- Email: [email protected]
Evaluation of MR safety of a set of canine ear defenders (MuttMuffs®) at 1 T
Radiography 19 (2013) 339e342
Contents lists available at ScienceDirect
Radiography journal homepage: www.elsevier.com/locate/radi
Evaluation of MR safety of a set of canine ear defenders (MuttMuffsÒ) at 1 T Martin A. Baker* Department of Veterinary Clinical Sciences, University of Liverpool, Small Animal Teaching Hospital, Neston, Wirral CH64 7TE, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 December 2012 Received in revised form 5 July 2013 Accepted 11 July 2013 Available online 1 August 2013
Previous studies have indicated that loud noise produced during MR scanning is hazardous for human patients. Although loud noise can also be harmful to canine patients in MRI, ear protection is not routinely provided. The purpose of this study was to test the safety of a set of commercially available canine ear defenders (MuttMuffsÒ) during MRI scanning at 1 T. A metal fastening ring was removed and replaced with a plastic washer prior to testing. Torque, translation, heating and artifact production were tested. No torque, translation, or excessive heating were detected. No artifacts were observed. Clinical use demonstrated additional benefits of improved immobilisation of the dog, with no effect on signal-tonoise ratio. Results from this study indicate that following replacement of the metal ring with one made of plastic, these canine ear defenders are suitable for use at 1 T. The author recommends the use of ear defenders during canine MRI scans in order to reduce the risk of hearing damage, reduce the dose needed for anaesthetic maintenance and reduce the need for repeated MRI sequences due to movement of the dog. Ó 2013 The College of Radiographers. Published by Elsevier Ltd. All rights reserved.
Keywords: Veterinary MRI Ear defenders Acoustic noise
Introduction MRI is being increasingly utiIised as a diagnostic tool for small animals in the UK, with many veterinary practices either having their own scanner or access to a mobile scanning unit. The commonest examinations are those of the brain and spine, for animals presenting with a range of conditions including tumours, seizures, inflammatory disease and disc problems. In this context the term ‘small animals’ refers to small mammals, birds and reptiles kept as pets. Acoustic noise in MRI is generated by currents within the gradient coils inducing Lorentz forces, which in turn cause the coils to vibrate.1 Average noise levels of 100 dB have been reported for animal scanning at 3 T.2 In addition, the small field of view and thin slice thickness required for small animal imaging can result in higher noise levels than for an equivalent adult human scan.3,4 Heavy sedation or general anaesthesia are routinely used during magnetic resonance imaging of dogs so that they will remain motionless during scanning, with one reason for movement being pain associated with loud noise.2,5
* Tel.: þ44 (0)151 7956136. E-mail address: [email protected].
Exposure to acoustic noise during magnetic resonance imaging (MRI) is a recognized hazard to human patients and staff members present in the scan room during imaging.3,6,7 The simplest and most common method is the provision of passive shielding in the form of ear defenders or ear plugs.4 Ear protection is therefore provided routinely in the human setting,8,9 and it has recently been proposed that animals would also benefit from this measure.2,3 Due to the difference in size and morphology, passive ear protection designed for humans is not suitable for animals. There are very few products available which provide passive ear protection for animals, and none have been tested for safety in the MR environment. Proper evaluation of equipment prior to use in the MR environment should take into account effects of translation, torque and heating, and also other effects such as artifact production where appropriate.10 Translation refers to a linear displacement force from the static magnetic field towards the centre of the scanner,11 whereas torque relates to a rotational force incumbent upon the object.12 Both mechanisms could potentially move or dislodge objects within the MR environment. Heating effects can occur as a result of electromagnetic induction or radio frequency energy absorption.13 Artifacts can cause a reduction in the diagnostic quality of images and so indirectly affect safety.10,13 The purpose of this study was to test the safety of a set of canine ear defenders (MuttMuffsÒ, Safe and Sound Pets, Westminster, Maryland) for MRI scanning with a 1 T unit
1078-8174/$ e see front matter Ó 2013 The College of Radiographers. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radi.2013.07.004
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M.A. Baker / Radiography 19 (2013) 339e342
Figure 1. Canine ear defenders, ‘Mutt muffsÒ’.
(Siemens Magnetom Harmony, Siemens AG, Erlangen, Germany) (Fig. 1). Methods and materials Prior to testing, a single steel rectangular loop used to fasten the chin strap was tested with a hand-held magnet, and found to be ferromagnetic. This was removed and replaced with a plastic washer to allow the ear defenders to be fastened. Translational force was tested using a deflection angle test.11 The ear defenders were suspended from a 30 cm length of lightweight string at a range of off-centre positions near the entrance to the bore of the MR scanner14 at a distance of 2.7 m from isocenter, which is the location of the greatest static magnetic field strength exposure to the animal. The string’s mass was less than 1% of the mass of the ear defenders. The apparatus was moved to the position described above, the string being held vertically and then released. Any deflection of the apparatus from vertical was measured using a protractor, to an accuracy of 0.5 . Torque was tested and qualitatively using a previously described method.14,15 The ear defenders were placed on a plastic dish, which was then moved to isocenter of the MR scanner, which is the location of maximum magnetic field strength, where the magnetic field is most uniform and where the greatest torque is likely to be experienced. The ear defenders were then directly observed to check for any rotation due to torque. This was repeated with the ear defenders being rotated at 45 increments compared to the original position until a complete set of positions over a 360 arc was achieved. Heating was tested using a fiberoptic thermometer and two temperature sensors (Neoptix Reflex, Neoptix Inc, Quebec City, Canada). The thermometer was located outside of the scan room and was attached to each sensor via an extension cable passed through a wave guide. The sensor had a resolution of 0.1 C and accuracy to within 1. The ear defenders were positioned directly on a pig head within a transmit-receive extremity coil, with the sensors positioned between the skin and the outer rim of the ear defenders. Due to the size of the pig head the ear defenders were positioned on the snout instead of the ears. The scanner’s ventilation fan was turned off to minimize cooling effects.16 The apparatus was allowed to reach thermal equilibrium with room temperature for 20 min prior to scanning. The head was then scanned for 20 min with a turbo spin-echo test sequence. Turbo spin-echo sequences
employ multiple 180 pulses and therefore deposit more RF energy than spin-echo or gradient-echo techniques,15,17 and are therefore associated with a high specific absorption rate (SAR). The scan time represented a longer scan time than is used for any clinical MR sequence. The sequence had the following parameters: TR: 210 ms, TE: 7.5 ms, FOV: 250, bandwidth: 16 kHz, slice thickness: 10 mm, turbo factor: 11, and a fast RF pulse mode. This resulted in an exposed body SAR of 2.3 w/kg. Artifact production was tested by positioning the ear defenders on a 20 cm 10 cm diameter cylindrical test phantom filled with magnesium sulphate and sodium chloride solution. The phantom was scanned in a transmit-receive extremity coil using a T2* gradient-echo sequence, and a T2-weighted turbo spin-echo. The field of view was increased to 200 mm in order to include the ear defenders on the images. Gradient-echo sequences have previously been demonstrated to be the most sensitive to artifacts for safety testing purposes.18 The modified ear defenders were then scanned with a helical CT scanner (Siemens Volume Zoom; Siemens AG, Erlangen, Germany), producing images with 1 mm slice thickness to rule out any further metallic components which may cause artifacts or other safety concerns. To test for effects on image quality, signal-to-noise ratios were calculated from T1-weighted spin-echo images of 40 clinical cases. The dogs were all of the same breed (Cavalier King Charles Spaniel), with half having been provided with the ear defenders and half which had not. Signal-to-noise ratio was calculated according to the following formula: SNR ¼ 0.655 S/R where S is the mean pixel intensity within a region of interest (ROI) placed in temporal muscle and R, standard deviation of noise within an ROI placed in air. Data were analysed using an independent samples two-tailed ttest assuming unequal variance. Results Results are presented in Table 1. The t-test showed no significant difference in mean signal-to-noise ratios when the ear defenders were used compared to when they were not (p ¼ 0.17). Discussion This study has shown that following a minor alteration, the ear defenders tested were suitable for use in MRI at 1 T. The author believes this is the first time such a device has been tested for the purpose of increasing safety for animals undergoing MRI, therefore addressing recent concerns regarding animal safety in MRI.2,3 Although there are no current guidelines concerning safe exposure levels to noise for small animals,2 it is known that acoustic noise is associated with temporary or permanent hearing loss, and increased anxiety in human patients.1 A study on dogs kept in kennels showed a rise in auditory thresholds where the peak sound
Table 1 Results from safety tests. Test
Result
Translation Torque Heating
No translation demonstrated No torque demonstrated Temperature rise <0.1 C
Artifacts T2-weighted turbo spin-echo T2* gradient-echo
No artifact seen No artifact seen
M.A. Baker / Radiography 19 (2013) 339e342
pressure levels reached 82e104 dB, with sound-reflecting walls adding to the problem.19 Such noise levels are directly comparable to those found in MR scanning. Similarly in MR scanning, reverberations are also present from the floor, walls and equipment such as the anaesthetic trolley present in the MR scan room.20 Hearing damage in companion animals can lead to stress, inability to hear commands or oncoming traffic, and aggressive behaviour,2 and these are important not only for pets but also for working dogs. Veterinary MR scans may also need to be longer to compensate for the loss in signal-to-noise ratio experienced when scanning smaller anatomical regions. The current failure to provide hearing protection for animals in MRI may be partly due to the belief that they will not be able to hear the noise because they are anaesthetised. However acoustic noise poses a risk during anaesthesia for two reasons, namely the increased chance of ear damage and potential for stimulation. The inner ear muscles are normally able to protect the ear from loud noises via the acoustic reflex. General anaesthesia can greatly reduce this protective reflex, and therefore increase the degree of noise-induced hearing damage.21 A permanent threshold shift has also been demonstrated in animals exposed to loud noises as a result of altered middle ear muscle function caused by anaesthesia.22 Conversely, some inhaled anaesthetic agents such as isoflurane have been shown to reduce noise-induced hearing loss23,24 and so may provide some degree of protective benefit, although this has only been demonstrated in mice. Despite this, anaesthesia with isoflurane was not sufficient to prevent chronic tinnitus in rats exposed to intense acoustic trauma, suggesting that protection is ineffective at greater noise levels.25 Acoustic noise has been shown to provide some degree of stimulation to anaesthetised animals.2 Substantial changes in blood pressure, heart rate and oxygen saturation have also been demonstrated in sedated infants undergoing MRI, probably as a result of sensory stimulation.26 A similar study which found no major physiological instability during MRI explained that this was due to lower noise levels whilst scanning.27 Auditory and sensory perceptions were also the most commonly reported sensations by children exhibiting awareness under general anaesthesia for a range of procedures including MRI.28 Reducing the effects of noise could therefore result in quicker scan times if less time is spent on maintaining anaesthesia in between sequences, and a lighter plane of anaesthesia could also be used.2,3 This is the case in paediatric MR scanning, where a combination of methods including ear protection allows scanning without sedation or anaesthesia in some younger patients.29 A study of anaesthetised children in MRI found that when ear defenders were used, less inhaled anaesthetic agent was required and recovery time was quickened,30 demonstrating a direct benefit of using ear protection. Noise-induced hearing loss is difficult to distinguish from agerelated hearing loss, and may be underestimated because it often affects specific frequencies.19 Another potential problem is that unilateral hearing impairment in dogs and cats is not usually behaviourally evident, and so often is overlooked by the owners.31 Potential adverse effects of translational and rotational movements of medical devices during MRI scanning include disconnection of anaesthetic equipment and physical movement of the subject. The former could have serious consequences if it were to lead to the animal gaining consciousness in the MR scanner. Movement of the animal during MR scanning, either as a result of stimulation or from movement of equipment, can also result in image artifacts and loss of diagnostic information. Pulsed magnetic fields employed in MR scanning can be responsible for the induction of an electrical current within a conducting material, resistance to which can cause heating.15,18 Heating has been demonstrated in ferrous materials, in non-
341
metallic conductors32 and within the patient themselves.33 The largest temperature rises from magnetic induction are to be expected at the surface of the body and at the periphery.17 Hence, heating considerations could be further complicated by a rise in body temperature rather than that of the ear defenders. Heat generated during scanning is usually reduced by the cooling effects of blood perfusion and thermoregulation.34 As the pig’s head was devoid of insulating fur between the ear defenders and the skin, and lacked any cooling by blood perfusion, it represents a more extreme model of heating than would occur in a living dog. Some non-metallic materials such as silicone are visible on MR scans due to the MR signal being similar to the signal obtained from fat.35 Such materials when incorporated into medical equipment could present a problem due to phase-wrap artifact overlying areas of clinical importance. The absence of signal from the ear defenders also means that the scan time will not be extended as a result of increased oversampling. Passive protection compares well to more sophisticated and expensive forms of noise reduction such as active noise cancellation (ANC) which may provide reductions of around 18e35 dB.36,37 Ear plugs and ear defenders are the most widely used method of passive noise control used in human MRI,4 but the degree of protection provided varies according to the noise frequency. Effectiveness of ear defenders and ear plugs increases with higher frequencies as these are better attenuated.4,36 The manufacturers of MuttMuffsÒ state that they provide 25e28 dB of protection at 20 kHz, but information for other frequencies was not reported.38 The pinnae of small animals and dogs in particular often protrude laterally, and this can cause problems due to phase-wrap artifact.5 This artifact can be reduced by oversampling with an associated increase in scan time. By compressing the pinnae against the dog’s head, the ear defenders reduce the need for oversampling and may therefore cut scan time. One potential difficulty is accommodating the ear defenders within the MR coil when large dog heads are being scanned in an extremity coil. Similar limitations have also been identified in human scanning as a result of the extra width required to accommodate the ear defenders.4 However, this would not be an issue when scanning other body regions. For brain scans the ear defenders assist in the immobilisation of the dog’s head within the coil. For dogs scanned for cervical spine in dorsal recumbency, the ear defenders make the head more stable and less prone to tipping during the scan. As this study was carried out at 1 T, further testing of this device is recommended to assess its suitability for use at higher field strengths. Funding No funding was received for this study. Financial interest The author declares no financial interest in the product described in this paper. References 1. McNulty JP, McNulty S. Acoustic noise in magnetic resonance imaging - an ongoing issue. Radiography 2009;15:320e6. 2. Lauer AM, El-Sharkawy AM, Kraitchman DL, Edelstein WA. MRI acoustic noise can harm experimental and companion animals. Journal of Magnetic Resonance Imaging 2012;36:743e7. 3. Baker MA. Reduction of MRI acoustic noise achieved by manipulation of scan parameters - a study using veterinary MR sequences. Radiography 2013;19: 11e6.
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4. De Wilde JP, Grainger B, Price DL, Renaud C. Magnetic resonance imaging safety issues including an analysis of recorded incidents within the UK. Progress in Nuclear Magnetic Resonance Spectroscopy 2007;51:37e48. 5. Baker MA. Technical note: considerations for MR imaging of small animals. Radiography 2011;17:171e4. 6. Morton G, Gildersleve C. Noise in the MRI scanner. Anaesthesia 2000;55:1213e5. 7. Nordell A, Lundh M, Horsch S, Hallberg B, Aden U, Nordell B, et al. The acoustic hood: a patient-independent device improving acoustic noise protection during neonatal magnetic resonance imaging. Acta Paediatrica 2009;98:1278e83. 8. Peden CJ, Menon DK, Hall AS, Sargentoni J, Whitwam JG. Magnetic resonance for the anaesthetist. Part II: anaesthesia and monitoring in MR units. Anaesthesia 1992;47:508e17. 9. Schmitz B, Nimsky C, Wendel G, Wienerl J, Ganslandt O, Jacobi K, et al. Anesthesia during high-field intraoperative magnetic resonance imaging experience with 80 consecutive cases. Journal of Neurosurgery and Anesthesiology 2003;15: 255e62. 10. Shellock FG, Woods TO, Crues JV. MR labeling information for implants and devices: explanation of terminology. Radiology 2009;253:26e30. 11. American Society for Testing and Materials. F2052e06: standard test method for measurement of magnetically induced displacement force on medical devices in the magnetic resonance environment. West Conshohocken, PA: American Society for Testing and Materials (ASTM) International; 2006. 6pp. 12. American Society for Testing and Materials. F2213-06: standard test method for measurement of magnetically induced torque of medical devices in the magnetic resonance environment. West Conshohocken (PA): American Society for Testing and Materials (ASTM) International; 2006. 7pp. 13. American Society for Testing and Materials. F2182-09: standard test method for measurement of radio frequency induced heating on or near passive implants during magnetic resonance imaging. West Conshohocken, PA: American Society for Testing and Materials (ASTM) International; 2009. 12pp. 14. Shellock FG, Kanal E, Gilk T. Regarding the value reported for the term ‘spatial gradient magnetic field and how this information is applied to labelling of medical implants and devices. American Journal of Roentgenology 2010;196:1e4. 15. Sawyer-Glover AM, Shellock FG. Pre-MRI procedure screening: recommendations and safety considerations for biomedical implants and devices. Journal of Magnetic Resonance Imaging 2000;12:92e106. 16. Baker KB, Tkach JA, Phillips MD, Rezai AR. Variability in RF-induced heating of a deep brain stimulation implant across MR systems. Journal of Magnetic Resonance Imaging 2006;24:1236e42. 17. Boss AB, Graf H, Berger A, Lauer UA, Wojtczyk H, Claussen CD, et al. Tissue warming and regulatory responses induced by radio frequency energy deposition on a whole body 3-Tesla magnetic resonance imager. Journal of Magnetic Resonance Imaging 2007;26:1334e9. 18. Shellock FG, Gounis M, Wakhloo A. Detachable coil for cerebral aneurisms: in vitro evaluation of magnetic field interactions, heating and artifacts at 3T. American Journal of Neuroradiology 2005;26:363e6. 19. Ter Haar G, Venker-van Haagen AJ, van den Brom WE, Smoorenburg GF. Effects of aging on brainstem responses to toneburst auditory stimuli: a crosssectional and longitudinal study in dogs. Journal of Veterinary Internal Medicine 2008;22:937e45. 20. Moelker A, Vogel MW, Pattynama PMT. Efficacy of passive acoustic screening: implications for the design of imager and MR-suite. Journal of Magnetic Resonance Imaging 2003;17:270e5.
21. Kato T. Zur Physiologie der Binnenmuskeln des Ohres. Pfluegers Arch 1913;150: 569. 22. Borg E, Nilsson R, Engstrom B. Effect of the acoustic reflex on inner ear damage induced by industrial noise. Acta Oto-Laryngologica 1983;96:361e9. 23. Chung JW, Ahn JH, Kim JY, Lee HJ, Kang HH, Lee YK, et al. The effect of isoflurane, halothane and pentobarbital on noise-induced hearing loss in mice. Anesthesia and Analgesia 2007;6:1404e8. 24. Kim JU, Lee HJ, Kang HH, Shin JW, Ku SW, Ahn JH, et al. Protective effect of isoflurane anaesthesia on noise-induced hearing loss in mice. Laryngoscope 2005;115:1996e9. 25. Norman M, Tomscha K, Wehr M. Isoflurane blocks temporary tinnitus. Hearing Research 2012;290:64e71. 26. Philbin MK, Taber KH, Hayman A. Preliminary report: changes in vital signs of term newborns during MR. American Journal of Neuroradiology 1996;17: 1033e6. 27. Battin M, Maalouf EF, Counsell S, Herlihy A, Hall A, Azzopardi D, et al. Physiological stability of preterm infants during magnetic resonance imaging. Early Human Development 1998;52:101e10. 28. Blusse van Oud-Alblas HJ, van Dijk M, Liu C, Tibboel D, Klein J, Weber F. Intraoperative awareness during paediatric anaesthesia. British Journal of Anaesthesia 2009;102:104e10. 29. Benavente-Fernandez I, Lubian-Lopez PS, Zuazo-Ojeda MA, Jimenez-Gomez G, Lechuga-Sancho AM. Safety of magnetic resonance imaging in preterm infants. Acta Pediatrica 2010;99:850e3. 30. Ogurlu M, Orhan ME, Cinar S, Piri E, Kurt E, Sut N, et al. Effect of headphones on sevoflurane requirement for MRI. Pediatric Radiology 2012;42: 1432e6. 31. Stevens-Sparks CK, Strain G. Post-anaesthesia deafness in dogs and cats following dental and ear cleaning procedures. Veterinary Anaesthesia and Analgesia 2010;37:347e51. 32. Westbrook C, Roth CK, Talbot J. MRI in practice. 3rd ed. Oxford: Blackwell Publishing Ltd; 2005. 33. Ito Y, Omoto Y, Habe K, Yamanaka KI, Hakamada A, Isoda KI, et al. Magnetic resonance (MR) imaging-induced deep second-degree burns of lower extremities by conducting loop. Journal of the European Academy of Dermatology and Venereology 2007;21:1140e1. 34. Collins CM, Liu W, Wang J, Gruetter R, Vaughan JT, Ugurbil K, et al. Temperature and SAR calculations for a human head within volume and surface coils at 64 and 300 MHz. Journal of Magnetic Resonance Imaging 2004;19: 650e6. 35. Shellock FG, Shellock VJ. Vascular access ports and catheters: ex vivo testing of ferromagnetism, heating and artifacts associated with MR imaging. Magnetic Resonance Imaging 1996;14:443e7. 36. Hall DA, Chambers J, Akeroyd MA, Foster JR, Coxon R, Palmer AR. Acoustic, psychophysical, and neuroimaging measurements of the effectiveness of active cancellation during auditory functional magnetic resonance imaging. Journal of the Acoustic Society of America 2009;125:347e59. 37. Zvyagintsev M, Klasen M, Mathiak KA, Weber R, Edgar JC, Mathiak K. Real-time noise cancellation for speech acquired in interactive functional magnetic resonance imaging studies. Journal of Magnetic Resonance Imaging 2010;32. 705e3. 38. Safe and sound pets. Available from: http://www.safeandsoundpets.com/page/ page/4851794.htm [accessed 20 Jan 2012].
Contents lists available at ScienceDirect
Radiography journal homepage: www.elsevier.com/locate/radi
Evaluation of MR safety of a set of canine ear defenders (MuttMuffsÒ) at 1 T Martin A. Baker* Department of Veterinary Clinical Sciences, University of Liverpool, Small Animal Teaching Hospital, Neston, Wirral CH64 7TE, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 December 2012 Received in revised form 5 July 2013 Accepted 11 July 2013 Available online 1 August 2013
Previous studies have indicated that loud noise produced during MR scanning is hazardous for human patients. Although loud noise can also be harmful to canine patients in MRI, ear protection is not routinely provided. The purpose of this study was to test the safety of a set of commercially available canine ear defenders (MuttMuffsÒ) during MRI scanning at 1 T. A metal fastening ring was removed and replaced with a plastic washer prior to testing. Torque, translation, heating and artifact production were tested. No torque, translation, or excessive heating were detected. No artifacts were observed. Clinical use demonstrated additional benefits of improved immobilisation of the dog, with no effect on signal-tonoise ratio. Results from this study indicate that following replacement of the metal ring with one made of plastic, these canine ear defenders are suitable for use at 1 T. The author recommends the use of ear defenders during canine MRI scans in order to reduce the risk of hearing damage, reduce the dose needed for anaesthetic maintenance and reduce the need for repeated MRI sequences due to movement of the dog. Ó 2013 The College of Radiographers. Published by Elsevier Ltd. All rights reserved.
Keywords: Veterinary MRI Ear defenders Acoustic noise
Introduction MRI is being increasingly utiIised as a diagnostic tool for small animals in the UK, with many veterinary practices either having their own scanner or access to a mobile scanning unit. The commonest examinations are those of the brain and spine, for animals presenting with a range of conditions including tumours, seizures, inflammatory disease and disc problems. In this context the term ‘small animals’ refers to small mammals, birds and reptiles kept as pets. Acoustic noise in MRI is generated by currents within the gradient coils inducing Lorentz forces, which in turn cause the coils to vibrate.1 Average noise levels of 100 dB have been reported for animal scanning at 3 T.2 In addition, the small field of view and thin slice thickness required for small animal imaging can result in higher noise levels than for an equivalent adult human scan.3,4 Heavy sedation or general anaesthesia are routinely used during magnetic resonance imaging of dogs so that they will remain motionless during scanning, with one reason for movement being pain associated with loud noise.2,5
* Tel.: þ44 (0)151 7956136. E-mail address: [email protected].
Exposure to acoustic noise during magnetic resonance imaging (MRI) is a recognized hazard to human patients and staff members present in the scan room during imaging.3,6,7 The simplest and most common method is the provision of passive shielding in the form of ear defenders or ear plugs.4 Ear protection is therefore provided routinely in the human setting,8,9 and it has recently been proposed that animals would also benefit from this measure.2,3 Due to the difference in size and morphology, passive ear protection designed for humans is not suitable for animals. There are very few products available which provide passive ear protection for animals, and none have been tested for safety in the MR environment. Proper evaluation of equipment prior to use in the MR environment should take into account effects of translation, torque and heating, and also other effects such as artifact production where appropriate.10 Translation refers to a linear displacement force from the static magnetic field towards the centre of the scanner,11 whereas torque relates to a rotational force incumbent upon the object.12 Both mechanisms could potentially move or dislodge objects within the MR environment. Heating effects can occur as a result of electromagnetic induction or radio frequency energy absorption.13 Artifacts can cause a reduction in the diagnostic quality of images and so indirectly affect safety.10,13 The purpose of this study was to test the safety of a set of canine ear defenders (MuttMuffsÒ, Safe and Sound Pets, Westminster, Maryland) for MRI scanning with a 1 T unit
1078-8174/$ e see front matter Ó 2013 The College of Radiographers. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radi.2013.07.004
340
M.A. Baker / Radiography 19 (2013) 339e342
Figure 1. Canine ear defenders, ‘Mutt muffsÒ’.
(Siemens Magnetom Harmony, Siemens AG, Erlangen, Germany) (Fig. 1). Methods and materials Prior to testing, a single steel rectangular loop used to fasten the chin strap was tested with a hand-held magnet, and found to be ferromagnetic. This was removed and replaced with a plastic washer to allow the ear defenders to be fastened. Translational force was tested using a deflection angle test.11 The ear defenders were suspended from a 30 cm length of lightweight string at a range of off-centre positions near the entrance to the bore of the MR scanner14 at a distance of 2.7 m from isocenter, which is the location of the greatest static magnetic field strength exposure to the animal. The string’s mass was less than 1% of the mass of the ear defenders. The apparatus was moved to the position described above, the string being held vertically and then released. Any deflection of the apparatus from vertical was measured using a protractor, to an accuracy of 0.5 . Torque was tested and qualitatively using a previously described method.14,15 The ear defenders were placed on a plastic dish, which was then moved to isocenter of the MR scanner, which is the location of maximum magnetic field strength, where the magnetic field is most uniform and where the greatest torque is likely to be experienced. The ear defenders were then directly observed to check for any rotation due to torque. This was repeated with the ear defenders being rotated at 45 increments compared to the original position until a complete set of positions over a 360 arc was achieved. Heating was tested using a fiberoptic thermometer and two temperature sensors (Neoptix Reflex, Neoptix Inc, Quebec City, Canada). The thermometer was located outside of the scan room and was attached to each sensor via an extension cable passed through a wave guide. The sensor had a resolution of 0.1 C and accuracy to within 1. The ear defenders were positioned directly on a pig head within a transmit-receive extremity coil, with the sensors positioned between the skin and the outer rim of the ear defenders. Due to the size of the pig head the ear defenders were positioned on the snout instead of the ears. The scanner’s ventilation fan was turned off to minimize cooling effects.16 The apparatus was allowed to reach thermal equilibrium with room temperature for 20 min prior to scanning. The head was then scanned for 20 min with a turbo spin-echo test sequence. Turbo spin-echo sequences
employ multiple 180 pulses and therefore deposit more RF energy than spin-echo or gradient-echo techniques,15,17 and are therefore associated with a high specific absorption rate (SAR). The scan time represented a longer scan time than is used for any clinical MR sequence. The sequence had the following parameters: TR: 210 ms, TE: 7.5 ms, FOV: 250, bandwidth: 16 kHz, slice thickness: 10 mm, turbo factor: 11, and a fast RF pulse mode. This resulted in an exposed body SAR of 2.3 w/kg. Artifact production was tested by positioning the ear defenders on a 20 cm 10 cm diameter cylindrical test phantom filled with magnesium sulphate and sodium chloride solution. The phantom was scanned in a transmit-receive extremity coil using a T2* gradient-echo sequence, and a T2-weighted turbo spin-echo. The field of view was increased to 200 mm in order to include the ear defenders on the images. Gradient-echo sequences have previously been demonstrated to be the most sensitive to artifacts for safety testing purposes.18 The modified ear defenders were then scanned with a helical CT scanner (Siemens Volume Zoom; Siemens AG, Erlangen, Germany), producing images with 1 mm slice thickness to rule out any further metallic components which may cause artifacts or other safety concerns. To test for effects on image quality, signal-to-noise ratios were calculated from T1-weighted spin-echo images of 40 clinical cases. The dogs were all of the same breed (Cavalier King Charles Spaniel), with half having been provided with the ear defenders and half which had not. Signal-to-noise ratio was calculated according to the following formula: SNR ¼ 0.655 S/R where S is the mean pixel intensity within a region of interest (ROI) placed in temporal muscle and R, standard deviation of noise within an ROI placed in air. Data were analysed using an independent samples two-tailed ttest assuming unequal variance. Results Results are presented in Table 1. The t-test showed no significant difference in mean signal-to-noise ratios when the ear defenders were used compared to when they were not (p ¼ 0.17). Discussion This study has shown that following a minor alteration, the ear defenders tested were suitable for use in MRI at 1 T. The author believes this is the first time such a device has been tested for the purpose of increasing safety for animals undergoing MRI, therefore addressing recent concerns regarding animal safety in MRI.2,3 Although there are no current guidelines concerning safe exposure levels to noise for small animals,2 it is known that acoustic noise is associated with temporary or permanent hearing loss, and increased anxiety in human patients.1 A study on dogs kept in kennels showed a rise in auditory thresholds where the peak sound
Table 1 Results from safety tests. Test
Result
Translation Torque Heating
No translation demonstrated No torque demonstrated Temperature rise <0.1 C
Artifacts T2-weighted turbo spin-echo T2* gradient-echo
No artifact seen No artifact seen
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pressure levels reached 82e104 dB, with sound-reflecting walls adding to the problem.19 Such noise levels are directly comparable to those found in MR scanning. Similarly in MR scanning, reverberations are also present from the floor, walls and equipment such as the anaesthetic trolley present in the MR scan room.20 Hearing damage in companion animals can lead to stress, inability to hear commands or oncoming traffic, and aggressive behaviour,2 and these are important not only for pets but also for working dogs. Veterinary MR scans may also need to be longer to compensate for the loss in signal-to-noise ratio experienced when scanning smaller anatomical regions. The current failure to provide hearing protection for animals in MRI may be partly due to the belief that they will not be able to hear the noise because they are anaesthetised. However acoustic noise poses a risk during anaesthesia for two reasons, namely the increased chance of ear damage and potential for stimulation. The inner ear muscles are normally able to protect the ear from loud noises via the acoustic reflex. General anaesthesia can greatly reduce this protective reflex, and therefore increase the degree of noise-induced hearing damage.21 A permanent threshold shift has also been demonstrated in animals exposed to loud noises as a result of altered middle ear muscle function caused by anaesthesia.22 Conversely, some inhaled anaesthetic agents such as isoflurane have been shown to reduce noise-induced hearing loss23,24 and so may provide some degree of protective benefit, although this has only been demonstrated in mice. Despite this, anaesthesia with isoflurane was not sufficient to prevent chronic tinnitus in rats exposed to intense acoustic trauma, suggesting that protection is ineffective at greater noise levels.25 Acoustic noise has been shown to provide some degree of stimulation to anaesthetised animals.2 Substantial changes in blood pressure, heart rate and oxygen saturation have also been demonstrated in sedated infants undergoing MRI, probably as a result of sensory stimulation.26 A similar study which found no major physiological instability during MRI explained that this was due to lower noise levels whilst scanning.27 Auditory and sensory perceptions were also the most commonly reported sensations by children exhibiting awareness under general anaesthesia for a range of procedures including MRI.28 Reducing the effects of noise could therefore result in quicker scan times if less time is spent on maintaining anaesthesia in between sequences, and a lighter plane of anaesthesia could also be used.2,3 This is the case in paediatric MR scanning, where a combination of methods including ear protection allows scanning without sedation or anaesthesia in some younger patients.29 A study of anaesthetised children in MRI found that when ear defenders were used, less inhaled anaesthetic agent was required and recovery time was quickened,30 demonstrating a direct benefit of using ear protection. Noise-induced hearing loss is difficult to distinguish from agerelated hearing loss, and may be underestimated because it often affects specific frequencies.19 Another potential problem is that unilateral hearing impairment in dogs and cats is not usually behaviourally evident, and so often is overlooked by the owners.31 Potential adverse effects of translational and rotational movements of medical devices during MRI scanning include disconnection of anaesthetic equipment and physical movement of the subject. The former could have serious consequences if it were to lead to the animal gaining consciousness in the MR scanner. Movement of the animal during MR scanning, either as a result of stimulation or from movement of equipment, can also result in image artifacts and loss of diagnostic information. Pulsed magnetic fields employed in MR scanning can be responsible for the induction of an electrical current within a conducting material, resistance to which can cause heating.15,18 Heating has been demonstrated in ferrous materials, in non-
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metallic conductors32 and within the patient themselves.33 The largest temperature rises from magnetic induction are to be expected at the surface of the body and at the periphery.17 Hence, heating considerations could be further complicated by a rise in body temperature rather than that of the ear defenders. Heat generated during scanning is usually reduced by the cooling effects of blood perfusion and thermoregulation.34 As the pig’s head was devoid of insulating fur between the ear defenders and the skin, and lacked any cooling by blood perfusion, it represents a more extreme model of heating than would occur in a living dog. Some non-metallic materials such as silicone are visible on MR scans due to the MR signal being similar to the signal obtained from fat.35 Such materials when incorporated into medical equipment could present a problem due to phase-wrap artifact overlying areas of clinical importance. The absence of signal from the ear defenders also means that the scan time will not be extended as a result of increased oversampling. Passive protection compares well to more sophisticated and expensive forms of noise reduction such as active noise cancellation (ANC) which may provide reductions of around 18e35 dB.36,37 Ear plugs and ear defenders are the most widely used method of passive noise control used in human MRI,4 but the degree of protection provided varies according to the noise frequency. Effectiveness of ear defenders and ear plugs increases with higher frequencies as these are better attenuated.4,36 The manufacturers of MuttMuffsÒ state that they provide 25e28 dB of protection at 20 kHz, but information for other frequencies was not reported.38 The pinnae of small animals and dogs in particular often protrude laterally, and this can cause problems due to phase-wrap artifact.5 This artifact can be reduced by oversampling with an associated increase in scan time. By compressing the pinnae against the dog’s head, the ear defenders reduce the need for oversampling and may therefore cut scan time. One potential difficulty is accommodating the ear defenders within the MR coil when large dog heads are being scanned in an extremity coil. Similar limitations have also been identified in human scanning as a result of the extra width required to accommodate the ear defenders.4 However, this would not be an issue when scanning other body regions. For brain scans the ear defenders assist in the immobilisation of the dog’s head within the coil. For dogs scanned for cervical spine in dorsal recumbency, the ear defenders make the head more stable and less prone to tipping during the scan. As this study was carried out at 1 T, further testing of this device is recommended to assess its suitability for use at higher field strengths. Funding No funding was received for this study. Financial interest The author declares no financial interest in the product described in this paper. References 1. McNulty JP, McNulty S. Acoustic noise in magnetic resonance imaging - an ongoing issue. Radiography 2009;15:320e6. 2. Lauer AM, El-Sharkawy AM, Kraitchman DL, Edelstein WA. MRI acoustic noise can harm experimental and companion animals. Journal of Magnetic Resonance Imaging 2012;36:743e7. 3. Baker MA. Reduction of MRI acoustic noise achieved by manipulation of scan parameters - a study using veterinary MR sequences. Radiography 2013;19: 11e6.
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4. De Wilde JP, Grainger B, Price DL, Renaud C. Magnetic resonance imaging safety issues including an analysis of recorded incidents within the UK. Progress in Nuclear Magnetic Resonance Spectroscopy 2007;51:37e48. 5. Baker MA. Technical note: considerations for MR imaging of small animals. Radiography 2011;17:171e4. 6. Morton G, Gildersleve C. Noise in the MRI scanner. Anaesthesia 2000;55:1213e5. 7. Nordell A, Lundh M, Horsch S, Hallberg B, Aden U, Nordell B, et al. The acoustic hood: a patient-independent device improving acoustic noise protection during neonatal magnetic resonance imaging. Acta Paediatrica 2009;98:1278e83. 8. Peden CJ, Menon DK, Hall AS, Sargentoni J, Whitwam JG. Magnetic resonance for the anaesthetist. Part II: anaesthesia and monitoring in MR units. Anaesthesia 1992;47:508e17. 9. Schmitz B, Nimsky C, Wendel G, Wienerl J, Ganslandt O, Jacobi K, et al. Anesthesia during high-field intraoperative magnetic resonance imaging experience with 80 consecutive cases. Journal of Neurosurgery and Anesthesiology 2003;15: 255e62. 10. Shellock FG, Woods TO, Crues JV. MR labeling information for implants and devices: explanation of terminology. Radiology 2009;253:26e30. 11. American Society for Testing and Materials. F2052e06: standard test method for measurement of magnetically induced displacement force on medical devices in the magnetic resonance environment. West Conshohocken, PA: American Society for Testing and Materials (ASTM) International; 2006. 6pp. 12. American Society for Testing and Materials. F2213-06: standard test method for measurement of magnetically induced torque of medical devices in the magnetic resonance environment. West Conshohocken (PA): American Society for Testing and Materials (ASTM) International; 2006. 7pp. 13. American Society for Testing and Materials. F2182-09: standard test method for measurement of radio frequency induced heating on or near passive implants during magnetic resonance imaging. West Conshohocken, PA: American Society for Testing and Materials (ASTM) International; 2009. 12pp. 14. Shellock FG, Kanal E, Gilk T. Regarding the value reported for the term ‘spatial gradient magnetic field and how this information is applied to labelling of medical implants and devices. American Journal of Roentgenology 2010;196:1e4. 15. Sawyer-Glover AM, Shellock FG. Pre-MRI procedure screening: recommendations and safety considerations for biomedical implants and devices. Journal of Magnetic Resonance Imaging 2000;12:92e106. 16. Baker KB, Tkach JA, Phillips MD, Rezai AR. Variability in RF-induced heating of a deep brain stimulation implant across MR systems. Journal of Magnetic Resonance Imaging 2006;24:1236e42. 17. Boss AB, Graf H, Berger A, Lauer UA, Wojtczyk H, Claussen CD, et al. Tissue warming and regulatory responses induced by radio frequency energy deposition on a whole body 3-Tesla magnetic resonance imager. Journal of Magnetic Resonance Imaging 2007;26:1334e9. 18. Shellock FG, Gounis M, Wakhloo A. Detachable coil for cerebral aneurisms: in vitro evaluation of magnetic field interactions, heating and artifacts at 3T. American Journal of Neuroradiology 2005;26:363e6. 19. Ter Haar G, Venker-van Haagen AJ, van den Brom WE, Smoorenburg GF. Effects of aging on brainstem responses to toneburst auditory stimuli: a crosssectional and longitudinal study in dogs. Journal of Veterinary Internal Medicine 2008;22:937e45. 20. Moelker A, Vogel MW, Pattynama PMT. Efficacy of passive acoustic screening: implications for the design of imager and MR-suite. Journal of Magnetic Resonance Imaging 2003;17:270e5.
21. Kato T. Zur Physiologie der Binnenmuskeln des Ohres. Pfluegers Arch 1913;150: 569. 22. Borg E, Nilsson R, Engstrom B. Effect of the acoustic reflex on inner ear damage induced by industrial noise. Acta Oto-Laryngologica 1983;96:361e9. 23. Chung JW, Ahn JH, Kim JY, Lee HJ, Kang HH, Lee YK, et al. The effect of isoflurane, halothane and pentobarbital on noise-induced hearing loss in mice. Anesthesia and Analgesia 2007;6:1404e8. 24. Kim JU, Lee HJ, Kang HH, Shin JW, Ku SW, Ahn JH, et al. Protective effect of isoflurane anaesthesia on noise-induced hearing loss in mice. Laryngoscope 2005;115:1996e9. 25. Norman M, Tomscha K, Wehr M. Isoflurane blocks temporary tinnitus. Hearing Research 2012;290:64e71. 26. Philbin MK, Taber KH, Hayman A. Preliminary report: changes in vital signs of term newborns during MR. American Journal of Neuroradiology 1996;17: 1033e6. 27. Battin M, Maalouf EF, Counsell S, Herlihy A, Hall A, Azzopardi D, et al. Physiological stability of preterm infants during magnetic resonance imaging. Early Human Development 1998;52:101e10. 28. Blusse van Oud-Alblas HJ, van Dijk M, Liu C, Tibboel D, Klein J, Weber F. Intraoperative awareness during paediatric anaesthesia. British Journal of Anaesthesia 2009;102:104e10. 29. Benavente-Fernandez I, Lubian-Lopez PS, Zuazo-Ojeda MA, Jimenez-Gomez G, Lechuga-Sancho AM. Safety of magnetic resonance imaging in preterm infants. Acta Pediatrica 2010;99:850e3. 30. Ogurlu M, Orhan ME, Cinar S, Piri E, Kurt E, Sut N, et al. Effect of headphones on sevoflurane requirement for MRI. Pediatric Radiology 2012;42: 1432e6. 31. Stevens-Sparks CK, Strain G. Post-anaesthesia deafness in dogs and cats following dental and ear cleaning procedures. Veterinary Anaesthesia and Analgesia 2010;37:347e51. 32. Westbrook C, Roth CK, Talbot J. MRI in practice. 3rd ed. Oxford: Blackwell Publishing Ltd; 2005. 33. Ito Y, Omoto Y, Habe K, Yamanaka KI, Hakamada A, Isoda KI, et al. Magnetic resonance (MR) imaging-induced deep second-degree burns of lower extremities by conducting loop. Journal of the European Academy of Dermatology and Venereology 2007;21:1140e1. 34. Collins CM, Liu W, Wang J, Gruetter R, Vaughan JT, Ugurbil K, et al. Temperature and SAR calculations for a human head within volume and surface coils at 64 and 300 MHz. Journal of Magnetic Resonance Imaging 2004;19: 650e6. 35. Shellock FG, Shellock VJ. Vascular access ports and catheters: ex vivo testing of ferromagnetism, heating and artifacts associated with MR imaging. Magnetic Resonance Imaging 1996;14:443e7. 36. Hall DA, Chambers J, Akeroyd MA, Foster JR, Coxon R, Palmer AR. Acoustic, psychophysical, and neuroimaging measurements of the effectiveness of active cancellation during auditory functional magnetic resonance imaging. Journal of the Acoustic Society of America 2009;125:347e59. 37. Zvyagintsev M, Klasen M, Mathiak KA, Weber R, Edgar JC, Mathiak K. Real-time noise cancellation for speech acquired in interactive functional magnetic resonance imaging studies. Journal of Magnetic Resonance Imaging 2010;32. 705e3. 38. Safe and sound pets. Available from: http://www.safeandsoundpets.com/page/ page/4851794.htm [accessed 20 Jan 2012].