MR SAFETY AND CEREBROSPINAL FLUID SHUNT VALVES

MR SAFETY AND CEREBROSPINAL FLUID SHUNT VALVES
Hydrocephalus is the accumulation of cerebrospinal fluid in the brain, resulting from increased production or, more commonly, pathway obstruction or decreased absorption of the fluid. CSF shunts have been used for decades for the treatment of hydrocephalus. CSF shunting involves establishing an accessory pathway for the movement of CSF in order to bypass an obstruction of the natural pathways.

The shunt is positioned to enable the CSF to be drained from the cerebral ventricles or subarachnoid spaces into another absorption site (e.g., the right atrium of the heart or the peritoneal cavity) through a system of small catheters. A regulatory device, such as a valve, may be inserted into the pathway of the catheters.

In general, the valve keeps the CSF flowing away from the brain and moderates the pressure or flow rate. Some valves are fixed pressure valves (i.e., monopressure valves), and others have adjustable settings. The drainage system using catheters and valves enables evacuation of the excess CSF within the brain and, thereby, reduction of the pressure within the cranium.

Several different types of CSF shunt valves and associated accessories are used for treatment of hydrocephalus. Information for certain CSF shunt valves is provided at the end of this monograph. In general, shunt valves that use magnetic components require following highly specific safety guidelines to perform MR procedures safely in patients with these devices.

CODMAN HAKIM PROGRAMMABLE VALVE

The CODMAN HAKIM Programmable Valve (Codman, Raynham, MA) offers the ability to optimize the opening pressure of a shunt system before and after implantation. This is considered to be an important feature because the shunted patient's condition will often change over the course of treatment. The use of a programmable valve allows the surgeon to noninvasively change the opening pressure, negating the need for revision surgery to alter the valve pressure. Furthermore, the programmability of the valve may allow for the development of specialized treatment regimens.

The opening pressure of the CODMAN HAKIM Programmable Valve is changed through the use of an externally applied magnetic field. The spring in the ball and spring mechanism of the valve sits atop a rotating spiral cam that contains a stepper motor. Applying a specific magnetic field to the stepper motor will cause the cam to turn slightly, increasing or decreasing the tension on the spring and ball, thus changing the opening pressure of the valve.

With regard to MRI, the product insert for the CODMAN HAKIM Programmable Valve states: "Note: Remember to verify valve pressure setting after an MRI."

DELTA SHUNT ASSEMBLY

The Delta Shunt Assembly (Medtronic Neurosurgery, Goleta, CA) combines the Delta valve with an integral, open-end, radiopaque peritoneal catheter. All Delta shunt assemblies incorporate the same product features as the Delta valves. These include injectable reservoir domes, occluders for selective flushing, and a completely nonmetallic design. The valves are fabricated of dissimilar materials -- polypropylene and silicone elastomer -- reducing the chance of valve sticking and deformation.

The normally closed Delta chamber mechanism minimizes overdrainage by utilizing the principles of hydrodynamic leverage. Because of the nonmetallic design, the Delta shunt is safe for patients undergoing MR procedures.

POLARIS ADJUSTABLE PRESSURE VALVE

The POLARIS Adjustable Pressure Valve (Sophysa USA, Costa Mesa, CA) has magnets made of Samarium-Cobalt, which are specially treated to preserve permanent magnetization even after repeated exposure to MR at 3T. The principle of the POLARIS Adjustable Pressure Valve is based on the variation in pressure exerted on a ball by a semicircular spring at different points along its curvature. The flat semicircular calibrated spring determines an operating pressure. Because of the unique design of the POLARIS Adjustable Pressure Valve, it is considered to be safe for patients undergoing MR procedures at 3T or less.

PULSAR VALVE

The Pulsar Valve (Sophysa USA, Costa Mesa, CA) for CSF drainage is a monopressure valve. Its principle is based on the play of a silicone membrane, calibrated in low, medium, or high pressure, ensuring a proximal regulation of CSF flow through the shunt system. The Pulsar Valve is safe for patients undergoing MR procedures.

SOPHY MINI MONOPRESSURE VALVE

The Sophy Mini Monopressure Valve (Sophysa USA, Costa Mesa, CA) for CSF drainage has a ball-in-cone mechanism. This device is safe for patients undergoing MR procedures.

STRATA VALVE

The PS Medical Strata Valve (Medtronic Neurosurgery, Goleta, CA) is an adjustable flow control valve in which the resistance properties of the valve can be changed noninvasively by the caregiver. It is designed to minimize overdrainage of CSF and maintain intraventricular pressure within a normal physiologic range, regardless of patient position.

Extensive testing of the Strata Valve was conducted using MR with a static magnetic field of 1.5T. Results of the testing indicated that the valve is "MR-Safe." That is, exposure of the valve to MR scanning will not damage the valve, but it may change the valve's performance level setting. Therefore, after MR exposure, the valve performance level setting needs to be confirmed and adjusted as necessary.

From a diagnostic standpoint, the presence of the Strata valve in a patient may disrupt or impair the use of MR if the area of interest is near the location of the valve (personal communication, 9/17/04, Karen Rhodes, manager, Medtronic Neurosurgery, Goleta, CA).

SOPHY ADJUSTABLE PRESSURE VALVE

The principle of the SOPHY Adjustable Pressure Valve (Sophysa USA, Costa Mesa, CA) resides in the variation in pressure exerted on a ball by a semicircular spring at various points along its circumference. The spring is attached to a magnetic rotor whose position can be noninvasively altered using an adjustment magnet. A series of indentations allows a variety of positions to be selected, each position representing a different pressure setting. The valve's ball-in-cone mechanism maintains the selected pressure constant without significant drift through the time.

Because a magnetic component is associated with this device, special MR safety precautions exist for scanning patients with the SOPHY Adjustable Pressure Valve:

The pressure settings should always be checked in case of shock on the implantation site.
Changing pressure settings must only be performed by a neurosurgeon.
The patient must be advised that carrying his or her patient identification card is important and necessary for follow-up of clinical conditions.
Patients undergoing MR exposure should be advised that they might feel a small yet harmless effect due to MR.
The pressure settings should always be checked before and after MR exposure or after strong magnetic field exposure.
The patient must be advised that in the case of implantation on the skull, vibrations due to CSF flow may be perceived.
Patients with implanted valve systems must be kept under close observation for symptoms of shunt failure.
[For these devices, MR healthcare professionals are advised to contact the respective manufacturer to ensure that the latest information is obtained and carefully followed in order to ensure patient safety.]

BIBLIOGRAPHY

Cerebral spinal fluid shunt valves and accessories. Raynham, MA: Codman. http://www.codmanjnj.com/CSFshunting.asp.

Cerebral spinal fluid shunt valves and accessories. Raynham, MA: Codman. http://www.codmanjnj.com/PDFs/Prog_ProcedureGuide.pdf.

Cerebral spinal fluid shunt valves and accessories. Goleta, CA: Medtronic Neurosurgery. http://www.medtronic.com/neurosurgery/shunts.html.

http://www.dimag.com/mrsafety/?articleID=49900269