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In our October 15, 2019 Patient Safety Tip of the Week Lots More on MRI Safety we noted that the FDA has now cleared 7T MR for clinical use. While stronger magnets can enhance the benefits of MRI scanning, they also bring some new safety concerns and other unintended consequences.
We already discussed some of the concerns in our October 15, 2019 Patient Safety Tip of the Week Lots More on MRI Safety and multiple other columns listed below. We noted that the most recent update of ACR (American College of Radiology) guidelines on MR safety (Greenberg 2019) reminded us of the need for safety guidelines that will be related to 7T MR. It notes that many implants, devices, and foreign bodies, that dont significantly overheat at lower strength MR, might heat excessively at 7T strength. And certain devices that may retain functionality at lower MR strengths might lose functionality at 7T. Another paper (Hoff 2019) outlined the safety risks and associated risk-avoidance strategies of clinical 7-T MRI. Of particular concern are metallic implants and the risk for thermal injury. They note that over 6,000 metallic devices that have undergone evaluation at 1.5- and 3-tesla imaging, but that only 300 or so metallic implants and RF transmit coils have been tested for safety at 7-tesla MRI. Hoff et al. are also concerned about patient bioeffects such as vertigo, dizziness, false feelings of motion, nausea, nystagmus, magnetophosphenes, and electrogustatory effects that are more common and potentially more pronounced at 7 T than at lower field strengths. They suggest that earplugs and headphones could help reduce acoustic noise and related inner-ear and vision abnormalities and that increasing the amount of time patients spend in the 7-tesla static magnetic field as they enter and exit the scanner might help alleviate any physical issues.
AuntMinnie.com recently reported on a presentation by Filiz Yetisir, PhD at the 2021 International Society for Magnetic Resonance in Medicine (ISMRM) virtual meeting on the potential effects stronger MRIs have on patients (Morton 2021). Perhaps the most obvious is the potential projectile effect. Any ferromagnetic item in proximity to the magnet can be captured by the magnetic field. The Morton article has a photograph of an office chair sucked into the MRI. Our February 19, 2008 Patient Safety Tip of the Week MRI Safety described the catastrophic case where a 6-year-old boy suffered a fatal head injury when an oxygen cylinder was sucked into the MRI bore. Our November 5, 2019 Patient Safety Tip of the Week A Near-Fatal MRI Incident discussed a similar potentially serious MRI accident in Sweden. Our January 7, 2020 Patient Safety Tip of the Week Even More Concerns About MRI Safety discussed a Swedish national survey on MR safety (Hansson 2019) that had some striking data on incidents involving projectiles. The following items were reported in 6 or more incidents in a 12-month period: scissors or knife, walker, wheelchair, bed, infusion pump, oxygen tank, and metal object in pocket. Projectiles reported in fewer incidents included ventilator or monitor, cart or cleaning cart, crutches, keys, phone, forceps, laryngoscope, sharp object, magnetic object, equipment part, glasses, hair clip, hair pin, basket lid, rescue stretcher, screw, vacuum cleaner.
But, in addition to the main magnet, Yetisir notes problems can arise from the gradient coils or the radiofrequency coils. The magnetic field produced by the gradient coils in the MR scanner can induce an electric field in the patient's body and this may form "hot spots", which can create enough electric potential to stimulate large nerves. This phenomenon is known as peripheral nerve stimulation (PNS). That can appear as an uncomfortable tingling or tapping sensation. If that occurs, it is possible to switch to another mode. But keep in mind that you must have an adequate method of communication with the patient to monitor for PNS.
The electric fields induced by the gradient coils can even stimulate the heart, leading to cardiac arrhythmias.
Another phenomenon related to the gradient coils is mechanical vibration that can cause acoustic noise and lead to acoustic trauma. Earplugs can be used to protect hearing but keep in mind, as above, youll need to have some other method of communication with the patient during the scan. If the mechanical vibrations are strong, she recommends use of table pads. And, if these are very discomforting, the scan should be stopped.
The radiofrequency coils have a magnetic field that can induce an electric current in conductive human tissue and this can lead to temperature elevation and thermal injury. Several of our columns have discussed how burns can result from currents in things like EKG leads, ferromagnetic medication patches, or almost any ferromagnetic item.
But Yetisir also points out that "limb loops" can increase local RF heating by up to a factor of three. Such loops caused by things like hands touching hips or clasping of hands.
Lastly, she comments on implants. Weve previously discussed importance of screening for implants and checking to see if any such implants are MRI-safe. But keep in mind that sometimes that safety has been determined for MRIs of lesser magnet strength.
These are some good observations and recommendations. As we move to magnets of greater strength, we need to be cognizant that there may be unwanted effects we did not see at lower strengths.
We refer you back to our multiple columns on MRI safety but especially to our Patient Safety Tips of the Week for October 15, 2019 Lots More on MRI Safety and January 7, 2020 Even More Concerns About MRI Safety.
And any time you upgrade your magnet, that is a good time to reorient your staff and anyone who might enter the MRI suite for any reason about the safety issues that must be observed. That includes working with your local first responders who might show up for a fire or other incident.
Some of our prior columns on patient safety issues related to MRI:
Greenberg TD., Hoff MN, Gilk TB, et al. for the ACR Committee on MR Safety. ACR guidance document on MR safe practices: Updates and critical information 2019. J Magn Reson Imaging 2019; published online July 29, 2019
Hoff MN, McKinney A, Shellock FG, et al. Safety Considerations of 7-T MRI in Clinical Practice. Radiology 2019 292: 3: 509-551
Morton W. As MRI strength increases, so do concerns about magnet safety. AuntMinnie.com 2021; May 20, 2021
Hansson, B., Olsrud, J., Wilιn, J. et al. Swedish national survey on MR safety compared with CT: a false sense of security? Eur Radiol 2019; Online first December 13, 2019
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