What are the risks of using a Radiofrequency (R) head coil for a brain Magnetic Resonance Imaging (MRI) in a patient with an implanted sacral nerve stimulator?

MRI Safety with Sacral Nerve Stimulators: Head Coil Only Protocol

Using only an RF head coil for brain MRI in a patient with an implanted sacral nerve stimulator is generally safe and represents the standard approach for these patients, as head-only MRI is the one body region where these devices have demonstrated acceptable safety profiles. 1, 2

Key Safety Principle

The critical distinction is that diagnostic MRI below the head is contraindicated with sacral nerve stimulators implanted 1, but head-only imaging has been studied and performed successfully. The use of a head-only RF coil is actually the recommended configuration because:

• The RF head coil keeps the imaging field localized to the head region, away from the implanted pulse generator (IPG) and leads in the sacral area 2
• When the area to be imaged is out of the isocenter of the MRI scanner, examinations have been found safe in patients with implantable devices 2
• Standard RF head coils intended solely for MRI can be used for head imaging, unlike body imaging where coil selection becomes more critical 3

Essential Safety Protocol

Before proceeding with brain MRI, the following steps must be taken:

• The IPG must be deactivated and set to "nominal" status before the patient enters the scanner 2, 4
• Continuous monitoring of the patient during the entire MRI procedure is mandatory 2, 4
• Device parameters should be recorded pre-MRI to allow proper reprogramming afterward 2
• The patient should be counseled that they may experience mild discomfort or warmth at the IPG site during scanning, though this is typically transient 5

Evidence Supporting Head-Only MRI Safety

Multiple studies have demonstrated that 1.5 Tesla MRI of the head can be performed safely in patients with sacral nerve stimulators:

• In a series of 8 MRI examinations at 1.5T in areas outside the pelvis (including head imaging), no patients required stopping the examination, devices functioned properly afterward, and no changes in bladder function occurred 2
• A study of 10 patients with InterStim Twin devices undergoing MRI in various body regions (with device deactivation) showed no pain, discomfort, or device malfunction, with stable impedances and stimulation parameters post-MRI 4
• Eleven patients who underwent 1.5T MRI (including head scans) showed no adverse events, unchanged device function, and no worsening of therapeutic efficacy at 1-month follow-up 5

Post-MRI Device Management

After completing the brain MRI:

• The IPG site should be examined for any changes 2
• The device must be reprogrammed to its previous settings using the programmer 2, 4
• Device impedances and battery life should be checked to ensure no significant changes occurred 5
• Patients should be monitored for any changes in stimulation perception or therapeutic efficacy 2, 4

Critical Caveats

The main risk is not from using the head coil itself, but from the MRI magnetic field interacting with the implanted device. Potential hazards include:

• Motion, dislocation, or torquing of the IPG 2
• Heating of the leads 2
• Damage to the IPG resulting in painful stimulation 2
• Radiofrequency interference (reported in 2 cases at 0.2T, but not at 1.5T) 6

However, these risks are minimized when imaging the head only because the device and leads remain outside the primary imaging field and isocenter 2.

Field Strength Considerations

1.5 Tesla systems have the best safety data for patients with sacral nerve stimulators 2, 6, 5. Higher field strengths (3T) should be avoided as safety data is lacking 3.