Abstract
Magnetic resonance imaging (MRI) is a widely accepted tool for the diagnosis of a variety of disease states. The presence of a metallic implant, such as a cardiac pacemaker (PM), or the use of conductive structures in interventional therapy, such as guide wires or catheters, are currently considered a strong contraindication to MRI (Kanal, Borgstede, Barkovich, Bell, Bradley, Etheridge, Felmlee, Froelich, Hayden, Kaminski, Lester, Scoumis, Zaremba, & Zinninger, 2002; Niehaus & Tebbenjohanns, 2001; Shellock & Crues, 2002). Potential effects of MRI on PMs’ implantable cardioverter defibrillator (ICDs) include: force and torque effects on the PM (Luechinger, Duru, Scheidegger, Boesiger, & Candinas, 2001; Shellock, Tkach, Ruggieri, & Masaryk, 2003); undefined reed-switch state within the static magnetic field (Luechinger, Duru, Zeijlemaker, Scheidegger, Boesiger, & Candinas, 2002); potential risk of heart stimulation and inappropriate pacing (Erlebacher, Cahill, Pannizzo, & Knowles, 1986; Hayes, Holmes, & Gray, 1987); and heating effects at the lead tip (Achenbach, Moshage, Diem, Bieberle, Schibgilla, & Bachmann, 1997; Luechinger, Zeijlemaker, Pedersen, Mortensen, Falk, Duru, Candinas, & Boesiger, 2005; Sommer, Vahlhaus, Lauck, von Smekal, Reinke, Hofer, Block, Traber, Schneider, Gieseke, Jung, & Schild, 2000). In particular, most of the publications dealing with novel MRI techniques on patients with implanted linear conductive structures (Atalar, Kraitchman, Carkhuff, Lesho, Ocali, Solaiyappan, Guttman, & Charles, 1998; Baker, Tkach, Nyenhuis, Phillips, Shellock, Gonzalez-Martinez, & Rezai, 2004; Nitz, Oppelt, Renz, Manke, Lenhart, & Link, 2001) point out that the presence of these structures may produce an increase in power deposition around the wire or the catheter. Unfortunately, this increased local specific absorption rate (SAR) is potentially harmful to the patient, due to an excessive temperature increase which can bring living tissues to necrosis. The most direct way to get a measure of the SAR deposition along the wire is by using a temperature probe: the use of fluoroptic® thermometry to measure temperature has become “state-of-the-art,” and is the industry standard in this field (Shellock, 1992; Wickersheim et al., 1987). When the investigation involves small objects and large spatial temperature gradients, the measurement of the temperature increase and of the local SAR may become inaccurate, unless several precautions are taken. It seems obvious to: (1) evaluate the error associated with temperature increase and SAR measurements; (2) define a standard protocol for probe positioning, which minimizes the error associated with temperature measurement.
Key Terms in this Chapter
Fluoroptic® Thermometry: A trademark patented by Luxtron’s researchers in the 1970s; the fluoroptic® thermometry determines the temperature of the sensor by measuring the decay time of the emitted light. It is a persistent property of the sensor that its decay time varies precisely with temperature.
RF Field: Electromagnetic field generated during MRI procedures. RF is absorbed and re-transmitted by hydrogen nuclei. Re-emitted energy is used to obtain the image. RF field is measured as Vm-1. The RF frequency is related to the static field; in most commercial systems (1.5 T), the frequency is 64 MHz.
Pacemaker (PM): A medical device designed to regulate the beating of the heart. The purpose of an artificial pacemaker is to stimulate the heart when either the heart’s native pacemaker is not fast enough, or if there are blocks in the heart’s electrical conduction system preventing the propagation of electrical impulses from the native pacemaker to the lower chambers of the heart, the ventricles.
Specific Absorption Rate (SAR): A measure of the rate at which radio frequency energy is absorbed by the body when exposed to radio frequency electromagnetic field.
Magnetic Resonance Imaging (MRI): An imaging technique based on the principles of nuclear magnetic resonance.
Static Field: Strong static magnetic field used in MRI systems to align the magnetization vector of the hydrogen nuclei. It is measured as Tesla (T). In most commercial MRI system, the static field is 1.5 T.
Implantable Cardioverter Defibrillator (ICD): A device that is implanted under the skin of patients that are at risk of sudden cardiac death, due to ventricular fibrillation. The rudiments of cardiac arrhythmia detection and treatment are incorporated into the implantable device. The device was designed primarily to deal with ventricular fibrillation. Its current use has, however, extended to include atrial and ventricular arrhythmias, as well as the ability to perform biventricular pacing in patients with congestive heart failure, and to pace, should there be any marked bradycardia.