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10. Implantable medical and non-medical objects

The use of MRI continues to expand in the UK, alongside advances in device technology.  In order to ensure good patient management and patient safety, departments should implement robust procedures to ensure the MRI safety status of any implanted device. 

SCoR and BAMRR recommend that departments obtain an up to date copy of the following publication Reference Manual for Magnetic Resonance Safety, Implants and Devices.24 This contains safety advice and guidance and a comprehensive list of implants alongside the field strength under which they have been tested. The list can also be viewed on the website www.mrisafety.com 25

However, as these are both American hosted publications they may not always contain information on a particular device.  In this instance, contact should be made directly to the device manufacturer to ascertain its MR safety status.

  • Departments should be aware that the MR compliancy listing of an implant may be changed and should ensure that their information is kept up to date.
  • All visitors and patients should be adequately screened by means of a safety questionnaire prior to entering the MR controlled access area and no-one should be allowed to enter if there is any doubt regarding the compliancy of any implants. Please also see section 3.1
  • Departments should have a mechanism for recording and storing details obtained regarding an implant’s MR compliancy.
  • For MR conditional devices departments may need to liaise with their MR safety expert and equipment manufacturer to ensure that the conditions can safely be met.
  • Radiographers should be aware of manufacturers’ features that assist in meeting conditional specifications, one such example is the Philips Scanwise Implant technology. 26
  • Manufacturers provide written conditions under which a conditional implant can be scanned. It is very important for the MR operator to understand in practical terms  the conditions of the spatial magnetic field gradients of their scanners and the SAR limitations
  • MR operators should be aware that the spatial field gradient of wide bore systems can be higher than the equivalent narrow bore system. This may alter the status of the implant in wide bore systems.
  • The field strengths and other relevant operating parameters at which specific implants have been tested should be noted when assessing the safety of implants.  Safety at one field strength may change at another.
  • It should be noted that many implants and devices have NOT been assessed at field strengths of 3T and above.
  • If there is any doubt about an implant, departments should proceed with caution. The SCoR and BAMRR recommend that written evidence regarding an implant's compliancy be obtained prior to scanning. If a department is unable to obtain the required information then scanning should not take place or should be delayed until a risk/benefit analysis and risk assessment has taken place. See also 3.4 and 10.3

Note: Manufacturers of implants and devices have a duty to supply safety information. If you are unable to obtain such information, please report it to the MHRA.

10.1 Active implanted medical devices (AIMD)

Mechanically, electrically and magnetically operated devices may malfunction in the presence of strong magnetic fields. This malfunction may not be obvious at the time of examination but may have serious consequences subsequently. Examples include:

  • cardiac pacemakers
  • cochlear implants
  • programmable hydrocephalus shunts
  • implanted neurostimulation systems
  • implanted drug infusion pumps.

Note: this is not an exhaustive list of examples.

  • Departments should ensure that their local rules contain information about active device implants which are MR unsafe.
  • The process for scanning patients with MR conditional AIMDs should be clearly documented.

10.1.1 Cardiac pacemakers
Most cardiac pacemakers are contra-indicated in MRI. Field strengths as low as 1mT may be sufficient to cause programming changes or to close reed switches. Such patients should not enter the MR environment. Departments should also be aware of patients who have had a pacemaker removed and should check for any remaining pacemaker wires which can act as antennae and cause induced currents. However, there are now a number of manufacturers in the UK that have developed MR conditional pacemakers and leads.

10.1.2 MR conditional pacemakers

Many manufacturers have developed MR conditional pacemakers, allowing for patients with these to undergo MRI scanning under certain stated conditions for safe operation.

Some examples include:

BIOTRONIK Pro MRI Pacing systems - further information available at:
http://www.biotronik.com/wps/wcm/connect/en_promri/biotronik/home/ 27 and the

MEDTRONIC SureScan - Advisa MRI Pacemaker and Ensura MRI Pacemaker systems. Further information available at:  http://www.medtronic.com/surescan/index.html 28

In addition to complying with the manufacturers’ conditions for safe operation, the SCoR advises that departments formulate a local policy to be included within the local rules.

Such a policy should include: a patient pathway, clearly defined roles and responsibilities of the radiology department, the radiographic staff and the cardiology department, a list of any contraindications, potential adverse events and emergency procedures, the manufacturers’ specific operating instructions and any local specific instructions.

  • This should include a mechanism for checking that both the pacemaker and the leads are MR conditional.

10.2 Non-active devices

There is a risk that implanted ferromagnetic devices will undergo attractive forces, such that they can dislodge, causing serious injury or discomfort to the person concerned. Such devices will be labelled as MR unsafe, MR conditional or MR safe.
Examples include:

  • coils, stents and filters
  • aneurysm clips
  • heart valves
  • orthopaedic implants.

Note: this is not an exhaustive list of examples.

  • Departments should ensure that their local rules include information about non-active device implants which are MR unsafe.
  • For those that are MR conditional, a process for scanning patients with such implants should be clearly documented.

10.2.1 Aneurysm clips

Many departments take the decision not to scan patients with aneurysm clips. MHRA2 advises: Scanning must not proceed unless there is positive documented evidence that the aneurysm clip is non-ferromagnetic. For example, titanium, tantalum and vanadium are non-ferromagnetic, whereas stainless steel has varying degrees of para- and ferromagnetism.

  • If your department has a policy to scan MR conditional aneurysm clips, a clearly defined process for ensuring accurate MR compliancy details of the clip and the process for scanning these clips should be documented within the local rules.

10.2.2  Recent implants

10.2.2.1 Great care must be taken with regard to recent MR safe or MR conditional ferromagnetic implants or clips, which are not anchored into bone, before they become embedded with fibrous tissue. Local rules should specify the time which should elapse prior to scanning, but should not be less than six weeks.

10.2.2.2 Those objects, such as bone screws or joint replacements which are firmly anchored, may safely be scanned, but should be monitored carefully because the object may be subject to temperature rise; scanning should be discontinued if discomfort occurs.  Image quality around the site will, in any case, be seriously degraded and examination may not be of diagnostic quality.

10.2.2.3 Passive implants, that is those that contain no electronic or magnetic components and are made of non-ferrous material, are safe to scan immediately at the field strength at which they were tested.24

10.3  Scanning patients with implants where MRI may be contraindicated

MHRA has issued guidance relating to the scanning of patients with implants where MRI may be contraindicated. It states ‘there may be a need in certain scenarios to perform an MRI exam on such a patient and advise a multi disciplinary team approach be taken with a full risk assessment’.2

  • SCoR and BAMRR advise that the risk assessment and decision to scan should be made on a case by case basis.

See also section 3.4

10.4  Intra orbital foreign bodies

10.4.1 Patients

Intra orbital foreign bodies (IOFB) are of particular concern and any patient, who presents with a history of an IOFB, should be treated with caution.  SCoR and BAMRR recommend that departments initiate a clinical screening process. This should be documented within the local rules.

An example of such a process provided by BAMRR is described below:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Note:  Referrals for orbital X- Rays should only be made by individuals entitled to act as a referrer by their employer. 29

  • Departments should also investigate the existence of any previous orbital imaging prior to  the patient undergoing an X- ray to exclude an IOFB.

10.4.2 Staff or carers accompanying a patient

Staff or carers accompanying a patient should not enter the MR environment nor should they undergo an X -Ray if they have a history of an IOFB.  In such an instance the department should make alternative arrangements if the patient requires a carer within the scan- room.

10.4.3  Volunteers

Volunteers who present with a history of an IOFB should not generally be scanned (although they should be advised that this would not necessarily preclude them for a clinical scan if they were to require one in future).  However, if there has been provision made within the ethical approval for exposure to ionising radiation then the volunteer may proceed and undergo the clinical screening procedure for IOFB as outlined in 8.4.1.

10.5  Non-medical objects

10.5.1 Bullets and shrapnel

Metallic objects in the body (such as shrapnel or a bullet) become fibrosed in tissue within six weeks. Beyond that time it may be safe for scanning. However, consideration must be given to the likelihood of heating effects and also the potential for larger, ferromagnetic objects to experience significant forces whilst being positioned within the bore of the magnet.

Any large (>1 cm3) metallic fragments should be assessed on a case by case basis. Such objects are of greater concern if they are located near significant soft tissue or vascular structures. Therefore, obtaining current and previous imaging is essential in the assessment of risk versus benefit required before the MR scan may proceed. Involvement of the patient, the referring clinician and a radiologist or consultant radiographer who can review and comment on related imaging is recommended in such cases.

  • Consideration should be given to utilising low SAR sequences during scanning of these patients.

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