Common Healthcare Related Instruments Subjected To Magnetic Attraction Study (CHRISTMAS): prospective in situ experimental study


Principal findings

By conducting our CHRISTMAS, we showed the projectile risk of objects commonly found near a clinical MRI scanner. Our findings suggested no clear relation between the weight of the objects and their minimal safe distance. Among the objects that were identified as projectiles, some had the potential to cause tissue penetration. This effect supports professional recommendations and best practices that patients and members of staff should receive extensive warnings and checklists before being allowed into the MRI room.

Strengths and limitations of this study

Preventable MRI incidents continue to occur around the world, and this study will help raise awareness of the dangers of common healthcare related objects in the vicinity of an MRI scanner. The study had several technical limitations. Firstly, although an FBI and NATO graded ballistic gel was used to simulate human tissue penetration, the gel was previously only validated in limited scenarios of a high impact wound penetration profile and tissue disruption, and therefore might not fully apply to the lower impact projectiles in this study. Also, other confounding factors, such as clothing, skin type, and thickness, are likely to reduce actual tissue penetration. A second limitation of the study was the angle of approach to the magnet bore. The selected objects were placed in line with the primary magnetic field, which would have increased SANTA values compared with real world scenarios where objects are more likely to be introduced at an angle. As such, the same objects would probably be under a lower magnetic attractive force at the same distance from the magnet bore.

Comparison with other studies

To the best of our knowledge, no study has evaluated the putative projectile risk of small metallic objects within the vicinity of a clinical MRI scanner.

Policy implications

Warning signs are usually visible from the registration areas to the changing areas, and even on the door of the magnet room. Staff members also routinely check if patients are carrying magnetic or metallic objects before entering the MRI room. At our institutions, physical checklists must be signed by a trained member of staff before the patient is allowed to enter the magnet room. As an added security measure, we have also trialled wall mounted ferromagnetic detection systems; handheld ferromagnetic detection systems are available at all scanner sites. Despite these efforts, small injuries and near misses have still occurred over the past 12 years, attributable to a multitude of human factors, including warning and alert fatigue.12

Some near misses originate from miscommunication between patients and staff. For example, in at least two incidents, patients reported that they were carrying only paper money and no metallic objects. In one instance, the member of staff prepared the patient for the examination and as the patient entered the magnet bore, coins were pulled from the patient’s pockets and became affixed to the magnet bore. On further questioning, the patient disclosed that their paper money referred to coinage they required to purchase a newspaper. We presume that these scenarios occur more frequently than reported by staff because coins are commonly found by service engineers under the patient’s bed during MRI servicing.

Other incidents might be related to a temporary lapse in judgment of patients and staff members. Patients are advised to change into hospital gowns for their examinations, but occasionally, at the patient’s insistence, they are allowed to wear their own pyjamas, sweat pants, or gym shorts if they confirm that their clothing has no metal objects. This scenario is common for a scan of an extremity where their torso will not be within the bore. Coins, paperclips, metallic microfibres woven into shirts and undergarments, and even a switch blade have been brought into the scanner bore in these instances.

Patients’ routines and habits can also contribute towards these incidents. Patients declaring that they are not wearing any jewellery or other metallic objects, but forgetting about a pendant or religious token which they always carry, is not uncommon, especially if the pendant is not attached by a metallic chain. These objects are usually detected after the patient enters the scanner because the patient feels the magnetic pull on the object or the radiographer notes distorted images.

That the highly ferromagnetic cutlery utensils (fork, spoon, knife, and teaspoon) could penetrate the skin is not surprising. We found that the knife had the deepest tissue penetration. Given the huge potential for harm, we strongly recommend that care is taken by the clinical teams to be extra vigilant when attempting to scan patients with altered mental states (eg, patients who are drowsy, confused, or unconscious). The use of handheld ferromagnetic detection systems should always be performed on these patients because cutlery could be unintentionally dropped and hidden within the folds of the bedsheets.

Many of the other objects in our study were included because they are commonly carried or attached to junior doctors, especially house officers and senior house officers. These doctors are often the first responders to a crash call in the hospital and, in their eagerness to help the patient, are at risk of forgetting to remove metallic objects before entering the MRI room. These metallic objects include pen torches, pens, and badge holders, which are often given to house officers when they join a medical defence union. We were pleasantly surprised at the resilience of the pen torch, badge holder, and thermometer in our study, which all survived intact. In contrast, previously, a pager belonging to one of the authors was unintentionally brought into the MRI magnet room and was irreparably damaged with near complete loss of audible volume.

The generic stethoscope as well as the nursing scissors, despite having large SANTA values, did not cause penetrating damage to the ballistic gel. We presume that the small plastic tip on the blunted end of the nursing scissors prevented penetration into the simulated tissue by reducing the force per unit area and potentially absorbing some of the impact kinetic energy. For the stethoscope, the actual SANTA outside of the experimental set-up could potentially be much higher. We believe that placement of the stethoscope within the acrylic tubing allowed for substantially increased surface contact between the ELF tubing surface and the rubber based tubing of the stethoscope, and hence substantially increasing friction. A very different scenario could occur if a junior doctor was wearing the stethoscope around their neck or if the stethoscope was lying on a patient’s bed. We were surprised to find that the iPhone 6 and credit card were still fully functional after several passes into the MRI scanner. We had anticipated finding some damage to the iPhone screen, speaker, or electronics based on forum threads on Apple13 and Quora.1415

One penny and two pence coins have been made from copper plated steel since September 1992,1617 and five pence and 10 pence coins have been made of nickel plated steel since 2012.1819 Given the steel base of the coins, we were not surprised that the one penny, two pence, five pence, and 10 pence coins were ferromagnetic. The 20 pence and 50 pence coins were made of cupro-nickel,2021 which is non-ferromagnetic, and showed no magnetic attraction. The £1 coin was made of nickel-brass,22 but although the magnetic attraction force to overcome the frictional resistance between the coin and the acrylic tubing was sufficient, the force was insufficient against gravity. The £2 coin was made from a combination of nickel-brass for the outer rim and cupro-nickel for the core.23 The weight of the coin at almost 12 g was likely far greater than the magnetic attraction force towards the 3 T magnet.

Although the two biscuit tins did not penetrate the simulated human tissue, sufficient force existed to cause a dent in the base of the tin as well as moving the whole ELF deeper into the bore of the magnet. Reports that as little as 375 J/m2 of surface tension is needed to break normal human tibial and femoral diaphyses24 suggests a high possibility of bone breakage for patients or staff in the way of these biscuit tins if unintentionally brought into the scanner room and accelerated towards the bore of the magnet. Given that the attractive force on a ferromagnetic object is proportional to the strength of the static magnetic field, the introduction of 7 T magnets (now available commercially) into clinical practice could have a higher potential risk of injury because the SANTA values of the objects could be more than double.


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