Deep Brain Stimulation (DBS): Activa (Neurostimulation) System (Medtronic, Minneapolis, MN)

Urgent Device Correction Change of Safe Limits for MRI Procedures Used With the Medtronic Activa Deep Brain Stimulation Systems

This letter is intended to inform you of a safety concern regarding the Medtronic Activa Deep Brain Stimulation (DBS) system instructions for performing MRI (magnetic resonance imaging). Medtronic's instructions have until now recommended a use of a displayed MRI head specific absorption rate (SAR) of not more than 0.4 W/kg. Medtronic recently determined that an SAR of 0.4 W/kg can cause excessive heating of implanted DBS electrodes during MRI. Medtronic is, however, not aware of any adverse events that resulted from this problem when MRI was conducted on DBS patients following Medtronic's MRI instructions.

Medtronic changed the instructions for use of the Activa DBS system. The recommended maximum displayed head SAR is now 0.1 W/kg. This is the only change to the instructions for use of the Activa DBS system. The revised instructions apply to the following Activa DBS system components:

Model 7424 Itrel II Neurostimulator (DBS applications only)
Model 7426 Soletra Neurostimulator
Model 7428 Kinetra Neurostimulator
Models 3387, 3389 DBS Leads
Models 7482, 7495 DBS Extensions

Explanation of the issue:

Medtronic's original testing relied on the MRI system's displayed head SAR value. We have now determined that the displayed value used in the original testing was inaccurate. This inaccuracy resulted in MRI instructions for our DBS systems that did not provide an adequate safety margin for MRI machines that have an accurate displayed SAR.

Medtronic recently retested the heating of Activa DBS electrodes by MRI scans at specific SAR values, using standardized methods and phantoms. At an actual applied SAR of 0.4 W/kg, the temperature of a DBS electrode embedded in a phantom increased by 5 degrees C. This temperature increase is near or at the level that can cause tissue injury. By contrast, the maximum temperature rise of the DBS electrode was less than 2 degrees C for an applied head SAR of 0.1 W/kg over a 15 minute scan. Therefore we have determined that an actual applied head SAR of 0.1 W/kg provides a reasonable thermal safety margin.

In the same series of tests, we evaluated the accuracy of the SAR display of three different 1.5 Tesla MRI machines using a transmit-receive (T/R) head coil. The actual SAR was measured in a calorimetric phantom. The two new machines had measured SARs corresponding to the displayed SARs. The older model MRI machine produced SARs as low as 1/4 of the displayed SAR.

If your MRI machine and specific software version develop actual SARs that are appreciably lower than the displayed SARs, then a displayed SAR of 0.1 W/ kg may not produce adequate images. Imaging problems resulting from a difference between actual SARs and displayed SARs would need to be addressed by the manufacturer of your specific MRI machine.

We appreciate your assistance with this matter and regret any inconvenience this may cause you and your patients. The Medical Community can further our understanding of Activa Deep Brain Stimulation Systems by reporting adverse events to Medtronic at 800-328-0810 or FDA at 800-332-1088. If you have any questions or comments, please contact your Medtronic representative or Medtronic Neurological Technical Services at 800-707-0933.


[*The information for the Activa System was reprinted with permission by Medtronic, Minneapolis, MN, www.medtronic.com.]

[For this device, MR healthcare professionals are advised to contact the manufacturer to ensure that the latest safety information is obtained and carefully followed in order to ensure patient safety relative to the use of an MR procedure.]

REFERENCES

Dormont D, Cornu P, Pidoux B, Bonnet AM, Biondi A, Oppenheim C, Hasboun D, Damier P, et al. Chronic thalamic stimulation with three-dimensional MR stereotactic guidance. AJNR Am J Neuroradiol 1997;18:1093-1097.

Finelli DA, Rezai AR, Ruggieri P, Tkach J, Nyenhuis J, Hridlicka G, Sharan A, Stypulkowski PH, Shellock FG. MR-related heating of deep brain stimulation electrodes: an in vitro study of clinical imaging sequences. AJNR Am J Neuroradiol 2002;23:1795-1802.

Finelli D, Rezai AR, Rugieri P, Tkach J, Nyenhuis JA, Shellock FG. Neurostimulation systems used for deep brain stimulation: in vitro assessment of MRI-related heating at 1.5-Tesla. Radiology 2002;222:586.

Gleason CA, Kaula NF, Hricak H, et al. The effect of magnetic resonance imagers on implanted neurostimulators. Pacing Clin Electrophysiol 1992;15:81-94.

Liem LA, van Dongen VC. Magnetic resonance imaging and spinal cord stimulation systems. Pain 1997;70:95-97.

http://www.medtronic.com/

Rezai AR, et al. Thalamic stimulation and functional magnetic resonance imaging: localization of cortical and subcortical activation with implanted electrodes. J Neurosurg 1999;90;583-590.

Rezai AR, Finelli D, Ruggieri P, Tkach J, Nyenhuis JA, Shellock FG. Neurostimulators: Potential for excessive heating of deep brain stimulation electrodes during MR imaging. Journal of Magnetic Resonance Imaging 2001;14:488-489.

Rezai AR, Finelli D, Nyenhuis JA, Hrdlick G, Tkach J, Ruggieri P, Stypulkowski PH, Sharan A, Shellock FG. Neurostimulator for deep brain stimulation: Ex vivo evaluation of MRI-related heating at 1.5-Tesla. Journal of Magnetic Resonance Imaging 2002;15:241-250.

Rise MT. Instrumentation for neuromodulation. Archives Of Medical Research 2000;31:237-247.

Shellock FG. Magnetic Resonance Procedures: Health Effects and Safety. CRC Press, LLC, Boca Raton, FL, 2001.

Shellock FG. MR imaging and electronically-activated devices. Radiology. 219:294-295, 2001.

Shellock FG. MR safety update 2002: Implants and devices. Journal of Magnetic Resonance Imaging 2002;16:485-496.

Shellock FG, Kanal E. Magnetic Resonance: Bioeffects, Safety, and Patient Management. Second Edition, Lippincott-Raven Press, New York, 1996.

Smith CD, Kildishev AV, Nyenhuis JA, Foster KS, Bourland JD, Interactions of MRI magnetic fields with elongated medical implants. J Appl Physics 2000; 87:6188-6190.

Smith CD, Nyenhuis JA, Kildishev AV. Chapter 16. Health effects of induced electrical currents: Implications for implants. In: Magnetic resonance: health effects and safety, FG Shellock, Editor, CRC Press, Boca Raton, FL, 2001; pp. 393-413.

Tronnier VM, Stauber A, Hahnel S, Sarem-Aslani A. Magnetic resonance imaging with implanted neurostimulators: an in vitro and in vivo study. Neurosurgery 1999;44:118-125.

Zonenshayn M, Mogilner AY, Rezai AR. Neurostimulation and functional brain stimulation. Neurological Research 2000;22;318-325.

MRI Guidelines for Medtronic Deep Brain Stimulation Systems
Medtronic Inc., Minneapolis, MN
Models:
Kinetra: 7428
Soletra: 7426
Itrel II: 7424
Activa PC Deep Brain Neurostimulator (Model 37601)
Activa RC Deep Brain Neurostimulator (Model 37612) Activa SC Deep Brain Neurostimulator (Model 37602)
Activa SC Deep Brain Neurostimulator (Model 37603)

MRI and Activa Therapy
Introduction
It is important to read this section in its entirety before conducting an MRI examination on a patient with any implanted Activa System component.

Due to the number and variability of parameters that affect MRI compatibility, the safety of patients or continued functioning of Activa Systems exposed to MRI cannot be absolutely ensured. MRI systems generate powerful electromagnetic fields that can produce a number of interactions with implanted components of the Activa neurostimulation system. Some of these interactions, especially heating, are potentially hazardous and can lead to serious injury or death. However, with appropriate control measures, particularly with respect to the selection of MRI parameters and RF coils, it is generally possible to safely perform an MRI head scan on an Activa patient. In addition, Activa System components can affect the MRI image, potentially impacting the diagnostic use of this modality. The following information describes the potential interactions and control measures that should be taken to minimize the risks from these interactions.

Contraindication
Implantation of an Activa Brain Stimulation System is contraindicated for patients who will be exposed to Magnetic Resonance Imaging (MRI) using a full body transmit radio-frequency (RF) coil, a receive-only head coil, or a head transmit coil that extends over the chest area. Performing MRI with this equipment can cause tissue lesions from component heating, especially at the lead electrodes, resulting in serious and permanent injury including coma, paralysis, or death.

Warnings
-Do not conduct an MRI examination on a patient with any implanted Activa System component until you read and fully understand all the information in this section. Failure to follow all warnings and guidelines related to MRI can result in serious and permanent injury including coma, paralysis, or death.
-In-vitro testing has shown that exposure of the Activa neurostimulator system to MRI at parameters other than those described in this guideline can induce significant heating at the lead electrodes or at breaks in the lead. Excessive heating may occur even if the lead and/or extension are the only part of the Activa System that is implanted. Excessive heating can result in serious and permanent injury including coma, paralysis, or death.
-MRI examinations of patients with an implanted Activa System should only be done if absolutely needed and then only if these guidelines are followed. MRI should not be considered for Activa patients if other potentially safer diagnostic methods such as CT, X-ray, ultrasound, or other methods will provide adequate diagnostic information.
-A responsible individual with expert knowledge about MRI, such as an MRI radiologist or MRI physicist, must assure all procedures in this guidelines are followed and that the MRI scan parameters, especially RF specific absorption rate (SAR) and gradient dB/dt parameters, comply with the recommended settings, both for the pre-scan (tuning) and during the actual MRI examination. The responsible individual must verify that parameters entered into the MRI system meet the guidelines in this section.
-Do not conduct an MRI examination if the patient has any other implants or limiting factors that would prohibit or contraindicate an MRI examination.

Precautions

-The neurostimulator, especially those without filtered feedthroughs such as the Itrel II Model 7424, may be reset or potentially damaged when subjected to an MRI examination. If reset, the neurostimulator must be reprogrammed. If damaged, the neurostimulator must be replaced.
-MRI images may be severely distorted or image target areas can be completely blocked from view near the implanted Activa System components, especially near the neurostimulator. If the MRI targeted image area is near the neurostimulator, it may be necessary to move the neurostimulator to obtain an image, or use alternate imaging techniques. Do not remove the neurostimulator and leave the lead system implanted as this can result in higher than expected lead heating.
-Carefully weigh any decision to perform magnetic resonance imaging (MRI) examinations on patients who require the neurostimulator to control tremor. Image quality during MRI examinations may be reduced, because the tremor may return when the neurostimulator is turned off.
-If possible, do not sedate the patient so that the patient can provide feedback of any problems during the examination.
-Monitor the patient during the MRI examination. Verify that the patient is feeling normal and is responsive between each individual scan sequence of the MRI examination. Discontinue the MRI immediately if the patient becomes unresponsive to questions or experiences any heating, pain, shocking sensations/uncomfortable stimulation, or unusual sensations.

Note: The MRI guidelines provided here may significantly extend the MRI examination time or prevent some types of MRI examinations from being conducted on Activa patients.

General Information on MRI
An MRI system produces three types of electromagnetic fields that may interact with implanted neurostimulation systems. All three of these fields are necessary to produce an MRI image. Each of these fields can also produce specific but different types of interactions with implanted neurostimulator systems. These fields include:
Static Magnetic Field. This is a steady state non-varying magnetic field that is normally always ON, even when no scan is underway. In a 1.5 Tesla MRI system, the static magnetic field is approximately 30,000 times greater than the magnetic field of the earth.
Gradient Magnetic Field. This is a low-frequency pulsed magnetic field that is only present during a scan. The gradient magnetic field can induce voltages onto the lead system that may result in unintended stimulation or functional interactions with the neurostimulator.
RF Field. This is a pulsed radio frequency (RF) field that is only present during a scan. It can be produced by a variety of transmission RF coils such as a whole body transmit coil or an extremity coil such as a transmit/receive head coil. Only a transmit/receive head coil should be used as the other RF coils can expose more of the lead system to RF energy, thereby increasing the risk of excessive heating and thermal lesions possibly resulting in coma, paralysis, or death.

MRI Interactions with Implanted Activa Systems
MRI/neurostimulation system interactions are various, and the risk to the patient can range from minimal to severe. These interactions include the following:
Heating – The MRI RF field induces voltages onto the lead system that can produce significant heating effects at the lead electrode-tissue interface or at the location of any breaks in the neurostimulator lead system. Component heating from the MRI RF field is the most serious risk from MRI exposure. Failure to follow these MRI recommendations can result in thermal lesions possibly resulting in coma, paralysis, or death.
Magnetic Field Interactions – Magnetic field interactions such as force and torque effects are produced by the static magnetic field. Any magnetic material will be attracted to the static magnetic field of the MRI. The force and torque effects may produce movement of the neurostimulator that can be uncomfortable to the patient, open a recent incision, or both. Activa System components are designed with minimal magnetic materials.
Induced Stimulation – Gradient magnetic fields may induce voltages onto the lead system that may cause unintended stimulation. The voltage of the induced stimulation pulses is proportional to the time rate of change (dB/dt) of the gradient pulses, the effective loop area created by the neurostimulator lead system, and the location of the lead system with respect to the gradient coils of the MRI.
Effects on Neurostimulator Function – The static, gradient, and RF fields of the MRI may affect the neurostimulator operation and programming. The static magnetic field may cause the neurostimulator to turn ON or OFF if the neurostimulator uses a magnetically controlled switch that allows the patient to control stimulation by the application of a handheld magnet. Additionally, the MRI RF, static, and gradient fields may temporarily affect or disable other functions, such as telemetry or stimulation pulses. Parameters will need to be reprogrammed if the MRI causes a POR (Power On Reset) of the neurostimulator.
Image Artifacts and Distortion – The neurostimulation system components, particularly the neurostimulator, can cause significant imaging artifacts and/or distortion of the MRI image, particularly if the neurostimulator components contain magnetic material. The neurostimulator can cause the MRI image to be completely blocked from view (i.e., signal loss or signal “void”) or severely distorted within several inches of the neurostimulator.

MRI Procedure
Scope
These MRI/neurostimulator exposure guidelines apply to Activa Systems comprising combinations of the following components:
-Neurostimulator Models: Itrel II 7424, Soletra 7426, Kinetra 7428, Activa RC 37612, Activa PC 37601
-Lead Extension Models: 7495, 7482, 7482A, 37085
-Lead Models: DBS 3387, 3389

Supervision
A responsible individual such as an MRI radiologist or MRI physicist must assure these procedures are followed. If the MRI is operated by an MRI technician, it is strongly recommended the responsible individual verifies that the MRI recommendations are followed.

Preparation
Do the following prior to performing an MRI examination on an Activa patient:
1) Inform the patient of the risks of undergoing an MRI.
2) Check if the patient has any other implants or conditions that would prohibit or contraindicate an MRI examination. Do not conduct an MRI examination if any are found.
3) Verify that all proposed MRI examination parameters comply with the “MRI Operation Settings” on Table 2. If not, the parameters must be modified to meet these requirements. If this cannot be done, do not perform an MRI.
4) If the patient has implanted leads but does not have an implanted neurostimulator, perform the following steps:
-Wrap the external portion of the leads/percutaneous extensions with insulating material.
-Keep the external portion of the leads/percutaneous extensions out of contact with the patient.
-Keep the external leads/percutaneous extensions straight, with no loops, and running down the center of the head coil.
5) If the patient has an implanted neurostimulator, perform the following steps:
-Review the neurostimulator with a clinician programmer and print out a copy of the programmed parameters for reference.
-Test for possible open circuits by measuring impedance and battery current on all electrodes in unipolar mode (see Table 1). If an open circuit is suspected, obtain an x-ray to identify whether the open circuit is caused by a broken lead wire. If a broken lead wire is found, do not perform an MRI.

Warning: An MRI procedure should not be performed in a patient with an Activa System that has a broken lead wire because higher than normal heating may occur at the break or the lead electrodes which can cause thermal lesions. These lesions may result in coma, paralysis, or death.

Table 1. Measurement Values Indicating Possible Open Circuits
Neurostimulator, Impedance
Itrel II Model 7424, >2000
Soletra Model 7426, >2000
Kinetra Model 7428, >2000
Activa RC Model 37612, >2000
Activa PC Model 37601, >2000

Note: For all devices, any measurement with a value of <250 Ω typically indicates a possible short circuit.

Warning: An MRI procedure should not be performed in a patient with an Activa System that has a broken lead wire because higher than normal heating may occur at the break or the lead electrodes, which can cause thermal lesions. These lesions may result in coma, paralysis, or death.
-If the Activa System is functioning properly and no open or short circuits are found, program the neurostimulator to the settings provided in Table 2.

Table 2. Recommended Neurostimulator Settings (for all programs) for MRI
Parameter, Setting
Stimulation output, OFF
Stimulation mode, Bipolar (Soletra and Itrel II only)
Amplitude, 0 Volts (Soletra and Itrel II only)
Magnetic (reed) switch, Disabled (Kinetra Model 7428 only)
Day Cycling, Disabled (Kinetra Model 7428 only)
Other parameters, Do not change

MRI Operation Settings
Prior to the MRI examination, a responsible individual such as an MRI radiologist or MRI physicist must assure the examination will be conducted according to the following MRI requirements. If standard MRI pulse sequences will be used, they must meet these requirements. If they do not, the pulse parameters must be adjusted so that they comply with these requirements:

Warning: In-vitro testing has shown that exposure of the Activa System to MRI under conditions other than described in this guideline can induce excessive heating at the lead electrodes or at breaks in the lead to cause lesions. These lesions may result in coma, paralysis, or death.
-Use only a 1.5 Tesla horizontal bore MRI (do not use open sided or other field strength MRI systems).
-Use only a transmit/receive head coil.

Contraindication: Implantation of an Activa Brain Stimulation System is contraindicated for patients who will be exposed to Magnetic Resonance Imaging (MRI) using a full body transmit radio-frequency (RF) coil, a receive-only head coil, or a head transmit coil that extends over the chest area. Performing MRI with this equipment can cause tissue lesions from component heating, especially at the lead electrodes, resulting in serious and permanent injury including coma, paralysis, or death.
-Enter the correct patient weight into the MRI console to assure the head SAR is estimated correctly.
-Use MRI examination parameters that limit the displayed average head SAR to 1/10 (0.1) W/kg or less for all RF pulse sequences unless the applied SAR is known. If known, an applied SAR up to 1/10 (0.1) W/kg may be used.

Warnings
-Ensure the SAR value is the value for head SAR. Some MRI systems may only display SAR, whole body SAR, or local body SAR. Make sure the value being limited to 1/10 (0.1) W/kg is for head SAR. Excessive heating may occur if the wrong SAR value is used.
-If MRI parameters must be manually adjusted after the initial automatic MRI prescan, do not make any adjustments that will increase the SAR value. Some MRI machines may not automatically update the displayed SAR value if manual adjustments are made. This may lead to higher than expected temperature increases in the Activa System, particularly at the lead electrodes.
-Limit the gradient dB/dt field to 20 Tesla/second or less.

Note: The recommendations provided are based on in-vitro testing and should result in a safe MRI examination of a patient with an implanted Medtronic Activa System. However, due to the many variables that affect safety, Medtronic cannot absolutely ensure safety or that the neurostimulator will not be damaged. The user of this information assumes full responsibility for the consequences of conducting an MRI examination on a patient with an implanted Activa System.

Prior to the MRI Examination
Prior to the scan examination, the responsible individual must verify the MRI examination parameters comply with these guidelines.
-Patients with implanted Activa Systems should be informed of the risks of undergoing an MRI.
-If possible, do not use sedation so the patient can inform the MRI operator of any heating, discomfort, or other problems.
-Instruct the patient to immediately inform the MRI operator if any discomfort, stimulation, shocking, or heating occurs during the examination.

During the MRI Examination
-Monitor the patient both visually and audibly. Check the patient between each imaging sequence. Discontinue the MRI examination immediately if the patient is unable to respond to questions or reports any problems.
-Conduct the examination using only the MRI pulse sequence that the MRI radiologist or physicist has confirmed meets the MRI requirements above.

Post MRI Examination Review
-Verify that the patient is feeling normal.
-Verify that the neurostimulator is functional.
-Reprogram the neurostimulator to pre-MRI settings.

[MR healthcare professionals are advised to contact the respective manufacturer in order to obtain the latest safety information to ensure patient safety relative to the use of an MR procedure.]

REFERENCES
http://www.medtronic.com/