British Journal of Radiology 74 (2001),1118-1120 © 2001 The British Institute of Radiology
A national survey of attitudes towards the use of MRI in patients known to have intracranial aneurysm clips
J C Evans, MRCP, FRCR
E T Smith, FRCR
and
T E Nixon, FRCR
Department of Neuroradiology, The Walton Centre for Neurology and Neurosurgery, Lower Lane, Fazakerley, Liverpool L9 7LJ, UK
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Abstract
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The objective of this study was to determine the attitudes and practice of neuroradiology centres across the UK regarding the use of MRI in patients known to have intracranial aneurysm clips. A postal survey comprising three questions and a comments section was sent to 35 neuroradiology centres across the UK. There were 32 (91%) respondents to the single questionnaire. 16 (50%) respondents said that they would not consider performing MRI on a patient with an intracranial aneurysm clip. Of the remaining 50%, all said that identification of the clip type and assurance of its safety would be needed prior to scanning the patient. The magnetic strength of the system did not appear to affect the decision regarding whether or not to perform MRI on such patients. There was a variation in attitude towards the use of MRI in such patients between different regions. Neuroradiology centres are equally divided in their attitude and practice about whether it is safe to use MR to image a patient known to have an intracranial aneurysm clip. This is most probably due to the conflicting literature, as well as uncertainty about the identification and ferromagnetic properties of individual clips.
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Introduction
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The risks of placing patients with intracerebral aneurysm clips into a MRI scanner are well known. Reports of fatal outcome in such circumstances have understandably resulted in most radiologists being reluctant to perform MRI on such patients, instead using alternative forms of imaging. Over the years, manufacturers have concentrated their efforts on trying to reduce the ferromagnetism of their aneurysm clips to a negligible level. Nevertheless, there is still considerable anxiety about the safety of exposing these patients to MRI. Attitudes vary between regions, within regions and even within departments. The aim of this study was to determine what percentage of neuroradiology departments would consider using MRI in patients with intracranial aneurysm clips, and whether or not identification of the clip and the strength of the magnet affected this decision. We were also interested to see whether there might be any regional trends in attitude towards using MRI in these patients.
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Methods
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A simple questionnaire was sent to consultants at 35 neuroradiology departments in the UK. It comprised three questions and a comments section, and was sent with an addressed envelope.
- Would you do a MR scan on a patient known to have an aneurysm clip? Y/N
- If YES
- Only if the clip had been identified and was known to be non ferro-magnetic Y/N
- If the type was unknown Y/N
- What strength is your magnet?
- Any other comments.
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Results
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32 (91%) of 35 centres responded to the questionnaire. One respondent failed to state the strength of their magnet. 16 (50%) of the 32 respondents said that they would not perform MRI on a patient known to have an aneurysm clip. Of the other 16 who would perform MRI, all said that the type of clip would need to be identified and known to be non-ferromagnetic. No centre would consider performing MRI on a patient with an unidentified clip, whatever the circumstances.
The strength of a department's magnet was not a significant factor in deciding whether or not to image a patient known to have an aneurysm clip (Figure 1
). Centres were grouped into regions (Figure 2) to see whether there were any significant inter-regional variations in attitude (Figure 3). Proportionally, the London centres were most likely to perform MRI on patients with aneurysm clips. Conversely, none of the Scottish or Northern Ireland centres were prepared to perform MRI on such patients.
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Figure 2. Regional boundaries used in Figure 3 . The numbers indicate those centres that responded in each region. Scot, Scotland; NI, Northern Ireland; NW, north-west England; NE, north-east England; Mid, Midlands; East, eastern England; South, southern England; L, London.
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Discussion
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Many patients who have intracranial aneurysm clips need follow-up imaging to evaluate aneurysm progression or when further symptoms develop. Other patients with aneurysm clips may require MRI of organs outside the central nervous system, but anxiety about placing these patients close to the magnet bore may prevent MRI from being used. Alternative imaging modalities such as CT and digital subtraction angiography (DSA) may be used, but image resolution [1, 2] and patient safety, respectively, may be affected. For example, with carotid DSA, the risk of a disabling neurological deficit is 1%, with a mortality of less than 0.1% [3]. Morbidity and mortality figures in patients with aneurysm clips undergoing MRI are not known. There has been considerable research into MRI safety, especially in the field of intracerebral aneurysm clips [4–6]. Earlier comparative studies looked at clip deflection within the MRI suite at various distances from the magnet bore [7]. Later, more refined means of quantification of ferromagnetism were described, allowing values of magnetism to be ascribed to each clip [8]. The electromagnetic unit per gram (EMU g-1) of a clip and the strength of the magnetic field in which it lies define the degree of translational force applied to the clip. It has been suggested that a clip value of 1 EMU g-1 or less should represent a negligible risk for MRI [8]. An aneurysm clip experiences both translational and rotational forces within a magnetic field. As already mentioned, the former is more easily calculated and measures the distance that the clip would be deflected if free to do so. Rotational force, or torque, is dependent on the shape of the clip. It is believed that vascular injury due to clip movement is most likely owing to this rotational force, causing the clip to either detach from the aneurysm neck or to erode into the adjacent vessel [9].
Many different metals and alloys have been used in the manufacture of aneurysm clips, and multiple comparisons of ferromagnetism have been performed over the last decade [10–12]. Titanium and its alloys are recognized as having a comparatively low magnetic susceptibility as well as causing less artefact on CT and MRI [13]. There is concern that considerable variability of ferromagnetism may occur between batches of the same alloy. This was cited in the comments section of the questionnaire as the reason for not performing MRI by 3 (9.4%) of the respondents. There may even be intrabatch variability leading many to suggest that every clip be tested prior to insertion.
There is also concern that clip magnetism is induced while handling the clip during manufacture and insertion, and that repeat MRI may induce clip magnetism in a previously non-ferromagnetic clip. The latter has serious implications for the follow-up of patients with MRI. Recent work, however, does not suggest that chronic exposure of a non-ferromagnetic clip to a static magnetic field causes any significant alteration in the magnetic properties of the clip [14].
The equal mix of opinions expressed in this survey demonstrates that different departments have interpreted the research data into MRI safety in different ways. Some radiologists feel that while uncertainty exists MRI should not even be considered, whilst others feel that the risk of complications with certain clips is so small that imaging should not be compromised and MR angiography should be used for follow-up. While accepting that individual radiologists should be allowed to make an informed decision about whether or not to scan a patient, this difference in policy between regions and even between neighbouring hospitals will inevitably lead to public confusion. It might seem a straightforward decision when the identity of the clip and its ferromagnetic properties are known, but doubt will always exist when clip identity is uncertain. One of the respondents said that their surgeon signs a form at the time of surgery, taking responsibility for the type and safety of the clip. It was clear from the survey that no centre would consider performing MRI on a patient with an unidentified clip, and this seems a sensible policy.
It is perhaps not surprising that there was no significant correlation between magnet strength and the decision to scan. Although it is known that a stronger magnetic field strength causes a greater rotational force, other variables such as scan time, and number and type of sequences will all affect the mechanics of potential vessel damage.
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Summary
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This postal survey has demonstrated that there is equal division of opinion about whether or not it is safe to use MRI when imaging patients with intracranial aneurysm clips. This is probably a result of the vast amount of conflicting literature on the subject and also the lack of adequate testing and documentation of the clips used. We suggest that there should be national guidelines to protect these patients from the serious risks of exposure to MRI. Quantification and documentation of the ferromagnetic properties of aneurysm clips at all stages of manufacture and insertion should be standardized. Finally, further work is needed into the long-term effects of repeated exposure of implanted aneurysm clips to MRI, thus ensuring patient safety during serial follow-up.
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Acknowledgments
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We would like to thank all of the respondents who took the time to answer the questionnaire.
Received for publication November 28, 2000.
Revision received July 10, 2001.
Accepted for publication July 20, 2001.
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References
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Holtas S, Olsson M, Romner B, Larsson EM, Saveland H, Brandt L. Comparison of MR imaging and CT in patients with intracranial aneurysm clips. Am J Radiol 1988;9:891–7.
-
Brothers MF, Fox AJ, Lee DH, Pelz DM, Deveikis JP. MR imaging after surgery for vertebrobasilar aneurysm. Am J Neuroradiol 1990;11:149–61.[Abstract]
-
Hankey GJ, Warlow CP, Sellar RJ. Cerebral angiographic risk in mild cerebrovascular disease. Stroke 1990;21:209–22.[Abstract/Free Full Text]
-
Becker RL, Norfray JF, Teitelbaum GP, et al. MR imaging in patients with intracranial aneurysm clips. Am J Neuroradiol 1988;9:885–9.[Abstract]
-
Shellock FG, Kanal E. Yasargil aneurysm clips: evaluation of interactions with a 1.5-T MR system. Radiology 1998;207:587–91.[Abstract/Free Full Text]
-
Dujovny M, Kossowsky N, Kossowsky R, et al. Aneurysm clip motion during magnetic resonance imaging: in vivo experimental study with metallurgical factor analysis. Neurosurgery 1985;17:543–8.[Medline]
-
Dujovny M, Dujovny N, Fiat D, et al. Magnetic field gradients in the MR suite and their effects on aneurysm clips. Neurol Res 1996;18:483–6.[Medline]
-
Dujovny M, Alp MS, Dujovny N, et al. Aneurysm clips: magnetic quantification and magnetic imaging safety. Technical note. J Neurosurg 1997;87:788–94.[Medline]
-
Klucznik RP, Carrier DA, Pyka R, et al. Placement of a ferromagnetic aneurysm clip in a magnetic field with a fatal outcome. Radiology 1993;187:855–6.[Abstract/Free Full Text]
-
Shellock FG, Crues JV. Aneurysm clips: assessment of magnetic field interaction associated with a 0.2-T extremity MR system. Radiology 1998;208:407–9.[Abstract/Free Full Text]
-
Ooka K, Shibuya M, Suzuki Y. Motion and image artefacts of various intracranial aneurysm clips in a magnetic field. Acta Neurochir Wien 1996;138:1241–5.[Medline]
-
Kanal E, Shellock FG, Lewin JS. Aneurysm clip testing for ferromagnetic properties: clip variability issues. Radiology 1996;200:576–8.[Abstract/Free Full Text]
-
Piepgras A, Guckel F, Weik T, Schmiedek P. Titanium aneurysm clips and their advantages in diagnostic imaging. Radiologe 1995;35:830–3.[Medline]
-
Kanal E, Shellock FG. Aneurysm clips: effects of long term and multiple exposures to a 1.5-T MR system. Radiology 1999;210:563–5.[Abstract/Free Full Text]
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Abstract
Introduction
, no; 
, yes.