A ventriculoperitoneal (VP) shunt is a medical device implanted to treat hydrocephalus, a condition characterized by an abnormal buildup of cerebrospinal fluid (CSF) in the brain. The shunt system safely drains this excess fluid from the brain’s ventricles to the abdominal cavity, where it can be absorbed. VP shunts are generally safe for Magnetic Resonance Imaging (MRI). However, specific precautions are mandatory, with the required safety measures depending entirely on the specific type and model of the implanted device.
The Ventriculoperitoneal Shunt and MRI Basics
A VP shunt is a system composed of three main parts: a proximal catheter, a one-way valve, and a distal catheter. The proximal catheter is placed within the fluid-filled space of the brain, and the distal catheter carries the CSF to the absorption site in the abdomen. The valve is the regulatory component, controlling the flow and pressure of the fluid drainage.
The interaction with an MRI machine centers on this valve component. The valve and its reservoir often contain metallic elements, such as titanium or stainless steel, used for strength and durability. An MRI machine generates an extremely powerful magnetic field, which creates detailed images of the body’s internal structures.
Any metal within this strong magnetic field can potentially be problematic, leading to two main concerns: movement and heating. Modern shunt valves are typically designed to be “MR Conditional,” meaning they have been tested to be safe under specific MRI conditions, such as a maximum magnetic field strength of 1.5 Tesla or 3 Tesla. The metallic valve components are the focus of all safety protocols.
Programmable vs. Fixed Pressure Shunts
The most important distinction for MRI safety is the difference between fixed-pressure and programmable shunts. Fixed-pressure shunts are the simpler design, operating at a single, pre-set pressure level chosen at the time of surgical implantation. Since these valves do not contain internal magnetic mechanisms for external adjustment, they generally pose the lowest risk during an MRI scan.
Programmable, or adjustable, shunts are more complex because they allow the neurosurgeon to non-invasively change the valve’s pressure setting after implantation using a specialized external magnet. To allow for this adjustment, these valves contain small, intricate magnetic components in addition to other metals. The presence of these internal magnets means the valve’s setting can be unintentionally altered by the strong magnetic field of the MRI scanner.
The specific manufacturer and model determine the level of risk and the required safety protocol. Newer, second-generation programmable shunts, such as certain Codman Certas or Miethke proGAV models, feature internal locking mechanisms designed to resist magnetic field changes up to 3 Tesla. Older, first-generation programmable shunts lack this feature, making them highly susceptible to accidental reprogramming during the scan.
Potential Complications During MRI Scanning
The primary risk associated with scanning a patient who has a programmable shunt is the unintentional change, or resetting, of the valve’s pressure setting. Studies involving valves without a magnetic locking mechanism have reported setting change rates as high as 56.7% after exposure to an MRI. If the pressure setting is inadvertently changed, the patient may suffer from over-drainage or under-drainage of CSF, leading to symptoms of shunt malfunction.
Another concern is thermal injury, or heating, of the metallic components due to the radiofrequency (RF) energy pulses used by the MRI machine. Significant MRI-induced heating has not been shown to be a problem with contemporary shunt valves. The small size and non-ferromagnetic nature of most modern shunt metals, like titanium, help mitigate this risk.
Minor migration or movement of the shunt valve, known as translational attraction or torque, is also a concern for any metallic implant in a strong magnetic field. For most modern, MR Conditional shunts, the force exerted by the magnetic field is negligible, often less than the force of gravity. This risk is why a patient’s specific shunt information must be verified against the scanner’s field strength and operational guidelines.
Essential Safety Procedures and Post-Scan Checks
Pre-Scan Verification
Before an MRI is performed, the first and most important step is pre-scan verification, which involves positively identifying the exact shunt model, manufacturer, and type. This information is typically found on a patient identification card or through a review of medical records. Medical staff must then check the manufacturer’s instructions to confirm the shunt’s MR Conditional status and verify that the proposed magnetic field strength of the scanner is safe for that specific device.
For patients with an adjustable shunt, the current pressure setting must be accurately determined before the scan, often using a specialized external programmer or a simple X-ray. Adhering to the manufacturer’s guidelines regarding scanning technique is also mandatory, including limitations on the magnetic field strength and the maximum Specific Absorption Rate (SAR).
Post-Scan Follow-up
The most critical safety measure is the post-scan follow-up, especially for patients with any type of programmable shunt. Immediately after the MRI, the shunt setting must be checked again to ensure no change has occurred. For older, more susceptible shunt models, this check should happen as soon as possible, ideally within four hours of the exposure to the magnetic field. If a setting change is detected, a neurosurgeon must promptly and non-invasively reset the valve to the intended pressure to prevent potential complications from shunt malfunction.