What is Specific Absorption Rate (SAR)

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.