MRI Head Coil Selection for Sacral Nerve Stimulator Patients
For brain MRI in patients with sacral nerve stimulators, a transmit/receive (T/R) head coil should be used rather than a body coil, as this configuration minimizes radiofrequency field exposure to the implanted device and has been demonstrated safe in clinical practice.
Rationale for T/R Head Coil Selection
The fundamental safety principle is that transmit/receive head coils produce only weak radiofrequency B1+ fields at the location of implanted neurostimulator pulse generators and leads, which are positioned in the sacral/pelvic region 1. This is in direct contrast to body coils, which generate stronger fields throughout the torso where the sacral nerve stimulator components are located 1.
Evidence from Neurostimulator Studies
While direct evidence specific to sacral nerve stimulators is limited, extensive safety data from vagus nerve stimulator (VNS) studies—which share similar implant configurations with pulse generators in the chest/upper torso—provides critical guidance:
- Phantom studies comparing T/R head coils versus body coils demonstrated minimal heating (0.4-0.8°C at leads, negligible at the pulse generator) when using T/R head coils at 3T 1
- Clinical series of 27 MRI scans in 25 patients with vagus nerve stimulators using head coils at 1.5T reported no adverse events, with only one patient experiencing mild transient voice change 2
- A controlled study of 17 patients with VNS devices scanned at 3T using a T/R head coil completed successfully without any adverse incidents, heating, or discomfort 1
Manufacturer-Specific Considerations
The InterStim II sacral nerve stimulator is specifically approved by the manufacturer for MRI scanning of the head only, with all other body areas contraindicated 3. This labeling directly supports the use of head coils for brain imaging 3.
Technical Implementation
Scanner and Coil Specifications
- 1.5T or 3T MRI scanners are appropriate, with 3T preferred for superior signal-to-noise ratios and spatial resolution when imaging the brain 4
- A phased-array head coil with high sensitivity and homogeneity should be used 4
- The coil should have minimum 8 channels for adequate coverage, with higher channel counts providing better image quality 4
Critical Safety Protocol
Before scanning, the sacral nerve stimulator must be turned off or set to the lowest amplitude setting 5. This is a universal precaution across all neurostimulator types to minimize potential device interaction with the MRI magnetic fields 5.
Patient Positioning
Appropriate patient positioning ensures the implanted pulse generator remains as far as possible from the MRI isocenter 6. For brain MRI with a head coil, the device naturally remains distant from the imaging field, which is a key safety advantage 6.
Common Pitfalls to Avoid
- Never use a body coil for transmission when imaging patients with sacral nerve stimulators, as this dramatically increases radiofrequency field exposure to the implanted device 1
- Do not extrapolate safety data from one MRI system configuration to another without supporting evidence, as coil design and field characteristics vary significantly between manufacturers 1
- Avoid scanning with the device turned on unless absolutely necessary and previously validated for that specific configuration 2
- Do not assume full-body MRI compatibility—the InterStim II is only approved for head imaging, unlike some newer neurostimulator models that have full-body MRI conditional labeling 5
Post-Scan Device Assessment
After MRI completion, the device should be re-examined and re-programmed to verify proper functioning 6. Patients should be monitored for any changes in device perception or therapeutic effect 6.