Magnetic field interactions in adjustable hydrocephalus shunts

Laboratory investigation

Restricted access

Object

Exposing patients with ventricular shunts to magnetic fields and MR imaging procedures poses a significant risk of unintentional changes in shunt settings. Shunt valves can also generate considerable imaging artifacts. The purpose of this study was to determine the magnetic field safety and MR imaging compatibility of 5 adjustable models of hydrocephalus shunts.

Methods

The Codman Hakim (regular and with SiphonGuard), Miethke ProGAV, Medtronic Strata, Sophysa Sophy and Polaris programmable valves were tested in a low-intensity magnetic field, and then translational attraction (TA), magnetic torque (MT), and volume of artifacts on T1-weighted spin echo (SE) and gradient echo (GE) pulse sequences in a 3-T MR imaging unit were measured.

Results

The ProGAV and Polaris valves were immune to unintentional reprogramming by magnetic fields up to 3 T. Other valves randomly changed settings, starting from the intensity of field: Sophy valve 24 mT, Strata valve 30 mT, and both Codman Hakim programmable valves from 42 mT. Shunt performances in the 3-T MR imaging unit are reported in the order of compatibility: 1) Codman Hakim regular, TA = 0.005 N, MT = 0.000 Nm, GE = 30 cm3, SE = 2 cm3; 2) Miethke ProGAV, TA = 0.001 N, MT = 1.4 × 10−3 Nm, GE = 231 cm3, SE = 13 cm3; 3) Codman Hakim with SiphonGuard, TA = 0.005 N, MT = 2.3 × 10−3 Nm, GE = 233 cm3, SE = 19 cm3; 4) Medtronic Strata, TA = 0.27 N, MT = 18.0 × 10−3 Nm, GE = 484 cm3, SE = 86 cm3; 5) Sophysa Sophy, TA = 0.82 N, MT = 38.9 × 10−3 Nm, GE = 758 cm3, SE = 72 cm3; and 6) Sophysa Polaris, TA = 0.80 N, MT = 39.6 × 10−3 Nm, GE = 954 cm3, SE = 100 cm3.

Conclusions

All valves, with the exception of the Polaris and ProGAV models, are prone to unintentional reprogramming when exposed to heterogeneous magnetic fields stronger than 40 mT. All tested valves can be considered safe for 3-T MR imaging. All valves generated a distortion of the MR image, especially the GE sequences.

Abbreviations used in this paper: CSF = cerebrospinal fluid; GE = gradient echo; SE = spin echo; TA = translational attraction.

Article Information

Address correspondence to: Marek Czosnyka, Ph.D., Academic Neurosurgery, Box 167 Addenbrooke's Hospital, Cambridge, United Kingdom. email: mc141@medschl.cam.ac.uk.

Part of this work (MR imaging compatibility) was presented as a poster during the XIIIth International Symposium on Intracranial Pressure and Brain Monitoring in San Francisco in 2007.

© AANS, except where prohibited by US copyright law.

Headings

Figures

  • View in gallery

    Upper: Schematic showing the shunt-testing rig. Lower: Graphs depicting examples of changes in shunt settings recorded in the testing rig. The Codman Hakim programmable valve increased by 1 step (0.8 mm Hg). The Sophy switched from a medium to a high setting. The Strata valve switched from 1.5 to 2.5 and then from 1.5 to 1. P = pressure.

  • View in gallery

    Photograph showing the cylindrical magnet (BN 021A, sintered NdFeB) mounted on the screw on the pendulum.

  • View in gallery

    Principles involved in testing 3-T MR imaging magnetic field interactions. Upper: Schematic revealing the physical principles of the deflection angle test. The valve (black sphere) is suspended by a lightweight thread. Devices with ferromagnetic properties exposed to a magnetic field (3 T) will be deflected from the plumb line by a TA, which is a function of W and α(TA = W ×tangent α). α= deflection angle; W = valve weight. Lower: Schematics demonstrating the torque meter, as seen from the top (Z) and from the side (X). Unit B is positioned in the center of the MR unit and is composed of a Perspex slab carrying a glass ball bearings pulley. The tested shunt valve can be secured on the Unit B pulley by a custom-made micrometric clamp. Torque (curved arrow) is transmitted to Unit A via a system of weightless, inextensible nylon wiring (~ 4 m). The system is completely static and the nylon wiring is kept in tension by 2 weights (M > m) suspended by the 2 pulleys of Unit A, which is built so that its pulleys can be aligned and leveled with the pulley inside the scanner. Weight M rests on a precision weight balanced outside the scanner, and the balance is zeroed at M − m (~ 0.2 kg for the present experiment). The shunt valve is then clamped on pulley B and adjusted to maximize the torque transmitted to the weighing balance outside the scanner. Torque is calculated as the weight transmitted to the balance × the pulley radius.

  • View in gallery

    Graphs demonstrating the incidence of reprogramming versus the magnetic field flux intensity for the Sophy (A), Strata (B), and the Codman Hakim programmable valves (C, the regular and SiphonGuard subtypes produced the same values).

  • View in gallery

    Gradient echo (GE) and T1-weighted SE sequences (T1) revealing shunt valve-generated artifacts. The slice with the greatest artifact area is shown. Ghost diameter = 20 cm.

References

  • 1

    Allin DMCzosnyka ZHCzosnyka MRichards HKPickard JD: In vitro hydrodynamic properties of the Miethke ProGAV hydrocephalus shunt. Cerebrospinal Fluid Res 3:92006

    • Search Google Scholar
    • Export Citation
  • 2

    Anderson RCWalker MLViner JMKestle JR: Adjustment and malfunction of a programmable valve after exposure to toy magnets. Case report. J Neurosurg 101:2 Suppl2222252004

    • Search Google Scholar
    • Export Citation
  • 3

    Czosnyka ZCzosnyka MRichards HKPickard JD: Laboratory testing of hydrocephalus shunts—conclusion of the U.K. Shunt evaluation programme. Acta Neurochir (Wien) 144:5255382002

    • Search Google Scholar
    • Export Citation
  • 4

    Czosnyka ZHCzosnyka MRichards HKPickard JD: Evaluation of three new models of hydrocephalus shunts. Acta Neurochir Suppl 952232372005

    • Search Google Scholar
    • Export Citation
  • 5

    Inoue TKuzu YOgasawara KOgawa A: Effect of 3-tesla magnetic resonance imaging on various pressure programmable shunt valves. J Neurosurg 103:2 Suppl1631652005

    • Search Google Scholar
    • Export Citation
  • 6

    Jandial RAryan HEHughes SALevy ML: Effect of vagus nerve stimulator magnet on programmable shunt settings. Neurosurgery 55:6276292004

    • Search Google Scholar
    • Export Citation
  • 7

    Kondageski CThompson DReynolds MHayward RD: Experience with the Strata valve in the management of shunt over-drainage. J Neurosurg 106:2 Suppl951022007

    • Search Google Scholar
    • Export Citation
  • 8

    Lindner DPreul CTrantakis CMoeller HMeixensberger J: Effect of 3T MRI on the function of shunt valves—evaluation of Paedi GAV, Dual Switch and proGAV. Eur J Radiol 56:56592005

    • Search Google Scholar
    • Export Citation
  • 9

    Mangano FTMenendez JAHabrock TNarayan PLeonard JRPark TS: Early programmable valve malfunctions in pediatric hydrocephalus. J Neurosurg 103:6 Suppl5015072005

    • Search Google Scholar
    • Export Citation
  • 10

    McGirt MJBuck DW IISciubba DWoodworth GFCarson BWeingart J: Adjustable vs set-pressure valves decrease the risk of proximal shunt obstruction in the treatment of pediatric hydrocephalus. Childs Nerv Syst 23:2892952007

    • Search Google Scholar
    • Export Citation
  • 11

    Schneider TKnauff UNitsch JFirsching R: Electromagnetic field hazards involving adjustable shunt valves in hydrocephalus. J Neurosurg 96:3313342002

    • Search Google Scholar
    • Export Citation
  • 12

    Turner SGHall WA: Programmable shunt-related suicide attempt. Acta Neurochir (Wien) 48:130713102006

TrendMD

Metrics

Metrics

All Time Past Year Past 30 Days
Abstract Views 214 214 24
Full Text Views 84 84 3
PDF Downloads 158 158 2
EPUB Downloads 0 0 0

PubMed

Google Scholar