FULL PAPER Magnetic Resonance in Medicine 00:00–00 (2014)
Impact of Capped and Uncapped Abandoned Leads on the Heating of an MR-Conditional Pacemaker Implant Eugenio Mattei,1* Giulia Gentili,2 Federica Censi,1 Michele Triventi,1 and Giovanni Calcagnini1 INTRODUCTION
Purpose: To assess the risk of radiofrequency (RF) -induced heating in patients with MR-conditional pacemaker (PM) systems, in the presence of another lead abandoned from a previous implant. Methods: Four commercial pacemaker leads were placed beside a MR-conditional PM system, inside a human trunk simulator. The phantom has been exposed to the RF generated by a 64 MHz body bird-cage coil (whole-body specific absorption rate [SAR] ¼ 1 W/kg) and the induced heating was measured at the tip of the abandoned lead and of the MRconditional implant. Configurations that maximize the coupling between the RF field and the leads have been tested, as well as realistic implant positions. Results: Abandoned leads showed heating behaviors that strongly depend on the termination condition (abandonedcapped or saline exposed) and on the lead path (left or right positioning). Given a whole-body SAR ¼ 1 W/kg, a maximum temperature rise of 17.6 C was observed. The presence of the abandoned lead modifies the RF-heating profile of the MRconditional implant: either an increase or a decrease in the induced heating at its lead tip can occur, mainly depending on the relative position of the two leads. Variations ranging from 63% to þ69% with respect to the MR-conditional system alone were observed. Conclusion: These findings provide experimental evidence that the presence of an abandoned lead poses an additional risk for the patient implanted with a MR-conditional PM system. Our results support the current PM manufacturers’ policy of conditioning the MR compatibility of their systems to the absence of abandoned leads (including leads from MRconditional implants). From a clinical point of view, in such cases, the decision whether to perform the exam shall be based upon a risk/benefit evaluation, as in the case of conventional PM systems. Magn Reson Med 000:000–000, C 2014 Wiley Periodicals, Inc. 2014. V
The effort to make pacemakers (PMs) less likely to cause harm in patients who undergo MRI examinations has resulted in the development of a new generation of devices. In February 2011, the United States Food and Drug Administration (FDA), using terminology adopted from the American Society for Testing and Materials (ASTM), approved the first PM system for use as MR-conditional. This approval was meant to convey that under specific conditions of use, there were no known hazards or risks to patients. Two years before this PM system received FDA labeling as MR-conditional in the United States, it was granted CE mark approval allowing its implantation in Europe. CE mark approval has since been given to the second-generation of the same PM brand, which allows performing MRI scan over the entire body (for the firstgeneration device the possibility to scan the thoracic region was excluded). Today, most of the PM manufacturers have received the CE mark approval for their MRconditional brands. Initial reports on the performances of these new generation devices (1,2), although still limited in number and observation time, do not reveal any MRI-related complications or MRI-attributed sustained ventricular arrhythmias, asystole episodes, or PM malfunctions. Thus, the enhancements in PM technology and the improved understanding of the interactions between device components and the electromagnetic fields generated by MRI scanners have finally allow more patients to receive the benefits of both these technologies. Nonetheless, caution is still warranted. For example, patients may have other implanted devices which, according to the indications provide by the manufacture, invalidate the MR-conditional clause. In particular, the presence of an abandoned lead from a previous implant represents a risk for patient safety that needs to be considered. Over the lifetime of the implant, pacemaker leads may be disconnected and replaced due to lead fracture, insulation breaks, dislodgement, or abnormalities in pacing or sensing (3–5). While these leads are no longer functional, they often remain in the patient due to the risks associated with lead extraction, including myocardial perforation (6). Abandoned leads are disconnected from the PM, and in most cases a new lead is implanted and connected to the PM, assuming the pacing role of the abandoned. The abandoned lead, even if not connected to the PM, still acts as an antenna and couples with the radiofrequency (RF) field generated by the MRI scanner. Possible consequences are as follows:
Key words: magnetic resonance imaging; radiofrequency heating; pacemaker; abandoned leads
1 Department of Technology and Health, Italian Institute of Health, Rome, Italy. 2 Department of Electronic Engineering, University "Sapienza", Rome, Italy.
*Correspondence to: Eugenio Mattei, BMEng, Ph.D., Department of , Viale Regina Elena Technologies and Health, Istituto Superiore di Sanita 299, 00161 Roma, Italy. E-mail: [email protected] Additional Supporting Information may be found in the online version of this article. Received 5 September 2013; revised 10 December 2013; accepted 10 December 2013 DOI 10.1002/mrm.25106 Published online in Wiley Online Library (wileyonlinelibrary. com). C 2014 Wiley Periodicals, Inc. V
1
2
(i) The resulting current induced in the abandoned lead manifests as heating focused at the PM lead tip, which is implanted in the cardiac tissue. The currents induced in the tissue couple to the lead as the path of least resistance, with increased current density at the electrode/tissue interfaces, as an antenna in a lossy medium. The heating arises due to ohmic loss caused by the impedance mismatch that exists between the highly conductive metallic components of the PM lead and the relatively low conductivity cardiac tissue. (ii) when a new lead is implanted, the abandoned lead modifies the electromagnetic field distribution inside the patient and affects the coupling mechanism between the new lead and the RF signal. An interesting study recently published by Langman et al (7) assessed the risk of RF-induced heating in PMattached and abandoned leads, using in vitro temperature measurements in a simplified electric field exposure at 1.5 Tesla (T). The study concludes that current recommendations for MRI pacemaker safety should highlight the possible increased risk for patients with abandoned leads as compared to PM-attached; in particular it is demonstrated how, for clinical lead lengths (40–60 cm), abandoned leads exhibit higher lead tip heating compared with PM-attached ones. Additionally, the amount of the induced temperature increase for abandoned leads strongly depends on their termination condition: the standardized pacemaker lead termination IS-1 connector of the abandoned lead is often covered with a plastic cap to electrically isolate the lead and to prevent from accidental electrical excitation of the heart through the lead itself. In some cases, the IS-1 connector remains uncapped and exposed to the surrounding bodily fluids. The presence or the absence of the cap definitely affects the degree of heating induced at the lead tip: for clinical lengths, the presence of the cap may result in a temperature rise up to 3 times higher than for the uncapped abandoned lead. Although the presence of an abandoned lead represents an acknowledged hazard for patients undergoing MRI, to date, other publications that systematically addressed this issue are very limited. In particular, no studies have been performed to investigate how the presence of an abandoned lead modifies the RF-induced heating profile for a MR-conditional implant. This work specifically focuses on the risks that an abandoned lead poses for patients with a MR-conditional implant. It represents an important clinical issue that has never been investigated so far. As the medical community continues to pursue MRI scanning of patients with pacemakers, the possibility to perform MRI on MR-conditional implants also in the presence of an abandoned lead represents one of the future challenges to widen the benefits of MRconditional technology. In this research, in vitro measurements were performed to evaluate the risks of RF-induced heating on patients with an MR-conditional PM implant, in presence of an abandoned lead. The main aims of the study were (i) to evaluate the RF-induced heating at the tip of the abandoned leads, exploring the effects of the termination conditions (capped or uncapped) and of the lead path; (ii) to evaluate how the abandoned lead modifies
Mattei et al.
the RF-heating profile at the tip of the MR-conditional implant lead, exploring the role of the implant configuration and of the relative position of the implant and the abandoned lead. METHODS ASTM Phantom A human trunk simulator was used to host the MRconditional implant and the abandoned lead. The simulator was designed according to the ASTM F218202a:2011 standard (8) for testing MRI heating of implants, and was filled with a saline solution to meet the general requirements of the standard (Fig. 1a). The NaCl concentration that was used results in conductivity at 64 MHz of approximately 0.5 S/m, corresponding to a salinity of approximately 0.4% by weight, and permittivity of 79. In the region surrounding the lead tips of the MR-conditional implant and of the abandoned lead, a gelling agent (hydroethylcellulose, HEC) was added to the saline solution. The amount chosen for the HEC (2% by weight) allows the implants to be placed in the gel, moved and replaced, but, at the same time, prevents from rapid thermal convection. A rectangular sponge (15 10 cm) was used to provide a barrier to the diffusion of the gel in the saline solution and to ensure the electrical continuity all inside the trunk simulator. The ASTM phantom was filled with a total amount of saline solution of approximately 26 L. A PVC grid was submerged in the saline solution to support the leads and to maintain a consistent separation distance between the leads, the phantom surface, and the temperature probes (Fig. 1b). The grid was adjusted so that the top of the implant was positioned 1 cm below the phantom surface. MR-Conditional Implant and the Abandoned Leads Lead tip heating measurements were acquired for a MRconditional PM implant, prescribed to operate under the following conditions: positioning of the isocenter of the transmit RF coil outside the thorax area, the magnetic field strength limited to 1.5T, the gradient system limited to a slew rate of 200 T/(m/s), a specific absorption rate (SAR) 2 W/kg. The PM was programmed in MRI mode (VOO, rate 60 bpm) and the lead was a 62-cm-long, passive fixation, bipolar lead. Temperature measurements were also acquired for an abandoned lead placed beside the MR-conditional implant. To examine the effect of different leads, four different PM leads were used as an abandoned lead: three conventional leads, chosen among a group of commercial leads tested in a previous study (9), with different RF-heating profiles; and one lead of an MRconditional implant. Table 1 summarizes the properties of the four leads. PM lead tip heating was evaluated using two termination conditions: abandoned-capped, and abandoned saline-exposed. For the abandoned-capped leads, the IS1 connector was covered with a silicone plastic cap. For the abandoned saline-exposed leads, the IS-1 connector was exposed to the saline solution that simulates bodily fluids.
MR-Conditional Pacemaker Implant and Abandoned Lead
3
FIG. 1. The ASTM phantom: physical dimensions (a); with the PVC grid inside to support the implant and the temperature probes (b).
Implant Configurations and Lead Paths The MR-conditional implant and the abandoned lead were tested in two main configurations: Worst-case configuration: the MR-conditional implant and the abandoned lead were arranged in the ASTM phantom to obtain the maximum coupling with the RF field: both the leads were placed straight along the border of the trunk simulator, as depicted in Figure 2, upper panel. Given the E-field and current distribution resulting from the exposure of the phantom to the RF field, it was demonstrated that such configuration represents the worstcase conditions in terms of heating induced at the PM lead tip (9,10). Additionally, this implant arrangement is also recommended in the ASTM standard. Temperature measurements were acquired at the tip of the MRconditional implant and of the abandoned lead. Given the four leads and the two termination conditions tested for the abandoned lead, a total of eight sessions were acquired. Realistic PM implant configurations: in clinical practice, the PM chassis may be typically located in the left or right pectoral region of the patient, immediately under the skin; the lead is inserted into the subclavian vein (left or right), brachoencefalic vein (left or right), Table 1 Characteristic of the Leads Tested as Abandoned Lead
a
Lead #
MRconditional
Polarity
Fixation
Length (cm)
Tip resistance (Ohm)a
1 2 3 4
No No No Yes
Bipolar Bipolar Bipolar Bipolar
Passive Active Passive Passive
52 65 60 62
53 22 36 33
Measured as the resistance between the distal tip electrode ant the proximal IS-1 tip connector.
superior cava, until it reaches the right atrium. These implant locations and lead paths were reproduced for the MR-conditional system and for the abandoned lead, with the help of typical X-ray images of implanted patients. When a new lead is implanted in presence of an abandoned lead, the former can be placed either on the same side of the previous implant or on the opposite one. Both the scenarios were considered in this study: temperature measurements were acquired with the abandoned lead located on the same side of the MRconditional implant (ipsilateral implant) and on the opposite one (contralateral implant). The combinations of the possible positions for the MR-conditional implant and the abandoned lead resulted in a total of 32 measurement sessions, given the four leads and the two termination conditions of the abandoned lead. During all the tests, temperature was measured at the lead tip of the MR-conditional implant, of the abandoned lead, inside the saline solution, and at the tip of a 25-cmlong straight metal wire (reference wire), which was kept always in the same position in the ASTM phantom. This wire provided us with the means to ensure the repeatability of the measurements and to prevent misleading results due to changes of the experimental conditions (e.g., gel deterioration, displacement of the phantom inside the RF coil, changes in the RF delivered power). Temperature Measurements Temperature measurements were performed using a FluR thermometer (Luxtron model 3100 with SMM oropticV probes, Lumasense Technologies, Santa Clara, CA) with four probes (model SMM). These plastic fiber probes (1 mm diameter) minimize perturbations of RF fields. The Luxtron system has a resolution of 0.1 C and was set to operate at eight samples per second. The background noise was in the range of Luxtron resolution.
4
Mattei et al.
FIG. 2. Sketches of the implant configurations tested. The four temperature measurement points are also reported. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
For passive-fixation leads, the terminal portion of the temperature probes was placed in transversal contact (i.e., the probe is perpendicular to the body of the leadwire axis) with the lead tip: this contact configuration was demonstrated to minimize the measurement errors associated to the physical dimensions of the probes (11). For the active-fixation lead, the temperature sensor was placed inside the helix tip of the lead (Fig. 3). Plastic plates were used to maintain the lead and the temperature probes in place and to ensure a good contact between the lead tip and the sensitive portion of the probes. Data acquisition was performed using the analog output of the Luxtron thermometer, an A/D converter and a 16-bit acquisition card (National Instrument, Austin, TX, DAQCard-AI-16XE-50), installed on a standard notebook. Exposure System and Test Protocol Temperature measurements were performed inside a full-size RF coil (length 112 cm, inner diameter 60 cm) with 16 rungs forming the classic birdcage configuration
(Ian T. Chesnick/Nuclear Magnetic Resonance Consultants, PA). Tuning capacitors are placed on each of the rungs, resulting in a low-pass structure. It reproduces the typical coils used in 1.5T MRI machines and provides a controlled exposure for MRI safety testing (12). A quadrature power divider fed the coil, to select an incident field with a circularly polarized H-field. The exposure was realized by a RF amplifier that delivers over 150 W at 64 MHz, and the output power was calibrated to produce a whole-body (WB) SAR of 1 W/kg inside the ASTM phantom. Such WB SAR level is typical of most of the RF sequences used in clinical practice and it is compliant with the limitations given by the MRconditional implant manufacturer (SAR 2 W/kg). A directional power sensor connected to a power meter (NRT-Z14, Rhode & Schwartz, Germany) was placed at the output of the RF amplifier to continuously monitor the RF power delivered to the coil and to ensure that it did not change during the measurement sessions. The system is installed at the Department of Technology and Health of the Italian National Institute of Health and has been specially designed to provide a controlled
MR-Conditional Pacemaker Implant and Abandoned Lead
FIG. 3. Schematic representation of the temperature probe positioning for active and passive-fixation leads. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
5
induced by the RF exposure. The calorimetry study was performed according to the indication provided in the standard IEC 60601-2-33 (16) and the same approach has been widely used in many other studies on MRI-induced heating (9–11,13,14). The amplitude of the signal delivered to the coil was set to produce a WB-SAR of 1 W/kg (60.1 W/kg), and was continuously monitored during the tests by the power meter to ensure that no variation of the RF input signal could occur. Determining the WBSAR allows us to extrapolate the data collected in the laboratory coil on the heating of PM leads to values that occur in phantoms placed in clinical MRI systems, when using imaging sequences with a similar WB-SAR. To evaluate the effect of the abandoned lead on the amount of induced heating at the lead tip of the MRconditional implant, temperature measurements at the MR-conditional lead tip have been reported both in terms of absolute temperature increases and normalized to the value acquired with the MR-conditional implant alone (without the abandoned lead) inside the ASTM phantom. In such manner, the results immediately highlight when the presence of the abandoned lead determinates an increase (normalized dT >1) or a decreased (normalized dT
Impact of Capped and Uncapped Abandoned Leads on the Heating of an MR-Conditional Pacemaker Implant Eugenio Mattei,1* Giulia Gentili,2 Federica Censi,1 Michele Triventi,1 and Giovanni Calcagnini1 INTRODUCTION
Purpose: To assess the risk of radiofrequency (RF) -induced heating in patients with MR-conditional pacemaker (PM) systems, in the presence of another lead abandoned from a previous implant. Methods: Four commercial pacemaker leads were placed beside a MR-conditional PM system, inside a human trunk simulator. The phantom has been exposed to the RF generated by a 64 MHz body bird-cage coil (whole-body specific absorption rate [SAR] ¼ 1 W/kg) and the induced heating was measured at the tip of the abandoned lead and of the MRconditional implant. Configurations that maximize the coupling between the RF field and the leads have been tested, as well as realistic implant positions. Results: Abandoned leads showed heating behaviors that strongly depend on the termination condition (abandonedcapped or saline exposed) and on the lead path (left or right positioning). Given a whole-body SAR ¼ 1 W/kg, a maximum temperature rise of 17.6 C was observed. The presence of the abandoned lead modifies the RF-heating profile of the MRconditional implant: either an increase or a decrease in the induced heating at its lead tip can occur, mainly depending on the relative position of the two leads. Variations ranging from 63% to þ69% with respect to the MR-conditional system alone were observed. Conclusion: These findings provide experimental evidence that the presence of an abandoned lead poses an additional risk for the patient implanted with a MR-conditional PM system. Our results support the current PM manufacturers’ policy of conditioning the MR compatibility of their systems to the absence of abandoned leads (including leads from MRconditional implants). From a clinical point of view, in such cases, the decision whether to perform the exam shall be based upon a risk/benefit evaluation, as in the case of conventional PM systems. Magn Reson Med 000:000–000, C 2014 Wiley Periodicals, Inc. 2014. V
The effort to make pacemakers (PMs) less likely to cause harm in patients who undergo MRI examinations has resulted in the development of a new generation of devices. In February 2011, the United States Food and Drug Administration (FDA), using terminology adopted from the American Society for Testing and Materials (ASTM), approved the first PM system for use as MR-conditional. This approval was meant to convey that under specific conditions of use, there were no known hazards or risks to patients. Two years before this PM system received FDA labeling as MR-conditional in the United States, it was granted CE mark approval allowing its implantation in Europe. CE mark approval has since been given to the second-generation of the same PM brand, which allows performing MRI scan over the entire body (for the firstgeneration device the possibility to scan the thoracic region was excluded). Today, most of the PM manufacturers have received the CE mark approval for their MRconditional brands. Initial reports on the performances of these new generation devices (1,2), although still limited in number and observation time, do not reveal any MRI-related complications or MRI-attributed sustained ventricular arrhythmias, asystole episodes, or PM malfunctions. Thus, the enhancements in PM technology and the improved understanding of the interactions between device components and the electromagnetic fields generated by MRI scanners have finally allow more patients to receive the benefits of both these technologies. Nonetheless, caution is still warranted. For example, patients may have other implanted devices which, according to the indications provide by the manufacture, invalidate the MR-conditional clause. In particular, the presence of an abandoned lead from a previous implant represents a risk for patient safety that needs to be considered. Over the lifetime of the implant, pacemaker leads may be disconnected and replaced due to lead fracture, insulation breaks, dislodgement, or abnormalities in pacing or sensing (3–5). While these leads are no longer functional, they often remain in the patient due to the risks associated with lead extraction, including myocardial perforation (6). Abandoned leads are disconnected from the PM, and in most cases a new lead is implanted and connected to the PM, assuming the pacing role of the abandoned. The abandoned lead, even if not connected to the PM, still acts as an antenna and couples with the radiofrequency (RF) field generated by the MRI scanner. Possible consequences are as follows:
Key words: magnetic resonance imaging; radiofrequency heating; pacemaker; abandoned leads
1 Department of Technology and Health, Italian Institute of Health, Rome, Italy. 2 Department of Electronic Engineering, University "Sapienza", Rome, Italy.
*Correspondence to: Eugenio Mattei, BMEng, Ph.D., Department of , Viale Regina Elena Technologies and Health, Istituto Superiore di Sanita 299, 00161 Roma, Italy. E-mail: [email protected] Additional Supporting Information may be found in the online version of this article. Received 5 September 2013; revised 10 December 2013; accepted 10 December 2013 DOI 10.1002/mrm.25106 Published online in Wiley Online Library (wileyonlinelibrary. com). C 2014 Wiley Periodicals, Inc. V
1
2
(i) The resulting current induced in the abandoned lead manifests as heating focused at the PM lead tip, which is implanted in the cardiac tissue. The currents induced in the tissue couple to the lead as the path of least resistance, with increased current density at the electrode/tissue interfaces, as an antenna in a lossy medium. The heating arises due to ohmic loss caused by the impedance mismatch that exists between the highly conductive metallic components of the PM lead and the relatively low conductivity cardiac tissue. (ii) when a new lead is implanted, the abandoned lead modifies the electromagnetic field distribution inside the patient and affects the coupling mechanism between the new lead and the RF signal. An interesting study recently published by Langman et al (7) assessed the risk of RF-induced heating in PMattached and abandoned leads, using in vitro temperature measurements in a simplified electric field exposure at 1.5 Tesla (T). The study concludes that current recommendations for MRI pacemaker safety should highlight the possible increased risk for patients with abandoned leads as compared to PM-attached; in particular it is demonstrated how, for clinical lead lengths (40–60 cm), abandoned leads exhibit higher lead tip heating compared with PM-attached ones. Additionally, the amount of the induced temperature increase for abandoned leads strongly depends on their termination condition: the standardized pacemaker lead termination IS-1 connector of the abandoned lead is often covered with a plastic cap to electrically isolate the lead and to prevent from accidental electrical excitation of the heart through the lead itself. In some cases, the IS-1 connector remains uncapped and exposed to the surrounding bodily fluids. The presence or the absence of the cap definitely affects the degree of heating induced at the lead tip: for clinical lengths, the presence of the cap may result in a temperature rise up to 3 times higher than for the uncapped abandoned lead. Although the presence of an abandoned lead represents an acknowledged hazard for patients undergoing MRI, to date, other publications that systematically addressed this issue are very limited. In particular, no studies have been performed to investigate how the presence of an abandoned lead modifies the RF-induced heating profile for a MR-conditional implant. This work specifically focuses on the risks that an abandoned lead poses for patients with a MR-conditional implant. It represents an important clinical issue that has never been investigated so far. As the medical community continues to pursue MRI scanning of patients with pacemakers, the possibility to perform MRI on MR-conditional implants also in the presence of an abandoned lead represents one of the future challenges to widen the benefits of MRconditional technology. In this research, in vitro measurements were performed to evaluate the risks of RF-induced heating on patients with an MR-conditional PM implant, in presence of an abandoned lead. The main aims of the study were (i) to evaluate the RF-induced heating at the tip of the abandoned leads, exploring the effects of the termination conditions (capped or uncapped) and of the lead path; (ii) to evaluate how the abandoned lead modifies
Mattei et al.
the RF-heating profile at the tip of the MR-conditional implant lead, exploring the role of the implant configuration and of the relative position of the implant and the abandoned lead. METHODS ASTM Phantom A human trunk simulator was used to host the MRconditional implant and the abandoned lead. The simulator was designed according to the ASTM F218202a:2011 standard (8) for testing MRI heating of implants, and was filled with a saline solution to meet the general requirements of the standard (Fig. 1a). The NaCl concentration that was used results in conductivity at 64 MHz of approximately 0.5 S/m, corresponding to a salinity of approximately 0.4% by weight, and permittivity of 79. In the region surrounding the lead tips of the MR-conditional implant and of the abandoned lead, a gelling agent (hydroethylcellulose, HEC) was added to the saline solution. The amount chosen for the HEC (2% by weight) allows the implants to be placed in the gel, moved and replaced, but, at the same time, prevents from rapid thermal convection. A rectangular sponge (15 10 cm) was used to provide a barrier to the diffusion of the gel in the saline solution and to ensure the electrical continuity all inside the trunk simulator. The ASTM phantom was filled with a total amount of saline solution of approximately 26 L. A PVC grid was submerged in the saline solution to support the leads and to maintain a consistent separation distance between the leads, the phantom surface, and the temperature probes (Fig. 1b). The grid was adjusted so that the top of the implant was positioned 1 cm below the phantom surface. MR-Conditional Implant and the Abandoned Leads Lead tip heating measurements were acquired for a MRconditional PM implant, prescribed to operate under the following conditions: positioning of the isocenter of the transmit RF coil outside the thorax area, the magnetic field strength limited to 1.5T, the gradient system limited to a slew rate of 200 T/(m/s), a specific absorption rate (SAR) 2 W/kg. The PM was programmed in MRI mode (VOO, rate 60 bpm) and the lead was a 62-cm-long, passive fixation, bipolar lead. Temperature measurements were also acquired for an abandoned lead placed beside the MR-conditional implant. To examine the effect of different leads, four different PM leads were used as an abandoned lead: three conventional leads, chosen among a group of commercial leads tested in a previous study (9), with different RF-heating profiles; and one lead of an MRconditional implant. Table 1 summarizes the properties of the four leads. PM lead tip heating was evaluated using two termination conditions: abandoned-capped, and abandoned saline-exposed. For the abandoned-capped leads, the IS1 connector was covered with a silicone plastic cap. For the abandoned saline-exposed leads, the IS-1 connector was exposed to the saline solution that simulates bodily fluids.
MR-Conditional Pacemaker Implant and Abandoned Lead
3
FIG. 1. The ASTM phantom: physical dimensions (a); with the PVC grid inside to support the implant and the temperature probes (b).
Implant Configurations and Lead Paths The MR-conditional implant and the abandoned lead were tested in two main configurations: Worst-case configuration: the MR-conditional implant and the abandoned lead were arranged in the ASTM phantom to obtain the maximum coupling with the RF field: both the leads were placed straight along the border of the trunk simulator, as depicted in Figure 2, upper panel. Given the E-field and current distribution resulting from the exposure of the phantom to the RF field, it was demonstrated that such configuration represents the worstcase conditions in terms of heating induced at the PM lead tip (9,10). Additionally, this implant arrangement is also recommended in the ASTM standard. Temperature measurements were acquired at the tip of the MRconditional implant and of the abandoned lead. Given the four leads and the two termination conditions tested for the abandoned lead, a total of eight sessions were acquired. Realistic PM implant configurations: in clinical practice, the PM chassis may be typically located in the left or right pectoral region of the patient, immediately under the skin; the lead is inserted into the subclavian vein (left or right), brachoencefalic vein (left or right), Table 1 Characteristic of the Leads Tested as Abandoned Lead
a
Lead #
MRconditional
Polarity
Fixation
Length (cm)
Tip resistance (Ohm)a
1 2 3 4
No No No Yes
Bipolar Bipolar Bipolar Bipolar
Passive Active Passive Passive
52 65 60 62
53 22 36 33
Measured as the resistance between the distal tip electrode ant the proximal IS-1 tip connector.
superior cava, until it reaches the right atrium. These implant locations and lead paths were reproduced for the MR-conditional system and for the abandoned lead, with the help of typical X-ray images of implanted patients. When a new lead is implanted in presence of an abandoned lead, the former can be placed either on the same side of the previous implant or on the opposite one. Both the scenarios were considered in this study: temperature measurements were acquired with the abandoned lead located on the same side of the MRconditional implant (ipsilateral implant) and on the opposite one (contralateral implant). The combinations of the possible positions for the MR-conditional implant and the abandoned lead resulted in a total of 32 measurement sessions, given the four leads and the two termination conditions of the abandoned lead. During all the tests, temperature was measured at the lead tip of the MR-conditional implant, of the abandoned lead, inside the saline solution, and at the tip of a 25-cmlong straight metal wire (reference wire), which was kept always in the same position in the ASTM phantom. This wire provided us with the means to ensure the repeatability of the measurements and to prevent misleading results due to changes of the experimental conditions (e.g., gel deterioration, displacement of the phantom inside the RF coil, changes in the RF delivered power). Temperature Measurements Temperature measurements were performed using a FluR thermometer (Luxtron model 3100 with SMM oropticV probes, Lumasense Technologies, Santa Clara, CA) with four probes (model SMM). These plastic fiber probes (1 mm diameter) minimize perturbations of RF fields. The Luxtron system has a resolution of 0.1 C and was set to operate at eight samples per second. The background noise was in the range of Luxtron resolution.
4
Mattei et al.
FIG. 2. Sketches of the implant configurations tested. The four temperature measurement points are also reported. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
For passive-fixation leads, the terminal portion of the temperature probes was placed in transversal contact (i.e., the probe is perpendicular to the body of the leadwire axis) with the lead tip: this contact configuration was demonstrated to minimize the measurement errors associated to the physical dimensions of the probes (11). For the active-fixation lead, the temperature sensor was placed inside the helix tip of the lead (Fig. 3). Plastic plates were used to maintain the lead and the temperature probes in place and to ensure a good contact between the lead tip and the sensitive portion of the probes. Data acquisition was performed using the analog output of the Luxtron thermometer, an A/D converter and a 16-bit acquisition card (National Instrument, Austin, TX, DAQCard-AI-16XE-50), installed on a standard notebook. Exposure System and Test Protocol Temperature measurements were performed inside a full-size RF coil (length 112 cm, inner diameter 60 cm) with 16 rungs forming the classic birdcage configuration
(Ian T. Chesnick/Nuclear Magnetic Resonance Consultants, PA). Tuning capacitors are placed on each of the rungs, resulting in a low-pass structure. It reproduces the typical coils used in 1.5T MRI machines and provides a controlled exposure for MRI safety testing (12). A quadrature power divider fed the coil, to select an incident field with a circularly polarized H-field. The exposure was realized by a RF amplifier that delivers over 150 W at 64 MHz, and the output power was calibrated to produce a whole-body (WB) SAR of 1 W/kg inside the ASTM phantom. Such WB SAR level is typical of most of the RF sequences used in clinical practice and it is compliant with the limitations given by the MRconditional implant manufacturer (SAR 2 W/kg). A directional power sensor connected to a power meter (NRT-Z14, Rhode & Schwartz, Germany) was placed at the output of the RF amplifier to continuously monitor the RF power delivered to the coil and to ensure that it did not change during the measurement sessions. The system is installed at the Department of Technology and Health of the Italian National Institute of Health and has been specially designed to provide a controlled
MR-Conditional Pacemaker Implant and Abandoned Lead
FIG. 3. Schematic representation of the temperature probe positioning for active and passive-fixation leads. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
5
induced by the RF exposure. The calorimetry study was performed according to the indication provided in the standard IEC 60601-2-33 (16) and the same approach has been widely used in many other studies on MRI-induced heating (9–11,13,14). The amplitude of the signal delivered to the coil was set to produce a WB-SAR of 1 W/kg (60.1 W/kg), and was continuously monitored during the tests by the power meter to ensure that no variation of the RF input signal could occur. Determining the WBSAR allows us to extrapolate the data collected in the laboratory coil on the heating of PM leads to values that occur in phantoms placed in clinical MRI systems, when using imaging sequences with a similar WB-SAR. To evaluate the effect of the abandoned lead on the amount of induced heating at the lead tip of the MRconditional implant, temperature measurements at the MR-conditional lead tip have been reported both in terms of absolute temperature increases and normalized to the value acquired with the MR-conditional implant alone (without the abandoned lead) inside the ASTM phantom. In such manner, the results immediately highlight when the presence of the abandoned lead determinates an increase (normalized dT >1) or a decreased (normalized dT
