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Cardiac Imaging |
1 From the Depts of Radiology (T.S., M.R., U.H., W.B., F.T., J.G., H.S.), Cardiology (C.V., W.J.), and Cardiovascular Surgery (C.S.), University of Bonn, Sigmund-Freud-Str 25, 53127 Bonn, Germany; Dept of Cardiology, Marienhof, Koblenz, Germany (G.L.); and Dept of Diagnostic Radiology, University Hospital of Zurich, München, Germany (A.v.S.). From the 1996 RSNA scientific assembly. Received April 5, 1999; revision requested May 20; revision received September 1; accepted November 22. Address correspondence to T.S. (e-mail: t.sommer@uni-bonn.de).
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Abstract |
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MATERIALS AND METHODS: Twenty-one models of pacemakers and 44 pacemaker electrodes were exposed to in vitro MR imaging with continuous registration of pacemaker output and temperature at the lead tip. Prior to MR imaging examination, pacemakers were programmed to an asynchronous mode (A00, V00, or D00). Pacemakers were examined before and after MR imaging. Forty-four patients with implanted pacemakers underwent 51 MR imaging examinations under cardiologic surveillance, continuous electrocardiography, pulse oximetry, and capnographic monitoring.
RESULTS: MR imaging was safely performed in all patients. None of the pacemakers displayed a pacing dysfunction at MR imaging. No changes occurred in the programmed parameters in any device tested in vivo or in vitro. Maximum increases in the temperature at the lead tips were 8.90°C at a specific absorption rate (SAR) of 0.6 W/kg and 23.50°C under a worst-case radio-frequency (RF) heating condition with an SAR of 1.3 W/kg.
CONCLUSION: MR imaging at 0.5 T can be safely performed in patients with implanted pacemakers in carefully selected clinical circumstances when appropriate strategies (programming to an asynchronous mode, adequate monitoring techniques, limited RF exposure) are used.
Index terms: Heart, pacemakers, 51.43, 51.456 • Magnetic resonance (MR), experimental studies, **.1214112, **.121412, **.121413, **.121416, 51.43, 51.456 • Magnetic resonance (MR), safety, **.121411, **.121412, **.121413, **.121416, 51.43, 51.456 • Phantoms
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Introduction |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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In addition, as many as four patients with pacemakers are reported to have died after they inadvertently underwent MR imaging (11–13). However, these fatal events remain poorly characterized in terms of the type of pacemaker, field strength of the MR imaging unit, imaging sequence, and pacemaker dependency of the patient; in particular, the cardiac rhythms of patients were not documented or were insufficiently documented. All the same, in the past years, a number of publications (14–20) reported the safe performance of MR imaging studies at field strengths of 0.35–1.5 T in patients with advanced cardiac pacemakers. However, the patient populations in these studies were small (the studies were mostly case reports or included groups of up to five patients), and no consistent strategy was evaluated. In this prospective study, the safety and feasibility of MR imaging of cardiac pacemakers at 0.5 T were evaluated in vitro and in vivo.
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MATERIALS AND METHODS |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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In Vitro Testing of Electrodes
Forty-four pacemaker leads were investigated (Table 1). To achieve geometric and electrophysiologic conditions similar to in vivo conditions, the following experimental set-up was established: Electrodes were inserted into the right ventricular septum of an isolated porcine heart, connected to a pacemaker (Thera S 8944), and placed in a plastic container (60 x 40 x 25 cm). This container was filled with a 0.9% sodium chloride solution to permit conductive fluid to surround the pacemaker and heart. The normal anatomic distribution of leads in a patient in the supine position with a pacemaker implanted in the left infraclavicular region was simulated; the leads were held in place with plastic fixation devices in the shape typical of implanted leads at the bottom of the phantom. A semicircle configuration of pacemaker leads in the coronal plane was chosen to achieve a maximal magnetic induction area.
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The circularly polarized 56-cm-long body coil of the MR imaging system was used in all in vitro experiments to transmit and receive RF energy. A maximum RF heating condition, which represented the worst-case scenario, was obtained with the following MR imaging parameters: repetition time msec/echo time msec, 2,399/192; turbo factor (number of profiles measured per excitation), 39; flip angle, 90°; magnetic transfer contrast, off resonance; and number of sections, 19. Imaging duration was 10 minutes. By assuming a body weight of 80 kg, this turbo SE pulse sequence produced a whole-body–averaged specific absorption rate (SAR) of 1.3 W/kg, which was similar to that used in previous experiments in the assessment of implant heating under worst-case conditions (21,22).
To assess temperature changes that were dependent on spatial variations in the RF field (23), the temperature at the lead tip was measured at seven locations of the lead loop relative to the center of the body coil (Fig 1). In addition, all electrodes at the position with the highest temperature increase under this worst-case RF heating condition were exposed to a standard T1-weighted spin-echo (SE) sequence (573/14; number of sections, 21; imaging duration, 10 minutes) with an SAR of 0.6 W/kg.
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The electrocardiograms (ECGs) were recorded on standard ECG paper at a chart speed of 25 mm/sec, and traces recorded during MR imaging were thoroughly evaluated to detect pacing defects or changes in pacing rate. The pacemakers were examined before and after MR imaging for programmed parameters, which included pacing mode, rate, sensitivity, pulse width, pulse amplitude, and lead polarity. After MR imaging, the pacemaker settings were compared with those programmed prior to MR imaging, and all pacemaker functions were checked to detect possible changes in pacemaker settings, reed switch malfunctions, or damage to the electronics.
Patients and Monitoring Techniques
A total of 51 MR imaging examinations was performed in 44 patients with permanent cardiac pacemakers. In each patient, the referring physician and the radiologist believed that MR imaging examination was an urgent diagnostic necessity, without an acceptable imaging alternative. Approval was given by the institutional review board. Informed consent was obtained from each patient after the procedure was explained, and the potential risks—including irreversible damage of the pacemaker, thermal injuries, and pacemaker malfunction leading to death—were fully explained to the patient.
Models of pacemaker electrodes, programmed modes in which the MR imaging examination was performed, regions imaged, and MR imaging sequences are given in Table 4. Patients who were dependent on pacemakers were excluded from MR imaging. Patients with pacemaker dependency were those who had an escape rhythm that was hemodynamically unstable. Furthermore, to ensure stable sensing and pacing thresholds, only patients in whom pacemakers had been implanted more than 3 months prior to the MR imaging examination were accepted. No restrictions on the region imaged or pacemaker model were imposed.
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MR Imaging in Patients with Pacemakers
Evaluation of all programmed and measured pacemaker parameters, including assessment of lead impedance, battery voltage, battery impedance, sensing, and capture threshold was performed before and after MR imaging. To evaluate reed switch function within the static magnet field prior to MR imaging, patients underwent continuous electrocardiography in the position for subsequent MR imaging in the magnet bore. Afterward, in 49 MR imaging examinations, pacemakers were programmed to an asynchronous mode (D00, n = 26; V00, n = 21; A00, n = 2) to avoid artificial inhibition and triggering by the pulsed magnetic fields. On the basis of the intrinsic rhythm of the patient, we attempted to program a pacing rate of 60–100 beats per minute that was different from the magnet rate specific to the pacemaker model (ie, the pacing rate in the asynchronous mode due to reed switch activation).
In all patients, the rate adaptive mode, when present, was deactivated. Whenever possible, bipolar sensing configurations were programmed for the atrial and ventricular electrodes to reduce interference with pulsed magnetic fields. Two patients had pacemaker models (Spectrax 5941 VVI, 5995 Xyrel VP) in which the pacing mode was not programmable. In these patients, therefore, MR imaging examinations were performed in the VVI mode.
Each patient was asked to immediately inform the investigator of any torquing or heating sensation about the pacemaker pocket, palpitations, dizziness, pain, or any other unusual symptoms during MR imaging. After completion of the MR imaging examination, each pacing system was reprogrammed to the parameters used before imaging. Follow-up of each patient was performed at 3 months, with repeat measurements of capture and sensing thresholds and investigation of all programmed and measured data.
To reduce the risk of thermal injuries during MR imaging, the RF exposure was restricted to the whole-body–averaged maximum SAR of 0.6 W/kg, the imaging time of each sequence was restricted to 10 minutes, and a pause of 5 minutes was established between each sequence to allow the lead tips to cool. In a series of in vitro pretests in five electrodes, it was determined that after completion of the MR imaging examination, the increase in temperature decreased to less than 10% of the peak value within 5 minutes. A pause of 5 minutes was, therefore, considered to be a good compromise between allowing cooling at the lead tips and keeping the investigation time reasonably short.
In general, MR imaging sequences similar to those used at routine examination at our unit were applied. These included T1- and T2-weighted SE, turbo SE, gradient-echo and inversion-recovery sequences (fluid-attenuated inversion recovery, or FLAIR; short inversion time inversion recovery, or STIR), including the use of spectral presaturation with inversion recovery, or SPIR, for fat suppression and presaturation pulses for suppression of flow artifacts). Fast SE sequences with high turbo factors (which produce higher levels of RF energy than conventional SE sequences due to the high repetition rate of 180° refocusing pulses) that exceeded the defined upper SAR limit were modified to reduce the SAR to 0.6 W/kg or less. Magnetization transfer contrast techniques, gradient-echo and SE sequences, and echo-planar imaging were not applied.
Statistical Analysis
Quantitative data (ie, sensing and stimulation threshold, electrode impedance, battery voltage, and battery impedance) obtained before and after MR imaging were statistically evaluated with use of a Student t test for paired data. The level of significance was set at .05.
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RESULTS |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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In Vitro Testing of Pacemakers
Reed switch activation due to the static magnetic field that resulted in fixed pacing in the asynchronous mode at the pacemaker-specific magnet rate was observed in all patients when the pacemakers approached the MR imaging unit. However, when the pacemakers were positioned in the bore of the magnet, the reed switch deactivated in four of 16 pacemakers, which resulted in asynchronous stimulation at the programmed rate (40 beats per minute) in the A00, V00, or D00 mode (Fig 3). None of the pacemakers displayed a pacing dysfunction in the center or at the edge of the body coil when it was exposed to the pulsed gradient and RF magnetic fields, regardless of the imaging sequence used. No changes occurred in the programmed parameters of any of the devices tested. The ability to fully examine each device remained preserved. At the conclusion of MR imaging, all pacemakers continued to operate normally.
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As shown in Table 4, repeat MR imaging examinations were performed in four patients without adverse effects in the patients or pacemakers.
In the vast majority of the MR imaging examinations, the pacemaker system was located outside the field of view. In these cases, image quality was not impaired. Even in the five patients who underwent cardiac MR imaging examination with pacemakers and pacemaker leads located within the field of view, image quality was severely impaired by signal void and image distortion only in areas that were close to the pacemaker. The susceptibility effects of the leads were much less pronounced (Fig 5), which allowed satisfactory imaging of the heart with fast SE, SE, and gradient-echo (fast field-echo) sequences in all patients.
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DISCUSSION |
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MR Imaging–induced Heating Effects
MR imaging–induced heating is a possible hazard in patients with biologic implants made of metallic materials (25–27). It is generally accepted that the thermal effects resulting from gradient switching during MR imaging are negligible (28,29). Heating that results from the RF radiation used in MR imaging procedures is caused by power deposition into an ohmic resistance. Maximum RF-induced heating occurs at the electrode-tissue boundary; that is, the area of endocardium and myocardium close to the tip of the electrode has a potential risk of thermal injury (30), which results in the deterioration of pacing thresholds or atrial and/or ventricular perforation. Heating is considered to be especially problematic when objects are configured in a loop or coil (31), as conducting loops are known to provide a high current density in low-impedance, metallic, conductive materials (23). In addition to the described loop currents, the presence of extended wires that act as dipole antennas will pick up the electrical component (E field) of the RF field (30).
Energy absorption is indicated by the SAR. Therefore, the amount of power deposited into the pacemaker leads during an MR imaging procedure is a complex function of numerous variables, including Larmor frequency (which, in turn, is determined by the main magnetic field strength), flip angle, and duty-cycle of the RF pulse (ie, the repetition rate of 90° and 180° pulses) (32–35).
The results of our in vitro study demonstrate the dependence of the increases in temperature at the lead tips on the transmitted RF power (respective SARs) and on the distance of the lead loops from the RF coil center. For low RF exposure with a maximum whole-body–averaged SAR of 0.6 W/kg, the heating effects at the lead tips are limited, with maximum values of 8.90°C in a worst-case position; these effects are considered to be inconsequential in terms of safety and biologic effects. In patient studies, the increase in temperature is expected to be even smaller than that of our static no-flow phantom because of heat convection due to intravascular blood flow and myocardial perfusion. The fact that the patients' sensing and pacing thresholds remained unchanged after MR imaging confirms that no relevant thermal injury took place at the lead tips.
However, in our phantom studies, we observed temperature increases in some leads of up to 23°C at higher RF exposures of 1.3 W/kg. This increase in temperature occurs in a range that is used for temperature-controlled RF catheter ablation of cardiac accessory pathways and is probably capable of inducing tissue injury at the lead tips (36,37). In cases in which distant regions were imaged (ie, when the center of the region to be imaged was located 30 cm or farther from the center of the lead loop in the craniocaudal direction), the increase in temperature at the lead tip decreased distinctly, with a maximum value of 4.45°C at an SAR of 1.3 W/kg (Table 1). This has important implications for MR imaging in patients, as increases in temperature that are large enough to raise concerns about safety are unlikely to occur when MR imaging of the brain, abdomen, pelvis, lumbar spine, or lower extremities is performed at 0.5 T. However, when the bulk of the transmitted RF power is deposited into the pacemaker leads (which, in particular, is the case in MR imaging examinations of the thorax, heart, or thoracic spine), extra precautions are necessary; these precautions include reduction of SAR levels and in vitro testing of the electrodes prior to MR imaging.
Reed Switch Function during MR Imaging
Reed switch activation by the static magnetic field of the MR imaging unit results in the switch to a nonsensing (ie, asynchronous) mode with fixed pacing. This situation is analogous to asynchronous pacing that occurs when the pacemaker is exposed to a hand-held magnet as part of a routine follow-up examination of the pacemaker system. In this mode, the pacemaker is insensitive to sensing artifacts due to MR imaging. Reed switches used in pacemaker are known to close in low, static magnetic fields of between 0.5 (27) and 2 mT (38). Therefore, reed switch activation at magnetic field strengths used for clinical MR imaging between 0.5 and 1.5 T has been generally expected and was reported in previous in vitro and in vivo MR imaging studies (39,40).
However, an important finding in our study (which, to our knowledge, has not been reported in the literature) is that reed switch activation in cardiac pacemakers does not necessarily occur in a static magnetic field at 0.5 T. Although the reed switch was initially activated in all patients when the pacemaker approached to the MR imaging unit, we observed reed switch deactivation in vitro and in vivo when the pacemaker and patients, respectively, were positioned in the bore of the magnet for MR imaging. In particular, in the highly homogeneous central region of the static magnetic field, it is possible that, in certain positions (depending on the orientation of the reed switch to the static magnetic field), the force due to the local magnetic gradient was not strong enough to close the reed relay. This finding has important safety implications. When a pacemaker in the DDD mode is brought into the magnet bore and when reed switch deactivation occurs, starting the MR imaging examination may lead to life-threatening inhibition of pacing function (Fig 6). This problem can be avoided by programming the pacemaker to an asynchronous mode (D00, V00, A00) prior to the MR imaging examination to ensure either asynchronous stimulation in the programmed mode in case of reed switch deactivation or asynchronous stimulation at the magnet rate specific to the pacemaker model in case of reed switch activation.
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(b) Another type of rapid pacing with rates of up to 300 beats per minute, which is unrelated to false sensing and triggering, has been reported in animal studies (41) and in one patient who underwent MR imaging (42). The mechanism of this phenomenon is unclear. One hypothesis is that interference of the RF pulses with pacemaker output circuits results in pacing the heart at a cycle length that is a multiple of the repetition time (41). Although rapid pacing was not noted in our in vitro and in vivo studies, one should be aware of this phenomenon. Prompt recognition of rapid cardiac stimulation is crucial, as immediate cessation of the MR imaging examination will terminate the pacing process.
Caution should be used before the results of this study are extrapolated to other pacemaker models and MR imaging units. The maximum slew rate of the gradient system used in this study was 6.8 T · m-1 · sec-1. With new fast imaging techniques such as echo-planar imaging, more powerful gradient systems with much higher slew rates of up to 120 T · m-1 · sec-1 are in use. The safety of these sequences with respect to MR imaging in patients with pacemakers cannot be deduced from our data and must be evaluated in further studies.
Our data indicate that an SAR level of 1.3 W/kg may pose a serious risk of thermal injury, depending on the model of the lead and the position of the lead loop within the RF coil. RF power deposition and SARs are much higher in MR imaging units that operate at higher static field strengths and higher resultant Larmor frequencies. For example, several new pulse sequences (eg, the latest versions of turbo SE and magnetic transfer contrast sequences) are known to use extremely high levels of RF energy at 1.5 T with whole-body–averaged SARs that exceed 4.0 W/kg (27). MR imaging at 1.5 T with high RF exposures and an SAR of 3 W/kg has been shown to induce severe necrosis in the mucous membranes of dogs with transesophageal pacing leads in situ (30).
Therefore, to reduce the risk of thermal injuries, we recommend that MR imaging in patients with pacemakers should be limited to a maximum field strength of 0.5 T and a maximum SAR of 0.6 W/kg, unless the safety of higher field strengths and SARs has been explicitly shown in vitro for the specific model of pacemaker lead.
The results of our study suggest that MR imaging at 0.5 T can be safely performed in patients with implanted cardiac pacemakers in carefully selected clinical circumstances when appropriate strategies are used. In addition to a strong clinical need for diagnostic information and the absence of an acceptable imaging alternative, prerequisites for MR imaging in patients with pacemakers are the lack of dependence of the patient on the pacemaker and the use of appropriate monitoring techniques, including pulse oximetry and, most important, continuous ECG recordings of sufficient quality for an evaluation of the patient's rhythm. Equipment for resuscitation, appropriate pacemaker programming devices, and assistance from experienced cardiologists during imaging must be present. Activation of the reed switch inside the static magnetic field is not predictable; programming to an asynchronous mode (A00, V00, or D00) is, therefore, mandatory. Familiarity with the expected power deposition and increases in temperature in the pacemaker electrodes, which are dependent on specific pulse sequences and positions of pacemaker leads, is essential to guarantee a reasonable level of safety.
The risk-benefit ratio must be individually evaluated in each patient, with an understanding that, as with many other medical procedures, the performance of an MR imaging examination poses the potential for serious harm. Therefore, MR imaging in patients with pacemakers is definitely not a routine procedure and should be performed only in centers with experienced staff. More studies are required, with the close collaboration of radiologists and cardiologists, to fully evaluate the effects of magnetic field strength, Larmor frequency, pulse sequences, and slew rates of gradient systems on a wide range of pacemaker models and electrodes.
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Acknowledgments |
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Footnotes |
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Abbreviations: ECG = electrocardiogram, RF = radio frequency, SAR = specific absorption rate, SE = spin echo
Author contributions: Guarantor of integrity of entire study, T.S.; study concepts, T.S., C.V., W.J.; study design, T.S., C.V., A.v.S., W.J., H.S.; definition of intellectual content, T.S., C.V., A.v.S., H.S.; literature research, M.R., U.H., A.v.S.; clinical studies, T.S., C.V., G.L., C.S.; experimental studies, T.S., C.V., M.R., G.L., C.S., A.v.S.; data acquisition, T.S., C.V., M.R., G.L., C.S.; data analysis, T.S., C.V., M.R., U.H.; statistical analysis, T.S., C.V., U.H.; manuscript preparation, T.S., C.V., M.R., W.B., F.T.; manuscript editing, T.S., M.R., U.H.; manuscript review, C.V., M.R., W.B., F.T., J.G., H.S.
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M. NIEHAUS and J. TEBBENJOHANNS Electromagnetic interference in patients with implanted pacemakers or cardioverter-defibrillators Heart, September 1, 2001; 86(3): 246 - 248. [Full Text] [PDF]
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F. Duru, R. Luechinger, R. Candinas, T. Sommer, and H. Schild MR Imaging in Patients with Cardiac Pacemakers Drs Sommer and Schild respond: Radiology, June 1, 2001; 219(3): 856 - 858. [Full Text] [PDF]
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F. G. Shellock MR Imaging and Electronically Activated Devices Radiology, April 1, 2001; 219(1): 294 - 295. [Full Text]
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T) at the lead tip during 10 minutes of MR imaging with two electrodes, (a) Medtronic 5033 and (b) Medtronic 5024M, at different positions and SARs (1.3 vs 0.6 W/kg).
) and image distortion due to an implanted one-chamber pacemaker (Dialog 728).