What is
Magnetic Resonance Imaging ?
Magnetic resonance imaging –
MRI – is a scanning technique uses strong magnetic
fields to form clear, detailed images of the
internal tissue of the body , that can be studied
by physicians . MR images come from signals
originating from protons in the human body .
Almost all of the signals come from one or two
sources : water protons or from the protons in fat
molecules . In a strong magnetic field, these
protons act like little magnets, with a tendency
to line up with the main magnetic field . When
these protons are excited by radio – frequency
energy, give off energy in the form of radio waves
of the same frequency, which is usually received
by the same coil that transmitted the original
pulse . The signal intensity from different
tissues is dependent on different tissue
parameters, like T1 , T2 and proton density in a
particular bit of tissue . These parameters are
inherent to the tissue. Different tissues are
different in one or more of these parameters and
these differences are the basis for tissue
contrast in MRI . Using appropriate pulse
sequences we can generate images that bring out
these differences . Finally, very sophisticated
electronic hardware and computer soft ware are
used to produce images that can be studied by
physicaus .
When
might MRI be needed ?
Your
physician may request an MRI study for a number of
reasons. MRI is often used to obtain specific
diagnostic information not already provided by
other imaging technologies such as ultrasound,
computed tomography, and nuclear medicine . MRI
has commonly been used as an important diagnostic
tool in medicine . MRI can be considered as the
best neuroimaging modality . It has also been very
useful in imaging of joints, musculoskeletal
structures, pelvic and abdominal cavities,
mediastinum , heart and breast . Thanks to
different developing hardware and software, MRI is
able to delineate vascular, biliary and urinary
tract systems, effectivly and noninvasively
.
What else is MRI
used for ?
In addition to
providing anatomical pictures, research is showing
that MRI potentially has other uses . For instance
with MRI spectroscopy, it is now possible to
measure the amounts of various chemicals within
the brain, which gives information about the
health and viability of brain cells . This helps
to determine how and where in the brain these are
affected in different pathologies
.
How should I prepare for the
procedure ?
You may eat or
drink as usual unless the MRI examination requires
the injection or ingestion of a contrast agent for
image enhancement, if sedation or anesthesia is
necessary, or if there are other special care
considerations . In these situations, specific
dietary restrictions and other instruction are
provided by your physician in advance of the
appointment . It is very important to adhere to
these instructions to insure your safety while
undergoing the MRI examination . Otherwise , the
procedure will be rescheduled to a time and date
that allows for the appropriate preparation . It
is also important to notify the MRI staff of any
active illness, allergy or previous drug reaction
that may prevent the patient from undergoing the
examination safely
.
What will happen
during the procedure ?
Upon
arriving at the MRI suit your appointment and
registration are confirmed by the MRI scheduling
coordinator .You will be asked to fill out the MRI
screening questionnaire . The MRI nurse then asks
a few questions and does a clinical screening
examination . The patient must remove all metal
objects ( e.g. jewelry ) and electronic devices (
e.g. watch ) and change into hospital attire or
wear clothing from home that is comfortable and
free of metal ( zippers , buttons , etc ) . The
nurse then prepares you for the MRI procedure,
which may include oral or intravenous sedation by
the radiologist, or anesthesia by an
anesthesiologist . The MRI Technologist positions
and secures you comfortably on a narrow bed within
the large, doughnut – shaped magnet and adjusts
the radiowave coil . The technologist then
performs the MRI examination with the radiologist
and is in contact with the patient at all times .
During the MRI procedure, the patient will hear
several series of repetitive pulsing noises . It
is particularly important for the patient to
remain still during these noises since the MRI
machine is obtaining images at those times . If
the first pictures are motion – free, this will
help shorten the total exam time and prevent
delays . An entire MRI examination may take from
twenty minutes to one and a half hours depending
on the type of information required by the
radiologist and your physician . All patient
undergoing sedation or anesthesia receive
continuous vital monitoring and support
.
Will I feel anything ?
If an injection is required, you may feel
minor discomfort from the needle . MRI itself is a
painless procedure, in that the patient is not
touched . Rather loud pulsations are routinely
heard but can be reduced with ear plugs or
earphones provided by the MRI staff .
Occasionally, claustrophobia is a problem . If
there is concern about this possibility, the MRI
staff should be contacted in advance of the
appointment for information and assistance . MRI,
like other imaging technologies in radiology, is
FDA approved for its safety and effectiveness .
MRI has been used to image patients since 1981 and
no adverse biological effects have been reported
from exposure to the magnetic fields or radiowaves
.
What will I experience during a scan and how long does the scan take ?
You will be asked to lie on a table and
this table will slowly move to the center of the
magnetic field . The scanner is a long tube opened
at both ends . The part of the body being scanned
must be in the middle of this tube . Some patients
find that they can be somewhat claustrophobic
during the scan . However, the Philips scanner has
a very short bore and claustrophobia is not
usually an issue . The scanner generates loud
knocking noises during the scan so the staff will
provide earphones during the procedure . You can
listen to your favorite radio station or CD . The
scan times are very short and every consideration
will be taken to make you comfortable during the
procedure . The procedure usually lasts 15 – 30
minutes and you must lie very still . Some
patients like to take a light sedative or pain
medication before they arrive for the test
.
What happens after the images are
obtained ?
After the images
are obtained, you are free to go and resume your
normal activity .
When will I get
the results ?
The exam will be
reviewed by your radiologist shortly after the
completion of your exam . Some procedures require
post processing and can take some time to complete
. So a report is not alway readily available .
Then, the images and report will be sent to your
referring physician within 24 hours of the
procedure . A complimentary CD-ROM or x-ray film
copies of the exam are available to you or your
doctor .
Are there any risks
involved ?
MRI is a painless
imaging procedure that has the advantage of not
using any x – ray radiation . There are no known
side effects of the scan . However, since the MRI
scanner is a large magnet, it is very important to
notify the technologist of metal implants and it
is necessary to remove all metal objects such as
keys and watches before entering the scan room . A
screening form must be completed before starting a
scan . In some cases, the exam cannot be performed
due to certain implants . The most common
contraindication for scanning are :
• Cardiac pacemaker
• Implanted
defibrillator
• Brain aneurysm
clips
• Metal in the eyes
• Implanted devices , such as insulin
pumps
• Certain types of ear implants
• Other contraindications
include the
hardware and support devices accompanying some
patients . Some ventilators and monitors are MR
compatible , whereas others are not . Credit
cards, wrist watches, beepers , and other metal
objects should be removed before the patient or
staff enter the scan room . The technologist will
give you a more complete list and can answer any
questions you may have before the scan . The MRI
exam may require an injection of a contrast
material . The most common contrast is called
gadolinium . This basically does the same thing
for MRI as iodine does for CT scans . The
complication rate for this contrast is very low
.
Contrast agents
for Magnetic Resonance Imaging :
The alteration of signal intensity in
diseased tissue forms the basis for MR imaging in
diagnostic radiology . Tissue signal intensity
observed in MR images is the result of a complex
interaction of numerous factors , which can be
classified as those that reflect intrinsic
properties of biologic tissue , e.g. , T1 and T2
relaxation times and proton density , and those
that are equipment related , e.g. , field strength
or pulse sequences . However, due to a wide
biologic variation, relaxation times of normal and
abnormal tissues overlap . This limits the ability
of plain MRI to detect and characterize abnormal
tissues . By using MR contrast agents, you can
alter tissue relaxation times in favor of better
characterization of abnormal tissues . One of the
commonly used extracellular contrast agents is
GD-DTPA . Due to the high hydrophilicity of the Gd
chelates and their low molecular weight, they
rapidly diffuse into the interstitial space after
IV injection and a short intravascular phase . The
protein binding is negligible . The elimination of
the unmetabolized Gd complexes from the body
occurs via renal excretion and with a plasma
half-life of about 90 min . The compounds are
completely eliminated after a maximum of 24 hr if
the glomerular filtration is not diminished . The
half-life is prolonged in patients with impaired
renal function, but, this does not change the
safety profile. It can be assumed that this class
of contrast agents is by far the safest compared
with other contrast agents . An overall incidence
of adverse reactions is 1-2% . This incidence is
about two or three times higher in patients with a
history of allergies or in patients with asthma .
The most frequent adverse reactions are as follows
: Nausea / vomiting , local warmth / pain ,
headache , paresthesia , dizziness , urticaria ,
focal convulsion ; all being less than 0.5% . The
most relevant adverse reaction which may occurs
after IV injection of Gd compounds is an
anaphylactoid reaction that also occurs with other
contrast agents . The incidence of anaphylactoid
reactions is about six times lower than with
nonionic X-Ray contrast agents . As far as it is
known, there is no relationship between adverse
reactions and doses of up to at least 0.3 µmol/kg
BW . Also, at no time and in no patient was there
an effect on renal function that was attributed to
the Gd injection in the standard doses . Nowadays,
different kinds of contrast agents are available .
There are also other contrast agents which are more tissue specific, particularly the liver or
lymph node – specific contrast agents
.
Pregnant
patients :
As yet, there are
no known biological effects of MRI on fetuses .
However, there are a number of mechanisms that
could potentially cause adverse effects as a
result of the interaction of electromagnetic
fields with developing fetuses . Cells undergoing
division, which occurs during the first trimester
of pregnancy , are more susceptible to these
effects . It has been suggested that any
examination of pregnant patients should be delayed
until the first trimester and then a written
consent form should be signed before the
examination, by the patient . Pregnant patients or
those who suspect they are pregnant should be
identified before undergoing MRI, in order to
assess the relative risks versus benefits of the
examination . Gadolinium enhancement is at present
best avoided when examining a pregnant patient .
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