- Division of Neurological Surgery, Barrow Neurological Institute, St. Joseph's Hospital and Medical Center, Phoenix, AZ, 85013, USA
- Division of Neuroradiology, Barrow Neurological Institute, St. Joseph's Hospital and Medical Center, Phoenix, AZ, 85013, USA
- Field Technical Support Group, Codman & Shurtleff Inc., Raynham, MA, 02767, USA
Joseph M. Zabramski
Field Technical Support Group, Codman & Shurtleff Inc., Raynham, MA, 02767, USA
DOI:10.4103/2152-7806.99171Copyright: © 2012 Zabramski JM. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
How to cite this article: Zabramski JM, Preul MC, Debbins J, McCusker DJ. 3T magnetic resonance imaging testing of externally programmable shunt valves. Surg Neurol Int 28-Jul-2012;3:81
How to cite this URL: Zabramski JM, Preul MC, Debbins J, McCusker DJ. 3T magnetic resonance imaging testing of externally programmable shunt valves. Surg Neurol Int 28-Jul-2012;3:81. Available from: http://sni.wpengine.com/surgicalint_articles/3t-magnetic-resonance-imaging-testing-of-externally-programmable-shunt-valves/
Background:Exposure of externally programmable shunt-valves (EPS-valves) to magnetic resonance imaging (MRI) may lead to unexpected changes in shunt settings, or affect the ability to reprogram the valve. We undertook this study to examine the effect of exposure to a 3T MRI on a group of widely used EPS-valves.
Methods:Evaluations were performed on first generation EPS-valves (those without a locking mechanism to prevent changes in shunt settings by external magnets other than the programmer) and second generation EPS-valves (those with a locking mechanisms). Fifteen new shunt-valves were divided into five groups of three identical valves each, and then exposed to a series of six simulated MRI scans. After each of the exposures, the valves were evaluated to determine if the valve settings had changed, and whether the valves could be reprogrammed. The study produced 18 evaluations for each line of shunt-valves.
Results:Exposure of the first generation EPS-valves to a 3T magnetic field resulted in frequent changes in the valve settings; however, all valves retained their ability to be reprogrammed. Repeated exposure of the second generation EPS-valves has no effect on shunt valve settings, and all valves retained their ability to be interrogated and reprogrammed.
Conclusions:Second generation EPS-valves with locking mechanisms can be safely exposed to repeated 3T MRI systems, without evidence that shunt settings will change. The exposure of the first generation EPS-valves to 3T MRI results in frequent changes in shunt settings that necessitate re-evaluation soon after MRI to avoid complications.
Keywords: Magnetic resonance imaging, programmable, reliability, shunt-valve, 3-Tesla, Testing
The last half-century has seen significant advancement in the management of hydrocephalus. In 1955, John Holter, an American engineer, invented the first cerebrospinal fluid (CSF) shunt valve for implantation into his son who had been born with hydrocephalus.[
The first generation of EPS-valves (Codman Medos®, Medtronic Strata® and Sophysa Sophy® valves) have no locking mechanism to prevent changes in the shunt settings by external magnets other than the manufacturers’ programmer. As a result, these valves are at risk of being reprogrammed during MR imaging, and must be reevaluated after any MRI study to assure that the valve setting has not changed. This can create significant issues for patient safety when a scan is performed on an emergency basis at a center unfamiliar with the shunt valve, or when the appropriate tools for reprogramming the valve are not readily available. At the very least, it is an inconvenience for the patient and physician. In addition, repeated exposure to MRI at 3T field strength may affect the ability to reprogram these first generation EPS-valves.
The second generation of EPS-valves includes a locking mechanism designed to help minimize the risk of unintentional changes in valve settings. The Codman Certas® and Sophysa Polaris® valves utilize a dual-magnet design, while the Miethke proGAV® valve uses a mechanical locking mechanism to prevent changes to valves settings by strong magnetic fields other than the programming tools.
Presently, there are an estimated 11,000 diagnostic MRI units in the United States, an increasing number of which are high-field strength 3T units. These units offer an improved signal to noise ratio, which in turn leads to improved image quality and reduced scan times; however, 3T MRI may increase the risks for problems in patients with EPS-valves. While multiple groups have published reports describing the results of MRI testing with EPS-valves, the availability of manuscripts evaluating the interaction of these valves with 3T MRI systems is limited. A search of the Medline data base identified only five peer-reviewed publications, examining the effects of 3T MRI on the second generation EPS-valves: three describe the results of testing the Polaris® valve,[
This study was conducted using a total of 15 new EPS-valves, 3 identical valves from each available product line [
Part II of the study evaluated the combined effects of the 3T magnetic field and the microwave radiation produced by the RF imaging coils on the shunt function. Two complete MRI scans were performed using the head coil and normal imaging sequences [
In Part III of the study, one valve from each shunt line was used to evaluate the extent of MRI artifact it produced. Testing was carried out by performing MRI with the valves placed inside of a gadolinium-doped, saline-filled plastic phantom (measuring 14 cm diameter × 16 cm depth) following aspects of the American Society for Testing Materials (ASTM) International Designation. The shunt-valves were secured to a nylon mesh frame to facilitate positioning and imaging within this phantom [
T1-weighted, spin echo pulse sequence; repetition time = 500 ms; echo time = 20 ms; matrix size, 256 × 256; section thickness, 10 mm; field of view 24 cm; number of excitations, 1.
Gradient-recalled echo (GRE) pulse sequence; repetition time = 100 ms; echo time = 15 ms; flip angle = 30°; matrix size, 256 × 256; section thickness, 10 mm; field of view 24 cm; number of excitations, 1.
These are two commonly used pulse sequences in MR imaging with the T1-weighted sequence producing the least, and the GRE sequence producing the greatest metallic induced artifact. The valves were placed with their long axis in the vertical plane, and axial and coronal images were obtained. Final image locations were selected from multiple “scout” MR images to represent the largest, or worst-case, artifacts for each valve. The planimetry software provided with the MR system was used to measure the cross-sectional areas for the artifacts [
The results for Part I (four 30 min exposures to 3T magnetic field) and Part II (two complete MRI scans with full RF load) are presented in Tables
Artifact testing results are presented in
While MR imaging has become the gold standard for the evaluation of numerous acute and chronic neurological conditions involving the brain and spine, it may present a risk to patients with EPS-valves. The permanent magnets used in these valves are potentially susceptible to damage when exposed to the strong magnetic fields used in MRI, particularly the high-field strength, 3T systems. Permanent magnets exhibit a characteristic called “coercivity”, which is the ability of a material to withstand being demagnetized by the application of a stronger magnetic field.[
In this study, exposure of the first generation EPS-valves to a 3T magnet resulted in changes in pressure settings during 70–80% of simulated scans. Similar results have been reported by other investigators, and emphasize the importance of evaluating patients as soon as possible after an MRI scan to avoid problems from over- or underdrainage of CSF.[
In general, 3T MRI is not recommended in patients with the first generation programmable shunt-valves. There have been conflicting reports regarding the tolerance of these valves to exposure to a 3T magnetic field;[
The results of this study confirm previous reports that second generation EPS-valves are compatible with high-field strength, 3T MRI systems.[
As expected, the magnets used in EPS-valves all created significant metallic artifact during imaging.[
All second generation EPS-valves with locking mechanisms safely tolerated repeated exposure to a 3T MRI field, without evidence of effect on shunt settings or programming function. Exposure of the first generation EPS-valves to 3T MRI results in frequent changes in valve settings that necessitate the re-evaluation of shunt patients soon after any MRI procedure; however, the shunt-valves maintained their ability to be readily reprogrammed.
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