A ventriculoperitoneal (VP) shunt is a medical device surgically implanted to manage hydrocephalus, a condition characterized by an accumulation of excess cerebrospinal fluid (CSF) in the brain. The shunt works by diverting this fluid from the brain’s ventricles to another part of the body, typically the peritoneal cavity in the abdomen, where it can be absorbed. Magnetic Resonance Imaging (MRI) is a powerful diagnostic tool that uses strong magnetic fields and radio waves to produce detailed images of organs and soft tissues within the body. While MRI offers significant benefits for diagnosis, patients with VP shunts need specific considerations to ensure their safety during the scan.
VP Shunts and MRI Basics
A VP shunt has three main components: a catheter in a brain ventricle, a one-way valve regulating fluid flow and pressure, and a distal catheter draining CSF, often into the peritoneal cavity. This system reduces intracranial pressure from hydrocephalus.
MRI scanners create powerful magnetic fields, typically ranging from 1.5 Tesla to 3 Tesla, and use radiofrequency pulses to generate detailed images. The strong magnetic fields of an MRI scanner can interact with metallic or electronic components within a VP shunt. Some shunt valves contain small magnets or materials like 316 stainless steel, titanium, or tantalum. These materials can be influenced by the magnetic field, requiring specific safety protocols during an MRI.
Ensuring MRI Safety with a VP Shunt
Understanding the type of VP shunt is important for MRI safety. There are two types: fixed-pressure shunts and programmable shunts.
Fixed-pressure shunts, also known as non-programmable shunts, operate at a predetermined CSF pressure and typically do not contain magnetic components, making them largely unaffected by external magnets. Many non-programmable shunt valves are considered MR Safe or MR Conditional up to 3T, often being made of non-metallic materials like silicone or polypropylene, or non-ferromagnetic materials such as titanium or stainless steel.
Programmable shunts, however, contain magnetic components that allow their pressure settings to be adjusted non-invasively after implantation using an external magnet. These magnetic components pose a potential risk in the MRI environment because the strong magnetic fields can inadvertently alter the shunt’s pressure setting. First-generation programmable valves, such as Codman Medos®, Medtronic Strata®, and Sophysa Sophy® valves, lack internal locking mechanisms and are particularly susceptible to reprogramming by external magnets, sometimes experiencing setting changes in 70-80% of simulated scans.
Risks with programmable shunts include unintentional reprogramming of valve settings, which can lead to over- or under-drainage of CSF. There is also a potential for heating of shunt components, especially older models, although significant MRI-induced heating has not been a widespread issue with modern shunt valves. In rare instances, movement of metallic components and demagnetization or permanent damage to the device, potentially affecting its ability to be reprogrammed, are also concerns. Modern programmable shunts, like Codman Certas®, Sophysa Polaris®, and Miethke proGAV®, often incorporate internal locking mechanisms designed to minimize setting changes from external magnetic fields and have shown resistance to such changes in MRI scanners up to 3.0T.
Preparing for an MRI with a VP Shunt
Preparation is necessary for patients with a VP shunt undergoing an MRI scan. Inform the referring doctor, radiologist, and MRI technologist about the VP shunt. This allows the medical team to assess compatibility and take precautions.
Carry a shunt identification card, which provides information on the shunt’s model, manufacturer, and MRI compatibility. This helps staff determine safety guidelines. Some organizations, like the Hydrocephalus Association, also offer mobile applications such as HydroAssist® to store and access shunt treatment history and imaging, which can be beneficial in emergency situations.
Prior to the scan, medical staff will conduct a pre-scan assessment based on the shunt identification card and the patient’s medical history. For patients with programmable shunts, a specific protocol is followed. This typically involves checking the shunt’s current pressure setting before the MRI, and for some models, adjustments may be made if necessary. During the MRI scan, patients are advised to remain still and communicate any discomfort immediately to the MRI technologist.
Post-MRI Shunt Management
After an MRI scan, steps are taken to ensure the VP shunt’s proper function. For programmable shunts, a neurosurgeon or trained professional must re-check and potentially re-set the shunt to its correct pressure. This re-evaluation should occur within four hours, especially for older programmable valves lacking internal locking mechanisms.
Modern programmable shunts, with improved magnetic field resistance, may allow rechecking within days if no symptoms of shunt dysfunction are present. Patients and their caregivers should closely monitor for any symptoms that could indicate shunt malfunction or changes in pressure settings.
Symptoms can include headaches, nausea and vomiting, unusual fatigue, irritability, personality changes, problems with thinking and memory, or trouble with balance or walking. In infants, increased head size and a bulging soft spot are also warning signs.
Follow-up appointments with the neurosurgeon confirm the shunt is functioning and address concerns. If symptoms arise or worsen, seek immediate medical attention by contacting a neurosurgeon or going to the nearest emergency room.