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

Models: IsoMed®: All models beginning with 8472
Reference: IsoMed Technical Manual (220666-001)

Exposure of IsoMed® pumps to Magnetic Resonance Imaging (MRI) fields of1.5 T (Tesla) has demonstrated no impact on pump performance and a limited effect on the quality of the diagnostic information.

Testing on the IsoMed pump has established the following with regard to MRI safety and diagnostic issues.

Implant Heating During MRI Scans

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Specific Absorption Rate (SAR): Presence of the pump can potentially cause a two-fold increase of the local temperature rise in tissues near the pump. During a 20-minute pulse sequence in a 1.5 T (Tesla) GE Signa Scanner with a whole-body average SAR of 1 W/kg, a temperature rise of 1 degree Celsius in a static phantom was observed near the pump implanted in the “abdomen” of the phantom. The temperature rise in a static phantom represents a worst case for physiological temperature rise and the 20-minute scan time is representative of a typical imaging session. Implanting the pump in other locations may result in higher temperature rises 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.

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Peripheral Nerve Stimulation
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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
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For magnetic fields up to 1.5 T, the magnetic force and torque on the IsoMed pump will be less than the force and torque due to gravity.

In the unlikely event that the patient reports a slight tugging sensation at the pump implant site, an elastic garment or wrap may be used to prevent the pump from moving and reduce the sensation the patient may experience.

Image Distortion
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The IsoMed pump will cause image dropout on MRI images in the region surrounding the pump. The extent of image artifact depends on the pulse sequence chosen with gradient echo sequences generally causing the most image dropout. Spin echo sequences will cause image dropout in a region approximately 50% larger than the pump itself, about 12 cm across, but with little image distortion or artifact beyond that region

Minimizing Image Distortion - MRI 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. These general principles should be followed:

  1. Use imaging sequences with stronger gradients for both slice and read encoding directions. Employ higher bandwidth for both RF pulse and data sampling.
  2. Choose an orientation for read-out axis that minimizes the appearance of in-plane distortion.
  3. Use spin echo (SE) or gradient echo (GE) MRI imaging sequences with a relatively high data sampling bandwidth.
  4. Use shorter echo time (TE) 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.