MRI: Use, Safety, and Patient Care
By Donald Woodward
Resonance Imaging (MRI) has evolved to become one of the most valuable
diagnostic tools ever developed. MRI's ability to provide unprecedented
visualization of internal organs and structures without using X-rays makes it
the modality of choice for many applications. In addition, MRI can help reduce
healthcare costs by eliminating the need for other diagnostic studies that are
more invasive to the patient.
One of the most important elements in delivering this valuable imaging
service involves maintaining a safe environment. For most patients and settings,
MRI is completely safe. For others, though, it can have serious complications.
How MRI Works
The human body contains millions of hydrogen protons found mainly in water.
These protons act as tiny magnets. During an MRI exam, these tiny magnets are
exposed to a large magnetic field (thousands of times stronger than the Earth's
magnetic field), causing them to align with the field. Radio waves, tuned to a
specific frequency, are then pulsed toward the patient, causing protons to tip
away from the magnetic field. Once radio waves are turned off, protons realign
with the magnetic field. As they realign, protons release energy in the form of
another radio signal. This signal is picked up by the antenna (called a coil)
and sent to a computer, which calculates the signal into an image, which is then
transferred to film for interpretation.
MRI began as a study restricted to the brain and spine because of limited
hardware and software capabilities. As time went on, the technology was improved
to allow imaging of the musculoskeletal and abdominal areas. Newer techniques
have recently been developed to assess acute brain stroke, tumors, and renal and
peripheral vasculature. One of the most exciting applications currently being
developed will assess not only heart structure, but also function. Soon refined
techniques will preclude the battery of tests required for a complete cardiac
In general, most MRI scans take between 20-60 minutes and patients should
experience no discomfort. They will, however, hear a loud knocking or tapping
sound when the scanner is actually acquiring information.
Although MRI has become a well accepted imaging modality, it does have some
degree of risk for the patient. There are three main elements of concern when it
comes to MRI safety--protecting the patient, protecting the equipment, and
protecting the image.
The first concern is the protection of the patient. Some metallic implants
are ferromagnetic (attracted to a magnet) and may cause harm if moved or
dislodged by the strong magnetic forces used during an MRI exam. In addition,
there are also implants that may be electronically affected by the radio waves
and changing magnetic fields used during the scanning procedure. The best method
to protect patients from either of these situations is to review medical and
surgical history thoroughly. This usually involves a dedicated MRI screening
questionnaire completed by the patient and reviewed by the facility prior to
The questionnaire must ask about previous surgical procedures, with
particular regard to head or heart surgery, since most absolute
contraindications involve these areas. For example, a pacemaker or ferromagnetic
aneurysm clip will prohibit a patient from having an MRI exam, but a hip implant
usually will not. Common absolute contraindications include, but are not limited
to: pacemakers, ferromagnetic intra-cranial aneurysm clips, metal fragments in
the eye, and cochlear implants.
There has been a substantial amount of research to determine which
contraindications are absolute and which would be considered relative (may
prohibit a scan or may alter your approach). These include pregnancy (which is
based on risk versus benefit), certain heart valves, bone stimulators, neuro-stimulators,
hearing aids, ocular implants, and certain dental implants.
Protecting the equipment helps to ensure the safety of patient. Items that
are ferromagnetic (attracted to a magnet). These may be drawn to the MRI magnet
and become projectiles. They can become dangerous, hurtling through the air from
the point of origination to the center of the magnet. Thousands of dollars have
been spent replacing the outer housing of the MRI magnet from damage caused by a
Therefore, it is very important to control the environment by establishing
zones of protection. MRI technical personnel must screen all persons and
equipment for ferromagnetic objects prior to entering these zones.
The goal of the MRI screening is to obtain a high-quality image to safely and
accurately diagnose a patient. However, this image may not be achieved if
certain objects interfere. Some implants or metal objects can cause severe
artifacts, making it difficult to interpret the image. In some cases, the
artifacts are so severe, the images are considered non-diagnostic. Examples of
objects that may cause artifacts are hip-implant hardware or Harrington rods.
Recommendations When Preparing Patients for MRI
In order to have an effective MRI screening, the patient must be thoroughly
prepared for the experience. One of the biggest concerns patients report is
claustrophobia. When patients know what to expect--that the MRI screening
involves being in a small, enclosed space and holding still during the
process--they approach the situation in a calmer manner.
The referring physician and nurse also need to provide as much clinical
history as possible to the MRI facility performing the exam. Background
information, such as seizures, pains, and other factors can help the radiologist
design the most effective study for the patient. If the patient does require
some medication to assist with anxiety or pain, it is best ordered prior to the
study, and clearly communicated to the MRI facility.
MRI screening provides patients and doctors with an invaluable diagnostic
resource, but as with any tool, it is most effective when used in accordance
with proper guidelines and safety measures. By properly training all personnel
who come into contact with the technology--from the referring doctor to the
scheduling nurse and perhaps most importantly, the patient-- MRI can provide the
answers to many vital medical questions.
Donald Woodward is national manager of the Applications and Education
Group for Alliance Imaging, Inc. in Anaheim, Calif.