Rapid responses are electronic comments to the editor. They enable our users
to debate issues raised in articles published on bmj.com. A rapid response
is first posted online. If you need the URL (web address) of an individual
response, simply click on the response headline and copy the URL from the
browser window. A proportion of responses will, after editing, be published
online and in the print journal as letters, which are indexed in PubMed.
Rapid responses are not indexed in PubMed and they are not journal articles.
The BMJ reserves the right to remove responses which are being
wilfully misrepresented as published articles or when it is brought to our
attention that a response spreads misinformation.
From March 2022, the word limit for rapid responses will be 600 words not
including references and author details. We will no longer post responses
that exceed this limit.
The word limit for letters selected from posted responses remains 300 words.
In addition to the dangers mentioned in the article, there are other
problems.
The hazards come not just from the static magnetic field, but from all
three elements of MRI, the time varying magnetic field, and radio
frequency signal as well as the static magnetic field. Metal objects can
be hazardous in a variety of ways. The static magnetic field can create a
"missile effect”, the terminal velocity of a paper clip in a 1.5 Tesla
magnet is about 40 miles per hour as is a pair of scissors or any
ferromagnetic object. A nine year old boy died last July when he was
struck by an oxygen cylinder.
The radio frequency signal and time varying magnetic field can both induce
currents in conductors. The human body, in particular limbs in loops, can
act as a conductor resulting in skin burns. Any piece of electrical
equipment such as a syringe driver can be damaged.
A pacemaker or pacing wires are a good illustration of the numerous
potential hazards. The static magnetic field can cause torque on the
pacemaker box. The combination of magnetic fields and radio frequency
signals could reprogram the pacemaker. Currents induced in the pacing
wires could either be sensed by the box or could stimulate the myocardium.
The heating effect could burn the myocardium. There are two dozen patients
with pacemakers known to have survived MR imaging and five known death. As
the technology has progresses the field strengths have increased and the
time varying fields have changed making predictions based on old reports
unreliable.
Other hazards come from the noise (often >90dB) and a quench, the
release of cryogens (usually liquid helium used to cool the magnet) with
the risk of hypoxia to staff and patients.
MRI Hazards
In addition to the dangers mentioned in the article, there are other
problems.
The hazards come not just from the static magnetic field, but from all
three elements of MRI, the time varying magnetic field, and radio
frequency signal as well as the static magnetic field. Metal objects can
be hazardous in a variety of ways. The static magnetic field can create a
"missile effect”, the terminal velocity of a paper clip in a 1.5 Tesla
magnet is about 40 miles per hour as is a pair of scissors or any
ferromagnetic object. A nine year old boy died last July when he was
struck by an oxygen cylinder.
The radio frequency signal and time varying magnetic field can both induce
currents in conductors. The human body, in particular limbs in loops, can
act as a conductor resulting in skin burns. Any piece of electrical
equipment such as a syringe driver can be damaged.
A pacemaker or pacing wires are a good illustration of the numerous
potential hazards. The static magnetic field can cause torque on the
pacemaker box. The combination of magnetic fields and radio frequency
signals could reprogram the pacemaker. Currents induced in the pacing
wires could either be sensed by the box or could stimulate the myocardium.
The heating effect could burn the myocardium. There are two dozen patients
with pacemakers known to have survived MR imaging and five known death. As
the technology has progresses the field strengths have increased and the
time varying fields have changed making predictions based on old reports
unreliable.
Other hazards come from the noise (often >90dB) and a quench, the
release of cryogens (usually liquid helium used to cool the magnet) with
the risk of hypoxia to staff and patients.
Competing interests: No competing interests