RAPID Response to questions from readers

(formerly Dear SIRS)

Why Do the Gauss Lines Matter?

Anesthesia equipment within the MRI suite is exposed to powerful static, gradient (pulsed), and radiofrequency electromagnetic fields.1-3 We describe the potentially lethal failure of an Aestiva MRI anesthesia ventilator (Aestiva/5 MRI, Datex-Ohmeda, General Electric (GE) Healthcare Madison, WI) within the confines of a 1.5 Tesla (T) MRI suite. During the preparation for a pediatric general anesthetic, the ventilator maintained more than 30 cmH2O of PEEP while in the pressure-controlled ventilation mode when PEEP was activated. “Sustained pressure” and “check flow sensor” alarms were appropriately triggered. Manual ventilation was able to be performed without expiratory obstruction or sustained expiratory pressure, and the manual adjustable pressure level valve functioned correctly. The machine was removed from service and no patient harm occurred. The ventilator malfunction was completely reproducible and sustained in a controlled environment distant from the magnetic field. GE technical support determined that the flow control valve required recalibration. The company stated that they had not encountered this malfunction previously.

Further inspection of the machine also revealed absence of a built-in side bar designed to prevent the integrated ventilation system from swinging laterally. Consequently, the electronic ventilator system was able to swing outside the footprint of the machine, into the 300 gauss zone, without triggering the built-in gauss indicator which is remote from the ventilator system. The machine’s flow control valve, computer processing unit, and side bar were all replaced. The machine passed a full inspection and was put back in service. Our other Aestiva MRI machines were fully inspected after this event. The manufacturer recommended no further action for these machines other than regularly scheduled inspections and maintaining them outside 300 gauss. An FDA/Medsun report was submitted following this incident. After a failure mode and effects analysis (FMEA), permanent 300 Gauss line markers were installed in all MRI suites.

Indications for pediatric MRI are increasing due to the absence of ionizing radiation, high image resolution, and capability for function or biochemical measure acquisitions in real time. Infants and children undergoing MRI frequently require general anesthesia with intubation and mechanical ventilation due to patient condition or to eliminate respiratory artifact, by inducing prolonged periods of apnea.

The magnetic field strength inside MRI Zones 3 or 4 present significant challenges to biomedical equipment. There is no standard definition of MRI compatibility, and sensitive internal components of MRI-compatible equipment may be degraded by acute or chronic exposure to high-gauss magnetic fields. In this case, we suspect that repeated exposure to greater than 300 gauss interfered with ventilator performance resulting in patient risk. Permanent gauss line markers help mitigate this risk by providing visual cues against an invisible threat. However, there remain many barriers to the delivery of safe anesthesia care within the MRI suite including noise, field avoidance, obstructed lines of sight, and projectile risk.1,2 The advent of 3T MRI presents new challenges associated with stronger magnetic fields. The American Society of Anesthesiologists has published multiple practice advisories for the provision of anesthetic care during magnetic resonance imaging.1,2 It is imperative that anesthesiologists, nurse anesthetists, and anesthesiologist assistants understand MRI associated risks and remain vigilant.

John P. Scott, MD, Assistant Professor George M. Hoffman, MD, Professor and Richard J. Berens, MD, Professor

Medical College of Wisconsin, Departments of Anesthesiology and Pediatrics, Sections of Pediatric Anesthesia and Pediatric Critical Care.


  1. American Society of Anesthesiologists: Practice advisory on anesthetic care for magnetic resonance imaging. An updated report by the American Society of Anesthesiologists Task Force on anesthetic care for magnetic resonance imaging. Anesthesiology 2015; 122:495–520.
  2. Williams EJ, Jones NS, Carpenter TA, Bunch CS, Menon DK: Testing of adult and paediatric ventilators for use in a magnetic resonance imaging unit. Anaesthesia 1999; 54:969–74.
  3. American Society of Anesthesiologists: Practice advisory on anesthetic care for magnetic resonance imaging: A report by the American Society of Anesthesiologists Task Force on Anesthetic Care for Magnetic Resonance Imaging. Anesthesiology 2009; 110:459–79.

Response from GE Healthcare:

Dear Drs. Scott, Hoffman, and Berens,

Thank you very much for reminding everyone of 2 important issues which are critical to maintain safe operation of medical devices, particularly in the challenging MRI environment.

  1. It is important to ensure that equipment is maintained in its original configuration with all parts and components intact. Parts, such as the side bar, that you mention may seem to be unrelated to the safe operation of the device, but in fact these components are generally there for a reason and do need to be maintained in their original configuration. Your example illustrates this very clearly. This reminder applies equally to equipment used both within and outside of the MRI environment.
  2. Equipment designed for use in the MRI environment will include instructions and labeling describing how and where it may be used within that environment. Your initiative to provide visual markers at the 300 gauss line is a good example that will help to ensure that equipment is used within specifications.

Once again thank you for providing this information.

Sincerely, Kevin Tissot Chief Engineer Anesthesia and Respiratory Care GE Healthcare

Editorial Note:

The American College of Radiologists have identified and labeled 4 zones within the MRI suite. Zone 1 is a “safe” area where gauss fields are typically less than 0.5 gauss, and is an uncontrolled area, accessible to the general public without any screening for ferromagnetic objects. Zone 2 is a buffer area between zone 1 and the hazards of zone 3 and 4. Typically screening of individuals occurs in zone 2 before being allowed into zone 3 or 4. Zone 3 is usually the area just outside the magnet room where the magnetic field is strong enough to present hazards to unscreened individuals. Zone 4 is the magnet room itself where the magnetic fields are strongest and from which ferromagnetic objects must be excluded.