Setting pressure can change the size and shape of MRI artifacts caused by adjustable shunt valves: a study of the 4 newest models

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  • 1 Departments of Neurosurgery and
  • 2 Radiology, Seirei Hamamatsu General Hospital, Hamamatsu;
  • 3 Department of Neurosurgery, Seirei Numazu General Hospital, Numazu;
  • 4 Department of Neurosurgery, Hamamatsu Medical Center;
  • 5 Department of Neurosurgery, Seirei Mikatahara General Hospital; and
  • 6 Department of Neurosurgery, Hamamatsu University School of Medicine, Hamamatsu, Japan
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OBJECTIVE

Adjustable shunt valves that have been developed for the management of hydrocephalus all rely on intrinsically magnetic components, and artifacts with these valves on MRI are thus inevitable. The authors have previously reported that the shapes of shunt artifacts differ under different valve pressures with the proGAV 2.0 valve. In the present study the authors compared the size and shape of artifacts at different pressure settings with 4 new-model shunt valves.

METHODS

The authors attached 4 new models of MRI-resistant shunt valve to the temporal scalp of a healthy volunteer: the proGAV 2.0; Codman Certas Plus; Polaris; and Strata MR. They set 3 different scales of pressures for each valve, depending on magnet orientation to the body axis. Artifacts were evaluated and compared among all valves on a 3.0-T GE scanner and 2 valves were also evaluated on a Philips scanner and a Siemens scanner. In-plane artifact sizes were evaluated as the maximum distance of the artifact from the expected scalp.

RESULTS

The sizes and shapes of artifacts changed depending on valve pressure for all valves on the 3 different MRI scanners. Artifacts were less prominent on spin echo sequences than on gradient echo sequences. For diffusion-weighted imaging and time-of-flight MR angiography, the authors matched image numbers within the same sequence and compared appearances of artifacts. For all valves, the number of images affected by artifacts and the image number showing the largest artifact differed among valve settings.

CONCLUSIONS

Artifacts of all adjustable shunt valves showed gross changes corresponding to pressure setting. Not only the maximum distance of artifacts but also the shape changed significantly. The authors suggest that changing pressure settings offers one of the easiest ways to minimize artifacts on MRI.

ABBREVIATIONS DWI = diffusion-weighted imaging; FLAIR = fluid-attenuated inversion recovery; T1WI = T1-weighted imaging; T2WI = T2-weighted imaging; 3D-MRA = 3D MR angiography.

OBJECTIVE

Adjustable shunt valves that have been developed for the management of hydrocephalus all rely on intrinsically magnetic components, and artifacts with these valves on MRI are thus inevitable. The authors have previously reported that the shapes of shunt artifacts differ under different valve pressures with the proGAV 2.0 valve. In the present study the authors compared the size and shape of artifacts at different pressure settings with 4 new-model shunt valves.

METHODS

The authors attached 4 new models of MRI-resistant shunt valve to the temporal scalp of a healthy volunteer: the proGAV 2.0; Codman Certas Plus; Polaris; and Strata MR. They set 3 different scales of pressures for each valve, depending on magnet orientation to the body axis. Artifacts were evaluated and compared among all valves on a 3.0-T GE scanner and 2 valves were also evaluated on a Philips scanner and a Siemens scanner. In-plane artifact sizes were evaluated as the maximum distance of the artifact from the expected scalp.

RESULTS

The sizes and shapes of artifacts changed depending on valve pressure for all valves on the 3 different MRI scanners. Artifacts were less prominent on spin echo sequences than on gradient echo sequences. For diffusion-weighted imaging and time-of-flight MR angiography, the authors matched image numbers within the same sequence and compared appearances of artifacts. For all valves, the number of images affected by artifacts and the image number showing the largest artifact differed among valve settings.

CONCLUSIONS

Artifacts of all adjustable shunt valves showed gross changes corresponding to pressure setting. Not only the maximum distance of artifacts but also the shape changed significantly. The authors suggest that changing pressure settings offers one of the easiest ways to minimize artifacts on MRI.

ABBREVIATIONS DWI = diffusion-weighted imaging; FLAIR = fluid-attenuated inversion recovery; T1WI = T1-weighted imaging; T2WI = T2-weighted imaging; 3D-MRA = 3D MR angiography.

Various types of shunt valve have been developed for the management of hydrocephalus. Among these, adjustable shunt valves are widely employed.2,3,10,13,15 All such valves contain intrinsically magnetic components that enable valve pressure to be changed using an external magnetic adjustment device.13 Furthermore, new valves include locking systems that reduce the risk of unintentional changes to valve setting during MRI.7,16

With these valves, artifacts on MRI are inevitable and can be problematic. One of the most serious problems with metallic implants is image distortion when placed in the static magnetic field of MRI. These metallic implants produce large artifacts that hamper the examination of brain structures. One report pointed out the difficulty in epilepsy assessment due to a significant influence of artifact on the frontal and temporal lobes.5 A different article reported that magnetic field inhomogeneity caused failure in CSF suppression that mimicked a subarachnoid hemorrhage.9 Furthermore, huge artifacts caused by the shunt valve can conceal the development of an underlying brain tumor.12 Although MRI is useful in evaluating cerebral parenchyma, with the increasing use of these new valves, options to reduce these artifacts need to be considered.

We have already reported that the shapes of valve artifacts differed under various valve pressures with the specific use of the proGAV 2.0 valve (Miethke).14 As magnets in the valve rotate in correspondence to the pressure setting, we hypothesized that magnetic field interactions between the MRI scanner and shunt valve would likewise change. While some reports have compared artifacts between different types of adjustable shunt valve, none appears to have paid attention to results at different pressure settings. This study therefore compared the size and shape of artifacts at different pressure settings using the most recent valves from four vendors.

Methods

After receiving permission from the institutional ethics committee of Seirei Hamamatsu General Hospital, this study was conducted with a single healthy volunteer.

Valves

We examined 4 of the newest adjustable shunt valves: the proGAV 2.0 (Miethke), Codman Certas Plus (Johnson & Johnson), Polaris (Sophysa), and Strata MR valve (Medtronic). None of these valves contains antisiphon components.

Each valve was placed on the identical left temporal area of the head, with it long axis parallel to the body axis. Valves were fixed tightly with a rubber sponge and elastic bandages, which had been proven to show no artifact on MRI.

We set 3 different pressures for each shunt valve, corresponding to the intrinsic magnet-field axis: 1) parallel to the static magnetic field (B0) of the MRI scanner; 2) perpendicular to B0; and 3) parallel to B0 and opposite the first position. If the valve did not completely fulfill the aforementioned conditions, the nearest position was selected. These positions were referred from each operation manual and confirmed with a compass before and after examinations. According to this rule, we used the following settings: the Miethke proGAV 2.0 at 5 cm H2O, 11 cm H2O, and 17 cm H2O; the Codman Certas Plus at positions 2, 4, and 6; the Sophysa Polaris at 30 mm H2O, 110 mm H2O, and 200 mm H2O; and the Medtronic Strata MR at positions 0.5, 1, and 2. Shunt valves were checked before and after MRI, with each shunt tool to confirm that setting pressures were unchanged.

MRI

All imaging studies were performed on various 3-T MR scanners: a GE scanner (SIGNA Pioneer 3.0-T MR scanner, GE Healthcare) with a 24-channel head coil; a Philips scanner (Ingenia 3.0-T MR scanner, Philips Healthcare) with a 15-channel head coil; and a Siemens scanner (Skyra 3.0-T MR scanner, Siemens Healthcare) with a 20-channel head neck coil. All 4 valves were examined with the GE scanner. The proGAV 2.0 and Certas Plus valves were also examined using the Philips scanner, and the proGAV 2.0 and Strata MR valves were examined with the Siemens scanner. Routine brain MR images were obtained with the following sequences: T1-weighted imaging (T1WI); T2-weighted imaging (T2WI); fluid-attenuated inversion recovery (FLAIR); diffusion-weighted imaging (DWI); and 3D MR angiography (3D-MRA). Table 1 shows these imaging parameters for each MRI scanner.

TABLE 1.

Imaging parameters and sequences used in this study

SequenceFOV (cm)Thickness (mm)Spacing (mm)TR (msec)TE (msec)TI (msec)ETLFABandwidth (Hz/pixel)Frequency MatrixPhase MatrixNEXPI
SIGNA Pioneer 3.0-T MRI scanner
 EPI DWI4040500073.319532242243Yes
 FSE  T2WI225240259618162.83203201No
 SE T1WI225260010162.83201921No
 FLAIR2252900090246918122.13521921Yes
 3D TOF MRA201.2212.418195.3320192Yes
SequenceFOV (cm)Thickness (mm)Spacing (mm)TR (msec)TE (msec)TI (msec)TFFABandwidth (Hz/pixel)Frequency MatrixPhase MatrixNEXPI
Ingenia 3.0-T MRI scanner
 EPI DWI2251.56000739014301281283Yes
 TSE T2WI2251.530007016488.34003202Yes
 SE T1WI2251.53656590.52881621No
 FLAIR2251.510,000125270028233.32881682Yes
 3D TOF MRA15243.5202172562101Yes
SequenceFOV (cm)Thickness (mm)Spacing (mm)TR (msec)TE (msec)TI (msec)ETLFABandwidth (Hz/pixel)Frequency MatrixPhase MatrixNEXPI
Skyra 3.0-T MRI scanner
 EPI DWI2241.28090641807551601601Yes
 TSE T2WI2151450087121501964483251Yes
 SE T1WI2251.26006702502562161No
 FLAIR2251.29000812500161502893202171No
 3D TOF MRA200.5−4213.43181863843311Yes

EPI = echo-planar imaging; ETL = echo train length; FA = flip angle; FOV = field of view; FSE = fast spin echo; NEX = number of excitations; PI = parallel imaging; SE = spin echo; TE = echo time; TF = turbo factor; TI = inversion time; TR = repetition time; TSE = turbo spin echo; TOF = time-of-flight.

Evaluations

Two neurosurgeons evaluated artifacts on MR images independently in the following manner without any information about the valve applied. The data were adopted when both neurosurgeons pointed out the existence of artifact on each image. Mean diameters of artifacts were calculated by an observer, and statistical analyses were performed. In-plane artifact sizes were evaluated as the maximum distance of the artifact from the expected region of scalp (Fig. 1), while artifact sizes along the z-axis—i.e., perpendicular to slices—were evaluated as affected slice numbers. Intracranial artifacts on T1WI, T2WI, DWI, FLAIR, and 3D-MRA were measured. In this study, we defined the range of the artifact as the area in which we could not recognize brain structures or vessels. Areas that were slightly shaded but in which these structures were possible to distinguish were not included in the range of the artifact. As for DWI and 3D-MRA, we matched image numbers of the 3 different pressure settings and compared the appearances of artifacts.

Fig. 1.
Fig. 1.

Intracranial artifacts were calculated by measuring the maximum distance perpendicular to the scalp on each image.

Results

For all valves, pressure settings did not change during examinations. Representative images obtained with the proGAV 2.0 are shown in Fig. 2. With the proGAV 2.0, a setting of 5 cm H2O showed the maximum in-plane artifact distance, and 17 cm H2O showed the minimum artifact distance. Sizes and shapes of artifacts changed markedly among the 3 different pressures. Representative MRA source images with the other valve are shown in Fig. 3. With the other valves, different pressure settings similarly resulted in different sizes and shapes of artifacts.

Fig. 2.
Fig. 2.

Representative MR images obtained when the proGAV 2.0 valve was used. Artifacts were largest at the 5–cm H2O setting for all pulse sequences. The shape and degree of artifacts changed markedly among different settings. These MR images were obtained from the GE MR scanner.

Fig. 3.
Fig. 3.

Representative MRA source images with each valve obtained in the GE unit. In this study, valves were set in the same region of the temporal scalp. Artifact sizes and shape differed depending on the valve pressure setting.

With all 4 new valve models, maximum artifact distance with each valve pressure differed depending on the valve pressure setting (Fig. 4). Table 2 summarizes the number of images affected by artifact and the range of artifact with all valves. Artifacts were less prominent on T1WI, T2WI, and FLAIR than on DWI or 3D-MRA with gradient echo sequences.

Fig. 4.
Fig. 4.

Comparison of maximum artifact distance obtained with each valve pressure. The maximum artifact size changed depending on the valve pressure setting.

TABLE 2.

Results for all valves examined with the GE scanner

Valve PressureMiethke proGAV 2.0Codman Certas PlusSophysa PolarisMedtronic Strata MR
5 cm H2O11 cm H2O17 cm H2O24630 mm H2O110 mm H2O200 mm H2O0.512 
No. of images affected by artifact
 SE T1WI424132444434
 FSE T2WI222111243333
 FLAIR322223254343
 EPI DWI17171112119182118181818
 3D TOF MRA826960255052808286807083
Median affect distance (25–75th percentile), mm
 SE T1WI20.8

(18.7–22.7)
18.6

(18.3–19.0)
15.6

(14.7–16.8)
16.0

(16.0–16.0)
14.4

(13.7–14.9)
16.7

(16.5–17.0)
16.6

(15.0–18.6)
20.2

(18.0–21.6)
21.4

(19.6–22.3)
15.7

(14.9–16.8)
19.3

(19.1–21.2)
19.8

(17.6–21.7)
 FSE T2WI19.4

(18.1–20.7)
17.7

(17.4–17.9)
14.6

(14.5–14.7)
15.0

(15.0–15.0)
16.0

(16.0–16.0)
17.7

(17.7–17.7)
18.8

(18.1–19.5)
16.6

(15.2–18.1)
19.7

(19.1–20.3)
15.7

(14.0–15.8)
20.1

(17.5–20.7)
19.2

(16.7–20.1)
 FLAIR19.7

(18.6–20.8)
18.2

(18.2–18.3)
15.4

(15.1–15.6)
15.0

(14.7–15.2)
14.6

(14.3–14.8)
15.1

(15.0–18.1)
18.9

(18.0–19.8)
18.7

(18.5–19.8)
19.1

(19.0–19.3)
14.7

(13.7–15.6)
19.1

(17.5–21.0)
20.1

(17.1–20.5)
 EPI DWI35.2

(31.4–41.7)
28.2

(23.8–29.7)
23.5

(22.7–25.8)
22.4

(18.8–26.3)
29.7

(28.2–32.1)
34.4

(28.3–34.6)
32.1

(29.9–34.0)
37.5

(35.3–38.0)
36.8

(34.9–42.3)
27.9

(25.4–30.9)
26.0

(22.9–35.4)
34.5

(31.3–39.8)
 3D TOF MRA21.7

(19.6–29.0)
20.7

(19.3–21.5)
17.4

(16.9–18.2)
16.6

(16.0–17.0)
22.0

(21.3–22.5)
22.1

(17.8–24.2)
18.7

(16.6–21.5)
23.4

(22.3–24.8)
28.1

(21.8–30.9)
18.4

(17.2–19.2)
20.5

(17.0–22.7)
22.9

(19.7–29.1)

As for DWI and 3D-MRA, in all valves, the number of images affected by artifacts and the image number showing the largest artifact differed between each valve setting (Fig. 5).

Fig. 5.
Fig. 5.

Artifact distance for each valve on DWI (A) and MRA (B) source images. Image numbers were matched for the same sequence to compare the appearances of artifacts. Appearances differed with setting pressures.

MR images obtained using the Philips and Siemens scanners also showed alteration of artifacts according to shunt pressure. However, the alterations in artifact size and shape with the Philips scanner were completely opposite those of the GE and Siemens scanners. With all Philips sequences, a setting of 17 cm H2O showed the maximum in-plane artifact distance, and 5 cm H2O showed the minimum artifact distance with the proGAV 2.0 valve. A setting of 2 showed the maximum artifact distance, and one of 6 showed the minimum artifact distance with the Certas Plus valve. The direction of B0 for the Philips scanner proved to be opposite that of the GE and Siemens scanners; that is, the patient side was magnetically the “north pole” with the Philips scanner, but “south pole” with the GE and Siemens scanners. The size and shape of artifacts examined were thus similar between the GE and Siemens scanners.

Discussion

These results confirmed that valve-induced artifacts on MRI change depending on the valve pressure, even though we set all valves at the same position on the left temporal scalp. Not only the maximal distance of artifacts but also the shape of artifacts changed significantly. This fact is important because, simply by changing valve pressure, we can confirm brain structures in areas previously obscured by valve-induced artifacts. The procedure is not harmful and very easy to perform.

Shunt technology for hydrocephalus has grown rapidly, and adjustable shunt valves are now common.7,11,13,16 Furthermore, the development of shunt valves with locking systems that eliminate the risk of unintentional valve pressure changes during MRI enables easy follow-up examination after shunt implantation.

MRI has become the gold standard for the evaluation of acute and chronic neurological conditions. In particular, DWI is useful in scanning for acute cerebral infarction, abscesses, and neurooncological lesions,1–3 and 3D-MRA is suitable for evaluating cerebrovascular diseases.4 However, susceptibility artifacts are more prominent on echo-planar imaging–based diffusion imaging and time-of-flight MRA based on gradient echo sequences than they are on sequences obtaining an echo that use a 180° pulse, such as T1WI, T2WI, and FLAIR.6,11,13,16 This means that patients with a shunt valve potentially pose a difficulty for evaluation of the aforementioned critical diseases. In fact, Singleton et al.12 reported a case in which a shunt valve artifact concealed the development of an underlying meningioma, highlighting the difficulty of detailed examination by MRI.

Following the increasing clinical use of MRI, several ingenious methods have been reported to reduce these artifacts. Some reports have suggested the optimization of MRI parameters to minimize artifacts, but none has proven fully satisfactory for clinical application.6,8

In this study, simply by changing valve pressures, the size and shape of artifacts were changed in all examined valves. These shunt valves contain intrinsic permanent magnets with their own magnetic fields. Rotating the magnets with an external adjustment tool allows valve pressure to be changed. Thus, the direction of shunt valve magnetic field changes corresponded to pressure settings.

On the other hand, a very strong static magnetic field exists inside the MR scanner and is modified with dynamically changing slice selection and frequency-encoding and phase-encoding magnetic gradient. We assumed that interference between the magnetic field of the shunt valve and the dynamically modified B0 of the scanner contributed to the size and shape of the distortion.

As for proGAV 2.0, we have previously reported that the artifact was largest when the outer side of the valve’s magnetic field was opposite the B0.14 In this study, we confirmed the same tendency with the other shunt valves. However, it is difficult to predict accurately the size of the artifact by reference to the setting of valves. Unfortunately, data on configurations of the intrinsic magnets have not been released, and accurately determining magnetic field directions for each valve is difficult. However, changes in artifact size according to the valve pressure (Fig. 3) strongly suggest a tendency toward polarity for each valve.

We confirmed that the directions of B0 differed among MR scanners. In this study, the patient table was the magnetic south pole for GE and Siemens scanners and the magnetic north pole for the Philips scanner. However, in institutes with multiple scanners, this may be changed to avoid interference between the magnetic fields of neighboring scanners.

Here we have identified the strong effect of intrinsic magnets, and further clinical studies are needed to determine a concrete way to reduce artifacts. In this fundamental study, shunt valves were placed on the temporal scalp of a volunteer, with the long axis parallel to the body axis. In actuality, shunt valve positions may vary between institutes and patients. We have to seek suitable valve pressures with smaller artifacts for each patient and each MRI scanner. At the present stage, we recommend changing the known intrinsic magnets of valve by 90° each time if an artifact conceals the lesion of interest. This is because mutual distortion of the magnetic field is dynamically changed by magnet rotation every 90°.

Some studies have reported comparisons of artifacts induced by programmable shunt valves, but most have mainly performed ex vivo assessments.8,10,11,16 Moreover, many studies have calculated volumes or gross areas of artifacts. From the clinical perspective, the most important information is the extent of brain structure concealed by artifacts and the ease of changing image results by changing pressure settings. However, previous studies have not examined these issues.

Some limitations to this study must be acknowledged. First, the data were derived by attaching shunt valves tightly to the surface of the scalp of a healthy volunteer and thus were not inside of the scalp. However, we have recently encountered a patient with a proGAV 2.0 in which the shunt valve produced different artifact sizes and shapes after changing pressure settings. Second, valves in this study were placed on the temporal scalp with the long axis parallel to the body axis; this shunt valve direction was determined to simplify interference between the magnetic field of the shunt valve and that of the MRI scanner. In reality, shunt valves should be placed aslant to the x-, y-, and z-axes in various sites of the skull.

Conclusions

Neurosurgeons must consider many aspects of shunting, such as cost, ease of adjustment, and reliability, when selecting a shunt valve for a patient. We had considered valve-induced artifacts as one important factor. In the present study, however, valve artifacts changed significantly with pressure settings. Based on this finding, identifying the pressure setting that yields the smallest artifact for a shunt valve familiar to the individual institute may be important.

Disclosures

Medtronic Japan Co., Ltd., B. Braun Aesculap Japan Co., Ltd., Sophysa, and Johnson & Johnson K.K. each provided their newest VP shunt valve for this study. No company had influence on or knowledge of the results of this study. The authors received no funding.

Author Contributions

Conception and design: Tanaka, Uchida. Acquisition of data: Uchida, Amano, Nakatogawa, Ando, Nakayama, Sato. Analysis and interpretation of data: Uchida, Nakatogawa, Masui. Drafting the article: Uchida. Critically revising the article: Tanaka, Masui. Reviewed submitted version of manuscript: Tanaka. Approved the final version of the manuscript on behalf of all authors: Tanaka. Administrative/technical/material support: Amano, Masui, Ando, Nakayama, Sato. Study supervision: Sameshima.

References

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Contributor Notes

Correspondence Tokutaro Tanaka: Seirei Hamamatsu General Hospital, Hamamatsu City, Japan. tokutarotanaka@sis.seirei.or.jp.

INCLUDE WHEN CITING Published online May 18, 2018; DOI: 10.3171/2017.12.JNS171533.

Disclosures Medtronic Japan Co., Ltd., B. Braun Aesculap Japan Co., Ltd., Sophysa, and Johnson & Johnson K.K. each provided their newest VP shunt valve for this study. No company had influence on or knowledge of the results of this study. The authors received no funding.

  • View in gallery

    Intracranial artifacts were calculated by measuring the maximum distance perpendicular to the scalp on each image.

  • View in gallery

    Representative MR images obtained when the proGAV 2.0 valve was used. Artifacts were largest at the 5–cm H2O setting for all pulse sequences. The shape and degree of artifacts changed markedly among different settings. These MR images were obtained from the GE MR scanner.

  • View in gallery

    Representative MRA source images with each valve obtained in the GE unit. In this study, valves were set in the same region of the temporal scalp. Artifact sizes and shape differed depending on the valve pressure setting.

  • View in gallery

    Comparison of maximum artifact distance obtained with each valve pressure. The maximum artifact size changed depending on the valve pressure setting.

  • View in gallery

    Artifact distance for each valve on DWI (A) and MRA (B) source images. Image numbers were matched for the same sequence to compare the appearances of artifacts. Appearances differed with setting pressures.

  • 1

    Al-Okaili RN, Krejza J, Wang S, Woo JH, Melhem ER: Advanced MR imaging techniques in the diagnosis of intraaxial brain tumors in adults. Radiographics 26 (Suppl 1):S173S189, 2006

    • Search Google Scholar
    • Export Citation
  • 2

    Antulov R, Dolic K, Fruehwald-Pallamar J, Miletic D, Thurnher MM: Differentiation of pyogenic and fungal brain abscesses with susceptibility-weighted MR sequences. Neuroradiology 56:937945, 2014

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 3

    Aronen HJ, Laakso MP, Moser M, Perkiö J: Diffusion and perfusion-weighted magnetic resonance imaging techniques in stroke recovery. Eura Medicophys 43:271284, 2007

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 4

    Blitstein MK, Tung GA: MRI of cerebral microhemorrhages. AJR Am J Roentgenol 189:720725, 2007

  • 5

    Costa AL, Appenzeller S, Yasuda CL, Pereira FR, Zanardi VA, Cendes F: Artifacts in brain magnetic resonance imaging due to metallic dental objects. Med Oral Patol Oral Cir Bucal 14:E278E282, 2009

    • Search Google Scholar
    • Export Citation
  • 6

    Gupta A, Subhas N, Primak AN, Nittka M, Liu K: Metal artifact reduction: standard and advanced magnetic resonance and computed tomography techniques. Radiol Clin North Am 53:531547, 2015

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 7

    Miyake H: Shunt devices for the treatment of adult hydrocephalus: recent progress and characteristics. Neurol Med Chir (Tokyo) 56:274283, 2016

  • 8

    Olsrud J, Lätt J, Brockstedt S, Romner B, Björkman-Burtscher IM: Magnetic resonance imaging artifacts caused by aneurysm clips and shunt valves: dependence on field strength (1.5 and 3 T) and imaging parameters. J Magn Reson Imaging 22:433437, 2005

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