Local cOinage and Hospital Equipment Index (Lo Hei): projectile distance of Singapore coinage and healthcare-related equipment in a 3T magnetic resonance imaging scanner

Abstract

Introduction:

Modern magnetic resonance imaging (MRI) scanners utilise superconducting magnets that are permanently active. Patients and healthcare professionals have been known to unintentionally introduce ferromagnetic objects into the scanning room. In this study, we evaluated the projectile risk of Singapore coinage as well as some common healthcare equipment within a 3 T MRI scanner.

Methods:

A rig termed ‘Object eNtry Guidance and Linear Acceleration Instrument’ (ONG LAI) was custom-built to facilitate safe trajectory of the putative ferromagnetic objects. A ballistic gel target was utilised as a human tissue surrogate to estimate tissue penetration. The point at which objects would self-propel towards the scanner was named ‘Huge Unintended Acceleration Towards Actual Harm (HUAT AH)’.

Results:

Singapore third-series coins (10-cent to 1-dollar coins) are highly ferromagnetic and would accelerate towards the MRI scanner from more than one metre away. Cannulas with their needles are ferromagnetic and would self-propel towards the scanner from a distance of 20 cm. Standard surgical masks are ferromagnetic and may lose their sealing efficacy when they are worn too close to the magnet. Among the tested objects, a can of pineapple drink (Lee Pineapple Juice) had the highest HUAT AH at a distance of more than 1.5 m.

Conclusion:

Some local coinage and commonly found objects within a healthcare setting demonstrate ferromagnetic activity with projectile potential from a distance of more than 1 m. Patients and healthcare professionals should be cognisant of the risk associated with introducing these objects into the MRI scanning room.

INTRODUCTION

The advent of magnetic resonance imaging (MRI) has revolutionised medical imaging by providing healthcare professionals with exceptional soft tissue visualisation without the need for ionising radiation. Modern clinical MRI scanners commonly utilise superconducting magnets that are permanently active, even when the scanner is switched off. Despite the presence of extensive visual warnings, verbal checks and training for healthcare professionals, incidents occur when patients and healthcare professionals unintentionally bring ferromagnetic artefacts into the MRI scanning room.

SUMMARY BOX

What is known?

With appropriate safety precautions, it is possible to evaluate the projectile risk of small objects within an MRI scanner.

What is new?

The new Singapore coins are highly ferromagnetic, while the older coins are not. Cannulas are ferromagnetic and have the potential to self-propel.

Clinical Implications

This study highlights the importance of adherence to the MRI safety checklist and the risk of potential harm from many commonly found local objects within a healthcare setting.

Any ferromagnetic object introduced into the MRI scanner’s magnetic field will immediately be attracted towards the scanner bore. Once the ferromagnetic attraction surpasses the point of frictional resistance, the object experiences a rapid acceleration towards the scanner bore due to an exponential increase in magnetic attraction.[1] The impact of such objects can deposit significant kinetic energy onto the gantry or unsuspecting patients, potentially causing extensive damage to the gantry and, in severe cases, even patient death from blunt force trauma.

While countless anecdotal stories exist among MRI radiographers, physicists and service engineers regarding small objects lodged inside MRI scanners, these are rarely documented or reported in the scientific or medical literature. A recently published study highlighted the possibility to safely evaluate the projectile risk of these smaller ferromagnetic objects under controlled conditions.[2] To provide local relevance to practice in Singapore, we performed a follow-up study adopting similar methodology. In this follow-up study, we have utilised our local coinage, healthcare-related equipment that have been previously introduced or nearly introduced into our MRI scanners, as well as objects often found on patients and healthcare staff.

METHODS

MRI scanners

The experiments were performed on a 12 year 3 month-old Siemens Magnetron Skyla 3T MRI scanner (Simens Medical Solutions USA, Inc., Malvern, PA, USA) and a 2-month-old GE SIGNA^™ Premier 3T MRI scanner (GE HealthCare, Chicago, IL, USA).

Object eNtry Guidance and Linear Acceleration Instrument

A customised rig, which we have named the ‘Object eNtry Guidance and Linear Acceleration Instrument (ONG LAI)’ for this follow-up study was built with minor modifications from the previously reported design.[2]

The outer structure of ONG LAI consisted of a slab of 1,200 × 600 × 50 mm high-density extruded foam (Art Friend, Singapore) wedged into the base of a 3-mm-thick prefabricated acrylic dome designed as a barrier shield for MRI scanners,[3] with the front panel removed. The back of the barrier shield was reinforced with a slab of 50-mm-thick high-density extruded foam cut to the contours of the barrier shield. The high-density foam slabs were then secured to the acrylic dome with 48-mm-wide duct tape (Duck Tape, Duck Brand, Cleveland, OH, USA).

The inner structural component of ONG LAI was made of 1,000 × 600 × 25 mm high-density extruded foam. The simulated human tissue equivalent target[4] consisted of a 200 × 150 × 150 mm block of North Atlantic Treaty Organization-rated 10% synthetic gelatine ballistic gel (Clear Ballistics, Greenville, SC, USA). This was surrounded on three sides by slabs of 200 × 150 × 25 mm high-density extruded foam affixed onto the sliding component with a combination of polyvinyl acetate glue (Elmer’s glue, Elmers, Westerville, OH, USA), bamboo skewers (HomeProud, NTUC FairPrice, Singapore) as dowels and reinforced with duct tape. The top of the ballistic gel was covered by a 2-mm-thick acrylic panel (Art Friend, Singapore) secured to the side panels with the duct tape. The ballistic gel target edge was recessed from the foam side panels and the acrylic top to ‘capture’ any potential errant projectiles.

The guidance tubing towards the target consisted of a 1,000-mm-long, 5-mm-thick acrylic tubing with an inner diameter of 110 mm (Kings Materials, Singapore) supported by custom-cut high-density extruded foam supports. The end of the guidance tube was wedged against the surface of the ballistic gel. The extension tubing consisted of 1,000-mm-long, 5-mm-thick acrylic tubing with an inner diameter of 50 mm telescoped into the larger-bore guidance tube. A second layer of protection was built on top of the ballistic gel-guidance tube interface region with three panels of 2-mm-thick clear acrylic panels measuring 300 × 450 mm, secured at the joints with the crystal tape (Gorilla Glue Company, Cincinnati, OH, USA) to allow for visualisation and with the duct tape on their joints to the high-density extruded foam base of the sliding component. A custom-cut sheet of 3-mm-thick corrugated plastic board (Art Friend, Singapore) was then affixed to the back of the second layer of protection and secured with the duct tape.

The ONG LAI was sited within the MRI scanner bore, with the impact interface of the ballistic gel at the entrance of the magnet bore [Figure 1]. For reproducibility of results, the position of the impact face of the ballistic gel would be cross referenced with the laser location guide at the front of the bore of the scanners before each experiment.

Figure 1.

Setup of the Object eNtry Guidance and Linear Acceleration Instrument (ONG LAI) without the guidance tubing in the GE SIGNA™ Premier 3T magnetic resonance imaging (MRI) scanner.

Objects tested

A total of 28 different objects were carefully introduced into the MRI scanners [Table 1]. These objects were chosen based on anecdotal historical precedence of having been accidentally worn or carried into the MRI scanner, or commonly found by engineers during servicing and repair of local scanners.

Table 1.

Objects introduced into the 3T magnetic resonance imaging scanning room via the ONG LAI.

No.	Object	Details of object	Remarks
1	Gold	22K gold zodiac pendant, Poh Heng Jewellery	Year of dog
2	Jade	Chopsticks, jadeite, Burmese	–
3	White gold	Love ring, Cartier	–
4	Straits Settlements silver half-dollar coin	King Geroge V, King and Emperor of India	Year 1920
5	First-series 10-cent coin	Seahorse design	Year 1980
6	Second-series 10-cent coin	Star jasmine design	Year 2000
7	Second-series 20-cent coin	Powder puff plant design	Year 1997
8	Second-series 50-cent coin	Yellow Allamanda design	Year 1996
9	Second-series 1-dollar coin	Periwinkle design	Year 1998
10	Third-series 10-cent coin	Public housing design	Year 2017
11	Third-series 20-cent coin	Changi Airport design	Year 2018
12	Third-series 50-cent coin	Port of Singapore design	Year 2017
13	Third-series 1-dollar coin	Merlion design	Year 2018
14	Ten-dollar coin	LKY100 commemorative coin	Year 2023
15	MRI contrast bottle	Dotarem, Guerbet, Chicago, IL, USA	Empty bottle
16	18G cannula	Introcan safety®, B. Braun Medical Inc., Bethlehem, PA, USA	Uncapped
17	22G cannula	Introcan safety®, B. Braun Medical Inc., Bethlehem, PA, USA	Uncapped
18	24G cannula	Introcan safety®, B. Braun Medical Inc., Bethlehem, PA, USA	Uncapped
19	Paperclip	Triangular shaped, generic, unbranded	–
20	Surgical mask	Kolmi OP-R type 2 medical mask	–
21	Pen	Staedtler black ball point	–
22	Android mobile Neffos	Model: C7s	–
23	Android mobile Sony	Model: Xperia F5321	–
24	TraceTogether token	Fourth generation, flat with battery change compartment screw	–
25	Salbutamol inhaler	Ventolin, 100 mcg/puff, GSK, London, UK	–
26	Pocketknife	Single-blade ‘Swiss Army knife’, unbranded	Used closed
27	Apple juice	Pokka Sparklin’ Fuji Apple less sugar, 325 mL	Empty tin
28	Pineapple juice	Lee Pineapple, 325 mL	Empty tin

MRI: magnetic resonance imaging, ONG LAI: Object eNtry Guidance and Linear Acceleration Instrument

The pen and the mobile phones were tested and checked for full functionality before use. It was not possible to test for the functionality of the TraceTogether token. The salbutamol inhaler was completely discharged before use to prevent any potential explosive event.

Huge Unintended Acceleration Towards Actual Harm

When the magnetic attraction force exceeds the gravitational and frictional force on the object at rest, the object would be pulled towards the bore of the magnet. Due to the inverse cube law of magnetic dipoles, the force of attraction would increase exponentially, resulting in increasing acceleration of the objects as they move closer to the bore. For this experiment, we have named the point at which the objects move independently towards the bore as the Huge Unintended Acceleration Towards Actual Harm (HUAT AH) point.

The smaller objects were slowly advanced in ONG LAI using a 100 cm × 1 cm diameter acrylic rod at a rate of approximately 5 mm/s. The canned drinks were too large to fit into the extension tubing. Their ring tabs were secured with 35.6-kg tension-rated ultralight-weight fluorocarbon line (Decathlon, Villeneuve-d’Ascq, France) with the other end of the line secured to the end of the scanner bed. The line was long enough to allow for free movement of the cans towards the scanner bore via the ONG LAI, but not long enough for the cans to hit the scanner bore. The cans were placed on top of the extension guide rails created by two parallel 3-mm-thick acrylic tubing with an outer diameter of 40 mm guiding the can into the large bore of the ONG LAI. The cans were gently guided forward until their HUAT AH point.

Ballistic gel penetration

Visual inspection was performed after every launch to assess for penetration of the ballistic gel. For any object that was deemed or suspected to have penetrated the ballistic gel, a study team member would firmly hold it with their dominant hand, anchoring their elbow on the base of the ONG LAI to prevent any further object movement. Two other team members would retrieve the ONG LAI by sliding out the inner structural component. Once outside, the first team member would gently release their hold on the object. If the object fell without causing any additional laceration to the ballistic gel, it was considered non-penetrating. The ballistic gel would be replaced if any penetration was found, to maintain the structural integrity of the impact surface.

RESULTS

Each object was introduced into ONG LAI at least three times. The HUAT AH value was taken from the trailing end of the object. The highest HUAT AH value among the passes of each object was taken as the final value. If the leading edge of the object reached the ballistic gel without any spontaneous movement, it was given a HUAT AH value of 0.

Among the 28 objects tested, 13 objects did not have a HUAT AH value in the experimental setup. These were gold (22K), jade (Burmese jadeite), white gold, salbutamol inhaler, MRI contrast bottle and all older and special editions Singapore coinage (including Straits Settlements silver half-dollar and LKY100 $10 coin). The third-series coins demonstrated large HUAT AH values. While the apple juice can did not demonstrate any HUAT AH, we could feel resistance of rotational movement on the can while trying to retrieve it.

For the objects with HUAT AH [Figure 2 and Supplemental Digital Video], the lowest values were noted in the cannulas at 19 cm and 20 cm, followed by the two android mobile phones at 35 cm (Neffos) and 44 cm (Sony). The ball point pen came in at 51 cm, the new surgical mask at 74 cm and the TraceTogether token at 105 cm. The third-series Singapore coins came in at between 119 cm and 124 cm. The paperclip came in at 145 cm and the ‘Swiss Army knife’ at 162 cm. Interestingly, the largest HUAT AH was for the can of Lee Pineapple Juice at 176 cm.

Figure 2.

Schematic diagram of the magnetic resonance scanner and the Huge Unintended Acceleration Towards Actual Harm (HUAT AH) values of some objects. Values of the tested objects were plotted to scale along the launch axis from which the objects were attracted towards the scanner bore. Objects that did not have a HUAT AH value are not included in the diagram.

Among all the objects tested in this experiment, there was no penetration of the surface of the ballistic gel. Mild indentation was noted on the gel edge by the third-series coins [Figure 3] and the rim of the pineapple juice can. None of the tested objects developed sufficient force to overcome the frictional resistance of ONG LAI against the MRI scanner bed.

Figure 3.

Photograph show third-series Singapore coins (10-cent, 20-cent, 50-cent, 1-dollar) indenting but not penetrating the ballistic gel at the bore of the 3T magnetic resonance imaging scanner. The 5-mm-thick acrylic guidance tubing can be seen wedged against the impact face of the ballistic gel.

The pen was still fully functional with no apparent change in writing quality. Both android mobile phones remained fully functional with no apparent defects. The TraceTogether token could not be evaluated due to the lack of specialist equipment. The MRI scanners remained fully functional with no discernible effects after the experiments, even on very sensitive sequences such as diffusion tensor imaging and fibre tractography [Figure 4].

Figure 4.

Axial diffusion tensor imaging of the brain obtained using the GE SIGNA™ Premier 3T magnetic resonance imaging scanner following the Local cOinage and Hospital Equipment Index experiment. There was no discernible artefact or loss of image quality.

DISCUSSION

Gold and jade jewellery are commonly worn by our elderly patients who may forget to remove these objects before entering the scanning room. In some instances, it may not be possible to remove gold or jade jewellery such as rings and bangles, due to degenerative changes and swelling of the joints over time since the jewellery was first worn. Gold and jade were included in the study, as they were both used as drugs in ancient China.[5] Jade was also considered as a preventative treatment for renal disease and calculi by the American Indians.[6] Even in this millennium, both jade and gold are being utilised in modern medicine.[7,8,9,10] It is, therefore, reassuring to know that there is negligible risk of these objects acting as projectiles within the MRI scanner.

Coins were tested as there were anecdotal reports that they were found wedged under the scanner beds by MRI radiographers and service engineers, especially when unexplained artefacts were noted on patient examinations. It is unsurprising that the Straits Settlements silver half-dollar coin was not attracted into the scanner, as silver is not ferromagnetic. The first- and second-series Singapore coins (excluding the second-series 1-dollar coin) were made of cupronickel.[11] The second-series 1-dollar coin and the LKY100 coin were made of aluminium bronze.[11,12] Aluminium bronze has no ferromagnetic properties. While nickel demonstrates magnetic properties, cupronickel alloys utilised for coinage do not have sufficient nickel composition to confer ferromagnetic properties.[13] The third-series coins are made from plated steel[11] and as such, it is unsurprising to see their large HUAT AH values.

Despite changing into gowns and declaring they have no metallic objects on them, patients have been known to bring their TraceTogether token into the scanning room. One of these cases was only detected when the radiographers noted significant artefacts on the scan. Given the high HUAT AH value of the TraceTogether token, we could only assume the patient did feel the pull of the token but did not report the token to the radiographers performing the scan.

Mobile phones and small personal effects such as pens, inhalers and even pocketknives may be carried by patients[14] and healthcare professionals. These may be accidentally introduced into the scanning room if there were any lapses of judgement. It is interesting to note that the HUAT AH equivalent value for iPhone 6 was 64 cm,[2] while android phones had lower values of 35 cm and 44 cm. We could only postulate that this is due to the different internal construction components of the mobile phones.

Previous near misses have occurred where healthcare professionals entered the scanning room with their vascular access/medication tray and noted the needles moving within the magnetic field before carefully retreating from the rooms. We attempted to measure the distance and potential harm of cannulas commonly used in our hospital and are also reassured to note that, while the cannulas had HUAT AH values, none of them had penetrated the ballistic gel.

The surgical face mask had a HUAT AH value of 74 cm. While the surgical face mask is extremely unlikely to cause any penetrative injury, this does suggest that the seal at the nose bridge of the face mask would likely be lost if the wearer came close to the bore of the scanner, reducing the filtration and protective efficacy of the mask. While there is minimal penetrative injury risk from face masks, there was a previous report of thermal injury from the use of masks with metallic component within an MRI scanner.[15] In our hospitals, to avoid these potential risks, we utilise surgical masks with polymeric (non-metallic) nose bridge wire whenever masks are required within the MRI scanning room.

Among the two drink cans, the apple juice was in a recyclable aluminium can while the pineapple juice was in a tin can. As aluminium is good conductor of electricity, it is, therefore, likely that the resistance felt by us when trying to remove the apple juice can from the bore was caused by electromagnetic induction forming electrical eddy currents within the tin, generating localised magnetic fields during can retrieval movements, rather than ferromagnetic attraction seen with the other objects. The largest HUAT AH value we obtained during our Lo Hei study was for the empty Lee Pineapple juice can at 176 cm, but there was no penetration of the simulated human tissue, and no damage or movement of ONG LAI.

While radiology and other healthcare professionals are often heavily drilled on MRI safety with relation to ferromagnetic objects, patients attending their appointments may not fully understand the importance of MRI safety protocols, as evidenced by multiple occasions where patients sneaked in or accidently introduced smaller objects into the scanning room. An anonymised public repository to learn from near misses in a no-blame culture within our local context, similar to the US Food and Drug Administration incident reporting system,[16] could be considered to help support MRI safety education for patients and healthcare professionals.

In Singapore, one of the biggest highlights during Chinese New Year is for friends and family to come together for a Lohei session, regardless of race or culture.[17] During Lohei, friends and family crowd around a large dish, adding in vegetables, fish, condiments and other ingredients one by one while reciting auspicious phrases loudly. Everyone then mixes the ingredients together by tossing the ingredients using chopsticks, often with boisterous shouts of ‘Huat Ah’, wishing prosperity to those present. It is believed that the higher the ingredients are tossed, the better the prospects for the coming year. Fruit, such as mandarin oranges (symbolising gold) and pineapples (symbolising the arrival of fortune in Cantonese and Hokkien dialects), are often found in abundance at these celebrations.

With the standing down of safe management measures and the switch to disease outbreak response system coordination framework from yellow to green in 2023,[18] the upcoming Chinese New Year would make for a perfect opportunity for healthcare professionals to come together for Lohei to celebrate the end of the difficult COVID-19 years and to look forward to better days ahead. For a Lohei session performed near the MRI scanners for any reason, we strongly suggest that chopsticks used be limited to those of jade, wooden or bamboo origin, ‘Huat Ahs’ to be limited to verbal well wishes and the auspicious pineapple cans (especially, our traditional Lee’s brand) to be kept as far away as possible from the magnet bores.

In conclusion, this follow-up study demonstrates the ferromagnetic activity of our new local coinage and many objects that may be carried by persons or found in the vicinity of an MRI scanning room. Some of these objects demonstrate ferromagnetic activity with projectile potential from a distance of more than 1 m. Patients and healthcare professionals should be cognisant of the risks associated with introducing these objects into the MRI scanning room. While MRI has been widely considered a safe imaging modality, as no ionising radiation is involved, it may not be that ‘safe’ after all if safety protocols are not closely adhered to.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

Supplemental digital content

Supplemental video is available online.