Spect Test

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Overview
A Single Photon Emission Computed Tomography
(SPECT) scan is a type of nuclear imaging test that
shows how blood flows to tissues and organs.
How does a SPECT scan work?
A SPECT scan integrates two technologies to view
your body: computed tomography (CT) and a
radioactive material (tracer). The tracer is what
allows doctors to see how blood flows to tissues
and organs.
Before the SPECT scan, you are injected with a
chemical that is radiolabeled, meaning it emits
gamma rays that can be detected by the scanner.
The test differs from a PET scan in that the
chemical stays in your blood stream rather than
being absorbed by surrounding tissues, thereby
limiting the images to areas where blood flows.
SPECT scans are cheaper and more readily available
than higher resolution PET scans.
The computer collects the information emitted by
the gamma rays and translates them into twodimensional
cross-sections. These cross-sections
can be added back together to form a 3D image of
your brain.
What does a SPECT scan show?
A SPECT scan is primarily used to view how blood
flows through arteries and veins in the brain. Tests
have shown that it might be more sensitive to brain
injury than either MRI or CT scanning because it
can detect reduced blood flow to injured sites.
SPECT scanning is also useful for presurgical
evaluation of medically uncontrolled seizures
(Figure 1). The test can be performed between
seizures (interictal) or during a seizure (ictal) to
determine blood flow to areas where the seizures
originate.
This type of scanning is also useful in diagnosing
stress fractures in the spine (spondylolysis), blood
deprived (ischemic) areas of brain following a
stroke, and tumors.
Single Photon Emission Computed Tomography (SPECT)
basic level
Figure 1: A SPECT scan of a patient with medically
uncontrolled complex partial seizures. The temporal
lobe on the left side of the brain shows less blood flow
than the right, confirming for the surgeon the
nonfunctioning area of the brain causing seizures.
How does the tracer work?
The radioisotopes typically used in SPECT to label
tracers are iodine-123, technetium-99m, xenon-
133, thallium-201, and fluorine-18. These
radioactive forms of natural elements will pass
safely through your body and be detected by the
scanner. Various drugs and other chemicals can be
labeled with these isotopes without changing their
properties.
The type of tracer used depends on what your
doctor wants to measure. For example, if your
doctor is looking at a tumor, he might use
radiolabeled glucose (FDG) and watch how it is
metabolized by the tumor.
Who performs the test?
A specially trained nuclear medicine technologist
will perform the test in the Nuclear Medicine
department of the hospital, or at an outpatient
imaging center. The nuclear medicine doctor will
review the images and report the findings.

How should I prepare for the test?
Wear comfortable clothing and be prepared to stay
for 1–2 hours.
What happens during the test?
First you will receive an injection of a small amount
of radioactive tracer. You’ll be asked to rest for
about 10–20 minutes until the tracer reaches your
brain. Next you’ll lie comfortably on a scanner table
while a special camera rotates around your head.
Be sure to remain as still as possible so that the
machine can get accurate pictures.
Once the scan is complete, you can leave. Be sure
to drink plenty of fluids to flush out any tracer left
in your body.
What are the risks?
The amount of radiation your body is exposed to is
less than you receive during a chest X-ray or CT
scan. Women who are pregnant or nursing should
not undergo a SPECT scan.
How do I get results?
The nuclear medicine doctor will promptly review
your images and communicate directly with your
referring doctor, who in turn will discuss the results
with you.
Sources & Links
If you have further questions about this diagnostic
test, contact the doctor that ordered the test or
visit:
www.radiologyinfo.org
www.nlm.nih.gov/medlineplus/
diagnosticimaging.html
Glossary
gamma rays: electromagnetic radiation emitted
during radioactive decay and having an extremely
short wavelength.
positron emission tomography (PET): a nuclear
medicine test in which tissue function can be
imaged. Damaged tissues have reduced metabolic
activity; therefore, gamma radiation from these
areas is reduced or absent.
photon: a particle that travels at the speed of light.
positron: an electrically charged particle that has
the opposite charge as an electron. It reacts with
an electron to produce gamma rays.
radiolabel: the technique of attaching, or
"tagging", a radioactive molecule to another
molecule (such as a protein) so that it can be
identified in the body. The radiolabeled substance
emits positrons that can be picked up by a special
scanner.
tomography: the technique of using rotating
X-rays to capture an image at a particular depth
in the body, bringing those structures into sharp
focus while blurring structures at other depths.
tracer: a substance, usually radioactively labeled,
which is injected into your body and can be
followed to gain information about metabolic
processes.
updated > 10.2004
reviewed by > Cheryl Stewart, MD

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