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Magnetic resonance imaging: safety aspects for the anaesthetist

image of Magnetic resonance imaging: safety aspects for the anaesthetist
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Abstract

Magnetic resonance imaging (MRI) is becoming more commonly available as a diagnostic tool in veterinary medicine. Heavy sedation or general anaesthesia is necessary in cats and dogs to reduce patient response to machine noise and provide the immobilization necessary for obtaining optimal diagnostic images. This chapters looks at how magnetic resonance images are obtained, conditions and hazards in the MRI unit and considerations for anaesthetic management

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Figures

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29.1 The strong magnetic field in an MRI system is always on, even when the system is not scanning.
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29.3 Example of a warning sign posted at the entrance to the MRI unit, outside the 5 Gauss line.
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29.4 Illustration of the ‘fringe’ magnetic field that extends beyond the magnet (labelled MRI) in all directions beyond the walls, floor and ceiling of the MRI unit (orange double-lined box). The black area indicates the limited access area inside the 5 Gauss line (yellow line). The distance of the 5 Gauss line from the magnet will vary depending on the magnet strength and shielding. (a) View from above. (b) View from the front.
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29.5 Symbols and their meanings to identify devices and equipment as safe, conditional or unsafe for use in the MRI room. Examples of each classification are included.
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29.6 Crash cart and associated equipment labelled with red tape to alert users that it must not be taken into the MRI room. Crash carts and emergency equipment should be available outside the MRI room in case of emergency.
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29.7 Carefully inspect any equipment before taking it into the MRI room. This MRI-compatible anaesthetic machine has a non-compatible capnograph on top of it (red arrow) that must be removed before the machine can safely be taken into the MRI room.
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29.8 Patient positioned on a long table, inside foam positioning devices, surrounded with radiofrequency coils. Next, the patient will be covered with insulating blankets and moved into the middle of the narrow bore of the MRI unit, which will compromise patient access and visualization throughout the imaging procedure.
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29.9 Copper sheeting is used to line an MRI unit to prevent radiofrequency (RF) interference from outside the room.
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29.10 This photograph of an MRI unit was taken outside the radiofrequency shielding (Faraday) cage built around it. Patients can be observed through the metal shielding during imaging. (Courtesy of The Veterinary Surgical Referral Practice of Northern Virginia, USA).
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29.11 Hearing protection styles for wear during MRI scanning. The noise reduction rating (NRR) indicates the extent to which the noise will be reduced when wearing the hearing protection.
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29.13 Patient placed on an absorbent pad to prevent urine or other fluids contaminating the table or leaking into components of the MRI system. Note the positioning of the intravenous catheter (yellow arrow) relative to the components of the magnet (red arrow indicates coil connection). Fluids from the intravenous infusion may damage coil connections.
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29.14 Large Wolfhound on an appropriately sized trolley just before transport into the MRI room. The red arrow indicates an ECG patch secured to the patient’s chest with a light wrap.
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29.16 (a) Non-compatible anaesthetic machine, vital signs monitors, fluid pumps and other equipment located outside the MRI room (magnet can be seen through the window), with MRI compatible hoses, cables and fluid lines passed through an radiofrequency waveguide portal (red arrow) into the MRI room. (b) Hoses and cables inside the MRI room connected to the patient. (Courtesy of Tanya Duke-Novakovski, Western College of Veterinary Medicine, University of Saskatchewan, Canada)
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29.17 Caution label on an MRI compatible Invivo monitor. The 5000 Gauss line would be inside the bore of either a 1.5 or 3.0 T magnet, making this monitor safe for these systems.
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29.18 Multiple monitors located inside the magnet room of this MRI trailer can be seen through the RF-shielded glass window. From this position, the patient, anaesthetic machine and monitors can all be observed from outside the room during imaging. (Courtesy of PetsDx Veterinary Imaging, USA)
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29.19 A remote vital signs monitor (on the far right) is located on the MR technologist’s desk outside the MRI room. The monitor in the middle shows a video camera view into the bore of the magnet to enable optimal patient visualization during imaging. (Courtesy of VCA Veterinary Referral Associates, USA)
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29.21 The large void (white arrow) seen on this MRI localizer image of a dog’s neck is an artefact caused by the small amount of metal in the pilot balloon of the endotracheal tube, which was under the dog’s neck. When positioning intubated patients, always ensure that the pilot balloon is moved away from the patient to avoid this artefact.
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29.22 The small circular area (white arrow) seen on this brain scan was found to be an artefact from the fluid in the intravenous tubing that was running beside the head coil. Objects adjacent to radiofrequency coils can be inadvertently brought into the MR images. To avoid this artefact, ensure that fluid lines are kept away from the area being scanned.
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29.24 Urine bag attached to a patient scheduled for an MRI scan. The bag has a metal clamp which is attracted to the small hand-held magnet, demonstrating that it is ferromagnetic. For safety reasons, before this patient enters the MRI area, the clamp should be removed from the bag, or the bag can be temporarily removed from the urinary catheter for the duration of the scan.
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29.25 Dog in the bore of a 1.5 T magnet, with the monitoring cables placed parallel to the bore with limited contact with the body. Red arrow = insulated ECG cable; Green arrow = pulse oximeter fibreoptic cable; Blue arrow = air-filled blood pressure line.
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29.26 A ‘snap’ or ‘click’ heat pad (a) before activation and (b) after activation. Pads of this type are magnet safe, but the metal activation snap inside the pad (black arrows) will cause an image artefact if it is close to the region being imaged. Note that these pads become very hot upon activation and should not be placed in direct contact with a patient, otherwise serious burns may occur.

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