- Email: [email protected]
Effect of MRI on breast tissue expanders and recommendations for safe use
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Journal of Plastic, Reconstructive & Aesthetic Surgery (2017) xx, 1e6
Effect of MRI on breast tissue expanders and recommendations for safe use Andrew A. Marano, Peter W. Henderson, Martin R. Prince, Stephen M. Dashnaw, Christine H. Rohde* Division of Plastic and Reconstructive Surgery, Department of Radiology, Columbia University Medical Center, New York, NY, USA Received 7 June 2017; accepted 26 July 2017
KEYWORDS MRI; Magnetic resonance imaging; MRI safety; Tissue expander; Breast tissue expander; Expander
Summary Introduction: Ferromagnetic port-containing breast tissue expanders are currently labeled MRI-unsafe because of the presumption that magnets should not enter the machine. However, designating these devices as MRI-unsafe can lead to unnecessary procedures or suboptimal imaging choices. This study provides an ex vivo analysis of how breast tissue expanders behave when subjected to strong magnetic fields to determine which variables might affect clinical risk. Methods: Three different brands of tissue expanders were evaluated in three MRI environments. Translational force was determined using the deflection angle method. Torque on empty, saline-filled, and air-filled expanders was evaluated on a 0e4 scale. Magnetic field was measured using a gaussmeter. The weight required to prevent displacement of the expanders was determined for both air- and saline-filled expanders. Temperature over time was measured using an alcohol thermometer. Results: Magnetic field strength, deflection angle, and torque were the greatest in 3T MRI environments and varied by device manufacturer (Sientra > Mentor > Allergan). Saline-filled expanders required 240 mL and air-filled required 360 mL volume to make the torque undetectable, and the effect of torque could be mitigated with prone positioning. A weight of 120 g was required to prevent displacement of a saline-filled tissue expander and 870 g for an empty expander. There were no appreciable changes in temperature. Conclusions: Previously described risks may be reduced by using a 1.5T MRI, device selection, filling expanders with saline, and prone positioning. MRI can be considered in patients with breast tissue expanders when appropriate peri-procedural choices have been made so that the benefits of undergoing MRI outweigh the risks. ª 2017 British Association of Plastic, Reconstructive and Aesthetic Surgeons. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (C.H. Rohde). http://dx.doi.org/10.1016/j.bjps.2017.07.012 1748-6815/ª 2017 British Association of Plastic, Reconstructive and Aesthetic Surgeons. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Marano AA, et al., Effect of MRI on breast tissue expanders and recommendations for safe use, Journal of Plastic, Reconstructive & Aesthetic Surgery (2017), http://dx.doi.org/10.1016/j.bjps.2017.07.012
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Introduction Magnetic resonance imaging (MRI) is an established diagnostic modality used across many specialties to evaluate a variety of tissue types throughout the body. Visualization is accomplished by placing the patient within a strong magnetic field and interrogating with a radiofrequency signal, using magnetic field gradients to localize the resulting signal for reconstructing an image. The presence of this strong magnetic field, radiofrequency signals, and magnetic field gradients has brought into question the safety of implantable devices made of ferromagnetic materials. The American Society for Testing and Materials has consequently established guidelines for determining the extent of magnetically induced displacement forces, torque, and device heating and standards for labeling products as “MR Safe,” “MR Conditional,” or “MR Unsafe.”1 The safety of medical implant devices during MRI is of particular importance to patients with breast tissue expanders. Placement of tissue expanders and exchange for permanent implants comprised approximately 73% of all breast reconstructions in 2014.2 During the expansion period, a patient may need an MRI. Common indications include surveillance of the contralateral breast,3 perforator mapping for possible transition to autologous reconstruction,4 and evaluation for brain5 and spinal metastases.6 Most breast tissue expanders have a ferromagnetic port. Surgeons have anecdotally noted image artifacts,7 tissue expander migration,8,9 a burning sensation,10 pain at the site of implant placement following MRI,11,12 magnet polarity reversal,13 and dislodgement of the magnetic ports.14 Until recently, there was no data to support the safety of tissue expanders during MRI, and thus, device manufacturers recommend MRI not be performed while these devices are implanted.15e17 As a result, MRI technicians and radiologists commonly refuse to perform MRI studies in patients with these devices, although other studies report that MRI with tissue expanders in place can be performed without complications.18 There are few options for patients with tissue expanders who must undergo MRI evaluation, each with its own shortcomings. In lieu of an MRI, the clinician may opt for a computed tomography (CT) scan; however, the use of a suboptimal imaging modality increases the potential for incorrect diagnoses. The clinician may also choose to remove the tissue expander or exchange it for a permanent implant, but doing so prior to the completion of the expansion process may compromise the quality of the reconstruction and expose the patient to additional, potentially unnecessary surgery and anesthesia. The operation to remove or exchange the tissue expander will also delay access to an MRI. Our group was recently involved in a large clinical series in which 71 patients with breast tissue expanders underwent MRI in a 1.5 tesla (T) environment.18 In this series, there were no reports of pain or sensation of device migration nor was there any evidence of tissue damage or complications upon removal of the expanders. To better understand these findings, we chose to investigate the behavior of these devices in MRI environments ex vivo. In doing so, we hope to elucidate which variables impart
A.A. Marano et al. greater risk and thus allow practitioners to make an informed decision when this important clinical dilemma arises.
Methods Four hundred-milliliter ferromagnetic port-containing breast tissue expanders from each of the three United States FDA-approved companies (Allergan: 133MV 400, Mentor: MH 400, and Sientra: 400ACX) were studied. Each device was subjected to three different but common MRI environments: 3T, 1.5T (actively shielded), and 1.5T (passively shielded). The preceding number refers to the strength of the magnetic field, measured in tesla, and typically ranges from 0.5T to 3.0T. Shielding refers to a process by which fringe fields, or areas where the magnetic field is present but outside the core of the magnet, are reduced. This can be achieved passively (through the use of a high-permeability material such as iron) or actively (using a superconducting conversely oriented coil wrapped over the top of the primary coil to cancel out the primary magnetic field).19 Translational force was measured for each device using a previously described method (Figure 1). Each device was affixed to the end of a 20-cm string that was suspended from a protractor. The deflection angle was measured as the displacement from a vertical line, which was measured at a location in the scanner where maximum displacement was achieved. The torque that was exerted by the magnet on each expander was qualitatively graded on a 0e4 scale, as described by Shellock et al.: 0, no torque; 1, mild torque or change in orientation but not fully aligned with the magnetic field; 2, moderate torque or slow alignment of the magnet with the magnetic field; 3, strong torque or rapid alignment with magnetic field; 4, maximal torque or forceful alignment with magnetic field.20 The scale was modified to contain finer degradations, with “þ” or “” indicating relative increase or decrease in torque,
Figure 1
Schematic of deflection angle.
Please cite this article in press as: Marano AA, et al., Effect of MRI on breast tissue expanders and recommendations for safe use, Journal of Plastic, Reconstructive & Aesthetic Surgery (2017), http://dx.doi.org/10.1016/j.bjps.2017.07.012
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Effect of MRI on tissue expanders respectively. Torque was measured for each of the three empty expanders. To determine the effect of filling the expander on torque, the Allergan expander was tested at incrementally increasing fill volumes of both air and saline until the recommended volume of 400 mL was reached in a 1.5T passively shielded MRI. The temperature of each device was measured using an alcohol thermometer at various time points: at initial baseline and then at six time points during MRI imaging with a high specific absorption rate fast spin echo pulse sequence. Subsequently, the magnetic properties of internal ports were determined by removing the ports from the rest of the device. Measurements were taken using a gaussmeter at maximum gauss in the y-direction, with the probe touching the magnet. Each measurement was repeated in triplicate, and the reported value is the mean of those three measurements. As a negative control, tissue expanders that contained non-magnetic ports (known as “external” or “tunneled” ports) were subjected to the same tests.
Results The deflection angle of the empty Allergan, Mentor, and Sientra devices respectively were 70, 80, and 90 in the 3T MRI; 43, 54, and 60 in the 1.5T actively shielded MRI; and 30, 40, and 50 in the 1.5T passively shielded MRI (Figure 2). The torque in each of the three environments for the Allergan, Mentor, and Sientra devices respectively was 3, 3þ, and 4 in the 3T MRI; 3-, 3, and 4 in the 1.5T actively
Figure 2 Torque and deflection angle by device manufacturer and MRI environment.
3 shielded MRI; and 2-, 3, and 4 in the 1.5T passively shielded MRI (Figure 2). When filled with saline and air, the Allergan 400 mL expander exhibited incrementally decreasing palpable torque with increase in volume. The volume at which torque was no longer detectable was 240 mL of saline and 360 mL of air (Figure 3). The amount of force required to prevent angular displacement was 870 g for an empty expander and 120 g for a saline-filled Allergan 400 mL tissue expander. There was no appreciable difference in the temperatures of the devices at any of the seven time points (recorded temperature was 24 C at all time points in all devices, except for the Sientra device at 358 s, which was recorded as 25 C, and then returned to 24 C at 778 s) (Figure 4). The ferromagnetic port magnetic field strengths were 56, 93, and 124 gauss for Allergan, Mentor, and Sientra magnetic ports, respectively (Figure 5). The non-magnetic breast tissue expander showed a magnetic field strength of 0 on the gaussmeter.
Discussion As MRI utilizes a superconducting magnet to create a strong magnetic field, it is reasonable to be cautious regarding the risk of undergoing MRI in patients with implanted devices that contain ferromagnetic materials. The theoretical risks, combined with statements from the breast implant device manufacturers, has led to the widely held misconception that breast tissue expanders are incompatible with MRI. However, there has been a recently published large clinical series showing tissue expander patients who were able to undergo MRI safely.18 The findings of this series refute the basis for the current practice of indiscriminate contraindication. Our ex vivo experiments expand on these clinical findings by providing an analysis of the magnetic interactions that occur with different variables and environments. Deflection angle and torque measurements represent the translational force exerted on the ferromagnetic portion of the device and therefore act as a surrogate
Figure 3 Palpable torque on Allergan 400 mL magnetic port with incremental increase in the volume of saline or air.
Please cite this article in press as: Marano AA, et al., Effect of MRI on breast tissue expanders and recommendations for safe use, Journal of Plastic, Reconstructive & Aesthetic Surgery (2017), http://dx.doi.org/10.1016/j.bjps.2017.07.012
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A.A. Marano et al.
Figure 4
Figure 5
Temperature by device manufacturer.
Magnetic field strength by device manufacturer.
measure for risk of expander migration. Sientra devices showed the strongest deflection angle and torque measurements, followed by Mentor and then Allergan. Our results also show that magnetic interactions are stronger with greater magnetic field strengths (3T greater than 1.5T) and active shielding. When interpreting the importance of the deflection angle, it is crucial to note that a 45-degree displacement represents magnetic forces that are approximately equal to the force of gravity on the magnetic port. Measurements equal to or less than 45 indicate forces no greater than the gravitational force that the device is constantly under the influence of and thus not likely to be of concern. Note that significant deviations from 45 only occurred in the setting of unfilled Sientra devices at 1.5T and all unfilled devices at 3T. The magnetic port within the tissue expander tries to orient perpendicular to the axis of the magnet, thereby creating torque within the device. This likely accounts for the reports of polarity reversal and cases of expander migration and damage following MRI. When incrementally filling the expanders with air and saline, it was evident that there was a decrease in palpable torque with an increase in expander volumes. When the tissue expander is empty, the magnet sits flush against the anterior and posterior aspects of the expander, and therefore, torque will be felt against both the overlying skin flaps and the underlying chest wall. As the tissue expander is filled, the magnet lifts off the posterior wall of the expander, and torque is eventually not palpable on the part that would lie against the chest wall (Figure 6). Thus, it seems the benefit of filling the tissue expanders is volume related rather than mass related. The observed difference between air- and saline-filled expanders is likely due to the fundamental differences between the materials used to fill the expander; gas is compressible and therefore decreases in volume in response to pressure, whereas saline is non-compressible
and maintains a constant volume. Furthermore, while 870 g of weight was required to prevent angular displacement of an empty expander, and 120 g was required for a full one, it should be noted that both these values are significantly less than the weight of a patient lying prone. The gaussmeter recordings represent the strength of the magnetic field produced by each ferromagnetic port, and each manufacturer uses a different port with distinct magnetic properties. Magnetic field strength is directly proportional to the strength of the MRI scanner force exerted on each device and thus serves as a predictor of port migration risk. Our measurements from each device show that the strongest magnetic field is produced by Sientra devices, followed again by Mentor and then Allergan, indicating a positive correlation with torque and deflection angle measurements. These findings suggest not only that certain devices and the 3T setting impose a greater risk of migration but also that the MRI environment and gaussmeter readings may be useful tools for stratifying the risk of expander migration in individual patients. Regardless of the device manufacturer or MRI environment, no device experienced a significant change in temperature. From these findings, in the context of recent supportive clinical evidence, we oppose sweeping contraindication and instead suggest that the decision to undergo MRI should be a patient-specific decision that weighs the risks of the imaging study against the risks of the alternatives. Our findings suggest that, when possible, a 1.5T should be preferred over 3T MRI. Additionally, the data in this paper suggest that Allergan devices are least influenced by the magnetic field produced during MRI, and correlating in vivo data suggest that these devices bear minimal clinical risks. Additional strategies may also be employed to reduce the risk of discomfort or migration. Prone positioning may provide benefit by taking advantage of the patient’s body weight to resist movements caused by magnetic forces. Similarly, greater tissue expander volumes provide more room for the magnet to rotate without impacting or stretching the tissue around the expander. The use of tissue expanders with MRI-compatible distant or tunneled ports may be an alternative strategy when the likelihood of needing an MRI in the subsequent 6 months is high. This strategy would not be useful, however, when the indication for MRI was not anticipated (i.e., new-onset symptoms concerning brain or spinal metastases).
Please cite this article in press as: Marano AA, et al., Effect of MRI on breast tissue expanders and recommendations for safe use, Journal of Plastic, Reconstructive & Aesthetic Surgery (2017), http://dx.doi.org/10.1016/j.bjps.2017.07.012
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Effect of MRI on tissue expanders
Figure 6
Schematic of torque when tissue expander is relatively empty (left) and when filled (right).
The ex vivo nature of this study does limit the strength of the clinical conclusions one can draw from these data. The benefit of this experimental model, however, is that we have removed any potential confounding factors that could influence the behavior of tissue expanders during MRI. In doing so, we could observe the purest form of this magnetic interaction, which is the fundamental cause of safety concern. The role of this study is to provide data that can be interpreted in tandem with clinical data to make an informed judgment of safety. This study includes a number of unique elements that have not before been published in the literature. First, we provide a detailed experimental analysis of the magnetic properties of breast tissue expanders from each of the three United States FDA-approved companies. Second, we offer a comparison of how these different devices respond to three different MRI environments. Finally, we propose recommendations for the safe use of MRI in patients with ferromagnetic port-containing breast tissue expanders. Subsequent studies are underway that involve the in vivo analysis of patients with a variety of manufacturers’ devices tested in multiple MRI environments.
Conclusion Recent clinical evidence suggests that MRI can be performed safely in patients with breast tissue expanders. On the basis of the findings in our study, the effect of the magnetic forces that expanders experience may be reduced by using 1.5T MRI, device selection (manufacturer), filling the device with saline, and prone positioning. Thus, MRI can reasonably be considered in patients with breast tissue expanders when appropriate peri-procedural choices have been made so that the benefits of undergoing MRI outweigh the risks.
Conflict of interest None.
Funding None.
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References 1. American Society for Testing and Materials (ASTM) International. Designation: F2503e05. Standard practice for marking medical devices and other items for safety in the magnetic resonance environment. West Conshohocken, PA: ASTM International; 2005. 2. Surgeons ASoP. 2014 plastics surgery statistics report. 2014 [Accessed 24 July 2016], https://d2wirczt3b6wjm.cloudfront. net/News/Statistics/2014/plastic-surgery-statistics-fullreport-2014.pdf. 3. Nava MB, Bertoldi S, Forti M, et al. Effects of the magnetic resonance field on breast tissue expanders. Aesthetic Plast Surg 2012;36(4):901e7. 4. Rozen WM, Stella DL, Bowden J, Taylor GI, Ashton MW. Advances in the pre-operative planning of deep inferior epigastric artery perforator flaps: magnetic resonance angiography. Microsurgery 2009;29(2):119e23. 5. Rostami R, Safarpour D, Tavassoli FA, Jabbari B. Intramedullary metastasis in breast cancerea comprehensive literature review. J Neurol Sci 2013;332(1e2):16e20. 6. Chan-Seng E, Charissoux M, Larbi A, et al. Spinal metastases in breast cancer: single center experience. World Neurosurg 2014;82(6):1344e50. 7. Feldman DR, Conway WF, Spicer KM. Ferromagnetic artifact from a tissue expander used for breast reconstruction. Plast Reconstr Surg 1995;95(2):426e7. 8. Zegzula HD, Lee WP. Infusion port dislodgement of bilateral breast tissue expanders after MRI. Ann Plast Surg 2001;46(1): 46e8. 9. Sowder LL. The case of the floating injection port. Plast Reconstr Surg 2002;110(3):991. 10. Duffy Jr FJ, May Jr JW. Tissue expanders and magnetic resonance imaging: the “hot” breast implant. Ann Plast Surg 1995; 35(6):647e9. 11. Liang MD, Narayanan K, Kanal E. Magnetic ports in tissue expandersea caution for MRI. Magn Reson Imaging 1989;7(5): 541e2. 12. Stueber K. A complication of tissue expander breast reconstruction. Plast Reconstr Surg 1997;99(5):1464e5. 13. Ascherman JA. Reversal of expander port polarity following magnetic resonance imaging. Plast Reconstr Surg 2004; 114(3):817. 14. Henderson PW, Dosanjh J, Rohde CH. Successful breast reconstruction despite dislodgement of tissue expander magnetic ports. Aesthet Surg J 2016;36(10):NP307e9. 15. Natrelle. Directions for use: Natrelle 133 tissue expanders with suture tabs. 2009.
Please cite this article in press as: Marano AA, et al., Effect of MRI on breast tissue expanders and recommendations for safe use, Journal of Plastic, Reconstructive & Aesthetic Surgery (2017), http://dx.doi.org/10.1016/j.bjps.2017.07.012
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6 16. Sientra. Directions for use: anatomic controlled tissue expansion matrix. 17. Mentor. Products insert data sheet: mentor CPX4 and CPX4 with suture tabs breast tissue expanders. 2013. 18. Thimmappa ND, Prince MR, Colen KL, et al. Breast tissue expanders with magnetic ports: clinical experience at 1.5 T. Plast Reconstr Surg 2016;138(6):1171e8.
A.A. Marano et al. 19. The American College of Radiology e Glossary of MRI Terms. https://www.acr.org/w/media/ACR/Documents/PDF/ QualitySafety/Resources/GlossaryOfMRTerms.pdf. 20. Linnemeyer H, Shellock FG, Ahn CY. In vitro assessment of MRI issues at 3-Tesla for a breast tissue expander with a remote port. Magn Reson Imaging 2014;32(3):297e302.
Please cite this article in press as: Marano AA, et al., Effect of MRI on breast tissue expanders and recommendations for safe use, Journal of Plastic, Reconstructive & Aesthetic Surgery (2017), http://dx.doi.org/10.1016/j.bjps.2017.07.012
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Journal of Plastic, Reconstructive & Aesthetic Surgery (2017) xx, 1e6
Effect of MRI on breast tissue expanders and recommendations for safe use Andrew A. Marano, Peter W. Henderson, Martin R. Prince, Stephen M. Dashnaw, Christine H. Rohde* Division of Plastic and Reconstructive Surgery, Department of Radiology, Columbia University Medical Center, New York, NY, USA Received 7 June 2017; accepted 26 July 2017
KEYWORDS MRI; Magnetic resonance imaging; MRI safety; Tissue expander; Breast tissue expander; Expander
Summary Introduction: Ferromagnetic port-containing breast tissue expanders are currently labeled MRI-unsafe because of the presumption that magnets should not enter the machine. However, designating these devices as MRI-unsafe can lead to unnecessary procedures or suboptimal imaging choices. This study provides an ex vivo analysis of how breast tissue expanders behave when subjected to strong magnetic fields to determine which variables might affect clinical risk. Methods: Three different brands of tissue expanders were evaluated in three MRI environments. Translational force was determined using the deflection angle method. Torque on empty, saline-filled, and air-filled expanders was evaluated on a 0e4 scale. Magnetic field was measured using a gaussmeter. The weight required to prevent displacement of the expanders was determined for both air- and saline-filled expanders. Temperature over time was measured using an alcohol thermometer. Results: Magnetic field strength, deflection angle, and torque were the greatest in 3T MRI environments and varied by device manufacturer (Sientra > Mentor > Allergan). Saline-filled expanders required 240 mL and air-filled required 360 mL volume to make the torque undetectable, and the effect of torque could be mitigated with prone positioning. A weight of 120 g was required to prevent displacement of a saline-filled tissue expander and 870 g for an empty expander. There were no appreciable changes in temperature. Conclusions: Previously described risks may be reduced by using a 1.5T MRI, device selection, filling expanders with saline, and prone positioning. MRI can be considered in patients with breast tissue expanders when appropriate peri-procedural choices have been made so that the benefits of undergoing MRI outweigh the risks. ª 2017 British Association of Plastic, Reconstructive and Aesthetic Surgeons. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (C.H. Rohde). http://dx.doi.org/10.1016/j.bjps.2017.07.012 1748-6815/ª 2017 British Association of Plastic, Reconstructive and Aesthetic Surgeons. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Marano AA, et al., Effect of MRI on breast tissue expanders and recommendations for safe use, Journal of Plastic, Reconstructive & Aesthetic Surgery (2017), http://dx.doi.org/10.1016/j.bjps.2017.07.012
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Introduction Magnetic resonance imaging (MRI) is an established diagnostic modality used across many specialties to evaluate a variety of tissue types throughout the body. Visualization is accomplished by placing the patient within a strong magnetic field and interrogating with a radiofrequency signal, using magnetic field gradients to localize the resulting signal for reconstructing an image. The presence of this strong magnetic field, radiofrequency signals, and magnetic field gradients has brought into question the safety of implantable devices made of ferromagnetic materials. The American Society for Testing and Materials has consequently established guidelines for determining the extent of magnetically induced displacement forces, torque, and device heating and standards for labeling products as “MR Safe,” “MR Conditional,” or “MR Unsafe.”1 The safety of medical implant devices during MRI is of particular importance to patients with breast tissue expanders. Placement of tissue expanders and exchange for permanent implants comprised approximately 73% of all breast reconstructions in 2014.2 During the expansion period, a patient may need an MRI. Common indications include surveillance of the contralateral breast,3 perforator mapping for possible transition to autologous reconstruction,4 and evaluation for brain5 and spinal metastases.6 Most breast tissue expanders have a ferromagnetic port. Surgeons have anecdotally noted image artifacts,7 tissue expander migration,8,9 a burning sensation,10 pain at the site of implant placement following MRI,11,12 magnet polarity reversal,13 and dislodgement of the magnetic ports.14 Until recently, there was no data to support the safety of tissue expanders during MRI, and thus, device manufacturers recommend MRI not be performed while these devices are implanted.15e17 As a result, MRI technicians and radiologists commonly refuse to perform MRI studies in patients with these devices, although other studies report that MRI with tissue expanders in place can be performed without complications.18 There are few options for patients with tissue expanders who must undergo MRI evaluation, each with its own shortcomings. In lieu of an MRI, the clinician may opt for a computed tomography (CT) scan; however, the use of a suboptimal imaging modality increases the potential for incorrect diagnoses. The clinician may also choose to remove the tissue expander or exchange it for a permanent implant, but doing so prior to the completion of the expansion process may compromise the quality of the reconstruction and expose the patient to additional, potentially unnecessary surgery and anesthesia. The operation to remove or exchange the tissue expander will also delay access to an MRI. Our group was recently involved in a large clinical series in which 71 patients with breast tissue expanders underwent MRI in a 1.5 tesla (T) environment.18 In this series, there were no reports of pain or sensation of device migration nor was there any evidence of tissue damage or complications upon removal of the expanders. To better understand these findings, we chose to investigate the behavior of these devices in MRI environments ex vivo. In doing so, we hope to elucidate which variables impart
A.A. Marano et al. greater risk and thus allow practitioners to make an informed decision when this important clinical dilemma arises.
Methods Four hundred-milliliter ferromagnetic port-containing breast tissue expanders from each of the three United States FDA-approved companies (Allergan: 133MV 400, Mentor: MH 400, and Sientra: 400ACX) were studied. Each device was subjected to three different but common MRI environments: 3T, 1.5T (actively shielded), and 1.5T (passively shielded). The preceding number refers to the strength of the magnetic field, measured in tesla, and typically ranges from 0.5T to 3.0T. Shielding refers to a process by which fringe fields, or areas where the magnetic field is present but outside the core of the magnet, are reduced. This can be achieved passively (through the use of a high-permeability material such as iron) or actively (using a superconducting conversely oriented coil wrapped over the top of the primary coil to cancel out the primary magnetic field).19 Translational force was measured for each device using a previously described method (Figure 1). Each device was affixed to the end of a 20-cm string that was suspended from a protractor. The deflection angle was measured as the displacement from a vertical line, which was measured at a location in the scanner where maximum displacement was achieved. The torque that was exerted by the magnet on each expander was qualitatively graded on a 0e4 scale, as described by Shellock et al.: 0, no torque; 1, mild torque or change in orientation but not fully aligned with the magnetic field; 2, moderate torque or slow alignment of the magnet with the magnetic field; 3, strong torque or rapid alignment with magnetic field; 4, maximal torque or forceful alignment with magnetic field.20 The scale was modified to contain finer degradations, with “þ” or “” indicating relative increase or decrease in torque,
Figure 1
Schematic of deflection angle.
Please cite this article in press as: Marano AA, et al., Effect of MRI on breast tissue expanders and recommendations for safe use, Journal of Plastic, Reconstructive & Aesthetic Surgery (2017), http://dx.doi.org/10.1016/j.bjps.2017.07.012
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Effect of MRI on tissue expanders respectively. Torque was measured for each of the three empty expanders. To determine the effect of filling the expander on torque, the Allergan expander was tested at incrementally increasing fill volumes of both air and saline until the recommended volume of 400 mL was reached in a 1.5T passively shielded MRI. The temperature of each device was measured using an alcohol thermometer at various time points: at initial baseline and then at six time points during MRI imaging with a high specific absorption rate fast spin echo pulse sequence. Subsequently, the magnetic properties of internal ports were determined by removing the ports from the rest of the device. Measurements were taken using a gaussmeter at maximum gauss in the y-direction, with the probe touching the magnet. Each measurement was repeated in triplicate, and the reported value is the mean of those three measurements. As a negative control, tissue expanders that contained non-magnetic ports (known as “external” or “tunneled” ports) were subjected to the same tests.
Results The deflection angle of the empty Allergan, Mentor, and Sientra devices respectively were 70, 80, and 90 in the 3T MRI; 43, 54, and 60 in the 1.5T actively shielded MRI; and 30, 40, and 50 in the 1.5T passively shielded MRI (Figure 2). The torque in each of the three environments for the Allergan, Mentor, and Sientra devices respectively was 3, 3þ, and 4 in the 3T MRI; 3-, 3, and 4 in the 1.5T actively
Figure 2 Torque and deflection angle by device manufacturer and MRI environment.
3 shielded MRI; and 2-, 3, and 4 in the 1.5T passively shielded MRI (Figure 2). When filled with saline and air, the Allergan 400 mL expander exhibited incrementally decreasing palpable torque with increase in volume. The volume at which torque was no longer detectable was 240 mL of saline and 360 mL of air (Figure 3). The amount of force required to prevent angular displacement was 870 g for an empty expander and 120 g for a saline-filled Allergan 400 mL tissue expander. There was no appreciable difference in the temperatures of the devices at any of the seven time points (recorded temperature was 24 C at all time points in all devices, except for the Sientra device at 358 s, which was recorded as 25 C, and then returned to 24 C at 778 s) (Figure 4). The ferromagnetic port magnetic field strengths were 56, 93, and 124 gauss for Allergan, Mentor, and Sientra magnetic ports, respectively (Figure 5). The non-magnetic breast tissue expander showed a magnetic field strength of 0 on the gaussmeter.
Discussion As MRI utilizes a superconducting magnet to create a strong magnetic field, it is reasonable to be cautious regarding the risk of undergoing MRI in patients with implanted devices that contain ferromagnetic materials. The theoretical risks, combined with statements from the breast implant device manufacturers, has led to the widely held misconception that breast tissue expanders are incompatible with MRI. However, there has been a recently published large clinical series showing tissue expander patients who were able to undergo MRI safely.18 The findings of this series refute the basis for the current practice of indiscriminate contraindication. Our ex vivo experiments expand on these clinical findings by providing an analysis of the magnetic interactions that occur with different variables and environments. Deflection angle and torque measurements represent the translational force exerted on the ferromagnetic portion of the device and therefore act as a surrogate
Figure 3 Palpable torque on Allergan 400 mL magnetic port with incremental increase in the volume of saline or air.
Please cite this article in press as: Marano AA, et al., Effect of MRI on breast tissue expanders and recommendations for safe use, Journal of Plastic, Reconstructive & Aesthetic Surgery (2017), http://dx.doi.org/10.1016/j.bjps.2017.07.012
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Figure 4
Figure 5
Temperature by device manufacturer.
Magnetic field strength by device manufacturer.
measure for risk of expander migration. Sientra devices showed the strongest deflection angle and torque measurements, followed by Mentor and then Allergan. Our results also show that magnetic interactions are stronger with greater magnetic field strengths (3T greater than 1.5T) and active shielding. When interpreting the importance of the deflection angle, it is crucial to note that a 45-degree displacement represents magnetic forces that are approximately equal to the force of gravity on the magnetic port. Measurements equal to or less than 45 indicate forces no greater than the gravitational force that the device is constantly under the influence of and thus not likely to be of concern. Note that significant deviations from 45 only occurred in the setting of unfilled Sientra devices at 1.5T and all unfilled devices at 3T. The magnetic port within the tissue expander tries to orient perpendicular to the axis of the magnet, thereby creating torque within the device. This likely accounts for the reports of polarity reversal and cases of expander migration and damage following MRI. When incrementally filling the expanders with air and saline, it was evident that there was a decrease in palpable torque with an increase in expander volumes. When the tissue expander is empty, the magnet sits flush against the anterior and posterior aspects of the expander, and therefore, torque will be felt against both the overlying skin flaps and the underlying chest wall. As the tissue expander is filled, the magnet lifts off the posterior wall of the expander, and torque is eventually not palpable on the part that would lie against the chest wall (Figure 6). Thus, it seems the benefit of filling the tissue expanders is volume related rather than mass related. The observed difference between air- and saline-filled expanders is likely due to the fundamental differences between the materials used to fill the expander; gas is compressible and therefore decreases in volume in response to pressure, whereas saline is non-compressible
and maintains a constant volume. Furthermore, while 870 g of weight was required to prevent angular displacement of an empty expander, and 120 g was required for a full one, it should be noted that both these values are significantly less than the weight of a patient lying prone. The gaussmeter recordings represent the strength of the magnetic field produced by each ferromagnetic port, and each manufacturer uses a different port with distinct magnetic properties. Magnetic field strength is directly proportional to the strength of the MRI scanner force exerted on each device and thus serves as a predictor of port migration risk. Our measurements from each device show that the strongest magnetic field is produced by Sientra devices, followed again by Mentor and then Allergan, indicating a positive correlation with torque and deflection angle measurements. These findings suggest not only that certain devices and the 3T setting impose a greater risk of migration but also that the MRI environment and gaussmeter readings may be useful tools for stratifying the risk of expander migration in individual patients. Regardless of the device manufacturer or MRI environment, no device experienced a significant change in temperature. From these findings, in the context of recent supportive clinical evidence, we oppose sweeping contraindication and instead suggest that the decision to undergo MRI should be a patient-specific decision that weighs the risks of the imaging study against the risks of the alternatives. Our findings suggest that, when possible, a 1.5T should be preferred over 3T MRI. Additionally, the data in this paper suggest that Allergan devices are least influenced by the magnetic field produced during MRI, and correlating in vivo data suggest that these devices bear minimal clinical risks. Additional strategies may also be employed to reduce the risk of discomfort or migration. Prone positioning may provide benefit by taking advantage of the patient’s body weight to resist movements caused by magnetic forces. Similarly, greater tissue expander volumes provide more room for the magnet to rotate without impacting or stretching the tissue around the expander. The use of tissue expanders with MRI-compatible distant or tunneled ports may be an alternative strategy when the likelihood of needing an MRI in the subsequent 6 months is high. This strategy would not be useful, however, when the indication for MRI was not anticipated (i.e., new-onset symptoms concerning brain or spinal metastases).
Please cite this article in press as: Marano AA, et al., Effect of MRI on breast tissue expanders and recommendations for safe use, Journal of Plastic, Reconstructive & Aesthetic Surgery (2017), http://dx.doi.org/10.1016/j.bjps.2017.07.012
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Effect of MRI on tissue expanders
Figure 6
Schematic of torque when tissue expander is relatively empty (left) and when filled (right).
The ex vivo nature of this study does limit the strength of the clinical conclusions one can draw from these data. The benefit of this experimental model, however, is that we have removed any potential confounding factors that could influence the behavior of tissue expanders during MRI. In doing so, we could observe the purest form of this magnetic interaction, which is the fundamental cause of safety concern. The role of this study is to provide data that can be interpreted in tandem with clinical data to make an informed judgment of safety. This study includes a number of unique elements that have not before been published in the literature. First, we provide a detailed experimental analysis of the magnetic properties of breast tissue expanders from each of the three United States FDA-approved companies. Second, we offer a comparison of how these different devices respond to three different MRI environments. Finally, we propose recommendations for the safe use of MRI in patients with ferromagnetic port-containing breast tissue expanders. Subsequent studies are underway that involve the in vivo analysis of patients with a variety of manufacturers’ devices tested in multiple MRI environments.
Conclusion Recent clinical evidence suggests that MRI can be performed safely in patients with breast tissue expanders. On the basis of the findings in our study, the effect of the magnetic forces that expanders experience may be reduced by using 1.5T MRI, device selection (manufacturer), filling the device with saline, and prone positioning. Thus, MRI can reasonably be considered in patients with breast tissue expanders when appropriate peri-procedural choices have been made so that the benefits of undergoing MRI outweigh the risks.
Conflict of interest None.
Funding None.
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Please cite this article in press as: Marano AA, et al., Effect of MRI on breast tissue expanders and recommendations for safe use, Journal of Plastic, Reconstructive & Aesthetic Surgery (2017), http://dx.doi.org/10.1016/j.bjps.2017.07.012