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Health Tip:
Imaging tests - How are they different?
Part 2 - MRI, Nuclear Scans, Ultrasound
Imaging tests produce "pictures" (images) of various parts of the body.
In producing these images, one of several forms of energy (X-rays, sound waves, radioactive particles, magnetic fields, etc.) is sent through the body.
In addition to showing normal body structure and function, imaging tests can demonstrate
abnormal processes such as cancer. Last week we looked at several different types
of radiologic procedures, such as x-rays. In today's Health Tip we'll consider three
other types of imaging studies, MRI, nuclear scans, and ultrasound.
 MRI
Magnetic
Resonance Imaging (MRI) uses a strong magnetic field to produce detailed images
of the body's organs and structures. Unlike CT scanning or x-rays, MRI is considered
to be free of radiation-associated risks. MRI is a particularly good imaging procedure
for evaluating soft tissue parts of the body that are less well seen with other
imaging tests.
The MRI scanner is a donut-shaped device that contains a magnet weighing
several tons. The person receiving the scan lies on a table within the "donut hole"
and is surrounded by a powerful magnet field. This magnetic force is so strong that
it causes protons within the nuclei of hydrogen atoms in the body to line up in
one direction. When they return to their original position, radio signals are produced
which are detected by the scanner. This information is then converted by a computer
into detailed black and white images of the area of the body being studied. Tissues
that contain little or no hydrogen, such as bone, appear black, whereas those tissues
that contain more hydrogen atoms produce a brighter image.
In part due to its expense,
MRI is most often used when information cannot be obtained with less expensive imaging
tests, such as x-rays or ultrasounds. It may be the test of choice, however, when
evaluating the brain or spinal cord for problems such as tumors or herniated discs.
It has also become a valuable tool for evaluating joints, such as the knee, hip,
or shoulder.
Nuclear Medicine Nuclear scans have a number of applications in medicine
including finding fractures in bones, looking for sites of infection, detecting
blockage to blood flow in the heart, and in looking for cancers that cannot otherwise
be found. They use substances called radionuclides that release low levels of radiation.
Body tissues affected by certain diseases, such as cancer, may absorb more or less
of the tracer than normal tissues. Special cameras pick up the pattern of radioactivity
to create images that show where the material travels and where it collects.
A bone scan involves injecting a radioactive material into a vein. The injected radiotracer
travels through the bloodstream and is attracted to areas where bone metabolic activity
is high, such as fractures, infections, or tumors. These areas are seen as "hot
spots" by a camera that records the image.
PET stands for positron emission tomography.
While a CT or MRI scan determines the shape or size of structures within the body,
the PET scan assesses function. Before the PET scan is performed, a radioactive
tracer is combined with a substance that is taken up by cells, such as sugar. This
radioactive "sugar" is then injected into the bloodstream. Cells within the body
will take up varying amount of this sugar, depending on how fast they are growing.
A special camera picks up rays from the areas with higher concentration of the radionucleotide
and creates pictures. Cancer cells, which grow rapidly, are more likely to take
up larger amounts of this radioactive sugar and will show up as brighter spots on
the PET scan image.
A thallium stress test is a nuclear imaging study that shows
how well blood flows to the heart muscle. It is done in conjunction with a treadmill
stress test. Near the end of a treadmill stress test, when the patient reaches his
or her maximum level of exercise, a radioactive tracer (thallium) is injected into
a vein. The thallium enters the circulatory system and travels to the coronary arteries
and heart muscle. If a portion of the heart is not receiving adequate blood flow,
less than a normal amount of thallium in the heart muscle is detected by a special
camera. An abnormal thallium test usually prompts the performance of an angiogram
to confirm blockage of the coronary arteries.
Other types of nuclear medicine scans
include the gallium scan
to look for "occult" infection, the radioactive iodine
uptake test
(RAIU) to check how well the thyroid gland is working, and the ventilation-perfusion
scan (also known as the V/Q or Lung scan) to look for evidence of a blood clot to
the lung.
Ultrasound An ultrasound device creates images
 called sonograms. Ultrasound
tests are probably best known for producing pictures of the developing fetus, but
they have a number of other applications in medicine also. In performing an ultrasound
test, high-frequency, inaudible sound waves are transmitted through the body using
an instrument called a transducer. As the waves bounce off of organs and tissues,
they create echoes which are detected and interpreted by a computer. This information
is displayed on a video monitor.
Ultrasound is very good at giving pictures of some
diseases of soft tissues that do not show up well on x-rays. In addition to obstetrical
ultrasound, other applications include evaluating the gall bladder, pelvic organs,
and breast lumps. In the evaluation of breast lumps, ultrasound is particularly
helpful in distinguishing solid tumors from cysts. It is also used to help doctors
know where to place a needle when performing a needle biopsy or aspiration of a
breast lump.
A special type of ultrasound known as the Doppler is able to evaluate
blood flow through the vessels. Doppler imaging is used to determine if blood flow
is restricted. Its most common application is with studying the blood flow to the
brain through the carotid arteries.
Certain imaging tests are better for detecting
specific problems than others. For example, a MRI is a much better test to evaluate
for a torn ligament in the knee than a CT scan. Whenever possible, doctors try to
order the least expensive imaging test that will provide the information that they
are after. Sometimes imaging tests are used in combination to gain additional information
or to confirm the presence of a suspected problem. An example of this would be the
use of a bone scan to confirm a fracture suspected on x-ray. While imaging studies
are widely used in evaluating for cancer, they often cannot distinguish between
benign and malignant. A tissue biopsy is typically required for a definitive diagnosis.
If you have questions about the many different types of imaging tests please sign into your account and send us your questions.
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