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Letters to the Editor |
Cardiac Arrhythmia Unit, University Hospital Zurich, Rämistrasse 100, CH-8091 Zurich, Switzerland; e-mail: firat.duru@dim.usz.ch
Editor:
We read with great interest the article by Dr Sommer and colleagues in the June 2000 issue of Radiology (1), in which they evaluated the safety and feasibility of magnetic resonance (MR) imaging at 0.5 T in patients with implanted cardiac pacemakers. MR imaging was safely performed in all patients examined, and there was neither a pacing dysfunction nor a change in the programmed parameters. These findings provide important information concerning the controversial topic of pacemaker and MR imaging compatibility and are of considerable interest for cardiologists and radiologists who are increasingly being consulted for decision making.
The findings are in accordance with the results of some previously published reports (2,3), which suggest that radio-frequency (RF)–induced heating may occur at the electrode-tissue interface. In our opinion, heating is one of the most challenging problems in MR imaging of pacemaker recipients. Certainly, the use of a 0.5-T system and MR sequences with a low specific absorption rate may decrease the heating effects. However, there are numerous factors, such as configuration of the leads in the body, that may influence the degree of heating.
While Dr Sommer and colleagues attempted to simulate worst-case scenarios to obtain maximum RF-heating conditions, it is practically impossible to test all possible combinations that can influence heating in vitro. With the presented results, the observed temperature increase to 23°C may be perfectly capable of inducing localized tissue injury. Even if such an effect is harmless as an acute event, the long-term complications, which may result from local burns and scars, are not known.
In this study, the investigators also noted an interesting observation that reed switch activation in cardiac pacemakers did not necessarily occur in a static magnetic field at 0.5 T. As a possible explanation for this unexpected observation, it was suggested that in certain positions in the central region of the MR imager, depending on the orientation of the reed switch, the force due to the local magnetic gradient was not strong enough to close the reed relay.
However, Luechinger et al (4) have previously shown that the reed switch of a pacemaker does not necessarily remain closed in the isocenters of 0.5- and 1.5-T MR imagers. The most likely explanation for this observation, however, is the opposing effects of magnetic force and torque in magnetic fields. The outer magnetic field induces magnetic dipoles in the ferromagnetic reeds in a highly nonlinear way (hysteresis). The magnetic dipoles of the reeds interact with each other (attraction force). In addition, each of the reeds interacts with the static magnetic field (torque). In low fields (<50 mT), the attraction of the dipoles is much stronger than the torque effect, and the reeds attract each other. The switch is closed in all orientations. In higher fields (>200 mT), the torque effects are stronger than the attraction force, leading to an open reed switch in approximately 50% of the cases. In clinical practice, however, it is possible to predict the state of a reed switch only if the exact orientations of the pacemaker in the MR imager, the reed switch in the pacemaker, and the reeds in the reed switch are known.
Certainly, it is desirable not to preclude pacemaker recipients from the diagnostic advantages of MR imaging, an important diagnostic modality. We agree with the authors’ opinion that if non–MR imaging modalities are not adequate to aid in a diagnosis, and after a careful risk-benefit assessment, it may be possible to perform MR imaging at 0.5 T safely in experienced centers with continuous monitoring during the procedure. However, the results of this and other studies should be interpreted with caution, and by no means should MR imaging in patients with pacemakers be generalized as being safe, since a future patient may have a pacemaker configuration and may undergo MR imaging with settings that correspond to those in one of the extreme scenarios. The potential for severe adverse effects can never be nullified in clinical practice, and, therefore, experienced cardiologic assistance and full resuscitation facilities should be available during imaging.
REFERENCES
Department of Radiology, University of Bonn, Sigmund-Freud-Strasse 25, 53127 Bonn, Germany; e-mail: t.sommer@uni-bonn.de
We appreciate the comments made by Dr Duru and colleagues and their interest in our study (1).
We agree that RF-induced heating is one of the main concerns in MR imaging of pacemaker recipients and that it is impossible to test all possible factors and combinations that can influence heating of electrodes. However, under worst-case conditions (lead loop within the center of the RF coil, maximum induction area of the leads, no flow phantom), the maximum temperature increase in our study at the lead tips was 8.75°C at a specific absorption rate of 0.6 W per kilogram of body weight, which was the upper limit of RF exposure in our patient investigations.
At a more peripheral position of the electrodes within the RF coil, representing approximately the position of the lead loop in the RF coil during MR imaging of the brain, heating effects distinctly declined, with maximum values of 0.6°C. Results of these in vitro investigations and the safe performance of MR imaging in a large group of patients—including follow-up investigations after 3 months to assess long-term complications such as development of local scars due to thermal injuries—indicate that a reasonable level of safety can be achieved when specific absorption rate values (ie, RF exposure) are limited to values less than or equal to 0.6 W/kg.
We noticed with interest that our observation was confirmed, that is, that reed switch activation did not necessarily occur in a static magnetic field of 0.5 T, and we appreciate the suggested hypothesis of the underlying mechanism with opposing effects of magnetic deflection forces and torque.
We understand that Dr Duru and colleagues agree with the conclusion of our investigations that MR imaging at 0.5 T should not necessarily be considered as absolutely contraindicated in patients with cardiac pacemakers and that it may be performed when appropriate strategies are implemented (programming to an asynchronous mode to avoid artificial inhibition and triggering of pacemaker stimulation independent of reed switch behavior in the static magnetic field, limited RF exposure to reduce heating effects at the electrode-tissue boundary, adequate monitoring techniques to detect inadequate pacemaker stimulation, and experienced cardiologic assistance).
We agree, and it has been explicitly pointed out in our article, that MR imaging in patients with pacemakers is definitely not a routine procedure, requires special precautions, and poses—as do many other diagnostic procedures—the potential risk for serious harm, which can naturally never be nullified in clinical practice. However, it is our firm belief that pacemaker recipients should not generally be excluded from MR imaging at 0.5 T, since in carefully selected clinical circumstances the definite diagnostic advantages of MR imaging may well outweigh the potential complications.
REFERENCES
This article has been cited by other articles:
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  C. A. Chen-Scarabelli, T. M. Scarabelli, C. Vahlhaus, T. Sommer, C. P. Naehle, A. Roguin, M. M. Zviman, G. R. Meininger, T. M. Dickfeld, A. Lardo, et al. Letters Regarding Article by Roguin et al, "Modern Pacemaker and Implantable Cardioverter/Defibrillator Systems Can Be Magnetic Resonance Imaging Safe: In Vitro and In Vivo Assessment of Safety and Function at 1.5 T" * Letters Regarding Article by Roguin et al, "Modern Pacemaker and Implantable Cardioverter/Defibrillator Systems Can Be Magnetic Resonance Imaging Safe: In Vitro and In Vivo Assessment of Safety and Function at 1.5 T" * Letters Regarding Article by Roguin et al, "Modern Pacemaker and Implantable Cardioverter/Defibrillator Systems Can Be Magnetic Resonance Imaging Safe: In Vitro and In Vivo Assessment of Safety and Function at 1.5 T" Circulation, June 14, 2005; 111(23): e390 - e392. [Full Text] [PDF]
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 J. Loewy, A. Loewy, and E. J. Kendall Reconsideration of Pacemakers and MR Imaging RadioGraphics, September 1, 2004; 24(5): 1257 - 1267. [Abstract] [Full Text] [PDF]
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A. Roguin, M. M. Zviman, G. R. Meininger, E. R. Rodrigues, T. M. Dickfeld, D. A. Bluemke, A. Lardo, R. D. Berger, H. Calkins, and H. R. Halperin Modern Pacemaker and Implantable Cardioverter/Defibrillator Systems Can Be Magnetic Resonance Imaging Safe: In Vitro and In Vivo Assessment of Safety and Function at 1.5 T Circulation, August 3, 2004; 110(5): 475 - 482. [Abstract] [Full Text] [PDF]
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 A. V. Proto Radiology 2004: Richard M. Friedenberg Completes His Service as Perspectives Editor Radiology, August 1, 2004; 232(2): 315 - 316. [Full Text] [PDF]
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