Magnetic resonance imaging and implantable devices.

M agnetic resonance imaging uses high-strength magnetic and electric fields to obtain multiplanar images with unrivaled soft tissue resolution. The image resolution and availability of various pulse sequences, each optimized for the evaluation of particular tissue attributes, make MRI the imaging modality of choice for numerous neurological, mus-culoskeletal, thoracic, and abdominal conditions. In addition, because of the absence of x-ray radiation, MRI is optimal for follow-up of chronic diseases that require repeat imaging and for diagnostic imaging in young patients and women of child-bearing age. Because of the advancing severity of disease and age of the population, and advances in device technology, the number of patients with permanent pacemakers and implant-able cardioverter defibrillators (ICD) continues to increase. It has been estimated that patients with a pacemaker or ICD have up to 75% likelihood of having a clinical indication for MRI over the lifetime of their device. When performed with appropriate supervision and following a protocol for safety, many studies over the past 10 years have reported the safety of MRI with selected devices. However, in older devices, catastrophic complications have also been reported. Familiarity with each device class and its potential for electromagnetic interaction is essential for cardiologists and electrophysiologists whose patients may require MRI. MRI uses high-strength magnetic and electric fields to evaluate tissue structure, heterogeneity, and motion. Within the MRI scanner, hydrogen nuclei (predominantly in water and fat) become aligned with or against the axis of the static magnetic field. A net magnetization vector is created because more protons are aligned with the static magnetic field than against it. The protons also rotate around their own axis and precess around the magnetic field lines at a rate dependent on the local magnetic field strength. Weaker gradient magnetic fields are then applied to introduce regional variability in the precession frequency of hydrogen nuclei, allowing for spatial localiza-tion. Radiofrequency pulses tuned to the precession frequency of hydrogen nuclei in the desired spatial location are then used to tip the net magnetization vectors of select nuclei from the equilibrium state. As the nuclei that received energy from the radiofrequency pulse relax to their equilibrium state, a weak current is induced in the radiofrequency receiver coil. The receiver coil current is converted to interpretable images via a Fourier transformation. The static and gradient magnetic fields and radiofrequency pulses described above are associated with several potential risks involving implanted devices. The most intuitive …