View as PDF version
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:
References:
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
https://pubs.rsna.org/doi/10.1148/radiol.2019182742
Morton
W. As MRI strength increases, so do concerns about magnet safety. AuntMinnie.com
2021; May 20, 2021
https://www.auntminnie.com/index.aspx?sec=rca&sub=ismr_2021&pag=dis&ItemID=132442
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
https://link.springer.com/content/pdf/10.1007%2Fs00330-019-06465-5.pdf
Print
PDF
version
http://www.patientsafetysolutions.com/