The Effects of Magnetic Resonance Imaging (MRI) on Medtronic Drug Infusion Systems

Models: SynchroMed® II: All models beginning with 8637
Reference: August 2008 -- Urgent: Medical Device Correction –Important Information on Potential MRI Effects

Programmable pump performance has not been established for greater than 1.5 T horizontal, closed-bore MRI scanners. Patients should not have MRI using >1.5 T horizontal, closed-bore scanners. The magnetic field of the MRI scanner will temporarily stop the rotor of the pump motor and suspend drug infusion for the duration of the MRI exposure. The pump should resume normal operation upon termination of MRI exposure; however, there is the potential for an extended delay in pump recovery after exiting the MRI magnetic field. While extended delays in pump recovery are unlikely, reports have indicated that there is the potential for a two to twenty-four hour delay in return to proper drug infusion after completion of an MRI scan.

The SynchroMed II pump detects motor stall and motor stall recovery. These events are recorded in the pump event log and can be reviewed using the clinician programmer. A motor stall will also cause the pump alarm to sound (two tone alarm). Medtronic does not recommend programming the SynchroMed II pump to “stopped pump mode” prior to an MRI because of the possibility of an increased delay in the detection of an extended motor stall. Note: in some cases, the SynchroMed II pump event log may not register motor stall recovery until after the pump has been interrogated a second time due to the effect of Electromagnetic Interference (EMI) on the pump.

Prior to MRI, the implanting or follow-up physician should determine if the patient can safely be deprived of drug delivery. If the patient cannot be safely deprived of drug delivery, alternative delivery methods for the drug can be utilized during the time required for the MRI scan. If there is concern that depriving the patient of drug delivery may be unsafe for the patient during or after the MRI procedure, medical supervision should be provided.

Patients receiving intrathecal baclofen therapy (e.g. Lioresal® Intrathecal) are at higher risk for adverse events, as baclofen withdrawal can lead to a life threatening condition if not treated promptly and effectively. For complete product information, refer to the Lioresal Intrathecal (baclofen injection) Package Insert. For information on other drugs, please refer to the product labeling for the drug being administered.

Upon completion of the MRI scan, or shortly thereafter as defined by the implanting or follow-up physician, the physician must confirm that therapy has properly resumed by interrogating the pump with the clinician programmer. For pumps programmed to deliver at least 0.048 ml/day, detection of the motor stall should occur within 20 minutes of MRI exposure. Detection of the motor stall recovery and recording of the recovery in the pump event log will typically occur within 20 minutes of the removal of the pump from the magnetic field of the MRI. Note: both the detection of the motor stall and detection of the motor stall recovery may each take up to 90 minutes if the pump is programmed to minimum rate mode (0.006 ml/day). In the unlikely event that electromagnetic interference from the MRI scan causes a change to “safe state”, the pump will automatically switch to minimum rate mode (infusion at 0.006 ml/day). The pump must be reprogrammed in order for proper drug infusion to resume.

Interrogation Guidelines (Back to top)

  1. At least 20 minutes after completing MRI exposure, interrogate the pump using the clinician programmer and select the check box to download event logs (see Figure 1). If the event log states “Motor Stall Occurred” and “Motor Stall Recovery Occurred”, normal function of the pump has returned (see Figure 2).
  2. If event log does not show stall and recovery, wait 20 minutes after the initial interrogation, re-interrogate the pump using the clinician programmer, and review the event logs again. (This will address the potential for event logging delays due to Electromagnetic Interference (EMI) from the MRI magnetic field.
  • If the event log states “Motor Stall Occurred” and does not state “Motor Stall Recovery Occurred”, there is a potential for an extended motor stall due to temporary gear binding. Contact Medtronic Technical services for further troubleshooting.
  • In all other cases, the pump has resumed its normal operation.

 

Figure 1

 


Figure 2

Testing on programmable pumps has established the following with regard to other MRI safety issues:

Tissue Heating Adjacent to Implant During MRI Scans (Back to top)

Specific Absorption Rate (SAR) — Presence of the pump can potentially cause a two-fold increase of the local temperature in tissues near the pump. During a 20-minute pulse sequence in a 1.5 Tesla (T) GE Signa scanner with a whole-body average SAR of 1 W/kg, a temperature increase of 1 degree Celsius in a static phantom was observed near the pump implanted in the “abdomen” of the phantom. The temperature increase in a static phantom represents a worst case for physiological temperature increase and the 20- minute scan time is representative of a typical imaging session. FDA MRI guidance allows a physiological temperature increase of up to 2 degrees Celsius in the torso. Therefore, the local temperature increase in the phantom is considered by FDA guidance2 to be below the level of concern. Implanting the pump more lateral to the midline of the abdomen may result in higher temperature increases in tissues near the pump. In the unlikely event that the patient experiences uncomfortable warmth near the pump, the MRI scan should be stopped and the scan parameters adjusted to reduce the SAR to comfortable levels.

Peripheral Nerve Stimulation During MRI Scans (Back to top)

Time-Varying Gradient Magnetic Fields — Presence of the pump may potentially cause a two-fold increase of the induced electric field in tissues near the pump. With the pump implanted in the abdomen, using pulse sequences that have dB/dt up to 20 T/s, the measured induced electric field near the pump is below the threshold necessary to cause stimulation. In the unlikely event that the patient reports stimulation during the scan, the proper procedure is the same as for patients without implants—stop the MRI scan and adjust the scan parameters to reduce the potential for nerve stimulation.

Static Magnetic Field (Back to top)

For magnetic fields up to 1.5 T, the magnetic force and torque on the programmable pump will be less than the force and torque due to gravity. The patient may experience a slight tugging sensation at the pump implant site. An elastic garment or wrap will prevent the pump from moving and reduce the sensation the patient may experience.

Image Distortion (Back to top)

The programmable pump contains ferromagnetic components that will cause image distortion and image dropout in areas around the pump. The severity of image artifact is dependent on the MR pulse sequence used. For spin echo pulse sequences, the area of significant image artifact may be 20–25 cm across. Images of the head or lower extremities should be largely unaffected.

Minimizing Image Distortion — MR image artifact may be minimized by careful choice of pulse sequence parameters and location of the angle and location of the imaging plane. However, the reduction in image distortion obtained by adjustment of pulse sequence parameters will usually be at a cost in signal-to-noise ratio. The following general principles should be followed:

  1. Use imaging sequences with stronger gradients for both slice and read encoding directions. Employ higher bandwidth for both radio-frequency pulse and data sampling.
  2. Choose an orientation for read-out axis that minimizes the appearance of in-plane distortion.
  3. Use spin echo or gradient echo MR imaging sequences with a relatively high data sampling bandwidth.
  4. Use shorter echo time for gradient echo technique, whenever possible.
  5. Be aware that the actual imaging slice shape can be curved in space due to the presence of the field disturbance of the pump (as stated above).
  6. Identify the location of the implant in the patient, and when possible, orient all imaging slices away from the implanted pump.
Please refer to the full prescribing information and system information for details or call Medtronic at 1-800-328-0810. Rx Only. Lioresal® is a registered trademark of Novartis Pharmaceuticals Corporation.