Magnetic Resonance Imaging (MRI) of the Prostate
What is Magnetic Resonance Imaging (MRI) of the Prostate? Back to Top
Magnetic
resonance imaging (MRI) is a noninvasive way of viewing organs, soft
tissues, bone and other internal body structures without the use of
x-rays. MRI uses a magnetic field and radio waves together with a
computer to create cross-sectional pictures of various parts of the
body. Detailed MR images allow physicians to better evaluate parts of
the body that may not be visible with other imaging methods such as
x-ray, ultrasound or computed tomography (also called CT or CAT scan).
MRI has proven valuable in diagnosing a broad range of conditions,
including cancer, heart and vascular disease, stroke, and joint and
musculoskeletal disorders.
The prostate is a walnut-size gland that is part of the male
reproductive system. It is located below the bladder, where urine is
stored, and surrounds the urethra, the tube that carries urine and
other fluids out of the body. The prostate helps make the milky fluid
called semen that carries sperm out of the body when a man ejaculates.
Ultrasound and MRI are the most commonly used techniques to image the
prostate gland.
What are some common uses of the procedure? Back to Top
The three most common prostate problems are:
- Infection (prostatitis)
- Enlarged prostate, called benign prostatic hyperplasia (BPH)
- Prostate cancer
The
prostate tends to grow bigger with age due to the development of BPH. A
tumor or infection can also enlarge the prostate. All of these
conditions may squeeze the urethra, causing urinary problems.
Diagnosing a prostate problem may require a series of tests
including an MRI exam, which can help identify abnormalities. If a
biopsy has confirmed cancer, a doctor may use MRI or other imaging
techniques to determine if the cancer is confined to the prostate, or
if it has spread outside the walls of the prostate gland and elsewhere
in the body.
How should I prepare for the procedure? Back to Top
Your MRI exam may involve
the use of an endorectal coil, a thin wire covered with a balloon,
placed inside the rectum. This coil helps focus on the prostate and
surrounding structures; it also enables your radiologist to perform
magnetic resonance (MR) spectroscopy, which provides additional
information on the chemicals present in the body's cells.
To prepare for an MRI with the endorectal coil, you should eat light
meals on the day prior to and on the day of your exam. This will help
make it easier to insert the coil. You may also be asked to use an
enema preparation prior to your exam. An enema involves injecting
liquid into your large intestine through your rectum to help clear the
bowel.
Prior to your exam, you may continue to take your usual medications,
unless you are told otherwise. You may be given a hospital gown to wear
during the exam, or you may be allowed to wear your own clothing if it
is loose-fitting and has no metal fasteners.
Jewelry and other accessories should be left at home if possible, or
removed prior to the MRI exam. Metal and electronic objects can
interfere with the MRI's magnetic field and are not allowed in the exam
room. These items include:
Jewelry, watches, credit cards and hearing aids, all of which can be damaged Pins, hairpins, metal zippers and similar metallic items, which can distort MR images. Removable dental work, pens, pocketknives, keys, coins and eyeglasses. In
most cases, an MRI exam is safe for patients with metal implants,
except for a few types. Tell the technologist if you have medical or
electronic devices in your body, such as:
- Artificial heart valves
- Implanted drug infusion ports
- Implanted electronic devices, including a pacemaker
- Artificial limbs or metallic joint prostheses
- Implanted nerve stimulators
- Metal pins, screws, plates or surgical staples
In
general, metal objects used in orthopedic surgery pose no risk during
MRI. However, a recently placed artificial joint may require the use of
another imaging procedure. If there is any question, an x-ray may be
taken to detect the presence of any metal objects. Sheet metal workers
and others who might have metal objects such as shrapnel in their
bodies may also require an x-ray prior to an MRI. Dyes used in tattoos
may contain iron and could heat up during MRI, but this is rarely a
problem.
People with the following implants cannot be scanned and should not enter the MRI area:
- Cardiac pacemakers
- Defibrillators
- Cochlear (ear) implants
- Clips used on brain aneurysms
Some
MRI procedures use oral or injected contrast material to help obtain
clearer images. For this reason, the radiologist or technologist will
ask if you have an allergy of any kind such as hay fever, hives,
allergic asthma, or allergy to food or drugs. However, the contrast
material used for an MRI exam, called gadolinium, does not contain
iodine and is less likely to cause an allergic reaction.
The radiologist should also know if you have any serious health
problems. Some conditions, such as kidney disease and sickle cell
anemia, may prevent you from having an MRI with contrast material.
What does the equipment look like? Back to Top
The traditional MRI unit is a
large cylinder-shaped tube surrounded by a circular magnet. The patient
lies on a moveable bed that slides into the magnet so the imaging can
be performed.
Other MRI units are open on all sides. These open units are
especially helpful for examining patients who are claustrophobic and
those who are very obese, but they may not provide the same image
quality.
How does the procedure work? Back to Top
Unlike conventional x-ray
examinations and computed tomography (CT) scans, MRI does not depend on
ionizing radiation. Instead, radio waves are directed at protons, the
nuclei of hydrogen atoms, in a strong magnetic field.
The strong magnetic field is produced by passing an electric current
through wire coils in the MRI unit. Other coils send and receive radio
waves. As you lie in the MRI machine, radio waves are directed at the
protons in your body. In the magnetic field, protons change their
position, producing signals that are detected by coils within the MRI
unit or placed around the body part being examined.
A computer processes these signals and generates a three-dimensional
representation of your body. The radiologist can then view any
two-dimensional plane (slice) from any direction on a video monitor for
evaluation.
Because protons are most abundant in water molecules (the "H" in
H20), MR images show differences in water content between various body
tissues. As a result, MRI is especially suited to detecting disorders
that increase fluid in the body, such as tumors, infection and
inflammation.
How is the procedure performed? Back to Top
If an endorectal coil is to be
used in your MRI exam, a nurse or physician will insert the wire coil,
which is enclosed within a small, flexible plastic tube, into your
rectum. Once inserted, a balloon is inflated to hold the coil in place
during the exam. When your exam in complete, the balloon is deflated
and the coil is removed.
You will lie on your back on a narrow table that can be moved into
and out of the MRI tunnel. Cushions are used to maintain the proper
position of your body and to make you as comfortable as possible.
The radiologist and technologist will leave the room while the MRI
examination is performed. An MRI exam generally takes 15 to 45 minutes,
although only a fraction of that time is needed for the actual imaging.
Imaging will be done in sequences, each lasting between a few seconds
and a few minutes. During imaging, you will be instructed not to move.
You will know when images are being recorded because you will hear
tapping or thumping sounds when the coils that create the magnetic
field are turned on. In between sequences, you will be able to relax.
If a contrast material is used to enhance the visibility of certain
tissues or blood vessels, it may be injected into your arm through an
intravenous (IV) line.
MR spectroscopy, which provides additional information on the
chemicals present in the body's cells, may also be performed during the
MRI exam and will add approximately 15 minutes to the exam time.
When your exam is completed, you will wait a short time on the table
while the images are examined to ensure no additional images are
needed.
What will I experience during the procedure? Back to Top
MRI of the prostate
is a pain-free procedure. During the exam, your prostate may feel
slightly warm, but this is normal and harmless. If contrast material is
injected, you may feel coolness for a minute or two.
If an endorectal coil is used, you will feel pressure in your rectum similar to that experienced during a digital rectal exam.
It is important to remain perfectly still when images are being
recorded. Some patients can find it uncomfortable to remain still
during MR imaging. People who tend to be nervous when in a confined
space (claustrophobic) may become uncomfortable when examined in an
enclosed MRI unit. Fewer than one in twenty patients will need a
sedative to relieve their anxiety and help them lie still. Another
option is to use an open MRI unit, if available, which is less
confining than a conventional unit.
MRI scanners are air-conditioned and well-lit. During your exam, you
will be able to talk to the radiologist and technologist, who will
observe you from an adjacent room throughout the exam. You may request
earplugs to reduce the thumping and humming noises of the MRI scanner.
Some scanners have music or a built-in television to help pass the
time. Many MRI centers allow a relative or friend to stay close by
during your exam.
If you have not been sedated, no recovery period will be necessary.
You may resume your usual activities and normal diet immediately after
the exam.
Who interprets the results and how do I get them? Back to Top
A radiologist,
a physician experienced in MRI and other radiology examinations, will
analyze the images and provide a report to your primary care or
referring physician. You will most often receive your results from your
primary care physician.
What are the benefits vs. risks? Back to Top
Benefits
- MRI is a totally noninvasive imaging technique that does not require exposure to ionizing radiation.
- MRI
provides more clear and detailed images of the soft-tissue structures
of the body than other imaging methods. The detail makes MRI an
invaluable tool in early diagnosis and evaluation of tumors.
- MRI images can help physicians evaluate the function as well as the structure of many organs.
- MRI
contrast material is less likely to produce an allergic reaction than
the iodine-based materials used for conventional x-rays and CT
scanning.
Risks
- The MRI examination poses no risk to the average patient when appropriate safety guidelines are followed.
- An undetected metal implant may be affected by the strong magnetic field.
- There
is a very slight risk of an allergic reaction if contrast material is
injected. Such reactions usually are mild and easily controlled by
medication. There also is a very small risk of skin infection at the
site of injection.
- If sedation is used, there are risks of
excessive sedation. The technologist monitors the patient's vital signs
to minimize this risk.
What are the limitations of MRI of the Prostate? Back to Top
Patients who are
claustrophobic may have to be sedated during an MRI exam or examined in
one of the newer open MRI units. A person who is very obese may not fit
into the opening of a conventional MRI machine.
The presence of an implant or other metallic object often makes it
difficult to obtain clear images, and patient movement can have the
same effect. A patient with severe pain may not be able to lie still
during imaging.
MRI may not always distinguish between tumor tissue and edema fluid. It cannot detect calcium present in a tumor.
The MRI findings by themselves do not establish an absolute
diagnosis, but in most situations the findings will suggest the correct
diagnosis. The images must be interpreted along with the patient's
history, physical findings, and information from other tests.
MRI may be more costly than other imaging methods including CT scanning. Back to Top |
