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Thermal Injuries Associated with MRI
Clinical Radiology (2001) 56: 457±465 doi:10.1053/crad.2000.0688, available online at http://www.idealibrary.com on
Review Thermal Injuries Associated with MRI M A RY F. D E M P S E Y, B A R R I E CO N D O N Southern General Hospital, Glasgow, U.K. Received: 14 July 2000 Revised: 6 November 2000
Accepted: 17 November 2000
Most physicians are aware of the absolute contraindications to magnetic resonance imaging (MRI). However, less familiar is the potential for an MRI-induced thermal or electrical burn associated with electrical monitoring devices. Although detailed studies concerning the burn hazard in MRI have not been reported, it is widely believed that direct electromagnetic induction in looped cables associated with the patient is responsible for the excessive heating and it is on this theory that present guidelines are based. Recent reports have however indicated that other mechanisms may cause the heating of metal, either in or on the patient. This document reviews numerous reported burn injuries sustained during MRI and addresses the underlying heating mechanisms possibly causing these events. Dempsey, M.F. and # 2001 The Royal College of Radiologists Condon, B. (2001). Clinical Radiology 56, 457±465 Key words: MRI, burns, thermal injuries, hazard.
There is unquestionable evidence of clinical magnetic resonance imaging-(MRI)-related reports of patient burns (thermal injuries/incidents) that strongly indicates the need for increased awareness, education and understanding concerning this rare, but real, MRI-related hazard. Burns to patients during MRI procedures are rare and are often ascribed to high currents being induced in metal loops in patient support equipment, particularly electrocardiogram (ECG) electrodes. Although there have been previous discussions regarding the various hazards associated with the use of monitoring devices during MRI procedures, and recommendations have been provided to prevent injuries, patients continue to be burned in the MRI environment. Due to the nature of these incidents, there are diculties obtaining details of the burns. Reports are often based on information collected through safety surveys or from the U.S. Food and Drug Administration's (FDA) medical products reporting system program MedWatch. Hence, while burn incidents are frequently referred to, and recommendations provided, there is little information oered about the burn received or the circumstances of the event. Editorial letters describing incidents occurring in the author's institution, together with published case reports Author for correspondence and guarantor of study: Mary F. Dempsey, Department of Nuclear Medicine, Western In®rmary, Dumbarton Road, Glasgow, G11 6NT, U.K. Fax: 44(0)141 211 6344. 0009-9260/01/060457+09 $35.00/0
and the FDA medical devices alert database provide more detailed accounts and have been the main information sources of the incidents described below. Much of the information has come from FDA `Centre For Devices and Radiological Health' medical device reporting ®les containing information from reports on devices, which may have malfunctioned causing a death or serious injury. These ®les contain reports received under both the mandatory Medical Device Reporting System (MDR)* and the Manufacturer and User Facility Device Experience (MAUDE){ voluntary reports. In the U.K., the Medical Devices Agency (MDA){ maintains a similar adverse incident database. Also, while the number of reported incidents described within exceeds 150, the reader should note that it is possible that, worldwide, many other unreported thermal injuries may have occurred.
POSSIBLE HEATING MECHANISMS DURING MRI
The mechanism responsible for the heating causing burns is often considered to be simple direct electromagnetic induction and it is commonly believed that heating can be *Medical Device Reporting Data Files internet address:http:// www.fda.gov/cdrh/mdr®le.html {Manufacturer and User Facility Device Experience internet address: http://www.fda.gov/cdrh/maude.html {Medical Devices Agency internet address: http://www.medicaldevices.gov.uk # 2001 The Royal College of Radiologists
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Table 1 ± Summary of FDA burn reports associated with ECG monitoring Burn description
Frequency and severity
Suggested causes
Burn area underneath electrode 84 FDA reports 19 third-degree, 10 second-degree and 55 severity not speci®ed
Formation of cable loop Close proximity of cable to inside surface of MR bore wall Electrode characteristics Large patients Higher power investigations
Burn on patient's ®nger
1 FDA report
ECG cable in contact with patient's ®nger
Burn at sites of ECG cable
1 FDA report
Loop formation Large patient Small surface area electrodes used
minimized by ensuring cables are not looped and electrodes are kept as close together as practicable. However, heating can be evoked by the time-varying magnetic ®elds used in MRI by several mechanisms:
Electromagnetic Induction Heating A change in the ¯ux of the magnetic induction through a ®xed circuit gives rise to an electromotive force (EMF) which lasts as long as the ¯ux is changing. The magnitude of this induced EMF is proportional to the rate of change of ¯ux with time [1]. Eddy currents are induced by the changing magnetic ®eld and result in joule heating of the conducting specimen [2].
Heating in a Resonant Circuit Maximum electromagnetic induction heating occurs when the circuit is in a resonant condition, resulting in induction of the maximum current [2]. A conducting coil exposed to time-varying magnetic ®elds is equivalent to an electric circuit having an inductance (L), a capacitance (C), a resistance (R) and a voltage oscillating at an angular frequency (o). Peak current in the circuit occurs when the circuit is in a resonant condition (o=1/(L/C)1/2).
Heating Due to the Antenna Eect When considering current induction in lengths of cable, the cable can be considered as a radiofrequency (RF) wire antenna. This type of antenna has a larger sensitivity for the electric component than for the magnetic component of the RF radiation. The additional electric ®eld induced by the currents in this antenna has a maximum ®eld line density at the antenna tip [3]. The following categories of incident will be reviewed: (1) burns associated with ECG monitoring; (2) burns associated with pulse oximeter monitoring; (3) burns associated with imaging coils; (4) Other burns in MRI. BURNS ASSOCIATED WITH ECG MONITORING
The nature of ECG leads makes it dicult to dierentiate between heating due to the antenna eect and heating due
to direct electromagnetic induction, as both are possible. A single lead at the resonant length may act as an antenna, however, two or more ECG leads could also form a conductive circuit and therefore be subject to direct electromagnetic induction. Most of the information regarding burns received during MRI studies associated with ECG monitoring has been collected from the FDA data ®les and these are summarized in Table 1 (reference keys for these reports are provided in Table 2). Generally, these reports do not oer a conclusive explanation of the event, however, some describe a series of possible contributory factors. As will be discussed, there are thought to be various possible contributory factors to these incidents and it is the purpose of this report to bring these aspects to the attention of the reader. In the majority of the cases shown in Table 1, burns have been received beneath one, some or all of the electrodes involved. Several of these reports indicated that the formation of an unintentional loop in the ECG cable would permit currents to be induced in the patient cable which may have resulted in thermal injury. Often, the reports indicated that loops are contraindicated in the product labelling. However, it should be noted that while only three of these reports mention that a cable loop was present, 13 of the reports indicated that the sta present were unaware of the formation of a loop. If this was indeed the case, 13 of these burn injuries could not be explained by the direct electromagnetic induction theory. Another suggested cause is thought to be that of the ECG cable being in close proximity to the inside surface wall of the MRI bore. RF-induced electromotive forces (EMFs) have been shown to be aected by ®eld inhomogeneity [4,5]. In the former study, the magnitude of the RF ®eld (B1) near the head coil was shown to increase symmetrically as a function of the distance from the central axial plane with a gain of approximately 60% near the edges of the coil. However, there are various electrode characteristics to be noted that could possibly play a role in these events: 1. Reddening or blistering under electrodes may be caused by allergic reactions to the electrodes or the electrode gel involved. 2. ECG electrodes being dry with no electrolyte gel ± permitting arcing of high current density (wet-gel
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THERMAL INJURIES ASSOCIATED WITH MRI
Table 2 ± Reference keys for FDA burn reports associated with ECG monitoring Medical device reporting (MDR) system ®les
Manufacturer and user facility device experience (MAUDE) reports
Key
Access number
Key
Access number
Key
MDR Report Key
Key
MDR report key
A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11 A12 A13 A14 A15 A16 A17 A18 A19 A20 A21 A22 A23
M268872* M406644 M435515 M442369 M591291 M709420 M709484 M710558 M733308 M734079* M734823 M737573 M741896 M742225 M743835 M749968 M749968 M757129 M757130 M757485 M758136 M759134 M765635*
A24 A25 A26 A27 A28 A29 A30 A31 A32 A33 A34 A35 A36 A37 A38 A39 A40 A41 A42 A43 A44 A45
M802581* M803804 M807051 M813378 M824983* M826647 M850232 M850681* M852129 M852688 M855463 M856923 M863718 M867068* M867582* M868235 M868237 M869962 M872898* M881147 M881148 M883924*
B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14 B15 B16 B17 B18 B19 B20 B21 B22 B23
26436 29312 33126 41501 42444 44811* 47973 48347 57003* 57135* 57434 57480 57539* 58368 61949 62465 63864 65794 66394 67603 72377 80358 81295
B24 B25 B26 B27 B28 B29 B30 B31 B32 B33 B34 B35 B36 B37 B38 B39 B40 B41 B42 B43
82476 85527 87150* 88797* 97005 97567 107438 110164 110167 132590 134050* 134185 141722* 143580 143589 145011 157094 192559 214364 215549
*Indicating patients described to have proportions larger than average.
electrodes are recommended to provide good skin contact). 3. Loosened or partially contacting ECG electrode ± permitting conduction of a high current density (over the small electrode surface area remaining in contact with the skin). 4. Use of ECG electrodes with smaller skin/electrode contact surface area ± again, high current density. 5. RF heating of the electrode gel. Some of the reports in Table 1 make a point of mentioning that the patient appeared diaphoretic (sweaty) on removal from the MR system ± this may be important as it may result in poor electrode skin contact ( permitting conduction of high current over small surface area). These reports do not always describe the patient involved in detail, however, several of the reports indicate that the patients involved were of large proportions (this was speci®ed in 20% of the accounts, as indicated in Table 2) and that this may be a contributory factor to the incident. Imaging of large patients may have several consequences: 1. High power RF investigation may be required ( possibly near the maximum levels) ± as the RF amplitude is increased (i.e. increasing RF power), increased current is induced in the monitoring cable. 2. Large patients are more likely to have a body part (e.g. hips or shoulders) in contact with the inside, electrically insulated, surface of the MRI bore wall. Contact of patient's skin or body part is reportedly contraindicated in some MRI product labelling. It is not clear why this should increase the risk of thermal injury, unless this allows increased RF energy to be coupled with the
patient's body or patient proximity to the RF coils is an important factor due to direct heat conduction. 3. The patient ECG cable is more likely to be in close proximity to the inside surface of the MR bore wall ± further explanation as to why this could increase the risk of injury shall follow. Additionally, many of the reported events involved imaging of the lumbar spine, an MRI examination which typically produces higher power levels and potentially increased current induction in the cable. In the majority of these incidents, subsequent biomedical inspection revealed no malfunction of the monitor, cable or electrodes. Heat damage was indicated in just a few of the reports. Of these, there are ®ve reports of patient cable ECG attachment clips being damaged by the heat [A7, A10, A21, B26, B31], three reports of electrodes being partially damaged by the heat [A13, B8, B31], and two reports of a patient ECG cable being damaged [B33, B39]. One of the ECG burn incidents involved a female patient undergoing a lumbar spine examination [A24]. This patient received second-degree burns under two of the four ECG electrodes and she was also reported to have received a small burn on her left hand ring ®nger where the ECG cable had passed by her hand. In this case, the operator had been able to position the cable in such a manner that loop formation could not occur which indicated that this burn must have been a consequence of the antenna eect. There are also three other reports of burns at the sites of the ECG cables [B6, B33, B40]. An interesting account involved an adult female patient undergoing a series of lumbar/cervical spine examinations [B33]. On completion of the examination, four skin blisters were observed on the
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patient chest where the ECG electrodes had been positioned. Additionally, a line approximately 24 inches long was observed beneath where the ECG cable had been positioned. This line was approximately 1 cm wide, and considered to be a second-degree burn area. An ECG cable loop was described as being in place across the abdomen, with evidence of this being observed where the line burn area was described by the operator. Paediatric (i.e. small surface area) ECG electrodes were used on at least one of the electrode sites and may have contributed to the burns. The patient was described as large (175 lbs) which may also have been a contributory factor. A case report in the literature discussed MRI burns associated with ECG monitoring [6]. This involved two patients undergoing MRI of the lumbar spine in a 1.5 Tesla unit. Cardiopulmonary monitoring was undertaken for both patients with equipment speci®cally designed for the MRI setting. Three electrodes were attached to these patients' chests. In both of these studies, at the end of the procedure, examination of the chest showed full-thickness burns to areas where the electrodes had been attached. The case reports inferred that current induction in an electrically conductive loop intercepting a changing magnetic ¯ux resulted in either a thermal or electrical burn. The former injury would result from heating of the skin and subsequent thermal radiation, whereas the latter may follow if the body tissues complete an electrical circuit with the wires as a result of capacitive coupling at the epidermis.
Thus, while these numerous reports provide support for the theory that looping in cables may be responsible for these burns, there is a strong indication that heating due to both the resonant eect and the antenna eect may also be playing a critical role in these incidents. BURNS ASSOCIATED WITH PULSE OXIMETRY
Contrary to ECG cabling, in which positioning of the leads may cause ambiguity in knowing whether the antenna eect or electromagnetic induction is the cause of excessive heating, pulse oximeter leads do not form large area circuits (unless the cable is looped) and therefore, it is likely that the antenna eect would be the cause of the following events. There are 36 reports in the FDA data ®les of burns associated with pulse oximeter monitoring during MRI, as summarized in Table 3 (reference keys for these reports are provided in Table 4). Only three of the above burn incidents involved the toe [C13, C17, C20], however, these reports failed to specify the area under investigation and, thus, it cannot be presumed that the sensor was outside of the RF transmit volume. There are also several reports in the wider literature regarding MRI burns associated with the use of pulse oximetry. The ®rst report involved a 59-year-old woman having an MRI evaluation of the thoracic spine in a 1.5 Tesla unit [7].
Table 3 ± Summary of FDA burn reports associated with pulse oximetry Burn description
Frequency and severity
Suggested causes
Burn to patient's ®nger
32 FDA reports* (4 third-degree, 18 second-degree burns and 10 severity not speci®ed)
Burns to patient's toe
4 FDA reports
Pulse oximeter within RF transmit volume High power scans Large patients
*One such burn was reported to have been of such severity as to require amputation of the ®nger [C2].
Table 4 ± Reference keys for FDA burn reports associated with pulse oximetry Medical device reporting (MDR) system ®les
Manufacturer and user facility device experience (MAUDE) reports
Key
Access Number
Key
Access Number
Key
MDR report key
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15
M291515 M297255 M316791 M366275 M371366 M371724 M372364 M376141 M376142 M376144 M376146 M376147 M376148 M376149 M376150
C16 C17 C18 C19 C20 C21 C22 C23 C24 C25 C26 C27 C28 C29
M376151 M376152 M409523 M442144 M455616 M487309 M487310 M525874 M525913 M570596 M592683 M702752 M729534 M742691
D1 D2 D3 D4 D5 D6
840 12152 14555 14556 16327 92511
Key
MDR report key
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THERMAL INJURIES ASSOCIATED WITH MRI
summarized in Table 5 (reference keys for these reports are provided in Table 6). Several of the FDA reports indicate that imaging coils are involved in these burn injuries, however, very little information about the actual coils involved is oered and it is therefore dicult to know which physical mechanism may have caused the excessive heating.
The patient was being monitored with a pulse oximeter attached to her ®nger. A third-degree burn approximately 3 mm in diameter was noted on the distal anterior and posterior aspects of the ®nger. It was suggested that the cable may have been looped during imaging and that the gradient and/or RF magnetic ®elds could have induced sucient current to heat the probe, causing the burn. Another report of two patients describes how they suered burns while being monitored with a pulse oximeter during MRI [8]. One patient, a man who had undergone imaging of the cervical spine, sustained a full-thickness burn requiring skin grafting of the tip of the little ®nger where the pulse oximeter sensor had been placed. The second patient underwent MRI of the head and had the pulse oximeter placed on the great toe with a loop of the connecting cable taped over the leg. Afterwards, a super®cial linear burn was found where the cable had been taped to the leg. Another case report involved a 59-year-old man who received burns during cervical MRI [9]. The oximeter probe was taped to the distal right ®fth digit, with the patient's arms at his sides. Cloth was placed between the hand and the oximeter cable, which was also examined to avoid loops. On removal of the oximeter probe, it was discovered that the patient had suered full thickness burns to the ®nger and ultimately required amputation.
OTHER BURNS IN MRI
The FDA data ®les contain 28 other reports of burns associated with MRI which could not be categorized as easily as before and these are summarized in Table 7 (reference keys for these reports are provided in Table 8). Several of these incidents may have been due to the MR system itself i.e. without patient monitoring devices, local coils or other objects present. Often, no description of the burn is oered and the report merely states that the patient was treated for a burn. Again, it is therefore dicult to know the process causing the heating. Aside from the FDA data ®les, there have been other reports of unusual types of burns published. One case involved a 22-year-old woman whose head was imaged with a 1.0 Tesla unit [10]. A magnetic artefact was encountered and was determined to be due to a ferromagnetically active pigment used as permanent eyeliner. Approximately 2 h after imaging, the patient experienced painless swelling and puness of her upper eyelids that peaked approximately 4 h after imaging and subsided over the next 48 h. Pigment used for eyeliner tattoos are an inhomogeneous group with no apparent standardization
BURNS ASSOCIATED WITH IMAGING COILS
The FDA data ®les contain 23 accounts of patient burns associated with MRI RF imaging coils and these are Table 5 ± Summary of FDA burn reports associated with imaging coils Burn description
Frequency
Suggested causes
Burn to patient's arm Burn to patient's thumb and thigh Burn to patient's hand and buttock Burn to patient's neck Burn to patient's leg Burn to patient's shoulder and arm Burns to point of skin contact with coil cable Burn area not speci®ed
2 1 4 4 2 3 1 6
Shoulder coil CTL coil CTL coil Cervical spine coil Knee coil CTL coil
FDA FDA FDA FDA FDA FDA FDA FDA
reports report reports reports reports reports report reports
Table 6 ± Reference keys for FDA burn reports associated with imaging coils Medical device reporting (MDR) system ®les Key
Access number
E1 E2 E3 E4 E5 E6
M307275 M359665 M490027 M813377 M834788 M839964
Key
Manufacturer and user facility device experience (MAUDE) reports Access number
Key
MDR report key
Key
MDR report key
F1 F2 F3 F4 F5 F6 F7 F8 F9
10418 17296 26403 32679 33299 89368 131095 132647 139452
F10 F11 F12 F13 F14 F15 F16 F17
153437 171569 171588 171643 179291 199042 203223 206322
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Table 7 ± Summary of FDA burn reports associated with imaging coils Burn description
Frequency
Burns to point of skin contact with inside surface wall of MRI bore Burn to patient's buttock Burns to patient's skin to skin contact point Heating around areas of skull pins of halo device Heating around hip screw Burn to patient's ®nger Heating in left groin Burn area not speci®ed
1 FDA report 1 FDA report 4 FDA reports 8 FDA reports 1 FDA reports 1 FDA report 1 FDA report 11 FDA reports
Suggested causes
Flesh loops Hip screw device Swan-Ganz thermodilution catheter device Metal implants
Table 8 ± Reference keys for FDA burn reports ± other burns in MRI Medical device reporting (MDR) system ®les Key
Access number
G1 G2 G3 G4 G5 G6 G7
M286228 M286257 M397218 M553719 M829924 M874396 M879842
Key
Manufacturer and user facility device experience (MAUDE) reports Access number
Key
MDR report key
Key
MDR report key
H1 H2 H3 H4 H5 H6 H7 H8 H9 H10 H11
15564 18471 25392 25547 46021 76997 83152 89363 89396 93213 98734
H12 H13 H14 H15 H16 H17 H18 H19 H20 H21
109464 116336 116362 121175 127333 171164 171244 177702 189703 214933
e.g. black may consist of carbon, iron oxide or logwood. A possible explanation of this event was that whatever the tattoo substance was, it may have been acting as a conductor causing increased energy deposition or heating in adjacent soft tissue. It is worth noting that in the safety manual of at least one MRI system, manufacturer warnings exist of some cosmetics containing ferromagnetic particles that can cause irritation to the patient under the in¯uence of a strong magnetic ®eld [11]. Also, the use of the system is contraindicated for patients who have undergone permanent eyelining, unless the physician can be certain that the product is not magnetically active. Another report of a burning sensation associated with MRI involved a 51-year-old woman with a breast implant [12]. The breast expander was made of a medical grade silicone with a metal-backed injection portal. Soon after the study began, the patient complained of a burning sensation over the area of the expander injection port in the region of the right chest. The burning swiftly subsided after the examination was aborted. The patient had never experienced any symptoms of burning before the MRI examination. It was thought that the conducting path between the needle strip in the injection port of the expander and the metallic casing of the venous access disk may have led to enough local heating for the patient to perceive a burning sensation. There is a further report in which a patient received a second-degree burn from the use of a temperature probe speci®cally designed for use during MRI [13]. In this case, a
10-year-old patient was undergoing MRI of the lumbosacral spine during general anaesthesia. Monitoring included an axillary skin probe. At the end of the 2-h procedure, examination of the axillary area revealed a large seconddegree burn, but no fault was detected in the probe. Although neither fraying nor looping of the cable was observed, it was thought that movement of the bed may have permitted formation of a loop in the cable.
PUBLISHED INVESTIGATIONS OF HEATING IN MRI
Little experimental work has been published discussing the physics involved in these burn incidents and therefore the cause of these injuries remains theoretical. However, various authors have investigated heating eects of conductive media during MRI. These published investigations of heating in MR have primarily been concerned with conductive implants and devices. Cochlear implants have been investigated, with a maximum temperature rise of less than 1 8C being measured [14]. Similarly, another investigation studied the heating eects of large, thin aluminium sheets with a temperature rise of 0.08 8C being measured [15]. The heating eects of changing magnetic ®elds and RF ®elds on small metallic implants (surgical clips and prostheses) was also studied with no ®ndings of signi®cant temperature rise [16]. Additionally, vascular access ports (IVAPs) and catheters have been examined; temperature changes ranged from no change (0 8C) to 0.3 8C. This relatively minor tempera-
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THERMAL INJURIES ASSOCIATED WITH MRI
ture increase associated with MR imaging of the IVAPs was not considered to be a potential hazard for patients [17]. Another investigation examined patient safety when recording electroencephalograms (EEG) during functional MRI ( fMRI) experiments [4]. This study concluded that voltages induced in loops formed by EEG wire resulting from the RF pulses were the single most important risk factor in EEG/fMRI. While necessary precautions were speci®ed, temperature measurements showed heating of the electrodes to be negligible. Temperature changes in nickelchromium intracranial depth electrodes during MRI scanning was also investigated [18]. Again, no signi®cant temperature increase was measured in these experiments. Thus, various investigations of heating eects of biomedical implants have found no cause for concern when considering temperature elevation. However, another investigation of the RF heating of an implanted spinal fusion stimulator found a maximum temperature rise of 2 8C [19]. In the same study, when one of the generator leads was broken at the middle section, the temperature rise was as high as 11 8C (the lead was placed inside a human phantom model, lead length not speci®ed). It was thought that the RF current at the tip of the broken electrode caused excessive heating. When the implant was intact, the RF current emanated through the entire length of the bare electrode and no signi®cant heating of the conducting electrode was observed. Also, a real-time temperature measurement of heating under ECG electrodes during MR imaging at 1.5 Tesla found a peak temperature increase of 12 8C [5]. In this study, the cable position for peak heating of the electrode was the bore wall. When considering electromagnetic induction (summarized previously), the geometrical con®guration of the conductive media de®nes its inductance. Thus, looping a wire will increase the inductance and therefore, it is commonly thought that the pulsed magnetic gradient ®elds and pulsed radiofrequency ®elds used in MRI may have induced sucient current in an electrically conductive loop to result in these thermal or electrical burn injuries [6,7± 9,20±22]. The former injury resulting from heating of the wire, whereas the latter may follow if tissues complete an electric circuit with the wire as a result of capacitive coupling at the epidermis. Wires that form a part of a conductive pathway, having a large enclosed area (and hence cutting many pulsed magnetic ¯ux lines), may allow higher current induction. Conductive loops may form by one of the following mechanisms: exposed lead wire in contact with patient; two leads in direct electrical contact; a single wire bending on itself; and current being able to ¯ow through the insulation at RF frequencies. The orientation of the loop is also important ± a loop lying in the plane perpendicular to the direction of the RF ®eld will cut the maximum amount of ¯ux lines and, therefore, have the maximum current induced. Although currents are induced in the human body under normal imaging conditions, conducting loops provide a concentration of current and therefore a high current density in the tissue under an electrode.
In MRI, only the magnetic radiation of the RF is needed to generate the magnetic resonance eect. Therefore, coils are designed to achieve a spatially homogeneous magnetic ®eld. The electric component of the RF ®eld is spatially inhomogeneous and low around the coil centre to minimize dielectric losses. This has implications if monitoring cables are considered as RF wire antennae. A safety investigation of transoesophageal cardiac pacing during MRI studied heating of the electrodes and pacing wire; when using a high RF radiation exposure protocol (higher than allowed in clinical MRI by regulation), the measured temperature increase was 36 8C [20]. Similarly, temperature rises exceeding 15 8C were measured while studying the heating eects of MRI on pacemaker leads [23]. In the same study, under special con®gurations, a maximal temperature increase of as much as 69 8C was reported. Other investigations of the safety of vascular guidewires revealed temperature rises of up to 17 8C [24] and 14 8C [25]. In one investigation of RF heating of actively visualized catheters and guidewires [26], heating was found to be due to electric ®eld coupling with the coaxial cable, and not due to magnetic ®eld coupling with the RF coil itself. The results of this study showed heating to be a function of many variables, including static magnetic ®eld strength, coil geometry, cable length, placement of the cables in the machine and the substance in which the device was immersed. Maximum temperature changes of around 20 8C were recorded. A further investigation of guidewire heating measured a temperature increase of up to 72 8C [27], depending primarily on guidewire length and tip position. Thus, with such high temperature rises being recorded in these studies (and such relatively low temperature elevations being reported for experiments involving direct induction), it is possible that extended wires (as used with monitoring devices) may cause thermal injury as a result of this antenna eect, rather than being a result of, what is more commonly reported, electromagnetic induction eect. Additionally, of the burns that have been attributed to cable looping, it must be remembered that the actual heating may be a result of resonance rather than direct electromagnetic induction. This hypothesis is under investigation at this centre.
DISCUSSION
Multiple reports of burn incidents and heating investigations of conductive media in MRI have been reviewed. While the direct electromagnetic induction theory cannot be excluded, strong support is provided to indicate that the resonant circuit and antenna eect may be involved in these burns. For example, the burn incidents in which the monitoring cables were not looped can only be explained by the antenna eect. Due to the incidence of these thermal injuries, various guidelines } have been published to prevent burns in the }Safety information is also available on Kanal's MR safety site: http://kanal.arad.upmc.edu/MR Safety
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MRI setting [6,7±9,20±22,28±31]. It is prudent that any discussion of burns associated with MRI should also mention the present guidelines. The main guidelines include taking special care and precautions to ensure that no loops are formed with conductive materials and the patient's tissues. Additionally, if possible, no potential conductors should touch the patient at more than one location, in order to decrease the chance of formation of a conductive loop that might involve the patient's tissues. All potential conductors should be checked before use on each patient to ensure the absence of frayed insulation, exposed wires and other hazard. Also, all electrically conductive material within the bore of the imaging system, that is not required for the study, should be removed from the bore before imaging. If possible, it is recommended that insulating material (such as air) should be placed between the wires coming away from the magnet system and the patient so that they do not come into direct contact. These preventative guidelines are based mainly on the assumption that direct electromagnetic induction is causing the excessive heating, despite that, to the authors' knowledge, there is no experimental evidence supporting this theory. However, as this theory cannot be excluded, it remains important to adhere to the present guidelines (this should also reduce the risk of achieving resonance). Additionally, if the antenna eect is involved in these thermal injuries, additional guidelines may be appropriate. For example, heating as a consequence of the antenna eect is a function of many variables such as cable length, position of cable in the scanner and the substances surrounding the cable and these factors should all be given consideration.
CONCLUSION
There have been many incidents of patients receiving thermal injuries while undergoing investigation employing MRI. The majority of these burns have been received while the patient is connected to some form of physiological monitoring device. Commonly, it has been suggested that electromagnetic induction currents in the monitoring cables are largely the cause of these events. However, in reviewing the many studies of temperature changes in conductive media in the literature, it also appears possible that these burn incidents could be a result of extended wires used in physiological monitoring behaving as antennae and capturing the electric ®eld component of the RF pulses used for imaging. If this is the case, additional safety guidelines may be appropriate. What is most evident is that further investigation of the underlying physics is required to allow preventative guidelines to be produced based on experimental ®ndings rather than theory. REFERENCES 1 Page L, Adams NI. Principles of Electricity: An Intermediate Text in Electricity and Magnetism, 2nd Ed. Princeton: D. Van Nostrand Company Inc, 1949.
2 Kraus JD, Carver KR. Electromagnetics, 2nd Ed. New York: McGraw Hill Book Company, 1973. 3 Balaris CA. Antenna Theory: Analysis and Design. John Wiley & Sons, 1982. 4 Lemieux L, Allen PJ, Franconi F, Symms MR, Fish DR. Recording of EEG during fMRI experiments: patient safety. Magn Reson Med. 1997;38:943±952. 5 Felmlee JP, Rochester MN, Hokanson DA, Zink FE, Perkins WJ. Real-time temperature measurement of heating under electrocardiographic electrodes during MR imaging at 1.5 Tesla. Radiology. 1995;197(Suppl.): 423, abstract. 6 Jones S, Jae W, Alvi R. Burns associated with electrocardiographic monitoring during magnetic resonance imaging. Burns. 1996;22:420±421. 7 Shellock FG, Slimp GL. Severe burn of the ®nger caused by using a pulse oximeter during MR imaging [Letter]. Am J Roentgenol. 1989;153:1105. 8 Bashein G, Syrovy G. Burns associated with pulse oximetry during magnetic resonance imaging [Letter]. Anesthesiology. 1991;75: 382±383. 9 Brown TR, Goldstein B, Little J. Severe burns resulting from magnetic resonance imaging with cardiopulmonary monitoring. Risks and relevant safety precautions. Am J Phys Med Rehabil. 1993;72:166±167. 10 Jackson JG, Acker JD. Permanent eyeliner and MR imaging [Letter]. Am J Roentgenol. 1987;149:1080. 11 Siemens ?. Magnetom SP/Magnetom SP4000 Version A2.5/Version A2.7, Part I: Registers A-K, Siemens AG, 1989. (Order no.: C2020.01.05.02.). 12 Duy FJ, May JW. Tissue expanders and magnetic resonance imaging: the `hot' breast implant. Ann Plast Surg. 1995;35:647±649. 13 Hall SC, Stevenson GW, Suresh S. Burn associated with temperature monitoring during magnetic resonance imaging [Letter]. Anesthesiology. 1992;76:152. 14 Teissl C, Kremser C, Hochmair ES, Hochmair-Desoyer IJ. Magnetic resonance imaging and cochlear implants: compatibility and safety aspects. J Magn Reson Imaging. 1999;9:26±38. 15 Buchli R, Boesiger P, Meier D. Heating eects of metallic implants by MRI examinations. Magn Reson Med. 1988;7:255±261. 16 Davis PL, Crooks L, Arakawa M, McRee R, Kaufman L, Margulis AR. Potential hazards in NMR imaging: heating eects of changing magnetic ®elds and RF ®elds on small metallic implants. Am J Roentgenol. 1981;137:857±860. 17 Shellock FG, Shellock VJ. Vascular access ports and catheters: ex vivo testing of ferromagnetism, heating, and artifacts associated with MR imaging. Magn Reson Imaging. 1996;14:443±447. 18 Zhang J, Wilson CL, Levesque MF, Behnke EJ, Lufkin RB. Temperature changes in nickel-chromium intracranial depth electrodes during MR scanning. Am J Neuroradiol. 1993;14: 497±500. 19 Chou CK, McDougall JA, Chan KW. RF heating of implanted spinal fusion stimulator during magnetic resonance imaging. IEEE Trans Biomed Eng. 1997;44:367±373. 20 Hofman MB, de Cock CC, van der Linden JC, et al. Transesophageal cardiac pacing during magnetic resonance imaging: feasibility and safety considerations. Magn Reson Med. 1996;35: 413±422. 21 Hazard ± thermal injuries and patient monitoring during MRI studies. ECRI problem reporting system. Health Devices. 1991;20: 362±363. 22 Kanal E. An overview of electromagnetic safety considerations associated with magnetic resonance imaging. Ann NY Acad Sci. 1992;649:204±224. 23 Luechinger R, Duru F, Scheidegger MB, Candinas R, Boesiger P. Heating eects of MRI on pacemaker leads: eect of lead types and positioning. Proceedings of the ISMRM, 7th Scienti®c Meeting. 1999;103. 24 Ladd ME, Zimmermann GG, Quick HH, et al. Active MR visualization of a vascular guidewire in vivo. J Magn Reson Imaging. 1998;8:220±225. 25 Liu C-Y, Farahani K, Lu DSK, Shellock FG. Safety of MRIguided endovascular guidewire applications. Proceedings of the ISMRM, 7th Scienti®c Meeting. 1999;1015.
THERMAL INJURIES ASSOCIATED WITH MRI
26 Ladd ME, Quick HH, Boesiger P, McKinnon GC. RF heating of actively visualised catheters and guidewires. Proceedings of the ISMRM, 6th Scienti®c Meeting. 1998;473. 27 Konings MK, Bakker CJG. Intolerable heating by resonating RF waves around guidewires. Proceedings of the ISMRM, 7th Scienti®c Meeting. 1999;1004. 28 Kanal E, Shellock FG. Patient monitoring during clinical MR imaging. Radiology. 1992;185:623±629.
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29 Kanal E, Shellock FG, Talagala L. Safety considerations in MR imaging. Radiology. 1990;176:593±606. 30 Shellock FG, Kanal E. Burns associated with the use of monitoring equipment during MR procedures [Letter]. J Magn Reson Imaging. 1996;6:271±272. 31 Shellock FG, Kanal E. Magnetic Resonance: Bioeects, Safety, and Patient Management. New York: Raven Press, 1994.
Review Thermal Injuries Associated with MRI M A RY F. D E M P S E Y, B A R R I E CO N D O N Southern General Hospital, Glasgow, U.K. Received: 14 July 2000 Revised: 6 November 2000
Accepted: 17 November 2000
Most physicians are aware of the absolute contraindications to magnetic resonance imaging (MRI). However, less familiar is the potential for an MRI-induced thermal or electrical burn associated with electrical monitoring devices. Although detailed studies concerning the burn hazard in MRI have not been reported, it is widely believed that direct electromagnetic induction in looped cables associated with the patient is responsible for the excessive heating and it is on this theory that present guidelines are based. Recent reports have however indicated that other mechanisms may cause the heating of metal, either in or on the patient. This document reviews numerous reported burn injuries sustained during MRI and addresses the underlying heating mechanisms possibly causing these events. Dempsey, M.F. and # 2001 The Royal College of Radiologists Condon, B. (2001). Clinical Radiology 56, 457±465 Key words: MRI, burns, thermal injuries, hazard.
There is unquestionable evidence of clinical magnetic resonance imaging-(MRI)-related reports of patient burns (thermal injuries/incidents) that strongly indicates the need for increased awareness, education and understanding concerning this rare, but real, MRI-related hazard. Burns to patients during MRI procedures are rare and are often ascribed to high currents being induced in metal loops in patient support equipment, particularly electrocardiogram (ECG) electrodes. Although there have been previous discussions regarding the various hazards associated with the use of monitoring devices during MRI procedures, and recommendations have been provided to prevent injuries, patients continue to be burned in the MRI environment. Due to the nature of these incidents, there are diculties obtaining details of the burns. Reports are often based on information collected through safety surveys or from the U.S. Food and Drug Administration's (FDA) medical products reporting system program MedWatch. Hence, while burn incidents are frequently referred to, and recommendations provided, there is little information oered about the burn received or the circumstances of the event. Editorial letters describing incidents occurring in the author's institution, together with published case reports Author for correspondence and guarantor of study: Mary F. Dempsey, Department of Nuclear Medicine, Western In®rmary, Dumbarton Road, Glasgow, G11 6NT, U.K. Fax: 44(0)141 211 6344. 0009-9260/01/060457+09 $35.00/0
and the FDA medical devices alert database provide more detailed accounts and have been the main information sources of the incidents described below. Much of the information has come from FDA `Centre For Devices and Radiological Health' medical device reporting ®les containing information from reports on devices, which may have malfunctioned causing a death or serious injury. These ®les contain reports received under both the mandatory Medical Device Reporting System (MDR)* and the Manufacturer and User Facility Device Experience (MAUDE){ voluntary reports. In the U.K., the Medical Devices Agency (MDA){ maintains a similar adverse incident database. Also, while the number of reported incidents described within exceeds 150, the reader should note that it is possible that, worldwide, many other unreported thermal injuries may have occurred.
POSSIBLE HEATING MECHANISMS DURING MRI
The mechanism responsible for the heating causing burns is often considered to be simple direct electromagnetic induction and it is commonly believed that heating can be *Medical Device Reporting Data Files internet address:http:// www.fda.gov/cdrh/mdr®le.html {Manufacturer and User Facility Device Experience internet address: http://www.fda.gov/cdrh/maude.html {Medical Devices Agency internet address: http://www.medicaldevices.gov.uk # 2001 The Royal College of Radiologists
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CLINICAL RADIOLOGY
Table 1 ± Summary of FDA burn reports associated with ECG monitoring Burn description
Frequency and severity
Suggested causes
Burn area underneath electrode 84 FDA reports 19 third-degree, 10 second-degree and 55 severity not speci®ed
Formation of cable loop Close proximity of cable to inside surface of MR bore wall Electrode characteristics Large patients Higher power investigations
Burn on patient's ®nger
1 FDA report
ECG cable in contact with patient's ®nger
Burn at sites of ECG cable
1 FDA report
Loop formation Large patient Small surface area electrodes used
minimized by ensuring cables are not looped and electrodes are kept as close together as practicable. However, heating can be evoked by the time-varying magnetic ®elds used in MRI by several mechanisms:
Electromagnetic Induction Heating A change in the ¯ux of the magnetic induction through a ®xed circuit gives rise to an electromotive force (EMF) which lasts as long as the ¯ux is changing. The magnitude of this induced EMF is proportional to the rate of change of ¯ux with time [1]. Eddy currents are induced by the changing magnetic ®eld and result in joule heating of the conducting specimen [2].
Heating in a Resonant Circuit Maximum electromagnetic induction heating occurs when the circuit is in a resonant condition, resulting in induction of the maximum current [2]. A conducting coil exposed to time-varying magnetic ®elds is equivalent to an electric circuit having an inductance (L), a capacitance (C), a resistance (R) and a voltage oscillating at an angular frequency (o). Peak current in the circuit occurs when the circuit is in a resonant condition (o=1/(L/C)1/2).
Heating Due to the Antenna Eect When considering current induction in lengths of cable, the cable can be considered as a radiofrequency (RF) wire antenna. This type of antenna has a larger sensitivity for the electric component than for the magnetic component of the RF radiation. The additional electric ®eld induced by the currents in this antenna has a maximum ®eld line density at the antenna tip [3]. The following categories of incident will be reviewed: (1) burns associated with ECG monitoring; (2) burns associated with pulse oximeter monitoring; (3) burns associated with imaging coils; (4) Other burns in MRI. BURNS ASSOCIATED WITH ECG MONITORING
The nature of ECG leads makes it dicult to dierentiate between heating due to the antenna eect and heating due
to direct electromagnetic induction, as both are possible. A single lead at the resonant length may act as an antenna, however, two or more ECG leads could also form a conductive circuit and therefore be subject to direct electromagnetic induction. Most of the information regarding burns received during MRI studies associated with ECG monitoring has been collected from the FDA data ®les and these are summarized in Table 1 (reference keys for these reports are provided in Table 2). Generally, these reports do not oer a conclusive explanation of the event, however, some describe a series of possible contributory factors. As will be discussed, there are thought to be various possible contributory factors to these incidents and it is the purpose of this report to bring these aspects to the attention of the reader. In the majority of the cases shown in Table 1, burns have been received beneath one, some or all of the electrodes involved. Several of these reports indicated that the formation of an unintentional loop in the ECG cable would permit currents to be induced in the patient cable which may have resulted in thermal injury. Often, the reports indicated that loops are contraindicated in the product labelling. However, it should be noted that while only three of these reports mention that a cable loop was present, 13 of the reports indicated that the sta present were unaware of the formation of a loop. If this was indeed the case, 13 of these burn injuries could not be explained by the direct electromagnetic induction theory. Another suggested cause is thought to be that of the ECG cable being in close proximity to the inside surface wall of the MRI bore. RF-induced electromotive forces (EMFs) have been shown to be aected by ®eld inhomogeneity [4,5]. In the former study, the magnitude of the RF ®eld (B1) near the head coil was shown to increase symmetrically as a function of the distance from the central axial plane with a gain of approximately 60% near the edges of the coil. However, there are various electrode characteristics to be noted that could possibly play a role in these events: 1. Reddening or blistering under electrodes may be caused by allergic reactions to the electrodes or the electrode gel involved. 2. ECG electrodes being dry with no electrolyte gel ± permitting arcing of high current density (wet-gel
459
THERMAL INJURIES ASSOCIATED WITH MRI
Table 2 ± Reference keys for FDA burn reports associated with ECG monitoring Medical device reporting (MDR) system ®les
Manufacturer and user facility device experience (MAUDE) reports
Key
Access number
Key
Access number
Key
MDR Report Key
Key
MDR report key
A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11 A12 A13 A14 A15 A16 A17 A18 A19 A20 A21 A22 A23
M268872* M406644 M435515 M442369 M591291 M709420 M709484 M710558 M733308 M734079* M734823 M737573 M741896 M742225 M743835 M749968 M749968 M757129 M757130 M757485 M758136 M759134 M765635*
A24 A25 A26 A27 A28 A29 A30 A31 A32 A33 A34 A35 A36 A37 A38 A39 A40 A41 A42 A43 A44 A45
M802581* M803804 M807051 M813378 M824983* M826647 M850232 M850681* M852129 M852688 M855463 M856923 M863718 M867068* M867582* M868235 M868237 M869962 M872898* M881147 M881148 M883924*
B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14 B15 B16 B17 B18 B19 B20 B21 B22 B23
26436 29312 33126 41501 42444 44811* 47973 48347 57003* 57135* 57434 57480 57539* 58368 61949 62465 63864 65794 66394 67603 72377 80358 81295
B24 B25 B26 B27 B28 B29 B30 B31 B32 B33 B34 B35 B36 B37 B38 B39 B40 B41 B42 B43
82476 85527 87150* 88797* 97005 97567 107438 110164 110167 132590 134050* 134185 141722* 143580 143589 145011 157094 192559 214364 215549
*Indicating patients described to have proportions larger than average.
electrodes are recommended to provide good skin contact). 3. Loosened or partially contacting ECG electrode ± permitting conduction of a high current density (over the small electrode surface area remaining in contact with the skin). 4. Use of ECG electrodes with smaller skin/electrode contact surface area ± again, high current density. 5. RF heating of the electrode gel. Some of the reports in Table 1 make a point of mentioning that the patient appeared diaphoretic (sweaty) on removal from the MR system ± this may be important as it may result in poor electrode skin contact ( permitting conduction of high current over small surface area). These reports do not always describe the patient involved in detail, however, several of the reports indicate that the patients involved were of large proportions (this was speci®ed in 20% of the accounts, as indicated in Table 2) and that this may be a contributory factor to the incident. Imaging of large patients may have several consequences: 1. High power RF investigation may be required ( possibly near the maximum levels) ± as the RF amplitude is increased (i.e. increasing RF power), increased current is induced in the monitoring cable. 2. Large patients are more likely to have a body part (e.g. hips or shoulders) in contact with the inside, electrically insulated, surface of the MRI bore wall. Contact of patient's skin or body part is reportedly contraindicated in some MRI product labelling. It is not clear why this should increase the risk of thermal injury, unless this allows increased RF energy to be coupled with the
patient's body or patient proximity to the RF coils is an important factor due to direct heat conduction. 3. The patient ECG cable is more likely to be in close proximity to the inside surface of the MR bore wall ± further explanation as to why this could increase the risk of injury shall follow. Additionally, many of the reported events involved imaging of the lumbar spine, an MRI examination which typically produces higher power levels and potentially increased current induction in the cable. In the majority of these incidents, subsequent biomedical inspection revealed no malfunction of the monitor, cable or electrodes. Heat damage was indicated in just a few of the reports. Of these, there are ®ve reports of patient cable ECG attachment clips being damaged by the heat [A7, A10, A21, B26, B31], three reports of electrodes being partially damaged by the heat [A13, B8, B31], and two reports of a patient ECG cable being damaged [B33, B39]. One of the ECG burn incidents involved a female patient undergoing a lumbar spine examination [A24]. This patient received second-degree burns under two of the four ECG electrodes and she was also reported to have received a small burn on her left hand ring ®nger where the ECG cable had passed by her hand. In this case, the operator had been able to position the cable in such a manner that loop formation could not occur which indicated that this burn must have been a consequence of the antenna eect. There are also three other reports of burns at the sites of the ECG cables [B6, B33, B40]. An interesting account involved an adult female patient undergoing a series of lumbar/cervical spine examinations [B33]. On completion of the examination, four skin blisters were observed on the
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CLINICAL RADIOLOGY
patient chest where the ECG electrodes had been positioned. Additionally, a line approximately 24 inches long was observed beneath where the ECG cable had been positioned. This line was approximately 1 cm wide, and considered to be a second-degree burn area. An ECG cable loop was described as being in place across the abdomen, with evidence of this being observed where the line burn area was described by the operator. Paediatric (i.e. small surface area) ECG electrodes were used on at least one of the electrode sites and may have contributed to the burns. The patient was described as large (175 lbs) which may also have been a contributory factor. A case report in the literature discussed MRI burns associated with ECG monitoring [6]. This involved two patients undergoing MRI of the lumbar spine in a 1.5 Tesla unit. Cardiopulmonary monitoring was undertaken for both patients with equipment speci®cally designed for the MRI setting. Three electrodes were attached to these patients' chests. In both of these studies, at the end of the procedure, examination of the chest showed full-thickness burns to areas where the electrodes had been attached. The case reports inferred that current induction in an electrically conductive loop intercepting a changing magnetic ¯ux resulted in either a thermal or electrical burn. The former injury would result from heating of the skin and subsequent thermal radiation, whereas the latter may follow if the body tissues complete an electrical circuit with the wires as a result of capacitive coupling at the epidermis.
Thus, while these numerous reports provide support for the theory that looping in cables may be responsible for these burns, there is a strong indication that heating due to both the resonant eect and the antenna eect may also be playing a critical role in these incidents. BURNS ASSOCIATED WITH PULSE OXIMETRY
Contrary to ECG cabling, in which positioning of the leads may cause ambiguity in knowing whether the antenna eect or electromagnetic induction is the cause of excessive heating, pulse oximeter leads do not form large area circuits (unless the cable is looped) and therefore, it is likely that the antenna eect would be the cause of the following events. There are 36 reports in the FDA data ®les of burns associated with pulse oximeter monitoring during MRI, as summarized in Table 3 (reference keys for these reports are provided in Table 4). Only three of the above burn incidents involved the toe [C13, C17, C20], however, these reports failed to specify the area under investigation and, thus, it cannot be presumed that the sensor was outside of the RF transmit volume. There are also several reports in the wider literature regarding MRI burns associated with the use of pulse oximetry. The ®rst report involved a 59-year-old woman having an MRI evaluation of the thoracic spine in a 1.5 Tesla unit [7].
Table 3 ± Summary of FDA burn reports associated with pulse oximetry Burn description
Frequency and severity
Suggested causes
Burn to patient's ®nger
32 FDA reports* (4 third-degree, 18 second-degree burns and 10 severity not speci®ed)
Burns to patient's toe
4 FDA reports
Pulse oximeter within RF transmit volume High power scans Large patients
*One such burn was reported to have been of such severity as to require amputation of the ®nger [C2].
Table 4 ± Reference keys for FDA burn reports associated with pulse oximetry Medical device reporting (MDR) system ®les
Manufacturer and user facility device experience (MAUDE) reports
Key
Access Number
Key
Access Number
Key
MDR report key
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15
M291515 M297255 M316791 M366275 M371366 M371724 M372364 M376141 M376142 M376144 M376146 M376147 M376148 M376149 M376150
C16 C17 C18 C19 C20 C21 C22 C23 C24 C25 C26 C27 C28 C29
M376151 M376152 M409523 M442144 M455616 M487309 M487310 M525874 M525913 M570596 M592683 M702752 M729534 M742691
D1 D2 D3 D4 D5 D6
840 12152 14555 14556 16327 92511
Key
MDR report key
461
THERMAL INJURIES ASSOCIATED WITH MRI
summarized in Table 5 (reference keys for these reports are provided in Table 6). Several of the FDA reports indicate that imaging coils are involved in these burn injuries, however, very little information about the actual coils involved is oered and it is therefore dicult to know which physical mechanism may have caused the excessive heating.
The patient was being monitored with a pulse oximeter attached to her ®nger. A third-degree burn approximately 3 mm in diameter was noted on the distal anterior and posterior aspects of the ®nger. It was suggested that the cable may have been looped during imaging and that the gradient and/or RF magnetic ®elds could have induced sucient current to heat the probe, causing the burn. Another report of two patients describes how they suered burns while being monitored with a pulse oximeter during MRI [8]. One patient, a man who had undergone imaging of the cervical spine, sustained a full-thickness burn requiring skin grafting of the tip of the little ®nger where the pulse oximeter sensor had been placed. The second patient underwent MRI of the head and had the pulse oximeter placed on the great toe with a loop of the connecting cable taped over the leg. Afterwards, a super®cial linear burn was found where the cable had been taped to the leg. Another case report involved a 59-year-old man who received burns during cervical MRI [9]. The oximeter probe was taped to the distal right ®fth digit, with the patient's arms at his sides. Cloth was placed between the hand and the oximeter cable, which was also examined to avoid loops. On removal of the oximeter probe, it was discovered that the patient had suered full thickness burns to the ®nger and ultimately required amputation.
OTHER BURNS IN MRI
The FDA data ®les contain 28 other reports of burns associated with MRI which could not be categorized as easily as before and these are summarized in Table 7 (reference keys for these reports are provided in Table 8). Several of these incidents may have been due to the MR system itself i.e. without patient monitoring devices, local coils or other objects present. Often, no description of the burn is oered and the report merely states that the patient was treated for a burn. Again, it is therefore dicult to know the process causing the heating. Aside from the FDA data ®les, there have been other reports of unusual types of burns published. One case involved a 22-year-old woman whose head was imaged with a 1.0 Tesla unit [10]. A magnetic artefact was encountered and was determined to be due to a ferromagnetically active pigment used as permanent eyeliner. Approximately 2 h after imaging, the patient experienced painless swelling and puness of her upper eyelids that peaked approximately 4 h after imaging and subsided over the next 48 h. Pigment used for eyeliner tattoos are an inhomogeneous group with no apparent standardization
BURNS ASSOCIATED WITH IMAGING COILS
The FDA data ®les contain 23 accounts of patient burns associated with MRI RF imaging coils and these are Table 5 ± Summary of FDA burn reports associated with imaging coils Burn description
Frequency
Suggested causes
Burn to patient's arm Burn to patient's thumb and thigh Burn to patient's hand and buttock Burn to patient's neck Burn to patient's leg Burn to patient's shoulder and arm Burns to point of skin contact with coil cable Burn area not speci®ed
2 1 4 4 2 3 1 6
Shoulder coil CTL coil CTL coil Cervical spine coil Knee coil CTL coil
FDA FDA FDA FDA FDA FDA FDA FDA
reports report reports reports reports reports report reports
Table 6 ± Reference keys for FDA burn reports associated with imaging coils Medical device reporting (MDR) system ®les Key
Access number
E1 E2 E3 E4 E5 E6
M307275 M359665 M490027 M813377 M834788 M839964
Key
Manufacturer and user facility device experience (MAUDE) reports Access number
Key
MDR report key
Key
MDR report key
F1 F2 F3 F4 F5 F6 F7 F8 F9
10418 17296 26403 32679 33299 89368 131095 132647 139452
F10 F11 F12 F13 F14 F15 F16 F17
153437 171569 171588 171643 179291 199042 203223 206322
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CLINICAL RADIOLOGY
Table 7 ± Summary of FDA burn reports associated with imaging coils Burn description
Frequency
Burns to point of skin contact with inside surface wall of MRI bore Burn to patient's buttock Burns to patient's skin to skin contact point Heating around areas of skull pins of halo device Heating around hip screw Burn to patient's ®nger Heating in left groin Burn area not speci®ed
1 FDA report 1 FDA report 4 FDA reports 8 FDA reports 1 FDA reports 1 FDA report 1 FDA report 11 FDA reports
Suggested causes
Flesh loops Hip screw device Swan-Ganz thermodilution catheter device Metal implants
Table 8 ± Reference keys for FDA burn reports ± other burns in MRI Medical device reporting (MDR) system ®les Key
Access number
G1 G2 G3 G4 G5 G6 G7
M286228 M286257 M397218 M553719 M829924 M874396 M879842
Key
Manufacturer and user facility device experience (MAUDE) reports Access number
Key
MDR report key
Key
MDR report key
H1 H2 H3 H4 H5 H6 H7 H8 H9 H10 H11
15564 18471 25392 25547 46021 76997 83152 89363 89396 93213 98734
H12 H13 H14 H15 H16 H17 H18 H19 H20 H21
109464 116336 116362 121175 127333 171164 171244 177702 189703 214933
e.g. black may consist of carbon, iron oxide or logwood. A possible explanation of this event was that whatever the tattoo substance was, it may have been acting as a conductor causing increased energy deposition or heating in adjacent soft tissue. It is worth noting that in the safety manual of at least one MRI system, manufacturer warnings exist of some cosmetics containing ferromagnetic particles that can cause irritation to the patient under the in¯uence of a strong magnetic ®eld [11]. Also, the use of the system is contraindicated for patients who have undergone permanent eyelining, unless the physician can be certain that the product is not magnetically active. Another report of a burning sensation associated with MRI involved a 51-year-old woman with a breast implant [12]. The breast expander was made of a medical grade silicone with a metal-backed injection portal. Soon after the study began, the patient complained of a burning sensation over the area of the expander injection port in the region of the right chest. The burning swiftly subsided after the examination was aborted. The patient had never experienced any symptoms of burning before the MRI examination. It was thought that the conducting path between the needle strip in the injection port of the expander and the metallic casing of the venous access disk may have led to enough local heating for the patient to perceive a burning sensation. There is a further report in which a patient received a second-degree burn from the use of a temperature probe speci®cally designed for use during MRI [13]. In this case, a
10-year-old patient was undergoing MRI of the lumbosacral spine during general anaesthesia. Monitoring included an axillary skin probe. At the end of the 2-h procedure, examination of the axillary area revealed a large seconddegree burn, but no fault was detected in the probe. Although neither fraying nor looping of the cable was observed, it was thought that movement of the bed may have permitted formation of a loop in the cable.
PUBLISHED INVESTIGATIONS OF HEATING IN MRI
Little experimental work has been published discussing the physics involved in these burn incidents and therefore the cause of these injuries remains theoretical. However, various authors have investigated heating eects of conductive media during MRI. These published investigations of heating in MR have primarily been concerned with conductive implants and devices. Cochlear implants have been investigated, with a maximum temperature rise of less than 1 8C being measured [14]. Similarly, another investigation studied the heating eects of large, thin aluminium sheets with a temperature rise of 0.08 8C being measured [15]. The heating eects of changing magnetic ®elds and RF ®elds on small metallic implants (surgical clips and prostheses) was also studied with no ®ndings of signi®cant temperature rise [16]. Additionally, vascular access ports (IVAPs) and catheters have been examined; temperature changes ranged from no change (0 8C) to 0.3 8C. This relatively minor tempera-
463
THERMAL INJURIES ASSOCIATED WITH MRI
ture increase associated with MR imaging of the IVAPs was not considered to be a potential hazard for patients [17]. Another investigation examined patient safety when recording electroencephalograms (EEG) during functional MRI ( fMRI) experiments [4]. This study concluded that voltages induced in loops formed by EEG wire resulting from the RF pulses were the single most important risk factor in EEG/fMRI. While necessary precautions were speci®ed, temperature measurements showed heating of the electrodes to be negligible. Temperature changes in nickelchromium intracranial depth electrodes during MRI scanning was also investigated [18]. Again, no signi®cant temperature increase was measured in these experiments. Thus, various investigations of heating eects of biomedical implants have found no cause for concern when considering temperature elevation. However, another investigation of the RF heating of an implanted spinal fusion stimulator found a maximum temperature rise of 2 8C [19]. In the same study, when one of the generator leads was broken at the middle section, the temperature rise was as high as 11 8C (the lead was placed inside a human phantom model, lead length not speci®ed). It was thought that the RF current at the tip of the broken electrode caused excessive heating. When the implant was intact, the RF current emanated through the entire length of the bare electrode and no signi®cant heating of the conducting electrode was observed. Also, a real-time temperature measurement of heating under ECG electrodes during MR imaging at 1.5 Tesla found a peak temperature increase of 12 8C [5]. In this study, the cable position for peak heating of the electrode was the bore wall. When considering electromagnetic induction (summarized previously), the geometrical con®guration of the conductive media de®nes its inductance. Thus, looping a wire will increase the inductance and therefore, it is commonly thought that the pulsed magnetic gradient ®elds and pulsed radiofrequency ®elds used in MRI may have induced sucient current in an electrically conductive loop to result in these thermal or electrical burn injuries [6,7± 9,20±22]. The former injury resulting from heating of the wire, whereas the latter may follow if tissues complete an electric circuit with the wire as a result of capacitive coupling at the epidermis. Wires that form a part of a conductive pathway, having a large enclosed area (and hence cutting many pulsed magnetic ¯ux lines), may allow higher current induction. Conductive loops may form by one of the following mechanisms: exposed lead wire in contact with patient; two leads in direct electrical contact; a single wire bending on itself; and current being able to ¯ow through the insulation at RF frequencies. The orientation of the loop is also important ± a loop lying in the plane perpendicular to the direction of the RF ®eld will cut the maximum amount of ¯ux lines and, therefore, have the maximum current induced. Although currents are induced in the human body under normal imaging conditions, conducting loops provide a concentration of current and therefore a high current density in the tissue under an electrode.
In MRI, only the magnetic radiation of the RF is needed to generate the magnetic resonance eect. Therefore, coils are designed to achieve a spatially homogeneous magnetic ®eld. The electric component of the RF ®eld is spatially inhomogeneous and low around the coil centre to minimize dielectric losses. This has implications if monitoring cables are considered as RF wire antennae. A safety investigation of transoesophageal cardiac pacing during MRI studied heating of the electrodes and pacing wire; when using a high RF radiation exposure protocol (higher than allowed in clinical MRI by regulation), the measured temperature increase was 36 8C [20]. Similarly, temperature rises exceeding 15 8C were measured while studying the heating eects of MRI on pacemaker leads [23]. In the same study, under special con®gurations, a maximal temperature increase of as much as 69 8C was reported. Other investigations of the safety of vascular guidewires revealed temperature rises of up to 17 8C [24] and 14 8C [25]. In one investigation of RF heating of actively visualized catheters and guidewires [26], heating was found to be due to electric ®eld coupling with the coaxial cable, and not due to magnetic ®eld coupling with the RF coil itself. The results of this study showed heating to be a function of many variables, including static magnetic ®eld strength, coil geometry, cable length, placement of the cables in the machine and the substance in which the device was immersed. Maximum temperature changes of around 20 8C were recorded. A further investigation of guidewire heating measured a temperature increase of up to 72 8C [27], depending primarily on guidewire length and tip position. Thus, with such high temperature rises being recorded in these studies (and such relatively low temperature elevations being reported for experiments involving direct induction), it is possible that extended wires (as used with monitoring devices) may cause thermal injury as a result of this antenna eect, rather than being a result of, what is more commonly reported, electromagnetic induction eect. Additionally, of the burns that have been attributed to cable looping, it must be remembered that the actual heating may be a result of resonance rather than direct electromagnetic induction. This hypothesis is under investigation at this centre.
DISCUSSION
Multiple reports of burn incidents and heating investigations of conductive media in MRI have been reviewed. While the direct electromagnetic induction theory cannot be excluded, strong support is provided to indicate that the resonant circuit and antenna eect may be involved in these burns. For example, the burn incidents in which the monitoring cables were not looped can only be explained by the antenna eect. Due to the incidence of these thermal injuries, various guidelines } have been published to prevent burns in the }Safety information is also available on Kanal's MR safety site: http://kanal.arad.upmc.edu/MR Safety
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MRI setting [6,7±9,20±22,28±31]. It is prudent that any discussion of burns associated with MRI should also mention the present guidelines. The main guidelines include taking special care and precautions to ensure that no loops are formed with conductive materials and the patient's tissues. Additionally, if possible, no potential conductors should touch the patient at more than one location, in order to decrease the chance of formation of a conductive loop that might involve the patient's tissues. All potential conductors should be checked before use on each patient to ensure the absence of frayed insulation, exposed wires and other hazard. Also, all electrically conductive material within the bore of the imaging system, that is not required for the study, should be removed from the bore before imaging. If possible, it is recommended that insulating material (such as air) should be placed between the wires coming away from the magnet system and the patient so that they do not come into direct contact. These preventative guidelines are based mainly on the assumption that direct electromagnetic induction is causing the excessive heating, despite that, to the authors' knowledge, there is no experimental evidence supporting this theory. However, as this theory cannot be excluded, it remains important to adhere to the present guidelines (this should also reduce the risk of achieving resonance). Additionally, if the antenna eect is involved in these thermal injuries, additional guidelines may be appropriate. For example, heating as a consequence of the antenna eect is a function of many variables such as cable length, position of cable in the scanner and the substances surrounding the cable and these factors should all be given consideration.
CONCLUSION
There have been many incidents of patients receiving thermal injuries while undergoing investigation employing MRI. The majority of these burns have been received while the patient is connected to some form of physiological monitoring device. Commonly, it has been suggested that electromagnetic induction currents in the monitoring cables are largely the cause of these events. However, in reviewing the many studies of temperature changes in conductive media in the literature, it also appears possible that these burn incidents could be a result of extended wires used in physiological monitoring behaving as antennae and capturing the electric ®eld component of the RF pulses used for imaging. If this is the case, additional safety guidelines may be appropriate. What is most evident is that further investigation of the underlying physics is required to allow preventative guidelines to be produced based on experimental ®ndings rather than theory. REFERENCES 1 Page L, Adams NI. Principles of Electricity: An Intermediate Text in Electricity and Magnetism, 2nd Ed. Princeton: D. Van Nostrand Company Inc, 1949.
2 Kraus JD, Carver KR. Electromagnetics, 2nd Ed. New York: McGraw Hill Book Company, 1973. 3 Balaris CA. Antenna Theory: Analysis and Design. John Wiley & Sons, 1982. 4 Lemieux L, Allen PJ, Franconi F, Symms MR, Fish DR. Recording of EEG during fMRI experiments: patient safety. Magn Reson Med. 1997;38:943±952. 5 Felmlee JP, Rochester MN, Hokanson DA, Zink FE, Perkins WJ. Real-time temperature measurement of heating under electrocardiographic electrodes during MR imaging at 1.5 Tesla. Radiology. 1995;197(Suppl.): 423, abstract. 6 Jones S, Jae W, Alvi R. Burns associated with electrocardiographic monitoring during magnetic resonance imaging. Burns. 1996;22:420±421. 7 Shellock FG, Slimp GL. Severe burn of the ®nger caused by using a pulse oximeter during MR imaging [Letter]. Am J Roentgenol. 1989;153:1105. 8 Bashein G, Syrovy G. Burns associated with pulse oximetry during magnetic resonance imaging [Letter]. Anesthesiology. 1991;75: 382±383. 9 Brown TR, Goldstein B, Little J. Severe burns resulting from magnetic resonance imaging with cardiopulmonary monitoring. Risks and relevant safety precautions. Am J Phys Med Rehabil. 1993;72:166±167. 10 Jackson JG, Acker JD. Permanent eyeliner and MR imaging [Letter]. Am J Roentgenol. 1987;149:1080. 11 Siemens ?. Magnetom SP/Magnetom SP4000 Version A2.5/Version A2.7, Part I: Registers A-K, Siemens AG, 1989. (Order no.: C2020.01.05.02.). 12 Duy FJ, May JW. Tissue expanders and magnetic resonance imaging: the `hot' breast implant. Ann Plast Surg. 1995;35:647±649. 13 Hall SC, Stevenson GW, Suresh S. Burn associated with temperature monitoring during magnetic resonance imaging [Letter]. Anesthesiology. 1992;76:152. 14 Teissl C, Kremser C, Hochmair ES, Hochmair-Desoyer IJ. Magnetic resonance imaging and cochlear implants: compatibility and safety aspects. J Magn Reson Imaging. 1999;9:26±38. 15 Buchli R, Boesiger P, Meier D. Heating eects of metallic implants by MRI examinations. Magn Reson Med. 1988;7:255±261. 16 Davis PL, Crooks L, Arakawa M, McRee R, Kaufman L, Margulis AR. Potential hazards in NMR imaging: heating eects of changing magnetic ®elds and RF ®elds on small metallic implants. Am J Roentgenol. 1981;137:857±860. 17 Shellock FG, Shellock VJ. Vascular access ports and catheters: ex vivo testing of ferromagnetism, heating, and artifacts associated with MR imaging. Magn Reson Imaging. 1996;14:443±447. 18 Zhang J, Wilson CL, Levesque MF, Behnke EJ, Lufkin RB. Temperature changes in nickel-chromium intracranial depth electrodes during MR scanning. Am J Neuroradiol. 1993;14: 497±500. 19 Chou CK, McDougall JA, Chan KW. RF heating of implanted spinal fusion stimulator during magnetic resonance imaging. IEEE Trans Biomed Eng. 1997;44:367±373. 20 Hofman MB, de Cock CC, van der Linden JC, et al. Transesophageal cardiac pacing during magnetic resonance imaging: feasibility and safety considerations. Magn Reson Med. 1996;35: 413±422. 21 Hazard ± thermal injuries and patient monitoring during MRI studies. ECRI problem reporting system. Health Devices. 1991;20: 362±363. 22 Kanal E. An overview of electromagnetic safety considerations associated with magnetic resonance imaging. Ann NY Acad Sci. 1992;649:204±224. 23 Luechinger R, Duru F, Scheidegger MB, Candinas R, Boesiger P. Heating eects of MRI on pacemaker leads: eect of lead types and positioning. Proceedings of the ISMRM, 7th Scienti®c Meeting. 1999;103. 24 Ladd ME, Zimmermann GG, Quick HH, et al. Active MR visualization of a vascular guidewire in vivo. J Magn Reson Imaging. 1998;8:220±225. 25 Liu C-Y, Farahani K, Lu DSK, Shellock FG. Safety of MRIguided endovascular guidewire applications. Proceedings of the ISMRM, 7th Scienti®c Meeting. 1999;1015.
THERMAL INJURIES ASSOCIATED WITH MRI
26 Ladd ME, Quick HH, Boesiger P, McKinnon GC. RF heating of actively visualised catheters and guidewires. Proceedings of the ISMRM, 6th Scienti®c Meeting. 1998;473. 27 Konings MK, Bakker CJG. Intolerable heating by resonating RF waves around guidewires. Proceedings of the ISMRM, 7th Scienti®c Meeting. 1999;1004. 28 Kanal E, Shellock FG. Patient monitoring during clinical MR imaging. Radiology. 1992;185:623±629.
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29 Kanal E, Shellock FG, Talagala L. Safety considerations in MR imaging. Radiology. 1990;176:593±606. 30 Shellock FG, Kanal E. Burns associated with the use of monitoring equipment during MR procedures [Letter]. J Magn Reson Imaging. 1996;6:271±272. 31 Shellock FG, Kanal E. Magnetic Resonance: Bioeects, Safety, and Patient Management. New York: Raven Press, 1994.