMR examination is different from chronic exposure. The induced currents from RF magnetic fields are unable to cause nerve excitations. One of the difficulties faced by medicine is proving that a procedure is not injurious because of anecdotal case reports of adverse events and publication bias toward nonneutral reports. This issue is also faced by such a well-established technology as ultrasound, where safety concerns have been raised over acoustic exposure.

In contrast to the insignificant thermal effects caused by switched gradients, however, thermal effects as a result of RF pulses are of significant concern. The main biologic effects induced by RF fields are therefore related to the thermogenic qualities of the RF field. A general point of discussion is the appropriate safety regulations for levels of magnetic field strength in CMR imaging. Application of the fundamental law of electrostimulation is well established, both on theoretical and experimental grounds. Application of this law, in combination with Maxwell's law, yields an equation called the fundamental law of magnetostimulation, which has the hyperbolic form of a strength-duration curve and allows an estimation of the lowest possible value of the magnetic flux density capable of stimulating nerves and muscles. Calculations have shown that the threshold for heart excitation is more than 200 times higher than for nerve and muscle stimulation, depending on pulse duration. However, in clinical practice, some precautions are necessary. First and most importantly, the specific absorption rate of the imaging sequence being operated is monitored by the scanner software and must be kept below limits set by such bodies as the US Food and Drug Administration (FDA). Second, circumstances that could enhance the possibility of heating injury should be avoided. This includes ensuring the prevention of loops that could act as aerials within the scanner and enhance the heating effect locally. Therefore, patients should not be allowed to cross their legs (loop via the pelvis) or clasp their hands (loop via the shoulder and upper chest). The simple use of pillows prevents such problems. Other possible loops include the ECG leads, which should always be run out of the scanner parallel to the main field, and not looped across the chest. Finally, pacemaker and ICD leads make excellent aerials. The pacemaker lead can heat significantly during CMR and become a potential hazard (discussed later). Another consideration in patients after cardiac surgery is the effect of retained epicardial pacemaker leads. These leads can be left in place after surgery, and they may therefore act as an antenna during CMR. Studies have suggested that such short retained epicardial wires do not pose a significant problem.

Finally, the use of ECG electrodes, which are essential for cardiac gating, must be considered. Metallic ECG electrodes may cause burns during CMR, but this risk can be reduced with the use of carbon fiber electrodes, and these have now become standard.