- Understand the potential risks, benefits, and uses of intraoperative MRI (iMRI).
- Summarize other types of intraoperative visualization techniques used in neurosurgery.
- Describe different available configurations for iMRI and how they affect the type of equipment available for safe use.
- Review MRI safety zones and MRI equipment ratings (MR-safe, MR-conditional, MR-unsafe) and their implications in the delivery of a safe anesthetic.
- Prepare for potential challenges to the anesthesiologist in the MR environment and additional preparation necessary for an anesthetic with iMRI.
- Know what needs to be included in the preoperative evaluation of a patient having an anesthetic with iMRI.
- Recognize the pitfalls of monitors in the MRI suite.
- Familiarize oneself with the concept of a checklist for the transition of the patient to and from the OR and MRI suite.
- Understand how to handle an emergency in the MRI suite.
- Define what it means to “quench” the magnet and potential risks.
A 48 year-old male with headaches presents for resection of a left parietal lobe tumor with iMRI. EEG and motor and sensory evoked potentials will be used.
His past medical history is significant for well controlled hypertension and type 2 diabetes.
No known drug allergies
BP: 127/64, HR: 71, RR: 18, SpO2: 100% on room air, T: 37.1oC
Height: 6.0 ft/183 cm
Weight: 196 lbs/88.9 kg
Mallampati III, TMJ distance of 3 cm, thyromental distance of 3 cm, prior intubation for a lumbar laminectomy was with a glidescope.
CBC, chemistry, blood glucose: all unremarkable
What is MRI? What is it used for? How does it work?
Magnetic Resonance Imaging (MRI) is a noninvasive method of obtaining detailed three dimensional images of structures within the body, and is especially useful for imaging soft tissue.1 In the brain, MRI differentiates white matter from grey matter and tumors and vascular abnormalities from healthy tissue.1 The powerful magnet produces a magnetic field, forcing protons in body tissue water to align with that field. Next, a radiofrequency (RF) current is pulsed through the patient, exciting the protons, and changing their rotational axis. After the RF current is then turned off, protons realign with the magnetic field and release energy on their way back into alignment. Different tissue types release different amounts of energy and take a certain amount of time to return to their “resting” state. Contrast dye (often containing Gadolinium) works to increase the speed at which the protons realign, brightening the image.1
Are there any associated risks of MRI?
In short, the risks related to the MRI are related to its magnetic fields: projectiles, interference with implanted devices, burns, and damage to hearing. MRI does not use ionizing radiation like x-rays and computed tomography.1
The three magnetic fields in the MRI suite include the following:
- The static magnetic field that ranges from 0.2T-3T and is orders of magnitude greater than the magnetic field of the earth. It has the potential to torque, attract, accelerate any ferromagnetic objects towards the opening of MRI bore and may interfere with implants, for example, pacemakers.2
- The radiofrequency field, which is produced by radiofrequency coils and has the potential to heat tissue and cause burns.2
- The fast-switching magnetic field gradients, which can heat implants, stimulate peripheral nerves. It is responsible for the noise generated in the MRI suite.2
The strong static magnetic field extends into to the entire room containing the MRI magnet and the surrounding rooms. The magnet can pull anything ferromagnetic into the scanner from across a room. Thus, care must be taken when entering an MRI area, as the magnet is always on. Anyone entering the MR environment must undergo screening and personnel must undergo MR safety training.3 Each institution has their own screening form and procedures. A sample screening form can be found here: http://www.mrisafety.com/ScreeningForm.html.4
Those with implants containing iron, such as pacemakers, implantable cardioverter-defibrillators, vagal nerve stimulators, deep brain stimulators, loop recorders, insulin pumps, cochlear implants, hearing aids, cerebral aneurysm clips, spinal cord stimulators, baclofen pumps, certain stents, bullets, and even endoscopy capsules are MR-incompatible. Only MR-safe or MR-conditional implants should enter the MR zone 3.2 Of note, many implants, medical materials and devices from the US and Europe over the last 30 years are made from nonferromagnetic materials and typically labeled MR-safe or MR-conditional.2 Surprisingly, even some tattoos and permanent makeup may contain ferrous material! There is a searchable online catalogue of devices and implants that can be helpful.4 Of note, dentures, braces and the spring in the endotracheal tube cuff will give an artifact.
MRI machines are noisy, and ear protection should be worn. The sound intensity can reach 120 decibels, which is the equivalent to loud sirens and louder than headphones turned to the maximum volume.5 Patients may also experience a twitching sensation due to the radiofrequency pulsing and the magnetic field stimulating peripheral nerves.1 Radiofrequency fields can also heat human tissue and may cause excessive heating and burns.2,3 The magnetic field can also generate currents in implants and pacemakers.3
Other considerations include the use of gadolinium-based contrast dye in patients with renal failure, especially those on dialysis as it can cause nephrogenic systemic fibrosis (NSF).2 NSF, originally named nephrogenic fibrosing dermopathy, is rare and presents one day to several years after gadolinium contrast exposure. Patients develop skin thickening and pruritis, with characteristic “cobblestoning” or indurated, raised papules. The “cobblestoning” spreads from the lower extremities to the upper extremities and can also fibrose internal organs.6,7 Cardiomyopathy, pulmonary fibrosis, pulmonary hypertension, diaphragmatic paralysis and death may result.8
Claustrophobia and the ability to lay still should also be considered. With regards to pregnancy, no effects have been demonstrated on fetuses, though it is recommended that MRI with gadolinium-based contrast are avoided as it is a pregnancy class C drug.2
How long has MRI been used for procedures in the operating room?
MRI has been used for diagnostic imaging since 1977.9 MRI has slowly made its way into the operating room since the first MRI-guided biopsy (non-neurosurgical) was done in the 1980s.10 Intraoperative MRI was first used in neurosurgery at Brigham and Women’s Hospital in Boston, Massachusetts in the 1990s.11
What are the uses for intraoperative MRI (iMRI) in neurosurgery? What are the benefits?
Intraoperative MRI (iMRI) is used in neurosurgery in both adults and children for awake and asleep tumor resections, DBS placement and epilepsy surgery.3 More recently, iMRI has been used for MR-guided high-intensity focused ultrasounds for ventral intermediate nucleus thalamotomies to treat medication-resistant essential tremor, Parkinson’s disease and other applications with good outcomes.12,13 Real-time imaging, improved tumor visualization and improved outcomes have been cited as benefits of iMRI.3 For example, neurosurgeons can visualize in real-time or near real-time the completeness of tumor resection and determine whether or not more tumor needs to be removed or if all of the necessary resection has been completed for a particular patient.3
What are other types of intraoperative visualization techniques?
There are also several types of intraoperative fluorescence techniques that allow direct, real-time visualization of a tumor or vascular structure. These include 5-aminmolevulinic acid (5-ALA), fluorescein, and indocyanine green (ICG). In general, these help the surgeon visualize difficult to see areas of tumor or vascular structures and/or differentiate these structures from normal tissue.
Gleolan or 5-aminolevulinic acid (5-ALA)
Gleolan or 5-aminolevulinic acid (5-ALA) is typically used for resection of high-grade gliomas, but its use for resection of medulloblastoma, melanoma, breast, and lung cancer has also been researched.14 The recommended dose is 20 mg 5-ALA HCl/kg PO, taken 3 hours prior to induction of anesthesia. The 5-ALA crosses the blood-brain barrier (BBB) and is taken up by the peptide transporter, Pept1. It has been shown to increase total tumor resection in a large multi-center trial.15 Of note, cadherin 12 can downregulate Pept1, causing some tumors not to fluoresce in those receiving 5-ALA.16 5-ALA is contraindicated in those who are hypersensitive to aminolevulinic acid or porphyrins and/or a history of acute or chronic porphyria.17 Adverse effects include pyrexia, nausea and vomiting, and hypotension. Elevation of liver enzymes is also possible postoperatively, specifically ALT and GGT. Photosensitivity may occur during the first 48 hours after administration; thus, the patient should be shielded from UV or room lights. Patients should also not receive drugs that may cause phototoxicity for 24 hours after administration. These include thiazides, sulfonureas, phenothiazines, sulfonamides, quinolones, tetracyclines and topical preparations that contain ALA.17 A special filter is used on the microscope and the tumor will appear red-violet, while healthy brain tissue will appear blue.17
Fluorescein also aids with tumor resection. The dose is 5 mg/kg IV, typically given at the time of induction of general anesthesia. It enters the tumor via the broken BBB. It is non-specific, as tumor, areas of edema, and blood vessels fluoresce in the presence of fluorescein and is bright yellow-green with the appropriate microscope filter.14 In one study, 95 patients with cerebral metastasis received a dose of fluorescein intraoperatively. Gross-total resection was achieved in 83% of those patients on postoperative contrast-enhanced MRI scans.18 Common side effects include yellow discoloration of skin and urine and nausea and vomiting. Hives, pruritis, bronchospasm and anaphylaxis have also been reported.18
Indocyanine green (ICG)
Indocyanine green (ICG) is often used for intraoperative angiography. Many doses have been described, but 12.5 mg to 25 mg IV is a typical dose. It absorbs light at 790 nm, with 98% of it binding to plasma proteins. It can help with visualization almost immediately after the dose circulates and remains in the blood for approximately 20-30 minutes due to its high protein binding.14 Under the appropriate microscope filter, it causes the blood vessels to turn bright white and the brain tissue is black when being used for angiography. Its ease of use and decreased cost make this more appealing than intraoperative catheter angiography.19 ICG is also taken up preferentially by many types of tumors and can aid in real-time tumor identification, not just high-grade gliomas, as is the case with 5-ALA. The “Second Window ICG” technique (SWIG) takes advantage of this differential uptake. ICG is administered 24 hours prior to surgery and accumulates in the tumor cells and aids in selective tumor visualization.20 After ICG is administered, it produces a temporary, dose-dependent decrease in SpO2 and increase in SctO2.21
What are the two types of MRI used for iMRI?
There are two categories of MRI used. They are “open” and “closed.” The “open” MRIs allow uninterrupted access by the surgeon to the patient and real-time imaging. This category can be further divided into two categories, horizontal gap and vertical gap.22 The names refer to the window where the patient is positioned and in which the surgeon operates. For the horizontal gap MRI, imagine two donuts stacked on top of one another. The patient will be positioned between the two donuts (magnets). The horizontal gap machines consist of low-field (0.2T-0.5T) and mid-field (0.5-1.0T) magnets. For a vertical gap MRI, imagine two vertical donuts stacked side by side.22 The patient will be “sandwiched” in between the two donuts (magnets). The vertical gap MRI uses a mid-field magnet (0.5T). The drawback is that open MRIs have limited field strength, poorer quality images and low availability. Both systems have a high set-up cost for the MRI machine itself and the need for all surgical and anesthesia equipment to be MRI-compatible.22
The “closed” category consists of either long bore or short bore magnets with are both high-field (1.5T or 3T). The patient is scanned inside of a “tube” or bore. They have high image quality, but do not allow patient access while scanning and require an operating room in addition to the room where the scanner is located for most procedures. They are expensive and all MRI systems require qualified personnel to operate these systems.22,23
What are some types of operating room/iMRI configurations?
In short, the operating room can be dedicated to surgeries with iMRI only, the MR system can come to the patient in the operating room on a rail from a separate room or shielded space, or the patient can leave the conventional operating room and enter an MRI suite.
When the MR scanner is located permanently in the operating room, it dedicates that operating room to iMRI cases only and the room must be shielded on all walls, the ceiling, and floor. Often, a high-field MR scanner is used in this configuration.9 Alternatively, a mobile MR scanner can enter an operating room. These are often low-field MRIs, and no special magnetic shielding of the operating room is required. However, a Faraday cage is used when the magnet enters the room and shields the magnet on all sides but the floor. The floor of the operating room under the operating table must be shielded permanently during construction.9 In this case, the shielding is to prevent radiofrequency interference from electronics, which degrades the images. When the magnet is not in the room, this type of operating room can be used as a regular operating room.9
Operating rooms used for iMRI are thoughtfully planned and constructed. The size of the operating room required if the MR magnet will enter the room is significantly larger than a regular operating room.9 All of the equipment must be MR-safe and the layout of the room must allow access to the patient and space for an anesthesia machine and the extra personnel necessary for procedures in this environment.9 It also needs to allow enough distance for other equipment beyond the 5-gauss line.9
What are the MRI safety zones?
|MRI Safety Zones2|
|Zone 1||Accessible to all staff and patients:
e.g., entrance to MRI facility
|Zone 2||Transition area: all patients must be screened and monitored by MRI personnel and MRI safety training is required for medical staff to enter this zone.
e.g., reception area, dressing rooms, MRI screening rooms
|MRI restricted area due to the proximity to the magnetic field and usually includes the control room for the MRI scanner, has physical barriers- e.g., coded access. MRI screening is also required for medical staff and patients to enter this zone. e.g., the MRI control room|
|Highest risk area as it is within the magnetic field. MRI magnet is here. Full MRI safety training required, and no ferromagnetic objects are permitted. The walls of the room contain the 0.5 mT (or 5-gauss) line of the fringe of the magnetic field|
How are devices, implants and equipment classified with respect to MRI safety?
There are three categories: MR-unsafe, MR-conditional, MR-safe. A useful infographic can be found on the FDA website.24 MR-unsafe items should not enter the MRI scanner room. MR-conditional items can enter the MRI scanner room under the specific conditions on their label. They have gauss line positioning restrictions or requirements to tether the device. MR safe items are not hazardous in the MR environment and have no restrictions.24
iMRI Case Questions:
How will you set up for this case? How might the set-up for an anesthetic for a neurosurgical procedure with iMRI differ from that of a regular neurosurgical procedure?
Considerations for set-up include whether the procedure will start in the OR, the MRI suite, or an induction room adjacent to the MRI suite and whether the patient will need to be moved between locations. Additional considerations include the type of MRI at your institution and its location with respect to the operating rooms.
The MR-compatible monitors, IV pumps and anesthesia machine need to be checked. An MR-safe stethoscope and emergency equipment such as an Ambu-bag, MR-safe oxygen tanks, emergency drugs, and specific drugs needed for the anesthetic should be prepared or readily available.
MR-conditional equipment needs to stay beyond the 5-gauss line. The 5-gauss line defines the line that, if crossed, the magnetic field could affect implanted devices.2 Within this line, the static magnetic field is higher than 5 gauss. A static magnetic field of less than 5 gauss is minimal risk to patients and staff.2
The distance of the patient from the anesthesia machine and pumps needs to be considered and extra extensions for the IV, arterial line and circuit should be available; a long end-tidal CO2 sampling line is helpful. Interestingly, set up time for iMRI cases has been previously studied and ranges from 0.5-3 hours but decreases exponentially as staff become familiar with the process.23
If the procedure will begin in the MRI suite, the patient can be induced on an MRI safe patient cart or on the MRI table, depending on airway concerns and how the patient will be positioned for the MRI. If the magnet is to come into the operating room, patient must have their head 180o from the anesthesia machine because the operating room bed in these rooms is stationary and should be taken into consideration prior to induction.3 If the patient is to move into the MRI scanner from the operating room, one must be prepared to have the patient moved onto the MR-compatible bed and freely move into the scanner without lines or the anesthesia circuit becoming caught on nearby objects.
Should anything additional be included in your preoperative evaluation given the fact that iMRI will be used?
In addition to information pertinent to the preoperative evaluation of a patient undergoing craniotomy, it is important to include questions regarding implants that may pose a danger if exposed to the MR environment. This includes, but is not limited to, the following: aneurysm clips, intracranial bypass clips, thalamic/bladder/vagal nerve stimulators, prosthetic heart valves, cardiac pacemakers and/or defibrillators, pacing leads or wires, gastric pacemakers, pain pumps, vascular ports/power ports, insulin pumps, continuous glucose monitors, vascular clips or stents, artificial limbs/joints, metal/shrapnel in body, metal in the eyes, dentures/partial plates, hearing aids or cochlear implants, medication patches, tattoos/permanent make-up or body piercings.9 This will be covered in the preoperative MRI patient safety questionnaire.
The patient should also be assessed for an allergy to gadolinium-based contrast. This is particularly important for those patients with renal disease, who are more susceptible to developing nephrogenic systemic fibrosis (NSF).9 Specifically, patients with acute or chronic severe renal insufficiency and a glomerular filtration rate of < 30 mL/min/1.73 m2, or those with acute renal insufficiency of any severity due to hepatorenal syndrome or in the perioperative liver transplantation period are at increased risk of NSF.25
If the patient is to be awake for the MRI portion of the procedure, they should be informed and assessed for claustrophobia or anxiety. They should also know that they will have ear plugs placed for the duration of the scan.
If there is any concern about the patient’s airway, induction and intubation should occur in a place where airway equipment, such as a glide scope or fiberoptic scope, is safe to use. This will be beyond the 5-gauss line in the MRI suite or in a controlled setting in the operating room or an induction room. MR-incompatible equipment used in proximity to the scanner should be tethered to the wall or secured per the institution’s guidelines.
Are there any special considerations for monitoring this patient?
Standard ASA monitors should be used. This includes EKG, blood pressure, end-tidal CO2, pulse oximetry and temperature monitors. The monitors used in the MR environment are MR-conditional. They are wireless and are susceptible to interference with the electronic noise generated by the MR RF pulses and magnetic gradients in the MR environment.9
The most susceptible monitor to electric noise interference is the EKG. The MR-conditional monitors have a filter for the noise produced in the MR environment, but the EKG waveforms may still be distorted and ST segment and rhythm changes may be masked.9 It has been recommended that the standard configuration of leads be used for iMRI and the filter on the EKG waveform be used.9
Temperature should also be monitored as the patient may overheat due to the RF waves from the magnet. The room is typically kept at a cooler temperature to prevent heating of devices/monitors from these RF waves, which may burn the patient.9 Patients may also become hypothermic and should be warmed with warm blankets and normothermic prior to extubation.26
Ventilation and oxygenation may be monitored by MR-safe end-tidal CO2 monitors and pulse oximeters. Additionally, breath sounds can be auscultated with a MR-safe stethoscope, though it may be difficult to hear breath sounds due to background noise in the MR-enviroment.9 Blood pressure monitoring can be done with either a non-invasive blood pressure cuff and/or arterial line without the risk of electric noise interference.9
Your patient has undergone their preoperative evaluation, preoperative MRI screening questionnaire, and the consents are signed. He is wearing a hospital gown without metal and is on an MRI-safe patient bed. The plan is for the patient to have a preoperative MRI under general anesthesia and then proceed to the operating room. What is your plan for induction and intubation?
After checking the anesthesia machines, equipment and drugs in both the MRI and OR and setting up these rooms, you can bring the patient back to the MRI suite. First, any non-MR safe monitors are removed from the patient prior to entering zone 4. MR-conditional standard ASA monitors should be placed and working/connected to the monitor. Due to dead-space in the long sampling line, the end-tidal CO2 waveform will take more time to appear. If the induction occurs in an “induction room” outside of zone 4, the process is the similar. Care must be taken when transitioning to MR-compatible monitors so that anesthetized patients are continuously monitored.
Induction may be done any number of ways, however if neurophysiologic monitoring precludes neuromuscular blockade, succinylcholine may be used. A small dose of nondepolarizing neuromuscular blocker may be beneficial for the initial scan to avoid the potential for motion artifact.
This patient had a glidescope intubation for past anesthetics. If this route is chosen, it must be done outside of the 5-gauss line. The pilot balloon on the endotracheal tube should be taped in place to avoid motion artifact generated by its metal coil. Once scanning has begun, there will be little access to the head. The airway should be well secured and with all connections checked. Only endotracheal tubes without metal should be used. Wire-reinforced or NIM tubes are not compatible with iMRI, as are neuromonitoring needles and wires.
You elect to intubate the patient on the bed in the MRI suite. He was last intubated for a lumbar laminectomy with a glidescope. After a smooth induction, you are able to mask ventilate. You decide to do a direct laryngoscopy given a favorable airway exam. You have a grade IV view and decide to return to mask ventilation. What will you do next?
Prior to starting the anesthetic, it is important to consider the location of the MRI suite in relation to the help that is available. Many MRI suites are in remote locations and help will take longer to arrive. It is reasonable to proceed with a glidescope intubation and call for help in advance. The glidescope should be outside of the 5-gauss line and anchored to avoid inadvertently crossing the 5-gauss line. If more airway equipment is required or more help is required, moving the patient from zone 4 into zone 3 should be considered. Another consideration is that remote locations may not have the supplies restocked frequently and it is very important to make sure all equipment is available and the room is stocked- including LMAs, oral airways, endotracheal tubes, etc.
What could happen if a ferromagnetic object, such as the glidescope, crossed the 5-gauss line and turned into a projectile, landing in or on the bore? What does it mean to quench the MRI? What are the indications for quenching the MRI? What are the dangers of quenching the MRI? How much does it cost to quench?
Quenching can be accidental or intentional. If a patient is unable to be removed from the bore due to equipment malfunction, if there is a fire and/or a life is at-risk, quenching the magnet is indicated.
The MRI contains superconducting coils that need to be cooled to allow an electrical current to flow through them. To achieve this, the coils are bathed in liquid helium, which remains liquid at absolute zero. When the magnet is quenched, there is a loss absolute zero of temperature in the magnet coils. Instead of superconducting, the coils become resistive and their temperature increases. Helium liquid boils to gas and rapidly escapes through helium vents. If the helium vents fail, helium will replace the oxygen in the room, the oxygen monitor in the MR suite will alarm and asphyxiation could result if personnel and the patient are not immediately evacuated. The room temperature will also acutely drop, with one source reporting a 10-15 degree Fahrenheit drop in temperature.27,28
The release of helium gas will also cause an increase in pressure in the scanning room compared to the control room, which may prevent the door between the two rooms from opening.27 In this scenario, the glass in the control room should be broken to release the pressure. This pressure increase may also cause tympanic membrane rupture.27 Newer MRI machines exist that have a low volume of helium and quenching does not cause a release of helium.28 Quenching the magnet is incredibly expensive. The cost includes repairs to the MRI and the resultant one to two months of magnet downtime. Helium is very expensive and will also contribute to the cost of the quench.
The patient already had a preoperative MRI. The surgeon will start the surgery in the operating room and then repeat the scan post-resection. How might this change your plan for induction?
Induction will be in the operating room, potentially on a special OR bed designed to transfer the patient onto an MRI exam bed. The bed type may cause the patient’s head to be further from the top of the bed, i.e., further from the anesthesiologist prior to induction/intubation. With certain set-ups, the operating room bed is in a fixed position and the orientation is 180o from the anesthesia machine (as with low-field MRIs that enter the operating room from a special “storage room”). Standard ASA monitors should be placed and should be replaced by MR-compatible monitors prior to transferring from the OR to the MRI suite. See The University of Kansas Operating Room to iMRI Protocol/Checklist below.
For maintenance of your anesthetic, what are your options?
Sevoflurane and isoflurane are options for inhaled anesthetics. The desflurane vaporizer is not MR-safe and cannot be used in MR environments.9 If a TIVA is planned, infusions must be on MR-compatible pumps. Depth-of-anesthesia monitors, such as BIS, are not MR-compatible.
The patient is pinned with MR-safe Mayfield frame and pins. They are positioned supine. Are there any concerns you have with regards to positioning specifically related to the MRI?
Positioning for the procedure may be more challenging in the MRI suite. First, the patient needs to be able to fit into the MRI bore. This can be checked with either a “dry run” prior to the day of the procedure or calipers that mimic the width of the MRI bore. Once this is confirmed, the patient will need to be positioned so that their arms are at their sides. They should be secured with Velcro or another device that will prevent the arms from slipping and the patient from shifting. All monitors and airway equipment should be positioned such that there are no loops or kinks to avoid induction of a current and a subsequent burn.2,11 Skin contact should be avoided by placing towels or foam between skin and cables and where there is skin to skin contact to avoid burns.2 The face should be free of pressure from the headframe or coils.
Scanning has begun and you notice a ST segment changes, and intermittent rapid heart rate, but his vitals are otherwise stable. What are your next steps?
As above, the MR-compatible monitors are wireless and are susceptible to interference with the electronic noise generated by the MR RF pulses and magnetic gradients in the MR environment.9 The most susceptible monitor is the EKG. The MR-conditional monitors have a filter for the noise produced in the MR environment, but the EKG waveforms may still be distorted and ST segment and rhythm changes may be masked.9
If there is a true emergency, one must clearly communicate this to the MRI personnel and the scan should be stopped. The patient should be promptly removed from MRI zone 4 for further intervention. Each institution should have a specific plan in place for such an emergency while ensuring that nobody inadvertently enters MRI zone 4 with a ferromagnetic object.
Several minutes later, you notice that you have lost end-tidal CO2 and the patient is beginning to desaturate. How will you proceed?
The main considerations are communicating the concern to the MRI personnel, stopping the scan, and removing the patient from the bore to assess the airway, patient, and circuit. If the airway needs to be re-secured, the considerations are the same as they were with the initial intubation. The exact next steps are unique to each scenario. Call for help early. It is not unreasonable to move the patient into MRI zone 3 and secure the airway accordingly. Of note, laryngeal mask airways and endotracheal tubes are MR-conditional.
The surgery has gone well and now it is time to transfer the patient from the OR to the MRI scanner. Do you have any concerns?
Each institution will have a protocol prior to the patient entering MRI zone 4 from the operating room or prior to the magnet entering the OR. The general idea is that anything ferromagnetic is removed from the patient, and only the patient on the MR-safe bed and MR-compatible monitors are moved from the operating room. Other equipment, such as IV poles, towel clamps, neuromonitoring wires/needles, etc. stay in the operating room. This is where practicing or simulating the transfer of the patient (or the magnet) into or out of the operating room could be incredibly beneficial. It also helps to have the entire team, even if not entering zone 4, trained in MRI safety. Checklists have been shown to improve communication between teams.3
The University of Kansas Operating Room to iMRI Protocol/Checklist:
|Prior to Induction:
After Induction- Prior to Start of Surgery:
Preparation for iMRI- prior to MRI table entering OR
MRI STAFF: Visual and verbal verification from Anesthesia team, Circulator, and Surgeon that nothing is contacting patient except MRI safe monitoring equipment. RN: Announce that ALL persons in the OR will remain in position as MRI staff bring in MRI table and move patient to the magnet room
MRI table is brought into OR
When the MRI doors open, the patient, anesthesia team, MRI staff and RN are THE ONLY personnel to enter the magnet room! After the patient enters the MRI suite, IMMEDIATELY close the MRI doors.
After going through the checklist, you realize that a temperature-sensing foley catheter has been placed by the new circulating nurse in the operating room prior to entering MRI zone 4. Does anything need to be done prior to entering the MRI suite?
The temperature-sensing foley is not MR-compatible. It should be replaced with a regular foley catheter. Temperature can be monitored with a separate MR-compatible temperature probe. When transferring the patient from the OR to the MR scanner, a sandbag (filled with sand, not ferromagnetic material) should be placed on top of the foley bag if it has a metal clip.
How might your plan change if this patient were presenting for an MRI-guided laser ablation of the tumor?
This procedure could be done entirely in the MRI suite under general anesthesia. The patient would enter the MR bore and exit the opposite end, where the surgery would be performed. This arrangement requires extensions on the anesthesia circuit and other tubing. In some institutions, the burr holes are made in the operating room and then the anesthetized patient is transferred to an MRI in another location outside of the operating rooms. It is important to communicate the with the surgery team and OR team because moving the patient from one location to the other (OR to MRI) requires setting up for the case in each location and checking two anesthesia machines.
The patient’s temperature matters for these procedures as the laser heats the brain tissue to ablate the tumor. If the patient is hypothermic, it will take longer to heat the tissue. Additionally, these procedures can take a long time. There are many periods of surgical stimulation followed by no stimulation, which lends itself to intraoperative hypotension that should be treated and may require a vasoactive infusion.
It is also important to remember a plan for DVT prophylaxis (sequential compression devices with long tubing, compression hose, etc.). Interestingly, one retrospective study showed no statistically significant difference in the incidence of DVT and PE in GBM patients undergoing tumor resection with iMRI when intermittent pneumatic compression devices were added to a regimen of graduated compression stockings, LMWH and postoperative physiotherapy.29
This patient instead has essential tremor and presents for MR-guided high-intensity focused ultrasound for ventral intermediate nucleus thalamotomy. How might this change your anesthetic plan?
These are done entirely in the MRI suite with little to no sedation as patients need to participate in these procedures. Patients should be assessed preoperatively for claustrophobia, their ability to lay flat and cooperate/participate. During this procedure, the patient lays supine with their head fixed to the MRI bed. Should there be an emergency, there are a few extra steps to removing the patient from the head-holder. There is a silicone or rubber membrane that the patient wears over their head and the space between the patient’s head and the ultrasound transducer is filled with water. In addition to releasing the frame from the MRI table, the water must be drained from above the membrane before the patient can be removed from the MRI suite.
One of the most common symptoms experienced by patients during these procedures is nausea. For this reason, suction should be available and able to reach the patient. Other common symptoms include headache and backache. Care must also be taken to avoid any medications that may interfere with the patient’s tremor.12
The procedure is complete, but it was all day long. The patient is not waking up. How will you proceed?
Assuming that vitals are stable, there is no residual anesthetic and adequate time has passed, it is important to check how much opioid and benzodiazepine (or any other drug) have been given. Additionally, the train-of-four (TOF) should be checked, and the patient should be properly reversed. The TOF monitor is not MR-compatible and should be used outside of the 5-gauss line. The patient’s temperature should be checked as they could be hypothermic. Bair huggers and fluid warmers are not MR-compatible, and temperature is typically managed with warm blankets, though patients tend to heat up inside the MRI bore. Interestingly, some hospital blankets are made with small copper fibers inside of them and are incompatible with MRI. There is one case report of this causing a fire in a PET/MRI imaging system.30
This patient is a diabetic, and as with any delayed emergence, their blood glucose should be checked. The glucometer is not MR-compatible and should not enter zone 4. An ABG with electrolytes should be sent to assess for hyper/hypoventilation and electrolyte abnormalities. As with glucometers, portable blood gas analyzers, such as iSTAT machines, are not MR-compatible.
The cause could also be related to the CNS, especially given the nature of the surgery. If emergence is truly delayed, communication with the neurosurgery team and obtaining a head CT are reasonable. If the patient needs more equipment (such as a warming device, etc.) then it may also be reasonable to move the patient out of zone 4 to decrease the chance of any ferromagnetic material from entering and becoming a projectile.
What are some specific challenges to the anesthesiologist while in the MRI environment?
The anesthesiologist must always maintain vigilance and continuously monitor the patient. With iMRI, the patient is often inside the bore of the machine, precluding direct visualization of the patient, their airway and lines. Simultaneously, they must know the institution’s MRI safety protocols and maintain awareness for potential hazards in the MRI suite, such as ferromagnetic projectiles. The MR environment is loud, and the acoustic noise may interfere with the ability to hear monitors or verbal communication. RF interference with MR-conditional monitors can be confounding.
Many MRIs are in remote locations. Checking to make sure all equipment, emergency equipment and drugs are properly prepared and available ahead of time is always important, and perhaps more important in a remote location. Help will take longer to arrive to a remote location. It is always wise to let colleague know that there is a case requiring anesthesia in a remote location and to let technicians or nurses know what number and/or colleague to call for help ahead of time, should an emergency arise.
Finally, items that an anesthesiologist may have with them that should be removed prior to entering the MRI suite include (but are not limited to) the following: pagers and phones (making communication with those outside of the MRI suite difficult), watches/jewelry, pens (especially those with springs), stethoscopes, laryngoscopes and advanced airway equipment, train-of-four monitors, glucometers, and certain pre-made medication syringes. When in doubt, always consult an MRI technician.
- Bioengineering NIoBIa. Magnetic Resonance Imaging (MRI). Accessed March 12, 2022. https://www.nibib.nih.gov/science-education/science-topics/magnetic-resonance-imaging-mri
- Sammet S. Magnetic resonance safety. Abdom Radiol (NY). Mar 2016;41(3):444-51. doi:10.1007/s00261-016-0680-4
- Berkow LC. Anesthetic management and human factors in the intraoperative MRI environment. Curr Opin Anaesthesiol. Oct 2016;29(5):563-7. doi:10.1097/ACO.0000000000000366
- Shellock FG, Ph.D. Pre-MRI Screening and the Pre-MRI Screening Form. Accessed March 12, 2022. http://www.mrisafety.com/ScreeningForm.html
- Disorders NIoDaOC. Noise-Induced Hearing Loss. Website. National Institutes of Health. Updated May 31, 2019. Accessed March 12, 2022. https://www.nidcd.nih.gov/health/noise-induced-hearing-loss
- Bhargava V, Singh K, Meena P, Sanyal R. Nephrogenic systemic fibrosis: A frivolous entity. World J Nephrol. May 25 2021;10(3):29-36. doi:10.5527/wjn.v10.i3.29
- Mathur M, Jones JR, Weinreb JC. Gadolinium Deposition and Nephrogenic Systemic Fibrosis: A Radiologist’s Primer. Radiographics. Jan-Feb 2020;40(1):153-162. doi:10.1148/rg.2020190110
- Nainani N, Panesar M. Nephrogenic systemic fibrosis. Am J Nephrol. 2009;29(1):1-9. doi:10.1159/000149628
- Bergese SD, Puente EG. Anesthesia in the intraoperative MRI environment. Neurosurg Clin N Am. Apr 2009;20(2):155-62. doi:10.1016/j.nec.2009.04.001
- Blanco RT, Ojala R, Kariniemi J, Perala J, Niinimaki J, Tervonen O. Interventional and intraoperative MRI at low field scanner–a review. Eur J Radiol. Nov 2005;56(2):130-42. doi:10.1016/j.ejrad.2005.03.033
- Rogers CM, Jones PS, Weinberg JS. Intraoperative MRI for Brain Tumors. J Neurooncol. Feb 2021;151(3):479-490. doi:10.1007/s11060-020-03667-6
- Sinai A, Katz Y, Zaaroor M, Sandler O, Schlesinger I. The Role of the Anesthesiologist during Magnetic Resonance-Guided Focused Ultrasound Thalamotomy for Tremor: A Single-Center Experience. Parkinsons Dis. 2018;2018:9764807. doi:10.1155/2018/9764807
- Quadri SA, Waqas M, Khan I, et al. High-intensity focused ultrasound: past, present, and future in neurosurgery. Neurosurg Focus. Feb 2018;44(2):E16. doi:10.3171/2017.11.FOCUS17610
- Ewelt C, Nemes A, Senner V, et al. Fluorescence in neurosurgery: Its diagnostic and therapeutic use. Review of the literature. J Photochem Photobiol B. Jul 2015;148:302-309. doi:10.1016/j.jphotobiol.2015.05.002
- Stummer W, Pichlmeier U, Meinel T, et al. Fluorescence-guided surgery with 5-aminolevulinic acid for resection of malignant glioma: a randomised controlled multicentre phase III trial. Lancet Oncol. May 2006;7(5):392-401. doi:10.1016/S1470-2045(06)70665-9
- Suzuki T, Wada S, Eguchi H, et al. Cadherin 13 overexpression as an important factor related to the absence of tumor fluorescence in 5-aminolevulinic acid-guided resection of glioma. J Neurosurg. Nov 2013;119(5):1331-9. doi:10.3171/2013.7.JNS122340
- Gleolan Prescribing Information. 2022. Accessed March 12, 2022. https://f.hubspotusercontent40.net/hubfs/20173990/full_prescribing_information.pdf
- Hohne J, Hohenberger C, Proescholdt M, et al. Fluorescein sodium-guided resection of cerebral metastases-an update. Acta Neurochir (Wien). Feb 2017;159(2):363-367. doi:10.1007/s00701-016-3054-3
- Hardesty DA, Thind H, Zabramski JM, Spetzler RF, Nakaji P. Safety, efficacy, and cost of intraoperative indocyanine green angiography compared to intraoperative catheter angiography in cerebral aneurysm surgery. J Clin Neurosci. Aug 2014;21(8):1377-82. doi:10.1016/j.jocn.2014.02.006
- Cho SS, Salinas R, Lee JYK. Indocyanine-Green for Fluorescence-Guided Surgery of Brain Tumors: Evidence, Techniques, and Practical Experience. Front Surg. 2019;6:11. doi:10.3389/fsurg.2019.00011
- Baek HY, Lee HJ, Kim JM, Cho SY, Jeong S, Yoo KY. Effects of intravenously administered indocyanine green on near-infrared cerebral oximetry and pulse oximetry readings. Korean J Anesthesiol. Apr 2015;68(2):122-7. doi:10.4097/kjae.2015.68.2.122
- Gandhe RU, Bhave CP. Intraoperative magnetic resonance imaging for neurosurgery – An anaesthesiologist’s challenge. Indian J Anaesth. Jun 2018;62(6):411-417. doi:10.4103/ija.IJA_29_18
- Barua E, Johnston J, Fujii J, Dzwonczyk R, Chiocca E, Bergese S. Anesthesia for brain tumor resection using intraoperative magnetic resonance imaging (iMRI) with the Polestar N-20 system: experience and challenges. J Clin Anesth. Aug 2009;21(5):371-6. doi:10.1016/j.jclinane.2008.09.004
- US Food and Drug Administration SfMRT. Understanding MRI Safety Labeling. Updated 11/23/2020. Accessed March 12, 2022. https://www.fda.gov/media/101221/download
- MAGNEVIST- gadopentate dimeglumine injection Drug Label Information. Website. U.S. National Library of Medicine. Updated July 23, 2019. Accessed March 12th, 2022. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=abef9e9b-a29f-4abe-aeff-88ae42648057
- Schroeck H, Welch TL, Rovner MS, Johnson HA, Schroeck FR. Anesthetic challenges and outcomes for procedures in the intraoperative magnetic resonance imaging suite: A systematic review. J Clin Anesth. May 2019;54:89-101. doi:10.1016/j.jclinane.2018.10.022
- Engineering UDoRaB. Magnet Quench. Accessed March 12th, 2022. https://radiology.ucsf.edu/patient-care/patient-safety/mri/quench
- Elster AD. What is a quench? September 9, 2022, 2022. Updated 2021. Accessed September 9, 2022, 2022. https://www.mriquestions.com/what-is-a-quench.html
- Ebeling M, Ludemann W, Frisius J, et al. Venous thromboembolic complications with and without intermittent intraoperative and postoperative pneumatic compression in patients with glioblastoma multiforme using intraoperative magnetic resonance imaging. A retrospective study. Neurochirurgie. Jun 2018;64(3):161-165. doi:10.1016/j.neuchi.2018.04.007
- Bertrand A, Brunel S, Habert MO, et al. A New Fire Hazard for MR Imaging Systems: Blankets-Case Report. Radiology. Feb 2018;286(2):568-570. doi:10.1148/radiol.2017162921